Note of caution, do not simply replace the IAT sensor with a static resistor or you will have difficulty starting the engine in cold weather. This is why I use a switch so I can have the stock circuit for startup. Then flip the switch if needed and adjust the pot to dial out the miss. You can hear the timing change in the engine when doing so as it becomes noticeably quieter when you retard the timing.
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Engine/Transmission (1998.5 - 2002) Miss only with edge on, FIXED!
- Thread starter timbo
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I find this matter interesting and I know of several members who have relocated the IAT sensor for different reasons.
1) In the stock location it can become fouled if or when an exhaust break is being operated.
2) Others have indicated that because of the location the temperature being sensed by the IAT is heat-soaked by the engine. Hence they move them to a more remote area of the air intake system to read a more true air temperature.
I believe Marco moved his to the air-horn intake by tapping the aluminum. I've recently seen some of the aftermarket intakes that have optional holes tapped and plugged for optional reasons.
I think most would agree that the location of the IAT is a slight PITA to R/R for cleaning but, in this thread, as I have reviewed... this could lead to additional complexities if one were to move the IAT to a "cooler" spot in the system?
Opinions???
William
1) In the stock location it can become fouled if or when an exhaust break is being operated.
2) Others have indicated that because of the location the temperature being sensed by the IAT is heat-soaked by the engine. Hence they move them to a more remote area of the air intake system to read a more true air temperature.
I believe Marco moved his to the air-horn intake by tapping the aluminum. I've recently seen some of the aftermarket intakes that have optional holes tapped and plugged for optional reasons.
I think most would agree that the location of the IAT is a slight PITA to R/R for cleaning but, in this thread, as I have reviewed... this could lead to additional complexities if one were to move the IAT to a "cooler" spot in the system?
Opinions???
William
A cooler location would certainly make this particular condition worse. When I was looking for relocation options I was looking for warmer locations. I stayed with the stock location primarily so the ECM can see a working intake grid heater, although additional reading on that seemed to indicate it was a non-issue.
I suppose that's the difference between us - you have, as you put it:
Personally, my nature is to not simply accept the "easy fix" just because it seems to work, but rather, try to understand the real underlying cause, and if possible or practical, try to treat THAT instead of its resulting symptom.
Please don't take this as a "dig" - it's not, only a comment meant to explain my own curiosity on a issue or "fix" you and a few others have already accepted as adequate.
Your theory apparently is that the entire root cause of the problem relates to the position of the camshaft or it's tone ring in relation to the sensor pickup - you may be absolutely correct - I'm not so sure.
Bear with my memory here, but It seems to me others HAVE tried mechanically changing injection pump timing by physically rotating the VP-44 in its mount by several different methods - only to discover the pump and electronic control immediately compensated for that shift, and returned to the programmed timing.
Apparently the VP-44 and related computer controls use the cam sensor primarily as only a BASIC timing point, but then uses that and other computer inputs to arrive at the PRECISE injection timing - if that is so, the cam timing sensor input is NEEDED as primary (rough?) reference, but not the total or ultimate defining element in controlling fuel injection timing.
Sure, I am a little late into this discussion on this board, but a few years back on a different board, and when I first was exposed to this issue, was pretty deeply involved in digging thru it's cause - The IAT function was considered as a cause, but as warmer weather arrived, further experimentation was dropped - and I personally had not been in a cold weather towing situation since then, and no longer active on that board.
OH, and by the way, I *do* drive the truck in cold weather - but the random miss/bucking is not evident when driving the truck unloaded or not towing.
BUT, regardless of whether the root cause IS the camshaft/sensor - or possibly tolerance variation in the electronic computerized control (my personal theory!), the easiest fix for most IS the resistor addition - but I already have so much extra added wiring garbage on my truck I'm reluctant to add yet ANOTHER switch and/or potentiometer to the heap that requires active monitoring or control.
I think my best approach will be to install a timer circuit that will have a small delay to allow normal engine starting, then activate a fixed resistance value to to the IAT for a set period of time, then be switched out of the circuit once enough time has passed for the engine to have reached operating temperature - all out of sight and automatic in operation.
I already have just such a device used for the VP-44 cooling setup I have installed on my truck, The circuit board seen here on the right:
#adOR, possibly an easier solution is a simple relay keyed to the ignition that electrically removes the added resistor from the IAT sensor, and once the engine starts, closes the contacts to add the resistor - and the resistor itself MIGHT be varistor/thermistor that provides the desired value when cold, but changes as the temperature increases within the 30-200 degree range to electrically "remove" itself from the circuit - I'll have to check to see what value varistors are commonly and easily avaiable.
Just in the "thinking" stage at this point - more to follow...
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This is true, I prefer the KISS principle and clearly you favor more elaborate solutions. Fwiw I did look into modifying the CAS signal and the electronic approach is not something I'm capable of doing while the mechanical approach (pull the cam and modify the sensor ring) is not something I'm willing to do.
Actually we feel the same in that regard. All of my investigative efforts pointed to injection timing and, as you indirectly addressed, the only way to shift the base timing is via modifying the keying of the sensor ring or by "fooling" the ECM into shifting it via the IAT sensor. Fwiw I used to have a TTPM which has a timing-only setting. With stock fueling and the added timing it had the miss, with the additional timing removed it didn't. Timbo spent some time driving around with a potentiometer in place of the IAT sensor so he could dial in the resistance needed to eliminate the miss. I tried that resistance and it worked so that's a good check. With the potentiometer in place I can adjust the resistance as needed. FYI discussions with Edge revealed that base timing varies slightly from one truck to another and this happens sometimes. It's infrequent enough that they will not change their product.
Correct, you cannot shift base timing via the VP44. It is referenced to the CPS sensor and the ECM will correct for the "error". Instead you need to shift the CPS sensor. One approach which has been demonstrated is modifying the sensor ring on the camshaft. I don't have the thread handy, but surely some time with the search feature can dig it up. My suggestion to you is to read up on what has already been done. No sense spending hours of your time rediscovering what is already known, but on the other hand it sounds like you're the kind of guy that thrives on that sort of thing
Perhaps what you need to look at is the affect of the IAT sensor. What changes based on a cold or hot reading of the IAT sensor? Injection timing IS affected by the IAT sensor and fwiw retarding the injection timing has been proven to eliminate this miss under load.
I suspect you won't believe me, but it's still there. You just don't notice it because the engine isn't under much of a load. Try a hard acceleration up a hill to put the engine under some load and you'll feel it.
BUT, regardless of whether the root cause IS the camshaft/sensor - or possibly tolerance variation in the electronic computerized control (my personal theory!), the easiest fix for most IS the resistor addition - but I already have so much extra added wiring garbage on my truck I'm reluctant to add yet ANOTHER switch and/or potentiometer to the heap that requires active monitoring or control.
My suggestion is first confirm that shifting the timing alleviates the condition (ie. when it's acting up unplug the IAT sensor and swap in the resistor and see if the miss goes away). Once you've confirmed that, then look for convenient ways to implement it. If at all possible I recommend the KISS principle.
oh, I absolutely believe you - it's just that my normal non-towing driving characteristics are not aggressive enough to cause the bucking effect to show itself.
And that's another puzzle in my case - if shutting down the Comp would stop the cold temp bucking, that's all I'd need do until the engine warmed up - but in my case, shutting off the Comp did NOT stop the bucking!
Undoubtedly another dumb question, but have you tried experimenting with a precise potentiometer setting that will eliminate your miss - but still allow normal engine starting? (yeah, you undoubtedly have!
) Sure would be great to be able to find a SINGLE resistance value that would work, and not need the extra wiring and switch... I've contacted an outfit that supplies thermistors, inquiring as to what they might have that fits what I'm trying to do - will report what their reply is...
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Undoubtedly another dumb question, but have you tried experimenting with a precise potentiometer setting that will eliminate your miss - but still allow normal engine starting? (yeah, you undoubtedly have!
We looked at a fixed resistance and the requirements don't close. You need the cold IAT resistance for cold starts (yes I tried cold starting with the resistor, hard to start and even then it sounds absolutely aweful) but a hot IAT resistance once it's warmed up so it won't miss.
On paper (excel actually) I looked at the effects of a resistor in parallel (with and without various additional resistors in series) to shift the IAT calibration to something I thought would work based on cold-start and hot-operation resistance requirements. I also searched for an alternative IAT sensor with a different/shifted calibration (so that it would read warmer than actual, but still vary with temperature). There is a GM IAT sensor that has the same connector with a shifted calibration but it didn't work out. The shift was a little too much when cold (reads too warm so it was still hard to start) but not quite enough when warm (miss was better, but still there). In the end my "best of both worlds" was a switch between the stock circuit, which I run all summer btw, and one with a pot in parallel. It's starting to get cold here in CO, but haven't needed it yet
I also think it is due to the variation in the tone ring "slot" machined into the camshaft. Some trucks have more timing from the factory than others. Mine is one of them. I also have been using the IAT fooling method for a year or two. As injectors get bigger its more of a problem. Another posible solution is to lower your compression by using a thicker headgasket.
I know some have tried to modify the tone ring slot in an effort to change the timing, however it is a trial and error process as the cam needs removed to do it.
As of right now, on a warm day, with my current huge injectors and the IAT temp fooled I still get the bucking at lower rpms (less than 2000 rpms). Can't wait to switch back to my winter injectors. The next time the head is off, its getting a bigger headgasket too.
I know one individual that actually fooled the IAT the opposite direction (colder) to gain timing for more hp on the dyno. Running a timing box as well. Laid down 740 hp on #2 alone...
My thoughts:
First off, another good discussion. and I should state that I am only able to detect cold weather timing changes only by engine sound changes.
My theory about advanced cold weather timing is some what different. Here is what I think and why.
I use a ScanGauge which reports the temperature that the ECM is seeing for both IAT and EWT (Engine Water Temperature).
What I notice is that the cold weather timing does not fully dis-engage when the IAT comes up. What I do see is that IAT will get to around 80-90F under most conditions fairly quickly after engine start. Specifically at idle or low power levels.
If the ECM were advancing the timing strictly based upon IAT it would likely also retard the timing fairly rapidly, but it does not. In fact, what I typically notice is that timing advance does not retard until after the EWT reaches normal operating temperature. I also notice that the timing does not get retarded immediately even after EWT is at normal operating temperature. It seems like maybe a timer of some sort is in play.
If I am using the block heaters the IAT and the EWT both will be 100F at startup. Under those conditions I still notice the advanced timing at start up. In fact, I notice it occuring for most starts almost any time of year. Another quirk for the IAT in this regard is that even if there is no WTS light, yet the grid heaters will still cycle after start even though IAT has never gone below 80F.
All this information kind of leads me to believe other factors are in place that I don't fully understand.
These other factors could be OAT, EWT, VP44 temperature sensor, engine startup timer or even possibly simply just the combustion dynamics of a cold engine.
I am not fully understanding this phenomena is an understatement, so any KISS method seems best for me. I plan on a few IAT bombs similiar to Timbo and ERuhl mostly to test out a few theories and to see if I can get my engine to run quieter.
Again like I said earlier, I have never had any timing stutter except when I was experimenting with the Fuel Saver SW. Which from what I can guess adds allot of base timing, if nothing else judging by the radical change in engine noise. I am estimating my truck would be in the 450+ Hp range @ around 40-55 psi of boost so that is also quite a bit of fuel and air. Under WOT conditions the ECM's timing advances as well. So running the Fuel Saver SW the base timing plus the ECM timing advance that was just too much. I only repeated the test a few times, fearing my head gasket would fail.
The stutter seem to occur in the 2000 to 2500 rpm range as I recall, but I am not sure. It only happened in 4th and 5th gears while I was pretty hard in it. The combination of "all of the above" was how I got in that condition. Just my SWAG, but that is what happened.
Jim
First off, another good discussion. and I should state that I am only able to detect cold weather timing changes only by engine sound changes.
My theory about advanced cold weather timing is some what different. Here is what I think and why.
I use a ScanGauge which reports the temperature that the ECM is seeing for both IAT and EWT (Engine Water Temperature).
What I notice is that the cold weather timing does not fully dis-engage when the IAT comes up. What I do see is that IAT will get to around 80-90F under most conditions fairly quickly after engine start. Specifically at idle or low power levels.
If the ECM were advancing the timing strictly based upon IAT it would likely also retard the timing fairly rapidly, but it does not. In fact, what I typically notice is that timing advance does not retard until after the EWT reaches normal operating temperature. I also notice that the timing does not get retarded immediately even after EWT is at normal operating temperature. It seems like maybe a timer of some sort is in play.
If I am using the block heaters the IAT and the EWT both will be 100F at startup. Under those conditions I still notice the advanced timing at start up. In fact, I notice it occuring for most starts almost any time of year. Another quirk for the IAT in this regard is that even if there is no WTS light, yet the grid heaters will still cycle after start even though IAT has never gone below 80F.
All this information kind of leads me to believe other factors are in place that I don't fully understand.
These other factors could be OAT, EWT, VP44 temperature sensor, engine startup timer or even possibly simply just the combustion dynamics of a cold engine.
I am not fully understanding this phenomena is an understatement, so any KISS method seems best for me. I plan on a few IAT bombs similiar to Timbo and ERuhl mostly to test out a few theories and to see if I can get my engine to run quieter.
Again like I said earlier, I have never had any timing stutter except when I was experimenting with the Fuel Saver SW. Which from what I can guess adds allot of base timing, if nothing else judging by the radical change in engine noise. I am estimating my truck would be in the 450+ Hp range @ around 40-55 psi of boost so that is also quite a bit of fuel and air. Under WOT conditions the ECM's timing advances as well. So running the Fuel Saver SW the base timing plus the ECM timing advance that was just too much. I only repeated the test a few times, fearing my head gasket would fail.
The stutter seem to occur in the 2000 to 2500 rpm range as I recall, but I am not sure. It only happened in 4th and 5th gears while I was pretty hard in it. The combination of "all of the above" was how I got in that condition. Just my SWAG, but that is what happened.
Jim
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FWIW I think that possibly Gary's problem seems based upon other factors as he is getting the stutter with the Edge off. I am thinking that the FCM in the VP44 might independently advance the timing based upon low fuel temperature.
Possibly Gary's mod's to cooling the VP44 confusing things?
Jim
Possibly Gary's mod's to cooling the VP44 confusing things?
Jim
Amen to that! :-laf
It's been over 4 years since my last expedition into this cold weather issue - and that one was cut short by the return of spring and summer temps. My own episodes are only when towing in freezing temps - and since we're "fair weather RVers", are not normally faced with the effect - at least not at a noticeable or objectionable level. .
Yeah, that's what I have pointed out a couple of times here - I've had the Comp since the truck was new. and only added other mods AFTER my first cold weather incident towing down in Texas and New Mexico - and even then, turning the Comp off didn't stop the stutter - but higher settings DID make it worse!
Now, I have added the larger injectors, and even with the Comp switched off, the stutter was as bad or worse than previously with the Comp on and set at highest level!
I suppose the addition of my VP-44 cooling MIGHT add it's own small degree of influence, but since that cooling was added long after my first encounter with the effect, doubt it's a major player.
Personally, I'm very happy to discover the progress others in this thread have made in at least theoretically locating the underlying problem, as well as a working solution to correct it.
At this point, I have to agree with ERuhl:
SO, until some possibly better solution arrives, will try the one he and others have used successfully - but will experiment with a relay/thermistor device that will be automatic in operation, other than possibly needing to be engaged/disengaged for the cold weather season - I've already ordered the thermistor for this project, and hope it's resistance range will do the job - more later as it progresses...
Here is another one for you.
Have you ever noticed the timing advance with light throttle higher RPM?
Say for example, the truck is 3rd just about to shift into 4th. A nice leisurely shift is what I am talking about. Just about the time the I start letting off the throttle the timing will advance. From the sound change it is pretty radically.
I wonder if the TPS, RPM and LOD (fueling rate) also have a significant effect?
The only reason I can think of this to happen is for emmisions, which might be the heart of all the other timing related issue(s) people are experiencing. On the other hand maybe its just a glitch or VP44 mechanically related.
Jim
Have you ever noticed the timing advance with light throttle higher RPM?
Say for example, the truck is 3rd just about to shift into 4th. A nice leisurely shift is what I am talking about. Just about the time the I start letting off the throttle the timing will advance. From the sound change it is pretty radically.
I wonder if the TPS, RPM and LOD (fueling rate) also have a significant effect?
The only reason I can think of this to happen is for emmisions, which might be the heart of all the other timing related issue(s) people are experiencing. On the other hand maybe its just a glitch or VP44 mechanically related.
Jim
YES, as a matter of fact, I notice it quite regularly, especially on a cold engine with the Smarty programming installed - even #2 that supposedly adds NO timing!
AND, since programmers and boxes obviously can ADD timing for improved performance, I'm wondering if a special cold weather program could be developed for the Smarty that would RETARD timing on one of it's settings for those of us with the cold weather timing issue?
For those considering the IAT mod, here's some info from a previous thread describing what I did:
Edit: While I'm at it, here's a table of IAT resistance vs. temperature
Since this is a topic of interest again hopefully that info will save some search time. The topic of relocating the IAT sensor has come up as well, and the IAT extension harness mentioned above would work for that as well.
Edit: While I'm at it, here's a table of IAT resistance vs. temperature
Code:
Deg F Ohms
32 34400
76 12800
96 6600
112 4800
122 3800
128 3000
140 2400
150 2100
160 1900
165 1700
190 870
212 625
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Circuit diagram
I was asked to clarify the circuit layout, so here's a quickie diagram showing the switch and potentiometer in parallel with the IAT sensor:
#ad
Again, with the switch open it's the stock configuration. Close the switch and the potentiometer is placed in parallel with the IAT sensor. The equation for determining the combined resistance of the potentiometer and the IAT sensor (ie. what the ECM sees) is:
#ad
Refer to the IAT calibration info in my previous post to convert resistance to temperature.
Once again, if you purchase an IAT extension harness you can tap into that to place the switch/potentiometer in parallel with the IAT sensor while leaving your stock harness unharmed. That's up to you.
I was asked to clarify the circuit layout, so here's a quickie diagram showing the switch and potentiometer in parallel with the IAT sensor:
Again, with the switch open it's the stock configuration. Close the switch and the potentiometer is placed in parallel with the IAT sensor. The equation for determining the combined resistance of the potentiometer and the IAT sensor (ie. what the ECM sees) is:
Refer to the IAT calibration info in my previous post to convert resistance to temperature.
Once again, if you purchase an IAT extension harness you can tap into that to place the switch/potentiometer in parallel with the IAT sensor while leaving your stock harness unharmed. That's up to you.
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One final note, the circuit above is just one way to skin this cat. Here's a few alternatives:
- Use a fixed resistor (~1k ohm, depends on what your truck needs) in place of the potentiometer in the circuit above. You lose the variable resistor, but it's easier to mount. In the circuit above I used a project box to mount the switch and potentiometer, but a switch by itself can be mounted anywhere.
- Use the potentiometer alone in place of the IAT sensor. Simply dial in the resistance you need for the given situation (>34k ohms for cold start, ~1k ohm when it's warm). Fwiw this is what Timbo used to determine the resistance needed to alleviate the miss in his truck.
- Use a double throw switch with two resistors in place of the IAT sensor. One switch position / resistor for cold starts and the other switch position / resistor for when it's warm.
- Use an auto-switching circuit as Gary described above.
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Good info and nice diagrams above!
As another potential fix for affected guys who are also using the Smarty, I have posted a request for possible programming for the Smarty that itself would provide a cold weather programming level for guys with this problem - I'd suggest those interested go to the "new generation Smarty" thread started by Bob Wagner, and add their comments and request.
As another potential fix for affected guys who are also using the Smarty, I have posted a request for possible programming for the Smarty that itself would provide a cold weather programming level for guys with this problem - I'd suggest those interested go to the "new generation Smarty" thread started by Bob Wagner, and add their comments and request.
Jim, have you noticed a IAT temp where the heating grid will NOT come on, when started?
My issue is this: When the weather gets colder, I end up with the timing surge, with the Comp box on. It actually is worse on low (1x2) settings, then on high (5x5). When drag racing as the weather get colder, the IAT is warm enough at the beginning of a pass (from heat soak), but during the run, the intake temp drops, and I will end up with timing surge at the top end of the track. The intake air horn actually feels cool, after a pass, and then warms up as the truck sits (heat soaks). If I run the Redline box (no timing), then the surge doesn't happen.
What I would like to try is to come up with an IAT value, that when using the Redline box, will advance the timing, but I don't want to go to far, and end up with the heater grids on for the 3 minutes, or 18mph period.
I would also like to come up with a value, that I can use with the drag comp, and reduce the timing. I'm going to try from 1K to 5K for that.
Thanks, Paul
Just one final addition related to ERuhl's post and diagrams up above.
First, I would think about a 10K potentiometer would give the low resistance needed for cold operation, but have enough total resistance to effectively be out of the circuit at it's maximum setting of 10K, eliminating the need for a switch. 10K in parallel with the max nominal IAT temp/resistance is close enough to the stock resistance value at maximum engine temp to be within reasonable tolerance range for the IAT to function properly after the engine is warmed up.
AND/OR,
Most potentiometers ALSO can be found that include their own off/on switch - so it would be easier to just use a single control for both functions...
First, I would think about a 10K potentiometer would give the low resistance needed for cold operation, but have enough total resistance to effectively be out of the circuit at it's maximum setting of 10K, eliminating the need for a switch. 10K in parallel with the max nominal IAT temp/resistance is close enough to the stock resistance value at maximum engine temp to be within reasonable tolerance range for the IAT to function properly after the engine is warmed up.
AND/OR,
Most potentiometers ALSO can be found that include their own off/on switch - so it would be easier to just use a single control for both functions...
A 10K resistance applied in parallel to the stock resistance at 32 degrees, would simulate a temp of about 92 degrees.
Paul
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