Engine/Transmission (1998.5 - 2002) Miss only with edge on, FIXED!

[ERuhl]

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 isn't enough. I attached a plot of total resistance vs. temperature for various resistors in parallel with the IAT sensor. As you can see, a 10k pot in parallel effectively bounds your IAT reading at a minimum of ~90°F. This is way too warm for a cold start. Even a 100k ohm resistor in parallel has a noticeable impact on the total resistance at cold temperatures. This is why I recommend a switch to completely remove the parallel resistor (infinite resistance).

Notes on reading the attached plot:

Recall the ECM is expecting a specific IAT calibration. Pick an actual IAT temperature and compare the resistance of any altered curve to the stock curve to back out what temperature the ECM sees. For example, an actual IAT temperature of 50°F with a parallel resistance of 20k ohms is equivalent to the stock circuit at ~80°F.
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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...

Now that would be nice, I didn't have any of those available at the local electronic parts store or I certainly would have gone that route.

 

[Gary - K7GLD]

10k isn't enough. I attached a plot of total resistance vs. temperature for various resistors in parallel with the IAT sensor. As you can see, a 10k pot in parallel effectively bounds your IAT reading at a minimum of ~90°F. This is way too warm for a cold start. Even a 100k ohm resistor in parallel has a noticeable impact on the total resistance at cold temperatures. This is why I recommend a switch to completely remove the parallel resistor (infinite resistance).

HMMmmm - one of us is missing something here as related to parallel resistance...



Or perhaps you are assuming I was suggesting a single setting of the potentiometer (which I was NOT!), rather than adjusting it as the engine warms up ( which I WAS!)?



FIRST, the potentiometer can be set from ZERO to 10K resistance - so the pot setting of 1K to match the fixed value you are already successfully using is no problem.



IF we next assume that 212 degree engine/IAT temp will generate a typical nominal 625 ohms as you supplied in the chart, placing the added potentiometer at it's maximum setting of 10K after the engine is warmed up will result in an effective parallel resistance of just slightly less than 600 ohms, or a IAT temp of somewhere slightly over 212 degrees - but close enough for our purpose for normal IAT and engine performance, I would think.



As another example, that same full-scale 10K potentiometer across the IAT at a temperature of 190 degrees - which with NO added resistance is 870 ohms according to your IAT resistance chart - now gives an effective resistance value of about 780 ohms, roughly half way between what would be 190 and 212 degrees with the stock IAT alone...



Just for grins, calculate the resulting ohm value of 10K ohms in parallel with 2400 ohms - which corresponds to an IAT equivelance of 140 degrees - and the resulting ohms value is about 1800 ohms, or the IAT equivalence of about 155 degrees - not a big deal, I wouldn't think...



Perhaps you assumed I was talking about setting the potentiometer at max, or some other value, and LEAVING it there?



I'm not - I AM suggesting setting the potentiometer at the setting required for proper engine operation when COLD, and setting it to it's HIGHEST ohm setting for starting or with a warm engine, which effectively electrically removes it from the circuit as far as IAT operation is concerned - and I doubt a switch would even be needed.



Correct me where I am wrong.

 

[ERuhl]

I AM suggesting setting the potentiometer at the setting required for proper engine operation when COLD

Which is a very high resistance value. As the plot shows, even a 100k ohm parallel resistance significantly reduces the combined circuit resistance at cold temps. Lower resistance = higher temperature to the ECM which is bad for cold starts. For cold starts you want the pot to disappear from the circuit (infinite resistance). With parallel resistors the combined resistance is never more than the component with the least resistance (and you only achieve that value if the remaining parallel resistors are set to infinite resistance). For example, if you set your 0-10k ohm potentiometer to 10k ohm then the combined resistance of the IAT/potentiometer pair can never be higher than 10k ohm. 10k ohm is approximately 80°F to the ECM and this is your best (coldest) case for cold starts. A reading of 80°F is too warm to support cold starts, but it gets worse. Since the IAT has less than infinite resistance we're subject to an even lower combined resistance. Let's say it's 32°F outside one morning so the IAT has a resistance of 34. 4k ohm. 10k in parallel with 34. 4k is only 7. 7k ohm which the ECM interprets as approximately 90°F... a far cry from the actual 32°F. Trust me, the engine does not like starting in this condition.

then setting it to it's HIGHEST ohm setting for starting or with a warm engine, which effectively electrically removes it from the circuit as far as IAT operation is concerned - and I doubt a switch would even be needed.

This is incorrect. When warm we want a LOW resistance and the stock reading is not sufficient... it's too high (results in the miss). Let's look at a condition where the IAT resistance is less than pot resistance of 10k ohm. At an IAT of 96°F the IAT resistance is 6. 6k ohm. 10k in parallel with 6. 6k is only ~4k ohm, or about 120°F as far as the ECM is concerned. This isn't warm enough, again Timbo and I needed a combined resistance of ~1k ohms to alleviate the miss. How do we fix it? Simple, reduce the pot resistance from 10k ohm to 1. 2k ohm. 1. 2k in parallel with 6. 6k is ~1k ohm.

IF we next assume that 212 degree engine/IAT temp

With a warmed up engine on a cold day (~32°F or so) I'd expect an IAT in the 90-100 degree range tops. I wouldn't assume it'll reach the same temp as the coolant (~190-212°F), in fact we know the stock IAT resistance is higher than 1k ohm (ie. colder than ~190°F) on a cold day or we wouldn't be having this discussion. Perhaps I misunderstood your statement in that regard? Maybe someone has some data on IAT vs. ambient air temp that they can share?

 

[Gary - K7GLD]

Which is a very high resistance value. As the plot shows, even a 100k ohm parallel resistance significantly reduces the combined circuit resistance at cold temps. Lower resistance = higher temperature to the ECM which is bad for cold starts. For cold starts you want the pot to disappear from the circuit (infinite resistance).

Which is exactly what I meant and SAID:

I AM suggesting setting the potentiometer at the setting required for proper engine operation when COLD, and setting it to it's HIGHEST ohm setting for starting or with a warm engine, which effectively electrically removes it from the circuit as far as IAT operation is concerned - and I doubt a switch would even be needed.

Clear enough?

With parallel resistors the combined resistance is never more than the component with the least resistance (and you only achieve that value if the remaining parallel resistors are set to infinite resistance).

YUP - all basic electronics! :-laf

For example, if you set your 0-10k ohm potentiometer to 10k ohms then the combined resistance of the IAT/potentiometer pair can never be higher than 10k ohms. 10k ohms is approximately 80°F to the ECM and this is your best (coldest) case for cold starts.

Which would be stupid - and NOT what I said! I would naturally set the pot to MAXIMUM ohm setting for starting a COLD engine, then reduce resistance to the value needed for best engine operation once the engine started - and THAT was exactly what I said!

A reading of 80°F is too warm to support cold starts, but it gets worse. Since the IAT has less than infinite resistance we're subject to an even lower combined resistance. Let's say it's 32°F outside one morning so the IAT has a resistance of 34. 4k ohms. 10k in parallel with 34. 4k is only 7747 ohms which the ECM interprets as approximately 90°F... a far cry from the actual 32°F. Trust me, the engine does not like starting in this condition.

I dunno WHY you keep harping on the 10K ohm as though it were a FIXED value - OR that I stated or intended to USE that setting for COLD starts - reread what I posted!

This is incorrect. When warm we want a LOW resistance and the stock reading is not sufficient... it's too high (results in the miss).

(SIGH!)



I KNOW that - reread what I SAID!

With a warmed up engine on a cold day (~32°F or so) I'd expect an IAT in the 90-100 degree range tops. I wouldn't assume it'll reach the same temp as the coolant (~190-212°F), in fact we know the stock IAT resistance is higher than 1k ohm (ie. colder than ~190°F) on a cold day or we wouldn't be having this discussion. Perhaps I misunderstood your statement in that regard?

THAT much is CERTAIN, pretty much in ALL regards! :-laf:-laf



It's clear that we both pretty well understand the electronis theory involved - and are having a simple problem comminicating our thoughts - and I STILL feel that using and setting a 10K ohm pot to about the 1K level you are using will correct the cold engine miss - AND that for STARTING a COLD engine, or operating a warmed up one, the full 10K setting COMBINED WITH the IAT's native resistance will still allow reasonable engine performance WITHOUT need to switch it out of the circuit - time will tell...

Maybe someone has some data on IAT vs. ambient air temp that they can share?

YUP - I've done lots of moving vehicle underhood temp testing - but NONE at the engine intake tract - but I do suspect the IAT and intake temps on a warmed up engine might be somewhat higher than you think...



Cheers!

 

[ERuhl]

Which is exactly what I meant and SAID:

Umm... are we reading the same posts? THIS is what you said:

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.

You've said this repeatedly, here it is again:

I would naturally set the pot to MAXIMUM ohm setting for starting a COLD engine

Taken in context with your self-imposed 0-10k ohm limitation, those statements are both FALSE. Yes you want maximum parallel resistance for cold starts, but 10k ohm ISN'T ENOUGH RESISTANCE! You do want it out of the circuit for cold starts, period, but out of the circuit means infinite resistance not a measly 10k ohm. I proved it to you with a pretty plot, I proved it to you with actual numbers, and I proved it to you with my own first-hand experience but apparently you are determined to disagree.

I dunno WHY you keep harping on the 10K ohm as though it were a FIXED value - OR that I stated or intended to USE that setting for COLD starts - reread what I posted!

Hey, don't get mad at me, the 10k ohm pot spec came from YOU. See your own text in the two quotes above (highlighted in red)! On more than one occasion you've identified this hypothetical 0-10k ohm potentiometer and your explicit intention to set it to 10k for cold starts. What I'm telling you, and what you're not hearing, is that 10k ohm is NOT ENOUGH RESISTANCE FOR COLD STARTS. It's not even close! Even 100k ohm is NOT ENOUGH RESISTANCE, so clearly your hypothetical 0-10k potentiometer isn't going to cut it without a switch to remove it from the circuit (infinite resistance).

I STILL feel that using and setting a 10K ohm pot to about the 1K level you are using will correct the cold engine miss

YES, by George he's got it #ad
. As I showed you're going to be setting it to around the 1. 2k ohm range.

AND that for STARTING a COLD engine, or operating a warmed up one, the full 10K setting COMBINED WITH the IAT's native resistance will still allow reasonable engine performance WITHOUT need to switch it out of the circuit - time will tell...

Doh!! Just when I think you're getting it, you go and miss the mark by a mile. First, the cold-start and cold-operation requirements are completely opposed. AGAIN, cold-start = infinite parallel resistance, cold-operation = ~1. 2k ohm parallel resistance. Run the numbers yourself since you clearly don't believe me, but an 80°F net reading (which is the coldest reading you can ever get with 10k ohm in parallel) is not going to work for sub-freezing cold starts.

YUP - I've done lots of moving vehicle underhood temp testing - but NONE at the engine intake tract - but I do suspect the IAT and intake temps on a warmed up engine might be somewhat higher than you think...

I can only relate my experience on a 3rd-gen which had a Juice w/attitude so my assumption is that the temp is relevant to the 2nd gen. Based on that experience, even on a warm day the IAT reading did not approach the CTS reading. Once again, feel free to prove this to yourself at your leisure.

 

[ERuhl]

Here it is in the simplest terms:

• Cold starts - The ECM needs to see a relatively high circuit resistance, 35k ohm or higher for sub-freezing temperatures.
• Cold operation - Once the engine is warm, the ECM needs to see ~1k ohm circuit resistance in order to prevent the "miss" that is associated with excessive injection timing in cold weather.

However you accomplish this is entirely up to you.

 

[Gary - K7GLD]

Here it is in the simplest terms:

• Cold starts - The ECM needs to see a relatively high circuit resistance, 35k ohm or higher for sub-freezing temperatures.
  
• Cold operation - Once the engine is warm, the ECM needs to see ~1k ohm circuit resistance in order to prevent the "miss" that is associated with excessive injection timing in cold weather.

However you accomplish this is entirely up to you.

Looks to me as though a 1 megohm pot in series with a 1K fixed, both connected in PARALLEL with the IAT.



Run the pot at it's full 1 meg for cold starting - then run it clear to zero for running.



That gives close to 35K ohms for starting, and 1K for cold temp running...



NO switch needed.



And I have a bunch of 1 meg pots...



Agree?

 

[Gary - K7GLD]

Only downside to the above, is the need for most of the potentiometer adjustability between hot/cold temps will occur at the LOW resistance end - so an audio taper pot wired "backwards" would be needed - will check my junk box to see what I have, and also Mouser Electronics...



It seems pretty certain my coming thermistor will be useless...

Run the numbers yourself since you clearly don't believe me, but an 80°F net reading (which is the coldest reading you can ever get with 10k ohm in parallel) is not going to work for sub-freezing cold starts.

You are correct - I find it hard to accept that "fooling" the ECM into thinking its 80 degrees or so rather than 32, is still not enough for decent starting - especially after using the intake heater grids...

 

[ERuhl]

Looks to me as though a 1 megohm pot in series with a 1K fixed, both connected in PARALLEL with the IAT.



Run the pot at it's full 1 meg for cold starting - then run it clear to zero for running.



That gives close to 35K ohms for starting, and 1K for cold temp running...



NO switch needed.



And I have a bunch of 1 meg pots...



Agree?

Sure, sounds like that should get you there.

I find it hard to accept that "fooling" the ECM into thinking its 80 degrees or so rather than 32, is still not enough for decent starting - especially after using the intake heater grids...

What heater grid? The ECM thinks it's 80°F, you don't get the heater grid. I tried cycling the grid repeatedly, and it's just not on long enough to make a difference. Now if the ECM thinks it's 32°F, like it actually is, then you get the nice long heater grid cycle.



You still haven't mentioned if you've confirmed that shifting the IAT signal will even alleviate your miss condition. Less than a dollar for the resistor, and a minute of your time to connect it, best of luck with whatever correction method you pursue.

 

[NoSeeUm]

Jim, have you noticed a IAT temp where the heating grid will NOT come on, when started?



Thanks, Paul

Not sure Paul, I seem to only notice when they come on, not vice versa. The weather is getting cooler I will try to take notice.



If you want to borrow my scan gauge you can for testing to see what temperature the ECM thinks the IAT is at when you get your stutter.



Send me a PM or call.



Jim

 

[NoSeeUm]

[*]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.

This is more less what I had in mind, but I was going to set about 90F in resistance in line for switch postion A and the IAT resistance in line for switch position B. I plan to remove the IAT from the manifold and hopefully install a second MAP sensor in its hole.



Not sure where I will mount the IAT sensor yet. I was thinking up near the engine side of the radiator, just so I still had some grid heater functionality. Like you say I could just as well discard the IAT sensor altogether and simply use resistors to fool the ECM. That actually might be the most predictable.



Jim

 

[Gary - K7GLD]

You still haven't mentioned if you've confirmed that shifting the IAT signal will even alleviate your miss condition. Less than a dollar for the resistor, and a minute of your time to connect it, best of luck with whatever correction method you pursue.

WELL, that was what cut my earlier cold weather problem experimenting short - to do heavy load/cold weather testing, you have to BE towing in cold weather, and we normally don't - that's the main reason we have only rarely experienced the effect. My primary interest in this thread is for guys who DO use their trucks in heavy service in cold climates, and yeah, for myself in those rare cases when I am affected.



The unknown issue with the 1 meg pot I suggest above, is what effect it will produce once the engine warms up if it is left in the circuit, and the OEM resistance curve is modified due to it's presence.



By the way, Mouser DOES carry 1 meg pots with audio taper - and an on/off switch! Oo.



OH - and for guys not familiar with the difference between audio and linear taper in a potentiometer, the linear taper is just that, 10% rotation of the knob produces roughly a corresponding 10% change is resistance. An audio taper is normally used in radio/TV volume controls, and the resistance is "bunched up" or more concentrated at one end of travel, so a small rotation at one end of travel will produce lots less change in resistance than the same rotation at the other end.



That might also be of value in this application, where a greater spread of resistance could be of value in making fine tuning of IAT resistance at one end or the other of engine operation vs temperature.



The reason the potentiometer in the circuit interests me, is the assumption that if timing and IAT resistance need "help" on the low temp end of the scale, it MIGHT also benefit from appropriate adjustment all across the operating range of temperatures.



In a manner of speaking, the 1K resistor "fixes" the cold weather miss - but the miss-timing is STILL there even once the engine warms up - so what additional adjustment benefit - fine tuning - might be of value even on a warmed up engine, or in warmer weather?

 

[NoSeeUm]

What minimum watt requirments for the resistors should I be looking at if any?



Jim

 

[Gary - K7GLD]

The half-watt jobbies should do the trick - and lots easier to work with than the smaller wattage versions...

 

[Gary - K7GLD]

HOOOKaaay - the thermistor has arrived - shows 1200 ohms in our freezer at a temperature of about 12* - and swings to 500 ohms at about 100* - didn't test further, since that would seem to cover the most pertinent temperature spread for our purpose...

 

[Gary - K7GLD]

ERuhl stated:

I purchased an IAT extension harness and modified it to place a potentiometer & switch in parallel.

I tried that outfit you pointed to, and was told the extension cable shown was for a Pontiac, and they had no clue as to other applications such as the Dodge/Cummins - you apparently found it to fit our Dodges? Or does it require modification to adapt?



I sure don't want to spend $20 bucks or so for the item + shipping, and then find it won't work properly! #@$%!

 

[WTBurke]

Gary,

I just received mine two days ago. It is very nice and works just fine. It was inexpensive and has the factory weather-boots. Yes the ZZP web-site is for Pontiac people but, I guess the IAT thing is some what standardized. Don't really know I'm just guessing.



I just relocated my IAT into the intake feed horn. It looks just as if Grandpa Chrysler did it originally. I plugged the hole with a proper metric bolt with a 22mm head. I cut off the excessive length with my Dremmel Tool. I applied a small o-ring to the bolt/plug and it seals just as if the IAT was in place. Tapping the air intake horn was a piece of cake.



William

 

[Gary - K7GLD]

THANKS for the info William - got my harness extender ordered, might consider relocating mine as well - keep us posted on any clear differences the relocation make...

 

[NoSeeUm]

I installed a 5. 6k resistor today. The IAT reads 100F. The truck runs incredibly quiet.



Jim

 

[Gary - K7GLD]

I installed a 5. 6k resistor today. The IAT reads 100F. The truck runs incredibly quiet.



Jim

Installed it permanently, or switchable in/out?



The "trick" in modding the IAT primarily for best cold temperature operation, is that of determining the closest resistance to what's needed for best OVERALL engine performance AFTER the engine warms up - power, economy. etc. Not sure exactly what, other than general engine sounds and power, to rely upon as a yardstick.