Engine/Transmission (1994 - 1998) less power at full throttle than 1/2 throttle

[Joe G.]

I rigged my fuel pressure gauge by tapping the banjo fitting in the side of the injection pump. I have a snubber and needle valve to keep the pressure spikes from wrecking the gauge sendind unit. There is a picture of it in my Reader's Rigs gallery. #ad

 

[paulsmith]

Well, I THOUGHT I had it fixed. To re hash, Truck is gutless. took it ot shop, all checked out, stumped and said he wanted to replace all the injectors as that was the "next step" in troubleshooting. I took it home, removed the AFC, found trash in the hose fitting, cleaned out and confirmed opration with regulated air source. Truck runs good again.



1500 miles later, and it is back to the same thing.



My truck has the EGR (which is blocked off now) if that makes a differance. My AFC connection goes like this: Short 1/8" POLY tube from intake Manafold to tee on back of AFC. Poly tube screws onto brass fitting with . 030 orafice in it (where it got stopped up before) which is screwed into the tee. The tee feeds this boost pressure to both the AFC, and is teed off to a long stell line that runs over to the waste gate.



To check it, I took loose the line to the waste gate and put a gauge it. this should give me a actual reading of pressure that the AFC is seeing, as this is "Down stream" of the same orafice.



At 55 mph floored, converter locked, I get 7 psi of boost



Truck runs great, smooth, etc, just no power. This is my "hired drivers" truck, so it is stock, and no gauges, etc. I just want it to run stock... .



I am going to try drilling and taping the fuel line fitting for 1/8" NPT and installing a temporay gauge soon... (when I I have time!!!!)



Any other suggestions? What did I do to make it work for 1000 miles?

 

[Joe G.]

A friend of mine had an AFC problem with his '97. He replaced the AFC with one from a '95. Any non-CA (non-EGR) AFC should work if replacing the fittings on yours does not work.

 

[Dave M]

A friend of mine had an AFC problem with his '97. He replaced the AFC with one from a '95. Any non-CA (non-EGR) AFC should work if replacing the fittings on yours does not work.

Joe,



I'm new at this but from what I've read... . could the fuel overflow valve cause this type problem ? Like you've said, it makes sense to get a fuel pressure gage to determine what the pressure is when the low boost occurs.



How about fuel return/supply line problems? Or am I off base on this because it only occurs at high boost and the overfow valve might always show low power all the time?



I'm just trying to understand how the boost, AFC, and fuel pressure are related... especially for this type of weird problem.



Thanks,



Dave

 

[EGILLILAND]

Joe, you mentioned pulling out the AFC to trouble shoot the problem. With your experience doing this... ... would you expect to see some higher than normal EGT's as well when running w/o the AFC? As an aside to this test, what are your thoughts about turning back the star wheel w/in the AFC instead of removing the whole unit?

 

[paulsmith]

from what I understand, if I remove the hose from the AFC, I will have no power, and would have symptoms like I am having. However, if I remove the entire AFC housing, and run the truck, this should give me lots of black smoke at take off, and would pin point a AFC problem, if power returns with it removed. I should Tape over the hole to prevent dust from geeting in the pump I suppose.

 

[Joe G.]

I have been off line for a few days. Trying to catch some salmon. Alas, the fish won.



Dave,



The OF valve is only concerned with fuel pressure. Fuel pressure and the AFC are independant things. Anytime there is a fuel problem, low power or whatever, you need to know fuel pressure. If it is low than a lot of symtoms may occur. It usually does not run well with low pressure. The description here is that the engine seems to run ok, but has no power.



EGILLILAND,



The AFC is ONLY in play before boost comes up to the proper pressure. So there should be no affect on EGT. Removing the whole part is a much more simple test. It takes the whole thing out of the picture. If it cures the low power problem then something is wrong with the AFC. If there is still low power then the AFC is probably ok. Messing with the star wheel is just changing an AFC adjustment. Once you know that the AFC is bad or good then that is the time to mess with adjustmentds.



Paul,

That is exactly how to do the test. It would be a good idea to tape over the hole for the test to keep crud out of it.

 

[Dave M]

Dave,

The OF valve is only concerned with fuel pressure. Fuel pressure and the AFC are independant things. Anytime there is a fuel problem, low power or whatever, you need to know fuel pressure. If it is low than a lot of symtoms may occur. It usually does not run well with low pressure. The description here is that the engine seems to run ok, but has no power.

Joe,



Thanks for educating me on that. I had a feeling I was off. Would a low power condition (like a low fuel pressure problem) cause the boost to be low or would boost still attain normal values? From what you are saying, it seems to me the answer is NO... the boost would be normal with the low power... they are unrelated. Does that mean I could still hit 20 psi boost with low power? That kinda seems weird to me. Just having trouble understanding this I guess.

EGILLILAND,

The AFC is ONLY in play before boost comes up to the proper pressure... .

Joe,

What level of boost gets the AFC out of the way?



Thanks,



Dave

 

[Joe G.]

Dave,



I'm not sure how much boost it takes to move the AFC. If the AFC does not move then the result is that the governor arm cannot move the rack very much. Kind of like having a block of wood under the throttle pedal. I don't think you can have enough boost to hit 20 PSI with a completely bad AFC. If this AFC is pushed all the way forward and other adjustments made it might be able to get up to 20 PSI boost. It takes fuel to make boost. Like I said above I would like to get my hands on this one to check it out. All I can do from this far away is to suggest tests to try to isolate the problem. Might be barking up the wrong tree, but some tests can point us in the right direction.

 

[Dave M]

Dave,



I'm not sure how much boost it takes to move the AFC. If the AFC does not move then the result is that the governor arm cannot move the rack very much. Kind of like having a block of wood under the throttle pedal. I don't think you can have enough boost to hit 20 PSI with a completely bad AFC. If this AFC is pushed all the way forward and other adjustments made it might be able to get up to 20 PSI boost. It takes fuel to make boost. Like I said above I would like to get my hands on this one to check it out. All I can do from this far away is to suggest tests to try to isolate the problem. Might be barking up the wrong tree, but some tests can point us in the right direction.

Joe,



Thanks for the reply. What you say is making sense to me. This is one for me to learn something from. Can't wait to see the results of more testing. Good luck Paul.



Dave

 

[paulsmith]

Well, Sunday and I fianlly got around to checking the fuell pressure. Tapped the fiitting at the pump 1/8" npt. 60 psi gauge. At idle I get about 17 psi of fuel pressure, at 60 mph cruise, about 20 psi, and floored at 65 mph, about 18 psi.



From memory, this seems low. Will this cause the 7 to 9 psi of boost floored at 50 mph, and total lack of balls????





I just replaced the spring loaded valve (not sure of the name) that the return line hooks to with a new one (had a spare). I have heard of people streathing the spring to get more pressure, but again, I had a new one, so I just swapped. No change! I will replace the lift pump if it is felt that would restore my power by increasing my pressure. I find it odd that fuel pressure remains pretty constant (within 5 psi) regardless of load or engine speed.

 

[paulsmith]

Well, I checked it again, I guess I had my "snubber valve" a little too tight, or it moved a little bit when I was changing the fuel over flow valve. Anyway, further checking with the new valve shows pressure at idle to be about 20 psi ,and in netural at 2200 rpm is about 26 psi, at 60 mph it is about 23 psi.



Also, further checking on this list shows that not to be too far from normal.



Paul Smith

 

[biggy238]

Most of the reports i've heard say that normal pressure is 31 at idle and down to 25 or 26 at throttle. With the miles you've reported, the lift pump may just be geting tired. I think even Piers said the same thing in a post I read about normal operating pressure.



I've spent part of the weekend trouble shooting a stuck overflow valve.



Good Luck on yours,

Kyle

 

[ABorchard]

As long as your fp doesn't drop below 18 psi it's fine

 

[paulsmith]

Well, I guess I will go out and pull the AFC housing. Of couse the EGR makes this that much more of a PTA. If this is the problem ,where would I go to get a replacment AFC? When I had it off last, I checked it for operation with 10 psi of air pressure and it worked back and forth with air.

 

[paulsmith]

Well, here we go again- Stock 96 3500 2wd auto:

To recap, truck has low power, Cant pull a 8000# load in OD, must run in 3rd, and still it drops to BELOW 40 mph on the gradual hills of the interstate 55 in St Louis.



I did the following:

Check for boost leaks- put rubber connector on turbo inlet and pressurised to 15 psi. Just like blowing up a tire- no leaks.

Replaced air filter and fuel filter.

Cut hole in bottom of Cat. Converter and knock out all the honeycomb. Block off plate on the EGR at intake.



Took it to Cummins shop, checked timing and set to 15 degrees, check fuel pressure, turbo, etc. All showed to be ok, recommended replacing all fuel injectors.



I thought that to be a shot in the dark. So I check the AFC, found a small piece of dirt in the boost line that could be the problem, cleaned it out, and slid the stock plate forward about . 010, and power seemed to return for a short time (I did not drive it loaded, but my driver felt it ran much better. )



One week later, I use the truck to pick up a load, and it "seems a little weak" but hard to tell. Within a few miles, it was obvious the truck was running just like before!



I assume that the afc is stopped up again (EGR dumping soot into my intake!) or low fuel pressure. Check fuel pressure, 20 to 26 psi. Today completely remove the AFC housing and drive truck with tape over the opening to keep the dust out. Lots of smoke if I nail it at a standng start, but still low on power.





I have two identical trucks, this one is stock, the other is bombed. Maybe I am just expecting too much rom a stock CTD???? Boost pressure with the waste gate hose unhooked is a measure of Horsepower correct? With it floored at 30 mph with engine reving to 2200 rpm, I am getting over 18 psi of boost, but as soon as the converter locks in, at 50 mph, rpms drop to 1500, and boost falls to 7 psi. As speed increases, boost will slowly climb, and by the time I am at 65 mph, I am running about 12 psi or better.



I know on my bombed truck, it will boost big at 1500 rpm, and feels to pull harder AFTER the converter locks up. This stock truck will loose power when the converter locks and engine rpm drop.



Is this normal boost pressure for a stock truck? If not, WHAT DO I DO? I leave out at 4:00 this afternoon on a 950 mile trip to pick up another load.



Paul Smith

 

[paulsmith]

Well, Its been a couple of days, no replies, So I guess I have Stumped the TDR!!!!!



A little more info. I made it back from the trip. Pulled a 5000# load, but 12 feet tall.



At 50 mph, when the converter locks in, (RPM drops to like 1400) boost floored is about 8 psi, at 55 mph, it is up to about 12 psi, and at 65 it is over 14 psi (around 1800 rpm) I am running a 15 psi boost gauge, and yet to peg it, and the waste gate hose is dissconnected from the turbo. Is this normal?



I was able to hold close to 65 mph with it floored, so long as was not in a big headwind, or even the slightest hill.



BTW, I slid the stock fuel plate up another . 030 when I had the AFC off, now it is almost fully to the front of the slots (towards front of truck)



I also noticed that I get the same boost reading whether I am at half throttle, or full throttle. Flooring the truck makes no differance.



What do I need to recheck? What else can I check? Who can I take it to to get it fixed? The local shops around here are a joke (Seneca, IL 61360)



Leaving out in the Morning For Detroit MI (about 450 miles) to pick up another load, then off to Arkansas for another 950 mile trip on Friday and Again Monday.



I have been thinking about a new truck, as I am not having much luck with this CTD, and the trans is slipping again in my other one. I am tired of getting passed by all those Fords and Chevys. I dont work on my Drag race car as much as I do this truck!

 

[Joe G.]

The first thing I see is that you are lugging the engine like crazy. 1800 with a load like that is not good. Wind resistance has to be really high. Keep it about 2000 at the minimum. These engines are engineered to run at 2300 all day. Even more RPM than that is ok. Why don't you take all the EGR stuff off? If the EGR is not connected to the AFC it can't mess it up. The only place that the EGR stuff was required in '96 was CA.

 

[CMckee]

i know i'm glad i subscribe to the mighty TDR!

 

[paulsmith]

I never hesitate to subscribe each year to the TDR, but I am really having a hard time keeping my two CTDs on the Road. This one is in perfect shape, and is infact a rust free CA truck. The other has a slipping trans again, I think I blew a head gasket, and the Power steering pump hose has rubbed a hole in The PS pump tank.

Anyway, back to the point.



The EGR is plated off. I either have to cover the hole in the intake and exhaust manifolds with a block off plate, or buy all new exhaust manifold and intake. The EGR does not hook to the AFC. But, the EGR dumps exhaust (soot) into the intake, which then works its way into all the hoses (Boost gauge, afc, waste gate)



Mine is auto trans. It is a 12 valve. Floored in neutral it will rev to about 2500 rpm. Under load, I can watch the boost drop as it begins to de-fuel at 2200 to 2300 rpm. With the Auto, I can run 55 mph in 3rd gear, tops, floored. Then it shifts into 4th, and engine rpms drop to 1500-- And that is if I lock it out of OD, otherwise it shifts into 4th at 47 mph and drops to 1400 rpm. I run about 2000 rpm at over 70 mph, 2300 at I think around or above 80 mph. So I either run 55 mph or 80 mph?



I was under impression that peak torque was well below 1800 rpm on the 12 valve?? Seems from memory that my other 96 was governed at about these same rpms until I installed the 3K gov kit.