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

[paulsmith]

I have a stock 1996 3500 2wd Dually, Auto, 3. 55, 188,000 mile truck from CA.

Not the truck in my SIG.



It seems down on power. In fact it "Seems" to pull better at part throttle than a full throttle. Floored, it cant pull 8,000# in overdrive. (it is a tall load, losts of wind) but at part throtle, it seems to be able to get up over 65 at times. I do KNOW it does not pull any stronger at past abotu 1/2 throttle. Also, it seems that if you can ever get up to about 72 mph, (2000 rpm?) it seems to pull better, and can stay up there.



What has been done:

New fuel filter, air fitler (KN drop in)

Pressure check intercooler (like airing up a tire!)

EGR valve has plate blocking it off.

Converter emptied



All resulted in no improvement, so:



Took it to diesel shop, Fuel pressure ok, Valves set, lift pump OK, Boost pressure ok, BUT timing was 4 degrees off (I think he said 10 and was supposed to be 14), hell, everthing he could check, and he says it is good to go. I dont know if he check fuel pressure going down the road or not, but he did say it was "OK"



My driver drove 17 hours yesteday, and said it "might" be better, but symptoms are still the same.



Maybe I am spoiled because my other truck has been bombed, but this one sure feels too slow. I want to leave it stock, as I am not going to get into the transmission on this one (still orginal at 188,000 miles)

But this just does not feel right...



Paul Smith

 

[Joe G.]

You might check to see if the EGR has sooted up the fitting to the AFC. A friend of mine had that happen to his '97. He put an AFC from a '95 on his truck when we finally figured out what was wrong. It is stock and really runs good now. Other than that I don't have a guess.

 

[ABorchard]

If it has 188k and still has the original cat it's likely clogged and will cause the symptoms you speak of.

 

[Nick2]

Looks like he already gutted the cat

 

[Andy Redmond]

Remove the shut down solenoid and see if the fuel lever can be lifted further than the solenoid raises it. Does the solenoid stay all the way up while truck is in operation (maybe zip tie it up for a test run) to assure this.



The AFC sticking can cause this.



I presume proper (not leaking) wastegate acuation was oberved in your innercooler/cac tesing.



The lift pump may be down on volume, but it sounds like its being flooded with fuel when its stomped.



Tranmission probs can cause issues (torq. converter plugged up), etc. .



If the injectors come out, perform a compression test. We would like to see real high 200 numbers even among the cylinders.



You may have internal pump issues if all the above ck. 's out.



Good luck and let us know what you find,

Andy

 

[paulsmith]

The AFC, or fuel shut off solinoidmight be the culprit...

I do not think it is over fueling, but I may be wrong. I dont see any smoke, but, under load, I have seen 9 mpg going 55 MPH.

The trans seems to be working perfect.

There were no leaks. I put a 4" hose over the turbo inlet, reduced it down and hooked it to a 10 psi regulated air source. It was like airing up a tire. The shop checked boost pressure, and said it was ok, so I assume the waste gate is regulating boost. I do not have a gauge installed, but have been considering it.

I have had the solinoid stick before, and it would not shut off. The rubber boot has been removed, and a quick shot of WD_40 allowed the plunger to drop down and shut off fuel. That was a year ago, and it has worked fine since then. You make a good point, and I will check this manually to insure that I am getting full lever stroke. The AFC Cover has been removed before as it has "normal" screws.



I will take any other ideas, and maybe some info on how to check the AFC for operation????

 

[Joe G.]

The AFC is easy to check. Take it off. Cover the hole so oil won't get out and dirt will stay out. Take it for a test ride. It should smoke like a forest fire when you step on it and go very well.

 

[Andy Redmond]

With the regulated air source, remove afc line at block/manifold and add short bursts of air with the afc plug removed. It should stroke back and forth easily and not "bind". Regulation is the key, more than about 40 psi and you'll likely blow out the diaphram at the back of the afc.



If the afc is removed stroke it with the reg. air supply and look for smooth operation. Also assure that it doesn't bind on the fuel plate when reinstalled on the engine. .



I sell boots for the solenoid and replacement solenoids, send me a pm if your interested. I wouldn't leave it exposed to the elements myself.



My . 02. Later, Andy

 

[TPCDrafting]

While you have the AFC off, pull up on the shut off solenoid and rock the throttle till you see the gov. arm move over toward the plate. It may not actually touch it, but it'll be close. Observe where the gov. arm hit's or would hit the plate vertically. If it's gonna go under the plate, it needs adjusting. If it's way up the plate, I think you would see more or as much performance at 1/2 pedal as you would at WOT. You could adjust it down for more normal performance.

 

[paulsmith]

Well, I pulled the fuel shut off Sol. today. It had about 1/4" of stroke left. (the arm was still about 1/4" from full up, when the truck is running). So I adjusted the rod to get the "full stroke" of the arm. Do you think 1/4" will make that big a differance?



Concerning the AFC. I put the Allen wrench in the plug, and it is just TOO tight. Cant get it with a normal Allen. I have a set of Allens I can put on my impact, just did not have them with me today.

As for pulling the complete AFC, I assume you mean the 4 screws on the top cover, two of which hold the fuel shut down solinoid?? This truck still has one of the tamper proof screws, and it is the one on the top left, right under the fuel lines... .



Do I really have to go through all that and pull the cover? I will later, just no more time today... . And if I pull the cover, it is going to be hard to not slide that plate, "just a little"



Paul Smith

 

[Joe G.]

1/4" might make a difference. The only way to tell is to take it for a drive and find out.



The reason for pulling the whole AFC off is to test to see if the power problem is the AFC. The EGR could have messed it up quite a while ago. If it is the problem then the truck should have plenty of power. It will be really smoky off the line, but what you want to see is how it acts at speed. If you still see a lack of power on the road, then the problem is something else.



With a problem like this you have to come up with some idea of what might be wrong and test it. No other way to do it really. You can spend a lot just buying new parts until the problem goes away, but that might just make your pocket lighter and not solve the problem.

 

[paulsmith]

Finally got around to checking out the suggestion for the AFC. With 10 psi of air pressure, it would stroke smoothly to about 2/3 the way across the slide. This was blowing air into the hose connection that feeds boost to the waste gate (not the boost supply fitting) I then blew through the boost supply line, and low and behold, it was stopped up! No boost pressure was reaching the AFC or Waste gate! There is a . 030 oriface in the fitting screwed to the AFC housing, which, with the EGR filling the intake with exahaust, had become stopped up.



I poked a . 030 wire through and cleaned out the hole. While I had it off, I slide the plate fowrard about 10 thousands of a inch (. 010, just a little, could not help it!) and boy, she is back where it is supposed to be, not as fast as my other CTD, but not a slug anymore either!



BTW, the local Cummins shop I took it to weeks ago checked timing, fuel and boost pressures, set valves and other things, to the tune of $400, then told me the truck ran better, but was not repaired. The shop said it would need a new set of injectors to cure the low power problem. I guess it does not need new injectors after all!



Paul Smith

 

[RMccurdy]

what is afc

 

[Joe G.]

The AFC is a throttle stop used to restrict fueling until boost is high enough to burn it without smokeing. It is moved out of the way by boost pressure. If the boost is blocked from the AFC that throttle stop stays in place and causes the kind of problem Paul had.

 

[SBozeman]

BTW, the local Cummins shop I took it to weeks ago checked timing, fuel and boost pressures, set valves and other things, to the tune of $400, then told me the truck ran better, but was not repaired. The shop said it would need a new set of injectors to cure the low power problem. I guess it does not need new injectors after all!



Paul Smith

Looks like those Cummins guys should subscribe to the TDR!

 

[paulsmith]

Ok, now I have another problem along the same lines, but a different truck. Still a 96, but with a #10 plate, 3000 Gov kit, open exhaust, big air filter, timing to 15 degrees, Auto trans, and 312,000 miles (most of those at 20 plus psi of boost, pulling the big loads!)



Anyway, at 60 mph, with the converter locked, If I give it half throttle, she will run up to about 26 psi of boost, and move out!. But, if I floor it, boost DROPS to about 20 or 22 psi, and you can feel it slow down (just a little). Not really a problem, as it still has plenty of power either way, and it is kinda nice to have a little limit when I have it in cruise, and it goes to the floor. Just wondering what causes this?

 

[Joe G.]

That sounds like turbo lag. A friend of mine with a '96 has that problem pretty bad. I've told him I would look at it and see if I could do something, but he has not brought it over here.

 

[paulsmith]

I mean on a pull from 60 mph to 90 mph (and beyond!) that if it is floored, it is 20 psi, 1/2 throttle is 26 psi. I can make the boost gauge dance up and down with the throttle, it just that boost pressure is "opposite" of what you would expect.

 

[Joe G.]

I really don't know. You are a little far away for me to get my hands on it and experiment. Have you got a fuel pressure gauge on it? That could give you some clues about what to look at.

 

[paulsmith]

Good point, and no, I have the boost, trans and EGT gauges only. I can try to rig up a gauge, but I dont have a port to hook it to... .