Engine/Transmission (1994 - 1998) Very low power, unusual symptoms. Please Help.

[Blacksdieselhp]

My friends 98' 12v automatic is having some trouble. Accelerates extremely solw. Like you are just barely mashing the gas pedal. But at WOT. It shifts normal, and runs smooth with no smoke. We have checked the just about everything. It has fuel pressure, but the guage jumped around at idle and smooths out when you rev the engine. it only shows about 6-7 psi. But still doesn't seem like that is the problem. We chaanged the fuel filter, but still nothing. I finally thought maybe the throttle cable was stretched out or broken, but its fine too. If any of you have seen or heard of this, please help me out. I'm stumped. Just when you think you know just about any problem, one comes up and bites ya. thanks for your help though.

 

[Signal73]

My friends 98' 12v automatic is having some trouble. Accelerates extremely solw. Like you are just barely mashing the gas pedal. But at WOT. It shifts normal, and runs smooth with no smoke. We have checked the just about everything. It has fuel pressure, but the guage jumped around at idle and smooths out when you rev the engine. it only shows about 6-7 psi. But still doesn't seem like that is the problem. We chaanged the fuel filter, but still nothing. I finally thought maybe the throttle cable was stretched out or broken, but its fine too. If any of you have seen or heard of this, please help me out. I'm stumped. Just when you think you know just about any problem, one comes up and bites ya. thanks for your help though.

Im no exspurt, But just make sure the fuel shut down solinoid is up all the way, Also what are the miles? thats AWFUL low FP for a 12V maybe the prefilter screen is clogged or the lift pump is dead. lots of folks in the 150-200K mile club have had lift pumps go bad.

 

[AMink]

Leak in the intake hoses?

 

[DBraunig]

Yeah, that is way too low. i would check the prefilter screen first. you can have it off pretty quick with a 17mm wrench. you can get a new screen and o-ring pretty cheap, $20 i think, or clean what you have. if that is not clogged i would suspect the overflow valve next. you need 18 psi minimum at idle and it should climb from there with engine rpm. you can clamp off the return line momentarily, while watching the fuel pressure gauge, just to verify the transfer pump is healthy enough to make normal pressure. if you get your fuel pressure normal, you may solve all your problems.



Dave.

 

[Joe G.]

Find the cause of the low fuel pressure. There are plenty of things that can leak air. Search this forum for a lot of discussions of the problem and how to fix it. There is a pretty good chance that the problem is cheap and easy to fix once you find it.

 

[AMink]

Where is the prefilter screen? I am embarassed to say I don't know.

 

[Blacksdieselhp]

There are no air leaks. We chacked all the hoses and clamps. Air filter is good. I haven't checked the prefilter though. Yea, I knew the fuel pressure was pretty low for a 12 v. But it just don't act like a dying or dead lift pump. On most 12v's with a dead pump, you get at least a little smoke. i will check the prefilter. I took the overflow valve off and stretched the spring a little. But it didn't change anything. I'm sure it will end up being something very simple, but this problem is kicking my butt. i hate throwing parts at a truck to try and fix a problem, but that may be what I have to do. I've worked on 100s of 12vs, but never seen one do this. I appreciate all your help though.

 

[Signal73]

Where is the prefilter screen? I am embarassed to say I don't know.

Right below the fuel filter in the heater bowl. I found it easiest to access from the bottom with a few extensions on my ratchet. Mine was full of flakes of stuff at 105K, just did that and a fuel filter change last weekend.

 

[Joe G.]

You don't know if there are any air leaks or not if all you have checked are hoses and clamps. The air filter has nothing to do with a fuel system air leak. The fuel heater can leak air. Bear to find if it does. The hoses can look really good and still suck air. All of the fittings in the system can leak air. The gaskets in the fuel heater/pre-filter assembly can leak air. The short curved hose between the pre-filter and the lift pump can leak air. A small air leak will wreck fuel pressure and the engine may run in spite of it. That happened to me.

 

[AMink]

Thanks Signal. That's a keeper of a diagram! So when you pull the canister that the filter is in, the prefilter is at the bottom?

 

[Signal73]

Yeah, once it unscrews and you get your diesel bath, its in the bottom of the bowl held in by a nut. I used some shop air to clean it using caution not to blow a hole in it.

 

[Blacksdieselhp]

I really appreciate all the input guys. And Joe G, your defunately right on the air leaks on the fuel side. I have fixed several of those kinda problems. A real headache to find. And I'm not 100% that this problem isn't from a an air leak in a fuel line, but it just doesn't seem like that is what it is. I know its hard to pcture exactly what a truck is doing without driving it, but this thing runs smooth and and idles smooth, just like everything is normal. It will rev up normally sitting still, but when you go down the road, it just doesn't get anywhere. its almost exactly like the wastegate is stuck open or an air hose has blown. I have checked everything fromt the turbo to the intake. I haven't checked the wastegate though. That just occured to me actually. But I will check all the fuel lines and wastegate, and clean the prefilter. So we'll see what happens. Again I appreciate all your very much. I hope i can help you guys in the future.

 

[Joe G.]

If it runs fine but you are worried about kids on bikes passing you then the problem may be the AFC is not working. A quick test for this is to remove the AFC, block the hole so oil doesn't get out or dirt in, and take if for a test ride. If the AFC is the problem it will smoke like a forest fire if you stand on it, but it will haul butt.

 

[AMink]

Does that year have a limp mode? We had a Chevy at work that was stuck in 3rd or 4th gear that felt like it had lost power, but I see you say it's shifting normally. Don't you love a challenge?

 

[Cooker]

After quickly reading this thread, I'm with Joe on the AFC may be the culprit. Also, as mentioned, do not overlook the FSS.

 

[Blacksdieselhp]

Well, when I was checking everything, I had the AFC housing off, so we went ahead and put a #10 plate in and adjusted the afc spring(turned it all the way to the front). But still no difference. I haven't checked the FSS. Guess I verlooked that too. Well, I'll check the FSS, afc, and prefilter tomorrow. I'll let you guys know if I find the problem. And yes Mink, I do love a challenge. :-laf

 

[Cooker]

You need to check that the AFC is functioning properly. The "foot" may not be moving.

 

[HFreeman]

Low power

If you disconnect the rubber line from the AFC to the wastegate metal line, you can plug the wastegate side of the rubber line. If all is well then you may have a blown wastegate diaphram. There is a procedure in the manual to test the wastegate using NOT MORE than 19 lbs of pressure. I've been there but am not saying that is the problem. LOL. Been there also ( Foot in mouth )

 

[HFreeman]

Ops

Read foot in mouth. I forgot it was a 2000. Thought it was a 12v. I apologize.

( Humbled )

 

[HFreeman]

Drat.

I can't read. Cancell last post.