Help me diaog problem w/ K1500 (fuel issue i think)

[Islandvic]

1995 GMC K1500 350TBI 168k miles. Problem started a year ago. The truck idles fine sometimes, then starts to idle rough then dies. It does not do this all of the time.

On the freeway (constant speeds of 55 and above) the truck will drive fine then the RPM's will drop (engine loses power). I have to give it more gas to stay at speed. If it is on cruise control, the trans downshifts and revs the motor up, then upshifts to continue cruise speed.

Previous owner replaced the fuel pump and screen (mechanic did not clean tank) and fuel filter.

I changed the plugs, wires, cap and rotor, PCV valve and fuel filter in the summer. Still no help.

I changed the fuel filter and drop'd the tank today, wiped down where I could in the center baffle of the tank, and installed a new screen. I will put the tank on in the morning.

I think its a fuel problem but it may be a vac leak also. The trans shifts fine and the trans would not cause the bad idle anyways.

Any suggestions of what I should check out?

 

[wapacz]

I had the tps go on my 95 k1500 305. Thought that through a check engine light. Also you could only really notice it under strong accerleration form me. Where stuff would get real rough and well I would have to stop my truck and turn it off and turn it back on and it would be fine.

Does your checke engine light still work?

 

[toneloc12345]

I'm going with your ignition module. Possibly coil but probably module. Have you checked fuel pressure?

 

[Islandvic]

Where is the ignition module?

I have not checked fuel pressure, don't have a pressure gauge. AZ does not have one in their loan a tool program.

What about the fuel pressure regulator?

 

[Islandvic]

I got the tank back on. Everything works!

I have not had time to test it idling or on the highway.

I have had 2 people tell me it could be the PCM.

Has anyone heard of bad PCM's before?

 

[sierrascout]

Check the EGR valve, and the ECM.

 

[racer12306]

i had a bad computer on my shadow one time. wouldnt start. replaced it with a junkyard computer and it was good till we got rid of it.

 

[Juan]

Hey Vic,
Check this out. I think it could be your problem... the fuel pressure regulator up in the throttle body...http://truckworld.tenmagazines.com/tenArticle.asp?aid=1324&sid=42&cat=2&sc=2

Scott stated that most GM trucks, equipped with the TBI injection engines, left the assembly line with as much as a 3-psi difference in fuel pressure. You can’t make more horsepower if you don’t give the engine the fuel it requires to run efficiently. According to CFM-Tech a TBI-equipped Chevy engine should see at least 14-psi at the throttle body. So he recommended our first upgrade be an adjustable fuel pressure regulator to ensure we were giving our old 5.7-liter engine all the fuel it wanted. Once we achieved this, we could then concentrate on bolting on parts for power

 

[1040 WreckerMan]

GM TBI's commonly wore out the throttle shaft bore. I've seen two that were worn so badly when you rocked the shaft up and down on the drivers side, you could see the end on the passenger side wiggle! This causes a bad vacuum leak; it can also cause the throttle to stick slightly. In addition to checking the fuel pressure regulator as mentioned above, also check the vacuum line that controls the regulator for leaks.

A stuck EGR valve can also cause a poor idle if it fails in the open position. If the truck was used on short trips often, it may very well have a coked up EGR. Easiest way to test the EGR is to pull the vacuum hose off the valve and plug it. You'll need some way to create a vacuum at the EGR valve (like using a MityVac) at this point. Start the truck (this should really be done on at least a slightly warm engine) and at low idle apply vacuum to the EGR valve, the engine should change speed, stumble and possible die. If there is no change in engine behavior, there is an EGR problem. If you do have a EGR problem, make sure to clean the carbon out of all the passages, don’t just bolt on the new valve!

 

[Islandvic]

Where can I find online diagrams for the TBI and vac line system?

Juan, cool site!

I will try to check for vac leaks when the weather clears up.

Thanks guys