Topic: 1988 Chev Truck Runs Rough

[Switz]

Hello,

I have a 1988 Chev full-sized truck with a TBI 5.0L V8 and the Turbo400 3-spd auto. transmission.  I just recently had the motor re-built (consumed about 1 litre of coolant per week = cracked head and blown head gasket).  I bought it used and put on over 100,000km until the rebuild.  Anyhow, since the rebuild by a local reputable mechanic (he has been fixing cars trucks large equipment forever), the engine stumbles, hesitates, bucks and snorts when asked to go with very small throttle application. It seems like the engine has too much fuel and is flooding.  As soon as I let off the throttle and the engine goes to idle, it is very smooth.  It doesn't matter if the truck is in gear or not, the engine stumbles with small throttle applications.  It does run better with larger throttle applications, but it still seems to be slightly rough.  The truck runs well when you first start it up with a cold engine, but as the engine warms up this stumble begins.  My mileage before the rebuild was about 14-15mpg.  Now its 12mpg.  If anything, the truck should have run rough before the rebuild because of all the problems it had.  It didn't.  It was an excellent starting and smooth running truck before the rebuild, except for its last day!  The engine should have run rough then and not now.

The mechanic is busy seeding (yes he's a farmer too) and he cannot look at the truck right now.  He says to keep driving it as is.  He says that it won't hurt anything.  Since it idles so nice there must be a different program in the computer for idling.  Anyone have an idea what might be the problem so that I could give him some ideas when he does have time to get to my truck?  I'm sure he hooked up all the wires to all the sensors correctly, but which wire to which sensor would cause this?

[articsteve]

If anyone can correctly diagnose this over the internet I must have there name. 

[barrie1]

I have seen in the past where the wire in the bottom of the distiributor is damaged and does similar things like this until it is replaced. Once on a scope and have a good reading of the codes that are probably stored the comp will tell you exactly which sensor or part to replace. 

[Switz]

I know this was an older post, but I have my truck fixed and I thought I would share the cure with everyone.

The engine was giving no trouble codes so the mechanic couldn't use that as a starting point.  He tried removing the temperature sensor and the truck ran fine as long as it didn't get hot.  Once that sensor got even a little bit warm the engine would run rough.  He thought that the computer perhaps didn't use the information properly from this sensor so he traded the computer for a used one he had on hand.  This still didn't solve the rough running motor.

Then he talked to some other mechanic friends of his and came to the conclusion that the EGR valve wasn't working right.  I don't know how you check if it is working right, but he said that the spring inside it was weak and that once it got the signal it just opened all the way causing the engine to stumble.  He only charged me for a new EGR valve and the time to replace it (not all the time he spent trying to figure out what it was) which was a good deal.  EGR valves are expensive though, $125.00.

So that's the end of my story.  I hope this satsfies those of you that read this and were anciously awaiting the solution to this mystery (ha ha ha... yeah right).

[barrie1]

Thank You for letting us know as I see a lot of these trucks and may be able to use this knowledge in the future. I suspected something like a sensor to begin with. Theres lots of the 305 engines out there which are a strong little workhorse. Sounds like you also have a honest mech as well.