88 TBI 350 stumbling and missing under load.

[NamesReed]

I really need help, this is my only vehicle, and I have to get to work every day.

Symptoms: Stumbling/missing (cant really tell weather its a fuel issue, or a spark issue) under high load/ higher rpm. All started about a week or so into really cold temps, not sure if that could have anything to do with anything. It almost sounds like irregular 2-step on a forced induction car.

Steps already taken: Replaced fuel filter, threw some heet/seafoam in it, checked fuel pressure (12 PSI at idle, couldnt really check it while actually having the problem), new ICM, new cap, new rotor, new plugs, new plug wires.

Side notes: The two screws holding the ICM onto the distributor are rusty, the distributor is quite rusty, and the two small positive and negative leads going into the ICM were quite corroded. I cleaned them up as best I could however. The lead on the coil that goes to the distributor was also corroded, i tried to clean that up as best i could aswell.

The truck engine wise is relatively stock, previous owner replaced camshaft (i dont know the lift and duration), lifters, and valve springs, and threw a tb spacer on it. Im about ready to get a beater and swap in a junkyard motor.

 

[RawbDidIt]

What's the fuel pressure do when you open the throttle body? Fuel pressure at idle is fine at 10 PSI (if I recall correctly for the TBI 10 works, but I do remember my old TBI running closer to 12 or 13), but if the pressure drops drastically during acceleration it could cause the problems you're having. If it drops and recovers, it's usually the fuel pressure regulator (I believe that's located on the driver's side of the block, attached to the fuel rail). If it drops and doesn't recover it may be a bad fuel pump. Before you do any tests, I would disconnect the ground for the fuel pump, clean it, and then do the pressure tests. The ground should be on the top if the frame, right below the fuel tank filler hose, it's a 13mm bolt on my truck.
Could also be a vacuum leak, but I'm assuming it's not that since you haven't mentioned a loud whistling/hissing noise.

I'm leaning towards fuel being the problem, not spark because it should be missing rather than sputtering if it were spark (excluding timing issues). If you find the fuel system to be in good order, I'd check timing, you may have jumped a link on your timing chain.

Hope this helps.

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[evilunclegrimace]

^^^1988 TBI does not have a fuel rail, the pressure regulator is in the TBI unit.

It sounds like your have a corrosion problem in your wiring. Cleaning it up the best that you can is not going to cut it, fix the corrosion problem correctly FIRST and then run the diagnostics

 

[96dually454]

Any corrosion is a bad thing. Pulls the voltage down considerably and creates heat which further pulls it down. ANY CORROSION in the distributor is not good either, especially on the pickup coil which is just below the distributors rotor. Its 2 wires that can be Ohm checked relatively easy. Below the rotor the pickup coil will have a bunch of points on it and be circular with 2 wires coming out of it. Test those 2 wires.

If you have internet there is a site called easyautodiagnostics.com that site will tell you how to test all these things. Hope this helps?

 

[RawbDidIt]

^^^1988 TBI does not have a fuel rail, the pressure regulator is in the TBI unit.

It sounds like your have a corrosion problem in your wiring. Cleaning it up the best that you can is not going to cut it, fix the corrosion problem correctly FIRST and then run the diagnostics

Thanks for the correction, Been over a decade since I've worked on a TBI. Corrosion is definitely an issue, as you mentioned it should be fixed, and those parts should be replaced, symptoms sound more like fuel or timing related from what I'm reading, but pictures would definitely help make a more concise diagnosis.

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[NamesReed]

Thanks for the correction, Been over a decade since I've worked on a TBI. Corrosion is definitely an issue, as you mentioned it should be fixed, and those parts should be replaced, symptoms sound more like fuel or timing related from what I'm reading, but pictures would definitely help make a more concise diagnosis.

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Ill pull the cap off again tomorrow and take some pictures of what im seeing.

 

[NamesReed]

It is also much better when the truck is still cold. But after about 5-10 minuites of run time it gets much worse.

 

[swervin]

I had similar issues with my 5.7 tbi only difference is mine would occasionally backfire befor dieing down.. first i checked my fuel pump, replaced the fuel filter, checked timing, replaced dist cap, rotor, spark plugs, and wires. I believe it was the dist. Cap and rotor replacement that solved it for me.

 

[samevo]

I had a similar issue with my 88. My truck would stutter going up hills or getting throttling on ramps or pulling a trailer and accelerating. My issue was a corroded pickup coil in my distributor. The connector was brittle and barley holding on. Wires were also brittle and cracked. She runs good now. I had a yellow spark when I tested the plugs.