88 GMC Intermittently runs rough

[tls56]

1988 GMC Sierrra C/K 1500 with 350 engine (replaced 9 years ago).

Started intermittently running rough 6-8 months ago.

I have a pretty good mechanic and we have been working on this for a while

At first we thought bad fuel or water in fuel, but no longer thinking that. (have refilled many times).

recently replaced fuel filter, plugs, wires, dist cap, rotor, smoke test - fixed vacuum leaks, rebuild kit for throttle body (gaskets, etc)

then replaced MAP sensor

still same issue -

sometimes the truck runs smoothly, as it should, other times it runs rough, as if it is having fuel issues.

the next thing mechanic thinks to try is replace injectors, which will be expensive. we have also thought about just switching to a carburetor

But this seems to me to be a MAJOR CLUE -

If I start the truck and start driving, and it is running rough, it will run rough until I turn off. If I start the truck and start driving and it is running smooth, it will run smooth until I stop. It doesn't matter if I am driving 1 mile, 10 miles, or 100 miles - the performance will be the same until I stop (turn off). So most days I am doing a lot of city driving. I might drive 2 miles rough, later 3 miles smooth, later 2 miles smooth, later 3 miles rough. Not really seeing a pattern except that it stays the same for that driving session. On Thanksgiving I drove 100+ miles from OKC to Tulsa - ran smooth all the way, and on the drive back it ran rough all the way. (I had not yet figured out the pattern or I would have restarted a few times until it was smooth)

I think this is a very early (1st gen?) computer that only runs the fuel injection. Mechanic does not suspect computer. It just seems like the symptom points to something to do with computer. (I wonder if those are
replaced or fixed?)

any ideas on this?

[Palf70Step]

How long has the fuel pump been in it? What is the fuel pressure?

[tls56]

fuel pump was replaced 7 years ago. symptom at that time - would not start, towed to mechanic (my regular was on vacation), started fine, they replaced fuel filter, a couple days later, would not start, towed back in, the they replaced
fuel pump and starter.

gas gauge quit working a year or so ago. mechanic says sending unit is on fuel pump and would be replaced when fuel pump is replaced. involves removing gas tank, so that's an expensive one.

I will ask about checking fuel pressure.

[stockypoo]

Has your mechanic connected a scan tool to read the data stream? Since you have a pattern, it would probably be worthwhile to invest in a scanner yourself that can datalog so you can record the sensor data when it is running smooth and also when it is running rough. Compare the two and see what's up. I had a fuel pump that died gracefully...some days it worked well, others it ran rough. Never fully died,

[tls56]

I don't know much about it, but my impression is that this is the very first computers they used and are very primitive, so not sure there is much or any data there - but I will ask about that. Also appreciate the info on the fuel pump.

[stockypoo]

Originally Posted by tls56

I don't know much about it, but my impression is that this is the very first computers they used and are very primitive, so not sure there is much or any data there - but I will ask about that. Also appreciate the info on the fuel pump.

If you've got a stock TBI, then you have access to some good data. LT and ST Fuel trims, MAP, TPS, IAC, Knock, ECT. Research WinALDL for the least expensive, but most complicated option. If your mechanic uses snap on, then any of their scanners will log it with the GM-1 adapter. A used Solus Pro on eBay goes for $300-$500 depending on all the bells and whistles. You've probably loaded that much in the parts cannon already.

[ChevyTech]

It sounds like the ignition module in the distributor could be bad. It has a base timing mode and an electronic spark timing mode. It sounds like it may not be switching out of the base timing mode at times.

When the engine starts the module is in the base timing mode, but as the engine start the module switches to electronic spark timing, and the timing adjusted according to a signal it receives from the spark (knock sensor) module while in the electronic spark timing mode.

Many of the bigger auto parts stores in my area will test the distributor module for free if you bring it in. Make sure when you install or reinstall the distributor module that you use heat sink compound under it. The compound helps dissipate heat from the module. Without it, the module will fail.

The free tests that are done to modules at auto parts stores don’t always find all bad modules. Some pass the test but are problem modules. Heat is a factor, and people experienced with testing modules will often test them a couple times to see if it fails as it heats up.

Also make sure the distributor shaft does not wiggle sideways if you try to move it. It is common for the housing to wear out.

Check to make sure no trouble codes are stored in the ECM. This can be done without a scan tool. I have instructions or a link to them in the FAQ thread.

I am extremely busy and have no idea if or when I can get back to this thread but will make a note to try to check back.

[tls56]

Good deal thanks. Will look into distributor ignition module. And this truck has OBD-1, which as I understand it, does not store any codes.

[ChevyTech]

I am a x dealer tech.

The OBD1 system on a 1988 does store trouble codes.

[ChevyTech]

Check codes with no scan tool instructions.

https://www.troublecodes.net/gm/

Edit
Here is a better site for checking codes:
https://www.chevytalk.org/fusionbb/s...php?tid/273660

I am Chevytech on that site too and have many old threads that can be searched for information.

[tls56]

Wow - that is some good info.
I jumpered A/B and got 2 codes - 33 and 45 -

33- MAP sensor circuit high (low vaccuum)

45 - rich exhaust

I went by the shop the other day when it was running rough so mechanic
could see it in that condition (though they have had it act that way for
them on other occasions) - at one point he gave it a lot of gas and had black smoke coming out of the exhaust. and it was after that that he replaced the MAP sensor.

I guess I should disconnect the bat and clear the codes and see if they
reappear later.

thanks

[ChevyTech]

You're welcome.

Make sure the vacuum hose to the MAP sensor is in good condition and has good vacuum at the MAP sensor.

Any time one of these trucks runs rich you should test the coolant sensor by the thermostat.

Your truck could have a bad connection to the coolant sensor which would explain why the problem comes and goes. Also make sure the grounds are clean and tight that terminate at the front of the intake.

The two top links in the link below have testing information that will work for your truck.

http://www.67-72chevytrucks.com/vboa...12&postcount=3

[3drburb]

If you haven't replaced the (IAC) Idle Air Control Valve. It may be something to consider.

[Second Series]

Originally Posted by ChevyTech

Any time one of these trucks runs rich you should test the coolant sensor by the thermostat.

Your truck could have a bad connection to the coolant sensor which would explain why the problem comes and goes. Also make sure the grounds are clean and tight that terminate at the front of the intake.

I had a similar problem with my '88 k2500. It would intermittently run rough, or hard to start. I used an ohm meter to measure resistance of the temperature sender. I measured it cold, and again measured it hot. It did have a range from cold to hot, however the range had shifted from what it was supposed to be. After I replaced the sender, the truck ran fine.