Any GM TBI experience?

[stang2be]

I'm trying to figure out an intermittent misfire in my 1995 C2500 5.7 TBI. 113k miles. So far I've done a major tune up (plugs, cap, wires, rotor, coil, air filter, fuel filter) and also replaced pcv valve, egr valve, and IAC. No signs of vaccum or intake leaks.

Fuel pressure is right at 13-15psi


I did a compression test this weekend but I think either the gauge was off or something wasn't right as it was reading 50-60psi in all 8 when factory spec is 150.

Any one have any other ideas to try or things to check or should I break down and take it to a shop?

[dville_gt]

How'd you do the comp. test?

Compression Test—Compression Gauge Check

Make sure oil in crankcase is of the correct viscosity and at the proper level and that the battery is properly charged.

Operate the vehicle until the engine is at normal operating temperature.

Turn the ignition switch to the OFF position, then remove all spark plugs.

Set throttle plate in the wide-open position.

Install a compression gauge such as Compression Tester in No. 1 cylinder.

Install an auxiliary starter switch in the starting circuit.

With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.

Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.

Compression Test—Test Results

The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading. Refer to the Compression Pressure Limit Chart. ~



If one or more cylinders reads low, squirt approximately one tablespoon of clean engine oil on top of the pistons in the low-reading cylinders. Repeat the compression pressure check on these cylinders.

[dee]

Tap on the PCM while running, seen several that are bad. Also change or check the MAP sensor.

[stang2be]

How'd you do the comp. test?

Compression Test—Compression Gauge Check

Make sure oil in crankcase is of the correct viscosity and at the proper level and that the battery is properly charged.

Operate the vehicle until the engine is at normal operating temperature.

Turn the ignition switch to the OFF position, then remove all spark plugs.

Set throttle plate in the wide-open position.

Install a compression gauge such as Compression Tester in No. 1 cylinder.

Install an auxiliary starter switch in the starting circuit.

With the ignition switch in the OFF position, and using the auxiliary starter switch, crank the engine a minimum of five compression strokes and record the highest reading. Note the approximate number of compression strokes required to obtain the highest reading.

Repeat the test on each cylinder, cranking the engine approximately the same number of compression strokes.

Compression Test—Test Results

The indicated compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading. Refer to the Compression Pressure Limit Chart. ~



If one or more cylinders reads low, squirt approximately one tablespoon of clean engine oil on top of the pistons in the low-reading cylinders. Repeat the compression pressure check on these cylinders.

When I ran the test the motor was cold and instead of adjusting the throttle plate we had one reading the gauge and another in the truck with the accelerator pedal to the floor. Sounds like I need to redo the test.

[stang2be]

Tap on the PCM while running, seen several that are bad. Also change or check the MAP sensor.

Hadn't thought about the PCM. MAP sensor was next on my list of things to try changing out/testing.

[shumpertdavid]

The map would likely make the engine run bad as a whole, not so much give you a misfire type condition. I've seen the pcm's go bad in those, but again, not generally giving a misfire type condition. The distributors do however give a lot of problems. Remove the cap and rotor and see if it has a lot of side to side shaft play. When the shaft bushings get bad, the reluctor wheel will hit the pickup coil points and can cause some misfiring. Also the magnet ring that's on the shaft below the rotor will crack over time and can cause erratic signals. The distributor is a very common issue.


As far as the compression test goes, as long as you held the throttle open for each cylinder test you should be fine.

[SEB]

I love my 94 TBI, Keep working at it. These are great motors & easy to work on

[shumpertdavid]

Almost forgot, the egr valves rarely failed, unless the vacuum orifice gets plugged and then they just don't open. However the Egr solenoid did fail commonly. It would give you really poor idle & very light throttle driveability problems, but highway speed and wide open throttle it'd run fine.

[stang2be]

The map would likely make the engine run bad as a whole, not so much give you a misfire type condition. I've seen the pcm's go bad in those, but again, not generally giving a misfire type condition. The distributors do however give a lot of problems. Remove the cap and rotor and see if it has a lot of side to side shaft play. When the shaft bushings get bad, the reluctor wheel will hit the pickup coil points and can cause some misfiring. Also the magnet ring that's on the shaft below the rotor will crack over time and can cause erratic signals. The distributor is a very common issue.


As far as the compression test goes, as long as you held the throttle open for each cylinder test you should be fine.

If just depressing the gas pedal from the cab qualifies then either I have a bad gauge or the motor has really low compression. I'm guessing it should be running like complete shit if it was really at 50-60 psi on 7 and only 70 on 1 cylinder? I'll try disconnecting the map sensor to see if it runs any better and also check the distributor and see if anything in there is loose or off.

Thanks again for the great tips so far. These do seem like decent motors. They may not be a ton for mods or power but seem to be workhorses

[stang2be]

Almost forgot, the egr valves rarely failed, unless the vacuum orifice gets plugged and then they just don't open. However the Egr solenoid did fail commonly. It would give you really poor idle & very light throttle driveability problems, but highway speed and wide open throttle it'd run fine.

My egr valve I replaced after when I would manually compress it and close off the port it would not hold a seal/vaccum and thought that meant it was worn out and not sealing properly and causing a leak. It did seem to improve a bit with the egr valve replaced but still havent found the culprit. I'm due for state inspection in December so should be interesting to see how it does on the sniff test

[stang2be]

Almost forgot, the egr valves rarely failed, unless the vacuum orifice gets plugged and then they just don't open. However the Egr solenoid did fail commonly. It would give you really poor idle & very light throttle driveability problems, but highway speed and wide open throttle it'd run fine.

The miss does seem more at idle/sitting at a light but wide open does not seem to be as noticeable. Looks like EGR solenoid could also contribute to a slow start? I know the TBI 350s don't fire up as fast as the vortecs

[SEB]

Does it mis fire & cut out from a dead stop. I had this problem & replaced the sensor under the Cap & it went away.

[stang2be]

Does it mis fire & cut out from a dead stop. I had this problem & replaced the sensor under the Cap & it went away.

It does it even sitting idle in park but does seem more when in gear sitting at a light.

Can also notice some pause/hesitation when getting back into the gas after coasting but if you get on it hard it pulls hard

[SEB]

It does it even sitting idle in park but does seem more when in gear sitting at a light.

Can also notice some pause/hesitation when getting back into the gas after coasting but if you get on it hard it pulls hard

Replace the sensor under the Dist, I bet thats your problem, They come loose after time & I had the same issues you did & replaced it & remember to use that Electric grease when you replace it.

[JC316]

Replace the sensor under the Dist, I bet thats your problem, They come loose after time & I had the same issues you did & replaced it & remember to use that Electric grease when you replace it.

+1. Nearly all problems on a TBI motor come down to the ignition control module. Otherwise, you might take the fuel pressure regulator apart and check for a pinhole leak. I just had one on my lot that would fall on it's face after 3K RPM, no codes. I threw about $200 at it and nearly a week trying to fix it. Come to find out, someone had replaced the fuel injector harness and had gotten the left and right reversed.