Help my 1987!!

[ChevyTech]

Backfiring is not a common problem on TBI trucks.

If you ruled out the ignition system, you should look for a mechanical cause for the problem.

An intake valve that is not sealing or a flat cam lobe can cause backfiring.

If the problem is an intake valve not sealing it should show up on a compression test.

To check for a flat cam lobe you could remove the rocker covers and see if one of the rockers is not moving like the rest of them.

[Gregs71]

Well today, I took the truck out again. When its cold I can rev it up over 2,000 rpm with out any issues. Once it started to warm up it would miss/sputter and have some backfiring.

When I got back to the driveway, after putting it in 'park', I revved up the motor. There was maybe a slight(maybe delayed reaction), but it revved up just fine with no noise. I revved it to 2000, let it drop to 1500 and back up, etc with no issues.

I guess it is only doing it under a load. I guess it is starting to look mechanical.

I guess I'll have to pull the valve covers and look for odd movement.

Greg

[ChevyTech]

My gut feeling on this is that it still sounds like an ignition problem.

[Gregs71]

Happened to be a Kragen the other day. Guy mentioned the Throttle position sensor. Any possibility of this??

Greg

[rfmaster]

Greg

You can always check TPS with a DVM. Just back probe Pin B of the TPS (should be dark blue wire). At idle TPS voltage should read 0.4 to 0.65 Vdc. Move throttle from idle to WOT and TPS voltage should increase smoothly to about 4.5 Vdc. TPS may develop a worn spot from idle to about 1/4 throttle - any sudden jumps in voltage are indicative of a worn spot. This check can be done with engine stopped, ignition on.

//RF

[ChevyTech]

Happened to be a Kragen the other day. Guy mentioned the Throttle position sensor. Any possibility of this??

Greg

Yes

Test it as rfmaster said.
I don’t like to back probe wires, and use jumpers when doing tests like this.

Keep in mind if a sensor fails, but the reading that is within a range that is a normal, the ECM may not (and usually won’t) detect the problem and no code will be set. So if the TPS contacts totally lose contact the ECM would quickly detect the problem. If the TPS failed in a way that the contacts did not move but never lose connection, the ECM may not ever detect the problem.

[ChevyTech]

Here is another thought about your truck.

The backfiring could have caused damage to the MAP sensor. This fits the symptoms better because the MAP sensor is what the ECM uses to sense engine load.

If it was the backfiring that caused the charcoal canister hose to come off, maybe it also damaged the MAP sensor.

[Gregs71]

Is there anyway to test the map sensor, or do you just replace it.

Greg

[ChevyTech]

I keep extra MAP sensors around so I can just try one.

Here is a reply I saved that I made to another post about MAP testing. I saved it under my TBI information so the wire colors may not be exactly the same but it should be close enough to use. You can see the service manuals don’t give very specific specs on this. Altitude has an effect on readings so the manuals are very vague.

MAP sensor testing TBI

Do you have a hand vacuum pump for testing the MAP sensor?

A normal Map sensor reading on the wire that connects to terminal “B” (possibly Lt green) would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

NOTE: if it runs poorly it could have much lower vacuum at idle which would cause a higher voltage reading.

Terminal “A” (possibly Gray) is voltage supply to the Map sensor. (Should be close to 5 volts)

Terminal “C” (possibly purple) is the ground for the Map sensor. (Should have no voltage)


On MAP sensors, high voltage readings are obtained at low vacuum levels, and low voltage readings are obtained at high vacuum levels.

A low voltage on the MAP sensor reading would indicate to the ECM that the engine has high vacuum.

[Gregs71]

PROBLEM SOLVED!!!!

Well I thought I'd bring closure to this thread

Finally got the plugs and wires changed. I guess there was a bad wire in the mix, because it runs good now.

So the problem ended up being the distributor module and a plug wire.

Thank you all for your advice, hopefully I can help you down the road.

Greg

[ChevyTech]

Glad to hear things worked out, and thanks for letting us know what the problem was.