My 1986 has a slight rough idle when cold and will back-fire at the throttle body or runners if you hit the throttle hard in neutral. After 3-4 minutes it is perfect. There is a heavy exhaust smell if you rev it hard for the first 3-4 minutes. I have no faulty computer codes comming up. The OHMS cheched out all right on all injectors,I replaced,plugs,MAS air flow senser and the EGR. The cold start valve also checked out. I keep coming back to injectors. Is there a test you can do to the injectors without pulling them out. Also could an injector be acking like this only when cold.Any ideas would be helpfull I"m just about out money .
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Re: 1986 tpi back-firing
You can do balance checks on the injectors to see if they are working, but I dont think that and injector would cause a back fire in the intake or TB. A worn camshaft or valve problem could. Ignition problems may also. -
Re: 1986 tpi back-firing
Rick:
Firstly, you MUST have the Shop Manual to track down this problem!
If the engine runs normally once it is in closed loop, then it points to an open loop problem - in other words, a transient condition that only occurs during warmup. I would initially suspect the cold start valve (supplemental injector). If the valve is dirty, then it would not necessarily set a code, since the voltage feedback to the ECM would be normal. This condition SHOULD trigger a code from the O2 sensor, but I am not sure how much leeway is built into the system for a fuel mixture condition during open loop.
Secondly, make sure you clear any codes in the ECM after any operation you perform. The easiest way to clear the ECM, is to disconnect the orange(?)ECM feed wire, WITH THE IGNITION OFF, for at least 10 seconds. This wire comes directly off of the "pos" batt. terminal. If you trace the wire towards the firewall, you will find a weatherproof connector. Even if no codes are flashed when you ground the ALCL "A" and "B" terminals (these are the 2 upper right terminals), the ECM can still store an "intermittent" code, which will affect engine operation.
Here is a test you should perform FIRST, this will put the engine in "field service mode":
1. Engine cold.
2. Clear ECM codes.
3. Start engine, and immediately short ALCL terminals "A" and "B".
4. Observe the "check engine" light. NORMALLY, in open loop operation (during engine warmup), the light should flash about 2 1/2 times per second. If the engine is too LEAN, the light will be out most of the time, if it is too RICH, it will be ON most of the time.
The results of the above will give you a clue as to what the problem is. After this test, go to the Field Service Mode Checklist. I still suspect a dirty cold start valve, but I am sure that there could be many other causes, too. You must follow the test procedures outlined in the manual, otherwise you will just be throwing money away.
Good luck, and keep us posted.
JoeComment
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Additional Information
I did some further reading and found that the cold start valve is only used during cranking. Once the starter is released, the cold start circuit is no longer energized. So that eliminates the cold start valve/thermo time switch.
Also, during field service test, run engine at 1600RPM, and it should go into closed loop after about 2 minutes. At this point, NORMALLY, the light should flash at about 1 per second.
Some of the things that you have done, lead me to think that you already have the Shop Manual. Perform the test that I showed you before. If you have a lean condition (code 44 - intermittent with no code set), or a rich condition (code 45-intermittent with no code set), then that will point you to such things as:
1. Fuel pressure too high.
2. Leaking injector(s).
3. Contaminated O2 sensor.
4. Saturated fuel canister.
5. TPS output signal out of range.
6. Fuel contamination
7. Faulty ECM
There are more, but I would look at the injectors as being dirty/leaking at this point.
JoeComment
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Re: Additional Information
since he smells fuel(rich) on startup i would guess the injectors are leaking down. would try some GM fuel injection cleaner from the dealer as this stuff is really good. i will check to see if i can find the part #. you can also pull some spark plugs before you try and start the engine after it set over night and see if they are wet or smell from gasoline and that will tell you that the injectors are leaking down.Comment
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GM part# 12345104 *NM*
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