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Old 01-11-2010, 06:38 PM   #1
corhey88
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Exclamation 96 GMC Jimmy black smoke, backfires, Timing?

Any help is much appredciated,
I have a 96 GMC Jimmy, it is hard to start. It has black smoke out tail pipe. It backfires. It has a bad smell of gas out the tail pipe. It runs horribly. The computer only reads one code, Multiple cylinder Missfire. and also gives me data. That My IGNITION ADVANCE IS: - 24, What does that mean? is my timing off? the Jimmy has 240,000 miles, i would suspect that the timing chain might be worn. I have replaced the following, The fuel regulator, and cleaned the spider. I replaced the plugs, wires, cap , rotor, new map sensor, new maf sensor. I cleaned the EGR valve. I believe that the computer has also put the transmsiion into "limp home mode" because it had all the gears but now, is only in second. Does anyone know as well, on the ECU there are three plugs that should be plugged in or two. Mine has a place for three plugs. and the center one was unpluged i just plugged it in, didnt change anhything????? ANyways ANBODY HAVE ANY IDEAS ??? Thanks SO MUCH Cory-
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Old 01-11-2010, 06:44 PM   #2
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

1) Do a smoke test on the EGR, PCV, and Intake.
Do a google ''Youtube Auto smoke test''
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Old 01-11-2010, 06:46 PM   #3
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

I know that the Jimmy is running too rich, i can smell the gas...
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Old 01-11-2010, 07:50 PM   #4
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

2) Do a compression test next
3) What about a upstream H02 sensor for the air/fuel mix.
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Old 01-11-2010, 08:19 PM   #5
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

I can do a compression test, i do not think that is the problem though. I have not got a code for the oxygen sensor, i beleive it would set one off.
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Old 01-11-2010, 08:21 PM   #6
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

I am going to pull the timing cover off tommorow. If it was not the problem at least i will have eliminated one variable.
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Old 01-11-2010, 10:21 PM   #7
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

You need to loosen the oil pan to remove the timing cover, and the shop manual recommends replacing the timing cover if removed to avoid oil leaks. Quick easy way to tell if the timing chain jumped, (as long as nothing has been taken apart since it started acting up).... Remove distributor cap, rotate the crankshaft manually with a socket and ratchet until the distributor rotor lines up with the "6" on the distributor housing. Then make sure the timing mark on the damper is lined up with the mark on the timing cover.

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Old 01-11-2010, 10:34 PM   #8
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

Where is the mark on the damper... And where is the Damper? I have not worked on a timing chain before. So i look to make sure these to lines line up with the crankshaft at TDC ??Thanks for the help..
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Old 01-11-2010, 10:52 PM   #9
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

I just thought of something, since the reading i got for the ignition timing was -24 could that be the reading for the camshaft retart? should i try replacingh the camshaft positioning sensor????
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Old 01-11-2010, 11:08 PM   #10
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

The damper is the large pulley on the crankshaft that the belt goes around. The mark is just a groove in the damper. There are two grooves but only one will be close to the mark on the timing cover. There is a tab with line on it on the timing cover and the two marks must line up. As you face the timing cover, the tab is approximately at the 2:00 position. The -24 degrees is the amount of retard the ECM is commanding. When a timing chain jumps time, valve timing retards along with ignition timing. The -24 degrees along with a lack of power might be an indication the chain has jumped and the ECM is tryng to compensate for it. Check the marks as mentioned, before replacing parts needlessly.
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Old 01-11-2010, 11:12 PM   #11
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

Thanks So Much !!
By turning the rotor to TDC do i need to turn it back to where it was after this test??
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Old 01-12-2010, 02:47 PM   #12
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

Nope, it turns when the engine is running. You're just putting it a "given" position so you can check the syncronization of the camshaft and crankshaft.
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Old 01-12-2010, 03:25 PM   #13
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

Ok, i did the test on the timing, And the rotor lined up perfectly with the 6.
So, i did not jump time?
After doing the test i tried starting it again, it just backfired throught the throtle body... I can get it to start and run for like a minute then it die out and backfires like the whole time it is running.
I amd wondering now if maybe the ECU computer is just FRIED or something.?????
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Old 01-12-2010, 04:05 PM   #14
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

If the rotor was on the "6" and the damper lined up with the tab on the timing cover, (at the same time ) the chain has not jumped.

Have you checked fuel pressure and leakdown?
What brand spark plugs and how old are they?
What brand and how old is the distributor cap & rotor?
What brand and how old are the plug wires?
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Old 01-12-2010, 04:53 PM   #15
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Re: 96 GMC Jimmy black smoke, backfires, Timing?

I used Bosh platinum plugs. I used Duralast cap and rotor.
I beleive they where duralast wires as well. I just put them on last week. And i put the new fuel regulator on last week too, and i cleaned the spider and poppet injectors . i put a new map sensor on it, and got a used mass air flow. I also cleaned the EGR valve with carb cleaner.. It also has a new fuel pump last year. Am i looking for low or too much fuel pressure. As it is backfiring and the fuel smell i assumed it was getting to much fuel,, or not neough air, spark??????
Thanks So much again !
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