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Old 11-16-2008, 03:50 PM   #1 (permalink)
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rough idle sounds like missfiring badly

got an 88 2.5L MJ, finally got it started (the power wire going to the starter was bare at the end and shorting against the block, fixed that and she fired right up) but now its back to its original problem of rough idle and it sounds like its missfiring. when i give it full throttle it bogs down severely but if i ease into the throttle it will gain rpm's but still sound like its missing. changed the dist, cap, rotor, coil, plugs and wires, crank sensor, tps, alt, battery.... its so bad that i cant even drive it. any ideas?
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Old 11-16-2008, 05:20 PM   #2 (permalink)
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sounds like you need to go back to the basics. check your compression, timing, fuel pressure, look for vacumn leaks and so fourth. check engine light on?? smoke from exhaust?? if so what color. you got some more diagnosing to do.
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Old 11-16-2008, 06:39 PM   #3 (permalink)
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sounds like you need to go back to the basics. check your compression, timing, fuel pressure, look for vacumn leaks and so fourth. check engine light on?? smoke from exhaust?? if so what color. you got some more diagnosing to do.
i did compression, it was fine, timing is set dead balls on, dont know how to check fuel pressure cuz i dont have a schrader valve on the fuel rail. vacum leaks? im sure theres a few, but nothing has changed in that regard since ive owned it and it used to run fine. so i dont think vacum is the issue. i got smoke coming out the exhaust thats kinda blueish and smells like gas horribly. its most deff running rich.
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Old 11-16-2008, 09:34 PM   #4 (permalink)
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well, if you are rich its either from the miss or that is causing the miss. maybe an O2 gone haywire???? unplug the O2s and see if it improves at all when you run it. you may have to pull the plugs and clean them up if you are running really rich. if one looks worse than the rest you may have a problem on one cylinder. if the are all fouled out you know it is on the control end of things going wrong. diagnostics on an 88 are worthless for the most part but do you have any codes or check engine light on??
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Old 11-18-2008, 07:54 AM   #5 (permalink)
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Red face Cross Firiing

I had rebuilt one of those engines for a customer. We had the same problem. It was crossfiring in the cap. Strangely enough, we advanced the distributor about 90 degrees, and let the computer take care of the rest, it smoothed right up.

Have you done a compression test to make sure there is no bad/open valves? You may have a open intake or closed exhaust causing the problem as well.
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Old 11-18-2008, 08:03 AM   #6 (permalink)
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I had the same problem on a 93 S-10. I did a tune up and got a couple plug wires mixed up.(Noob mistake but I was only 17) The truck was down for a week before I got it back the right way. It may sound dumb but its worth giving them a 2nd look.
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Old 11-18-2008, 09:56 AM   #7 (permalink)
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how long has it been settin? its prbably a long shot but drive it some and it might clear up on its own ir your lucky
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Old 11-18-2008, 12:19 PM   #8 (permalink)
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Originally Posted by cdeclercq View Post
I had rebuilt one of those engines for a customer. We had the same problem. It was crossfiring in the cap. Strangely enough, we advanced the distributor about 90 degrees, and let the computer take care of the rest, it smoothed right up.

Have you done a compression test to make sure there is no bad/open valves? You may have a open intake or closed exhaust causing the problem as well.
compression is good. how do i advance the dist? its non adjusting and has the tab that bolts to the block to keep it from turning
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Old 11-18-2008, 05:41 PM   #9 (permalink)
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take the o2 out and run it, if it runs better the cat is plugged
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Old 11-18-2008, 05:55 PM   #10 (permalink)
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take the o2 out and run it, if it runs better the cat is plugged
cat is completley hollowed out already
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Old 11-20-2008, 05:16 AM   #11 (permalink)
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If it's the TBI motor, then the fuel pressure regulator is integrated in the rear passenger side of the throttle body... what can happen (and did on mine) is the diaphram gets a little tear and fuel leaks out a little vent hole straight into the throttle body. The funny things is, your fuel pressure might still be dead on.

Pull the intake hose off the throttle body and have someone watch that little drain hole while you turn the key (don't start it) and look for fuel squirting out.

FWIW, to check the fuel pressure, you remove the little plug near where the fuel line meets the throttle body (pressure side) and install a schrader valve type of fitting to hook the gauge up to.
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