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1995 Pontiac Trans Am Rough Idle / miss in engine

Tiny
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  • 1995 PONTIAC TRANS AM
  • V8
  • 2WD
  • MANUAL
  • 218,873 MILES
Well, I've had this car for roughly 6 months. Previous owner really didn't take too good care of it, never replaced plugs/wires, clutch was shot, etc. Price (3500) for a Firehawk that just happens to be built on my birthday (9-20) was just too good to pass it up, esp. With the LT1/T56/T-tops combo.

Either way, I replaced all 8 plugs and wires, replaced the belts/hoses (incl. The PITA heater core hose that Does Not Exist) about 3 weeks to a month ago, and also did vacuum lines (they were completely rotted/cracked).

The miss/rough idle did not occur immediately after the repairs, instead it showed up in the past few days after most major repairs were performed. The car now shakes heavily (only occasional, makes it a pain to diagnose) and it triggers a CEL (although nobody in the town can diag. An OBD 1.5 GM, after the dealer moved out, and the other dealer won't touch anything over 10 y.O. Or over 150,000mi) during the rough idle, then it goes out and the car resumes normal operation.

Some repairs performed recently (all by me, all since purchasing the car in March/April-ish):
- Optispark (PITA), 2 months ago
- clutch/trans work (unrelated)
- all vacuum lines/hoses, belts, etc. (1 month ago)
- EGR fully cleaned, PCV replaced, etc. Etc. (5 mo. Ago)
- Throttle body cleaned, new 1LE intake elbow, cleaned MAF, new CAI installed all done 4-5 months ago
- shorty headers installed
- All new brake booster (old one was still leaking vacuum.)
- cleaned all electrical connectors, replaced some (really dirty/gunky) about 2-3 months ago

One other problem I do know if is it has been seeping oil out of the head gasket area (b/t head and block, mostly the driver's side bank, although the other bank's doing it as well), I figure it's just a hairline fracture allowing oil to seep slowly out, doesn't seem to mix with coolant (HC test and pressure test checked out fine) and it also holds compression in cylinders (10.5:1 down to 10.1:1) very well for a 15 y.O. Motor with as many miles, and oil pressure's steady at 40psi idle/cruising, stays b/t 25/30 and 50 most of the time (occasionally creeps up towards 60 on cold night starts). Don't know if maybe it got worse or not, but I figured on just leaving well enough alone until I get the money I need for a full head/cam/stroker setup with a total engine overhaul, and then deal with the oil seepage (it's maybe 1/8 qt. Per month, I change oil every other month).

All I haven't touched, yet, is the fuel system, internals, and the ECM. No real test for a failed ECM other than diag equipment I neither have nor have funds for. Really just looking for advice as to how to diagnose this, and I'm not going to trust the same Advance Auto Parts store where the guys spent 20 minutes trying to locate a radiator for a 1972 VW Beetle (AIR COOLED!) Or couldn't figure out that an ignition cylinder is not an internal engine part.
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Tuesday, October 19th, 2010 AT 4:16 PM

11 Replies

Tiny
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What was the question?
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Tuesday, October 19th, 2010 AT 8:55 PM
Tiny
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Car has a rough idle condition. Drops to 500-600rpms, then jumps up to 1000, cyclically (basically, you can watch the tach jumping all over like a ballet dancer w/i that range). Only occasional (thus, PITA to diagnose, since I cannot just start the car and start looking for the cause. Since it happens randomly), and when it does happen, it trips a CEL (SES. Whatever you want to call the check engine/service engine light) although I cannot get the codes, as no shop in the immediate vicinity can work on OBD 1.5 (and I don't feel like getting a 60 mile tow just yet.). CEL does go off after it resumes normal idle, so I'm hoping that limits/restricts what codes it could be.

Listed parts I did and didn't replace yet (for other problems that came up / things I noticed, i.E. Cracking vacuum lines = replace before the car leaks too much vacuum, variable pressure on the brake pedal, pedal gets stiff = check the brake booster, found a nasty vacuum leak there, etc. Etc.)

All I'm looking for is tips on what else this could be, and how to go about diagnosing those w/o an oscilloscope or code reader, although I do have a decent multimeter (AC/DC, resistance, capacitance, inductance, current, voltage).
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Wednesday, October 20th, 2010 AT 6:03 AM
Tiny
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Are there commonalities for the symptom?
IE. Is the engine hot or cold?
Damp weather?
Did the old plugs show any signs of abnormal burning.

Have you pulled the plugs out again to see if any are burning oil or showing too rich or too lean?
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Wednesday, October 20th, 2010 AT 7:44 AM
Tiny
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Old plugs were original, so we really don't know from those. A couple were bad (bad gap, shorted, etc.), All were fouled, but otherwise, they looked decent. No oil, coolant, or lean, just fouled from 15yrs and 215,000 miles of wear and tear.

Damp might be it; it did start getting really humid (and cold. So frost) the past couple days. Engine's always hot when running/driving, I don't jump in the car and head out (easy way to put excess wear on the car).

I don't know if you've worked on an LT1 or LS1 F-body before, but you don't "just pull the plugs". It took me 6-7 hours to change, labor's $600 for a shop, so yeah.
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Wednesday, October 20th, 2010 AT 11:05 AM
Tiny
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When you changed the wires did you use dielectic grease?

If damp is a possiblility, then mist water on the ignition wires when the problem is not happening and it may flush out any that are arcing.

Warranty book time is 1.8 hours, 2.7 flat. First I have come across plugs that went that long.
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Wednesday, October 20th, 2010 AT 3:41 PM
Tiny
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I saw a '74 charger with original plugs about 2 years ago. Old lady driving it, said she bought it new in '74, granted, only 84,000 miles on it (318) but yeah.

As far as the pontiac, we figure it was essentially dieseling. Would explain a Firehawk that should run mid/low 13's at the quarter only managing a 15.6 @ 92mph.

Yes, dielectric grease (a small amount) and also replaced wires (I don't think anyone would replace just wires w/o plugs. Could be wrong). Wires were all good and tight, sprayed a little water, nothing.

I'm almost positive it's a sensor starting to go. Gonna try the TPS later today, as that might have a few dead spots, always a possibility (friend of mine out here recommended that).
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Wednesday, October 20th, 2010 AT 5:20 PM
Tiny
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Update: TPS and O2 sensors both checked out.

Next step will be to get an AKM cable for the car, then download freescan, and see if I can get anything out of that (interfaces fully with the ECM, hopefully will give me some clue of what's going on). If it gets worse or I can't get anything out of freescan and AKM, I might come back here.

- JR @ WVU
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Thursday, October 21st, 2010 AT 5:55 PM
Tiny
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Check for vacuum leaks. Use carb cleaner and spray around the intake and any other sources of unmetered air. See if the idle changes when you hit a particular spot.
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Thursday, October 21st, 2010 AT 6:12 PM
Tiny
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Pointless, but done. Nothing.

As I said earlier, I replaced all vacuum lines with new ones. Intake's sealed (new intake manifold gaskets when I cleaned the intake manifold and throttle body. Had that disassembled, so figured, wtf, might as well).

Gonna try snaking out the EGR again, see if maybe that got gunked up again after another several thousand miles with those old, junk spark plugs in there. I'll laugh if it's that simple, but it's still going to wait until the AKM cable arrives and I get a quick scan/diag on it to see what that tells me about it. In the meantime, I got the new headgaskets in my '94 chevy, so I have a vehicle to drive to/from work and school.

Once again, thanks for (trying) to help out. PITA without knowing what code it's giving, and with it only being an occasional problem. And, of course, it only does it on the highway, when I sit around for 3 hours cursing my car "break down dangit!" It runs smooth as silk. Lol.
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Thursday, October 21st, 2010 AT 8:58 PM
Tiny
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Pointless?

Good luck.
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Friday, October 22nd, 2010 AT 5:10 AM
Tiny
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Already said I'd replaced vacuum lines, and quite a few other vacuum-related systems.
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Friday, October 22nd, 2010 AT 7:23 AM

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