another low rpm stumble thread
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Joined: Jan 2008
Posts: 3
From: Maryland
another low rpm stumble thread
i looked over a bunch of the old threads, i found lots of symptoms and possible culprits but no solutions that seem to work for me. The car is an 86 rx7 with a 95 lt1. car was running great, then took it down for some body work (nothing mechanical). ran fine when i first took it out then on the way to dinner it started the low rpm stumble/miss. basically any high-ish throttle low rpm (1500-3000) the car will buck and shake hard as if there's several cylinders missing. seems to run ok at WOT, maybe down alittle on power, it's hard to tell because it's been warmer here recently but i can't roast the tires as easily. the car as new plugs (gapped tr6's), opti and wires.
i've tried:
-unplugging both o2 sensors to see if it was a lean reading -> retard timing issue. no effect.
-unpluging the knock sensor (lifter/rockers are loud and the exhaust is close to frame rail). no effect.
-running off the MAP only to see if it's an erratic MAF reading, no effect
-the car has no cats, and a pretty much straight through exhaust. i unbolted the mufflers to see if there was some wierd backpressure issue with them and still didn't seem to have much of an effect.
-the fuel filter was recently replaced, and the car seems to run fine at WOT so it doesn't seem to be a fuel issue
the coil is an MSD, and reads 5.5k ohms across. could this still be the culprit even though it reads out ok? the problem occurs when the engine is cold or warm so i don't think it's an overheating ICM issue, although it does not have the spacer mod. other than that the only other possibility that i can think of is that one of the lifters is pretty loud at startup. it calms right down, but maybe at low rpm there isn't enough pressure to keep that lifter pumped up? the valves are adjusted. but even with one cylinder down i don't see how it would shake as violently as it does. i have the diagnostic connector wired up, but have no way of pulling the codes. i tired autozone but they will only do obdII. any help is appreciated!! thanks.
i've tried:
-unplugging both o2 sensors to see if it was a lean reading -> retard timing issue. no effect.
-unpluging the knock sensor (lifter/rockers are loud and the exhaust is close to frame rail). no effect.
-running off the MAP only to see if it's an erratic MAF reading, no effect
-the car has no cats, and a pretty much straight through exhaust. i unbolted the mufflers to see if there was some wierd backpressure issue with them and still didn't seem to have much of an effect.
-the fuel filter was recently replaced, and the car seems to run fine at WOT so it doesn't seem to be a fuel issue
the coil is an MSD, and reads 5.5k ohms across. could this still be the culprit even though it reads out ok? the problem occurs when the engine is cold or warm so i don't think it's an overheating ICM issue, although it does not have the spacer mod. other than that the only other possibility that i can think of is that one of the lifters is pretty loud at startup. it calms right down, but maybe at low rpm there isn't enough pressure to keep that lifter pumped up? the valves are adjusted. but even with one cylinder down i don't see how it would shake as violently as it does. i have the diagnostic connector wired up, but have no way of pulling the codes. i tired autozone but they will only do obdII. any help is appreciated!! thanks.
Last edited by slow231; Jan 10, 2008 at 02:18 PM.
Thread Starter
Registered User
Joined: Jan 2008
Posts: 3
From: Maryland
wide fenders and new hood. nothing mechanical had to be altered/removed. i just tried a new stock coil (20 bucks from advance auto) and it seemed to make the situation worse, more missing surging, so it really seems to be an ignition issue.
The opti I got was from ebay, the auction was listed as a NIB delco unit, however i suspect that it may not be (the cap and rotor look different). when this problem first started, a guy on another forum said that he had an ebay opti start acting up on him after about a month or so of use (same timeframe as mine). He ended up having to replace the cap and rotor and all was well (the opticical part is fine). so i went back in and swapped out the cap and rotor with my old used cap and rotor. perhaps i should have gone new? seems like a weak spark situation. the MSD 8.5's look perfectly fine and the plugs are pretty new. the headers i'm running don't allow access to some of the plugs so i haven't swaped them out yet. any other advice? I also went ahead and did the ICM spacer mod while i was swapping back the MSD coil but haven't driven it again since it's starting to rain here.
The opti I got was from ebay, the auction was listed as a NIB delco unit, however i suspect that it may not be (the cap and rotor look different). when this problem first started, a guy on another forum said that he had an ebay opti start acting up on him after about a month or so of use (same timeframe as mine). He ended up having to replace the cap and rotor and all was well (the opticical part is fine). so i went back in and swapped out the cap and rotor with my old used cap and rotor. perhaps i should have gone new? seems like a weak spark situation. the MSD 8.5's look perfectly fine and the plugs are pretty new. the headers i'm running don't allow access to some of the plugs so i haven't swaped them out yet. any other advice? I also went ahead and did the ICM spacer mod while i was swapping back the MSD coil but haven't driven it again since it's starting to rain here.
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