88irocz28

My 94 TA has developed a miss/hiccup that happens only under light load conditions between 1300 - 1700 rpm. It's most noticeable cruising down the highway between 55 and 65 mph - I can feel the engine shake and miss a few beats every few seconds. If I get on the gas pedal enough to make it accelerate, the miss goes away. It also hiccups hard if I don't take off gently from a dead stop. It also seems worse when the air is cooler, around the high 60s.

The motor is stock except for CAI and no cat, and I have been tuning it, which made me wonder if it has anything to with the tune. No radical changes anywhere, just 3 degrees of spark advance added to the timing tables with a little bit taken out where ever I saw any knock retard. The MAF tables were tweaked a little to get BLMs up into the 126-130 range. The last time I tried to datalog it, it drove fine, but since then it has started doing it again. I checked for DTCs too but there are none.

Since the problem has come and gone, and come back again with the same tune loaded, I'm tempted to think that doesn't have anything to do with the problem. Although all ignition parts except the ICM were replaced a couple of year ago, I still initially suspected the opti, but it doesn't have the tell tale high rpm high load miss that a bad opti causes. In fact, it runs greats when I get on it. The plugs look good too. The intermittent nature of the miss leads me to think there's either a bad connection somewhere in the ignition setup, or it could be the ICM. Even the ICM was spaced out from the cylinder head to prevent heat soak.

Any ideas what I should be looking for? Meanwhile, I'm going to start looking in the most obvious places like the plug wires and other ignition system connections.

byrons1502

cruise at 50-65mph in overdrive. when you lightly accel the converter stays engaged and the engine load is at the highest. Sounds like u have a engine miss. I would guess tune up parts. First place i would look is ignition. Not the opti

88irocz28

Not to sound ungrateful, but it's a manual, therefore no converter. Besides, engine load, as reported in all my datalogs, measured by MAP sensor in kPa, is consistently in the high 30s to low 50s, at all times when cruising, except on the steepest hills. High engine load conditions would be 75+ kPA and that hardly ever happens unless it's at WOT or close to it.

By the way, it's running a lot worse now. It rained today and now the motor is missing randomly all over the place at all RPMs regardless of throttle position. The sensitivity to humidity makes me think it might be the opti after all. I checked all my plugs and wires day before yesterday and the car ran fine for about 150 miles until it started raining this morning.

88irocz28

Well I tore into it today and it turns out one of the vent hoses had slipped off the opti, and it sucked in water and oil from the leaking front main seal. The cap is toast and the base, which I had replaced a couple of years ago, was heavily contaminated with oil. I got everything cleaned up, resealed, and put a new cap on it too. I went ahead and replaced all the front cover gaskets and seals, and also found an excessively loose timing chain so I put a new one in too. Going to reassemble everything early in the AM and find out if the beast lives.

EDIT: Moderator, please move this thread to General F-body Discussion, as it will be of greater use there. I would have posted it there earlier but I thought my problem might have been related to PCM tuning.

88irocz28

UPDATE:

Motor was still missing on cyl 3, so I went ahead and threw new NGK TR55 plugs in it. The plugs weren't that old, but they were badly fouled up, thanks to the opti hardly providing any spark. I could have cleaned them up and reused them but it was just easier to get new ones. Also got new plug wires since I had a lifetime warranty on the old set. Runs like a champ now, no miss!