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'88 W-body, W-engine cuts out off-idle


GSGregg
04-11-2011, 03:27 AM
My recently-acquired '88 Cutlass Supreme FWD 2.8L V6 mpfi runs flawlessly 90-95% of the time, but sporadically throws a fit when I try to accelerate. When the throttle is advanced beyond curb idle, it begins cutting out and in rapidly at about two to four cycles per second; the more throttle applied, the quicker the cycle (and the more severe the lurching, so I try to use a 'light foot'). No matter how bad it seems to get, however, it returns to a smooth idle once the throttle is released. The exasperating thing is, this can occur almost every drive cycle on a given day or it can go days without a hitch, and a fifty-mile freeway trip can be either perfect or a series of undesired stops waiting for it to clear itself up---because it always does.

This isn't one or more individual cylinders missing; either they all work, or all die, so it's a loss of either ignition or fuel, and I suspect fuel. I've ruled out water in the fuel (it idles on the same fuel that it gulps on the freeway) and a plugged filter (70+ mph with usually no issues).

And one weird thing; the tachometer (part of the vacuum-fluorescent cluster) reads appropriately when the engine is idling or pulling, but spikes when the engine is losing rpm---more throttle = higher spike. Could a higher-frequency signal be leaking into the EST wire and causing multiple ill-timed spark discharges---but only at random times? That might explain power loss without the presence of unburned fuel......hmmm.

I don't own a graphing multimeter, an oscilloscope, or a live-data-capable OBD1 scantool, but all steady-state sensor readings that I can extrapolate from a DVOM (MAF freq from Tach, INJ duration from Dwell, etc.) are pretty much what you'd expect, and fuel pressure is steady even when the engine is bucking, so that leaves either the DIS system or the ECM. Trouble is, guesswork gets expensive---so, does anyone recognize this as being 'such-and-such'?

Thanks for reading.

GSG

maxwedge
04-11-2011, 08:30 AM
The ecu's were very bad in these years, look for bad/corroded connections at the unit, also the ign module can give the same symptoms, tap the ecu firmly see if the tach moves around to confirm.

inafogg
04-11-2011, 01:28 PM
also try tapping on the mass air flow sensor

GSGregg
04-12-2011, 03:18 AM
Thanks to maxwedge and inafogg for their replies. Before I had the time to access the ECM and take readings, I thought the MAF might be the culprit and I picked up a reman unit (when I started the car to leave the parts house, it didn't act up again for over a week. Only after the next episode did I install it) but it made no difference, so I elected not to turn in the original---may as well keep a spare, right?

When I pulled the ECM, everything looked brand new (with only 119,000 miles in 23 years, most things probably are); it's nice that nobody 'lost' the splash cover that helped keep them that way. Tapping the case and flexing the harnesses had no effect on the idling engine but of course, idle isn't affected anyway; I should have blocked the throttle open a bit. Next time. After the thorough inspection of the coils and DIS module.

GSGregg
05-30-2011, 08:37 AM
Update: The verdict is apparently in; four weeks since replacing the DIS module, and the symptoms haven't recurred. It just irks me that it couldn't be nailed down by anything other than guesswork.

According to a rudimentary diagram supplied by Mitchell Publications, the negative primary side of one coil has a parallel lead (white wire) for RPM reference.....and the factory manual shows an orange wire (circuit 461) from ECM term. B5/ALDL term. M---but neither circuit is shown complete, so 'Thanks a lot, GM'! Apparently, an intermittent defect in the DIS module applied signals from one or both of the other coils, causing the tach to sense higher RPM while the coils were prevented from discharging spark.

Since When does anything solid-state go intermittent!?

So long, gang; it's been fun!:sarcasmsign:

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