m20 severe stumble @ low power when hot.

[Gene in NC]

'89 525 5s 184k has m20 w severe intermittent but very frequent stumble, and effect of low power when hot, especially under load. Starts and idles fine, even when hot. Before hot it runs fine. Test from home with cold engine ran strong and smoothly. 1.2 mi to freeway, 2 mi at up to 70mph OK. 1 mi on freeway return OK. City street at 35 mph stumbles so bad was happy to get home.

First started 135 mi from home while on loan to my son. He reported problem, but then reported OK when switched to Shell 93. Met him !/2 way home to swap cars. Surprized to find condition as below.

Stumble feels like cross firing in the distributor. Sharp break like ignition but engine acts like it is being "stopped" by cross fire with back and forth load on the driveline. Snatches drive line back and forth. Worse than just flicking ignition on and off. 5th speed impossible. 4th making progress at 70 on I 40 W but more and more rougn as we head west from the coast toward Raleigh, encountering more grades as we complete the 360 ft elevation change from sea level . By I 95 can't keep up with slowest traffic. Talked to indy who thought that even if cross firing another 30 mi wouldn't hurt anything.

Took off with condition worsening, tow seemed only way to get home. Max speed 45/50 and concerned about accident in 80 mph traffic.

Then what appeared in the rear view mirror but something that was almost as slow, one of those GM RVs, based on the Toronado, towing a Honda. Caught his draft on a down hill and drafted that sucker the last 25 freeway miles. A little dicy keeping up a couple of times, but MacGuyver would have been proud.

Dropped off the draft with 7 mi of city streets to go. Problem went away unitil the last mi when grades become more frequent and longer.

So what have I done?

Tested as above.

Pulled plugs (11k m on W8LCR) and they are a mess. No two have same appearance. Common condition is insulator clean on one side with heavy deposits on the other. #1 & 2 are sooty, while # 3-6 look a little wet. Oil or fuel? Can't tell but 24 hours out of engine appearance unchanged.

Five plugs have some of the deposit (about 25%) broken off down to bare insulator. Break goes from tip of insulator up in a roughly "v" shape. Son put about 5k on the car over the winter and has been known to use the cheapest gas that the car can possibly run on.

Compression unchanged.

Aha, must be a bad dist cap. Nope, dist cap looks new. Must have been replaced just before I bot the car about 11k m and 12 mos ago.

Any ideas? Would like to drive it to Vintage at the Vineyard 5/27. Oh man, would I like to have a digicam to post photos of the plugs.

[DaveVoorhis]

If you've got a working "Check Engine" light, do the stomp test and see if the ECU has recorded any codes.

If it's engine heat related, it might be electrical. I'd suspect the crank position sensor, coil, ECU, and AFM. These are wild guesses, mind...

[joshua43214]

It does sound alot like a bad cap or wires. If I am reading you right, you have what is known as a rolling misfire. start the engine and spray the wires down with a pump bottle, then spray the cap. If it misfires, you need wires. The caps can look good and still be bad internaly. The rotor can also fail, I have never been a fan of Bosch rotors, but its really the only game in town for your car. Don't bother to do a resistance check on the wires, its a waste of time. You said the cap looks likethe PO replaced it, check the brand of the wires and the cap and rotor, I have never had any luck with any thing other than OEM wires on BMW's, I would replace them on the spot if they are aftermarket, since they will just make trouble later on. I have also seen alot of aftermarket caps short out internaly, the Autozone ones can fail in a few hours of running and the CarQuest ones often fail in a few months.

The problem could be also related to fuel supply or spark management, but start with the basics before digging into other things. This kind of problem is frequently caused by faulty secondary ignition, so make totaly sure they are ok before proceeding.

I am not sure I understand your description of the plugs. do you mean the fouling is broken off or the electrode?

[632 Regal]

excellent discription! I have no clue but guesses, first things first get new parts...lol. Seriously, new car no clue, start with fuel filter/s plugs, wires and cap. That cap could have micro cracks or something, i saw this stuff before.Recommenced plugs? maybe someone cracked the insulator on install, very very hard to find. Hopefully it's something simple bro.

[Gene in NC]

Joshua, you may be on to something. We have two '89 525s and the other one drove me nuts for months before replacing the rotor, which also looked good. Luckily I saved the old one. Brand is Bremi. Is that OEM.
Will follow up on your suggestions.

The fouling broke off the insulators. Electrodes are fine.

[Gene in NC]

My wires have no indication of manufacture except "made in west Germany" on the connectors.

[Morgenster]

BREMI is usually cheaper than Bosch right?
I have some ignition coils on my M50 that are BREMI and I suspect they misfire in low RPM ranges.

My e30 had some similar symptoms and the ignition wires were just bad (as in low tech cheapo bad). Managed to improve performance by stripping and shortening slightly to get decent reconnection at the spark plugs. Changing them would have been easier, but if money is an issue...

[Gene in NC]

Checked dist cap ( Bosch that looked new although at least 13k m in the year that I've owned it) and wires (unknown/unidentified but not Bosch). Installed new W8LCR plugs and misted the wires with water while at idle. Slowed a bit but no sharp miss or stumble like really bad wires.

35 mile test of in town, freeway, flat, grades, with speed variation from 20 to 75 mph. Not a single stumble or miss. Sometimes idle smooth, but others a little irregular. Later on, idle had occasional mini clank/rattle that sounded like dual mass flywheel rattle.

Suspect reinstallation of old plugs might bring back the rolling miss problem. However, I have had enough of aftermarket dist caps and wires to be a true believer.

Summary: very poor plug condition must have put pressure on aftermarket ig wires causing cross firing.

[joshua43214]

With good wires, you will get no cross firing at all regardless of the plug condition. Sounds like you have it going again, I would budget in a set of new wires as soon as possible.

[Gene in NC]

One more for Joshua. Genrally avoid bragging bout what a dumb ass I was but the on the road situation was like "trying to remember, when you're up to your ass in alligators, that the original objective was to drain the swamp".

Now I recall that several rainy, soggy mornings over the winter the car wouldn't start. Plenty of crank but no fire up. Classic case of condensation on the ig wires. Duh.

Problem with wires is they are burried in that loom so the condensation is not obvious. Second problem with the loom is wires running parallel are especially susceptible to cross fire. Right?