m20 severe stumble @ low power when hot.

[Rustam]

'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.

coolant temperature sensor?

[Gene in NC]

Prob cleared. Replaced wires with "known good" Bosch with maybe 90k + mi. Last symptom of stumble at 4k rpm in 2nd cleared. Interesting to note now that the 2nd gear hard accel stumble was the original indication

"Known good " wires were from the our other '89 that showed wire like probs on heavy acceleration. Replacing wires no help. Went through an awful lot of time and expense before very careful improvement of grounds, including use of Stabilant, gave some relief. Finally cleared up with new dist cap and rotor.

Note we are talking about two '89 525s here. One long term from 36k to 150k, the contributor of the used wires and related experience and the 186k 5s that is the subject of this thread.

Thanks to Joshua for tips and to Jeff for reminder in line with wisdom of the ages on cars, "If you think prob is fuel related, start with the ignition."

Take away: $250 for dist cap, rotor and wires is not cheap but if the prob "Looks ..., walks... , and quacks like a duck (ignition miss) it might just be .........". Might have gone for aftermarket before but never again.

[ajt3nc]

I went to advance auto parts and bought cap and button for a friends 525, ( Sunday, gotta be done deal).
It started a no start miss deal after I was trying to cure a stumble.
I am a huge K.I.S.S. fan and kept messing with it. There rotor button is a cross reference and is actually 1/2 inch shorter than stock.
BMA cured that issue.