Missing during hard accelleration with high electrical load

[Chris Ryan]

I haven't posted for awhile, but last night I noticed a curious phenomenon. While accellerating onto the highway (2nd gear, 3rd gear) my car started missing and coughing, felt almost like the rev-limiter but rougher. It happened as revs hit 4000+.

This was at night in the rain, so I had lights & wipers on. Next day, no lights, no wipers, it seemed fine. So I Turned on the lights to test, and it happened again today.

Has anyone had this experience. The car is a stock 1984 911 Carrera with 175K miles. Top end rebuild at 130K just to fix a broken head stud; it has run perfectly ever since. I've owned it for 6 years and it has never acted like this before.

Any ideas ? Electrical issue?

Thanks, Chris

[psalt]

How old are your plug wires ?

[pete3799]

Alt. OK?
Check your voltage at the battery ( motor running, lights and no lights)

[LJ851]

put someone in the passenger seat with a volt meter attached to cig lighter and do some acceleration runs. this will prove if there is a connection between voltage levels and misfires.

[Chris Ryan]

OK
Tried it again yesterday and today. Dry weather, daytime driving. I pulled over on the hiway, turned on lights, heater (blower), wipers, and booted it. Couple of times - No problems. Pulled strong to redline...
When I noticed this originally, it was raining. I have a K&N cone shaped air intake (removed stock air box) Water getting sucked into intake causing this ?

[911pcars]

Difficult to diagnose over the air waves, but I'd put a voltmeter on the battery just to verify there's sufficient voltage from the charging system.

It could also be incipient fuel pump failure - random issues in fuel delivery.

Sherwood

[Walt Fricke]

How about the cylinder head temperature sender? Have you ever replaced this? If not, you are due. You can check that with an ohmmeter, though best to do that when it is acting up, not when on its best behavior.

I don't think you have quite the classic CHT symptoms, but worth checking nonetheless.

[PatrickB]

+1 on the CHT.
Also, if the problem is intermittant, and happening with no consistancy at all, it could be injector (s.) I had this problem several years ago, and it drove me nuts for over a year. Turned out I had 2 bad injectors. Replaced them all, and it was like I put a new motor in!
Is your gas mileage off as well?

[T77911S]

if you suspect electrical power, places like advance will load test your system. it will at least eliminate it...for.....FREEEEEEEE!

[johnsjmc]

This is classic symptoms of spark plug wires and possibly plugs also. The missfire in the rain and no problem dry should tell you. It often occurs when the sparkplug gap has grown a bit with miles. The ign system continues to fire with higher voltage at the wider gap but the chance of spark leakage increases with wet wires. Seen it often.