I am hoping someone could assist me in identifying the cause of the 'check engine' light coming on. It has happened randomly after the last three track weekends - usually after four to eight runs (plus the drive there and back). The car performs beautifully all day with no apparent problems and no engine hesitations. Then about 5 minutes from home when I was driving leisurely at 120 km/hr, the light came on.

My mechanic diagnosed it after the latest incident and said the computer diagnosis was "random misfire", whatever that means. Disengaging and reconnecting the battery (remebering the radio code)resets the computer and the light goes out until the next random occurence. This is a pain. Any ideas would be sincerely appreciated.

1997 C2 (not chipped but planning to).

 

If your mechanic had the diagnostics tool there was no need to connect& disconnect the battery to the best of my limited knowledge. Do a search on this as there have been multiple threads about it, the word <FONT color=red>carbon build up</font> might pop up. I bought the software listed on p-car.com pretty good tool allows you to read the code(s) and reset it.

 

My '96 suffered from random misfires as well, for no apparent reason. Over one 2 month period, I had 8 check engine lights, always when the engine was warm, and usually when I was at idle.

[knocking on wood] I have not had a check engine in the last 3 months, and I don't know for certain why it has stopped, but one change to my driving habits that I made was to lengthen my normal commute in the morning (driving to a train station farther from my house) which allows the engine to heat up and run at a higher speed for a longer amount of time. I also did a Techron treatment. And I also give my gas cap a couple of extra turns.

Good luck.

 

[quote]Originally posted by Patrick M:
<strong>I am hoping someone could assist me in identifying the cause of the 'check engine' light coming on. It has happened randomly after the last three track weekends - usually after four to eight runs (plus the drive there and back). The car performs beautifully all day with no apparent problems and no engine hesitations. Then about 5 minutes from home when I was driving leisurely at 120 km/hr, the light came on.

My mechanic diagnosed it after the latest incident and said the computer diagnosis was "random misfire", whatever that means. Disengaging and reconnecting the battery (remebering the radio code)resets the computer and the light goes out until the next random occurence. This is a pain. Any ideas would be sincerely appreciated.

1997 C2 (not chipped but planning to).</strong><hr></blockquote>

Hi Patrick:

Sorry to hear of your troubles but this is not uncommon; OBDII cars are VERY sensitive to these things.

Random misfires can be caused by:

1) Old spark plugs

2) Old plug wires

3) Occasional arcing inside distributor caps and across rotors

4) Dirty fuel injectors

5) Heavily carboned up intake valves

Like Mr. Becker said, if one reads and resets the code with the Hammer or PST-2, there is no need to disconnect the battery.

You didn't indicate how many miles on your car or how long it been since a major tuneup, but you might need to replace some ignition components if they have more than 20K miles on them. Pay particular attention to the resistance of the plug connectors.

Hope this helps,

 

Steve:

Thank you for your insights. I should clarify however that the mechanic did not disconnect the battery... actually I did it at the track on Sat in order to get rid of the pesky light before I could get the car back to the mechanic's shop on Mon. A DE friend suggested this and also suggested that the cause might be the gas cap (as per Kevin's suggestion) ... so I applied vaseline around the groves to assist in blocking any air leakage. Thought it worked until the latest diagnosis about 'random misfire'. My point is that this seemed to be ad hoc fix and the appearance of the check engine light is a pain. (ps. My 98 M3 has had the check engine light on for 150 k kilometers and no mechanic or BMW knows why).

The 993 has 74 k kilometers or about 45 k miles. Just had a tune up and the plugs and wires were fine. I will ask the mechanic to check for dirty fuel injectors. How do you test for heavy carbon on the intake valves?

Also on another note, I have seen comments about reprogramming the ECU for a 15-18 hp increase, and would like your advice if there are differences between shops who perform this. I am bringing it in on Wed and my mechanic will send it to the US, but I don't know which shop. Will ask.

Thanks for your help.

 

This is a question for Steve.

You mention several solutions for a random misfire. I have a 1996 with 20K and the Dealer replaced the DME. I also replace plugs, wires etc. Well to make a long story short the Check Engine Light has returned. Now the Dealer is suggesting that the Flywheel might be the problem. Does this sound right? Can is cause random misfires? Thank in advance.

 

Hi Patrick;

If your car has never had any Techron run through it and it sees city traffic, its a good bet that you have a carbon buildup on the injector tips and intake valves. This leans out the fuel mixture that can cause misfires at certain throttle openings.

I'd sure run 2 big bottles of that stuff through there while driving it aggressively,.....

How old are the oxysensors? Have your mechanic check how they are cycling and observe real-time lambda values from the front pair.

Regarding chips,......It would compromise my credibility and be a conflict of interest to recommend anyone as we offer this service. Ask around here in the Forum and get some opinions & feedback.

MG993:

The random misfire issue can be a tough one to resolve and some cars have been harder than others.

IMHO, the flywheel will NOT cause that problem unless the crank position sensor and ignition trigger are intermittent.

 

Steve:

Thank you for all your help. I will follow your advice and contact you directly about the chip.

Patrick