Re: Intermittent Check Engine Light

[zm@mhcnet.att.com (Zafer Mehmood [209])]

The following is applicable to the 86-88 5000 turbos. Your 89 200T may be
different, so take it with a grain of salt:

One of the reasons the check engine light comes on is to indicate that
the engine computer is detecting knocking, ie., the fuel is not supplying
the octane rating that the engine is demanding. Considering that your
light comes on under heavy acceleration, it is very likely that this is
the case. What octane fuel are you using? If you are using 87 or 89 (pump
octane) try going to 91 or higher to see if the problem disappears. A turbo
Audi will take advantage of the higher octane and perform better. If you are
already using 91+, then you have to try and locate the cause of knocking and 
eliminate it. It could be a faulty knock sensor, engine overheating,
carbon deposits in the cylinders, etc.

Start by dumping the error codes from the computer. On my '87 5000TQ,
no special equipment is required. Bentley shows the procedure - put a
fuse on top of the fuel pump relay and the "check engine light" will
start flashing. The number of flashes should be counted to see what error
code (if any) it corresponds to. There should be a separate code for the O2
sensor. I have found that you can manage without some of the sophisticated
test jigs that are shown in the factory manuals - on one of my other cars,
a Mitsubishi Starion, I found that an ordinary voltmeter is all I needed to
read codes from the computer though the manual showed a complicated looking
piece of equipment. You may find that an LED or a voltmeter may do the trick.

I haven't replaced the O2 sensor on my Audi, (has about 125K miles) and as
far as I know, the previous owner didn't either. BTW, my Audi owner's manual
says it is normal for the check engine light to come on briefly under
heavy acceleration, although it doesn't on my car.

Zafer


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Zafer Mehmood				   AT&T Bell Laboratories
zm@mhcnet.att.com			   Murray Hill, NJ