CGT NG Check Engine light. Need help mentally troubleshooting

[Jonathan Monetti]

I am looking at an 87.5 CGT.  Car was OK on the test drive, but I thought it
was not making great power--felt like it was hesitating a little, and
generally weaker than expected throughout the powerband.  Less than 10
minutes into the drive, the check engine light came on.

The right side bulb behind the digital dash was out, so I had no way of
seeing the coolant temp.

Stopped the car, opened the hood--radiator return hose very warm to the
touch, so thermostat appeared to be functioning properly.  Did a general
eyes & hands check--no oil in coolant/no funny bubbles on dipstick, no
smoke, drips, or other unusual stuff.

Started the car up, check engine light was dark for about 1/8 mile, then
came back on.  I drove the car back (very gently), and the owner and I
popped the hood and played with it a bit.  It was idling roughly, but no
other obvious signs besides the check engine light.

He said had not had this problem, but a few days before had pressure cleaned
the engine compartment (ugh) and put on a new set of ignition wires.

He's going to have it checked out, but I was hoping to get some suggestions
on some of the most basic things to look at on the NG, and hopefully rule
out.  Since its not my car (yet?) I only have the luxury of thinking through
possible causes.

My '86 had many issues, but I had almost _no_ engine problems, so I am
pretty green at diagnosis. Especially with an NG.

Any thoughts welcome

Thanks
Jonathan Monetti
2002 A4 1.8tq which aint worth a damn and maybe an 87.5 CGT if I can sort
this out.