I have a 1989 Corvette with 31,000 miles. Last summer the service engine soon light came on and stayed on, I reset it but it came back.
It was code 33, so I replaced the MAF and power relay and burn off relay.
The problem came back, however now its a intermitten problem and the light goes out about 30 seconds after it comes on. It repeats itself and happens 2-3 times on a 20 mile trip to work. It almost always happens
on the highway 65-75 MPH, not on the 4 miles about 40MPH trip to the highway. Its error code 33 still. I understand to look for vacuum leaks
and check voltage on MAF power relay is what to do. I was wondering if anything else could be checked with the GM OTC OBD1 scan tool. I was wondering is it safe to drive around with it connected to the ECM?. It has many modes but MAF or voltage is not one of them that I can see. Would mode 5 Oxygen sensor mode be helpful to watch? Any other mode
to watch mode 15 would be nice but it doesn't work with system code 17
which the 1989 uses for the tool PFI V8 engine. Its actaully a OTC 86
tool but works on a 1989 Corvette because it has the same engine. So the question is is it safe to drive around with the OTC scan tool connected and is there any mode to monitor while driving to watch
right before problem will happen. When it happens you get a slight
hesitation in engine, light comes on then, stays on for 30 seconds
then goes out. Thanks for any advice.

Jim

 

Update:

Checked for open circuit on MAF.
Checled power on burn off relay with car running.
Checked for bad grounds.
Checked for vacuum and and intake leaks.
Checked the oxygen sensor.

All checked out. The oxygen sensor I can also monitor the voltage
through the ECM GM OTC OBD! tool.

Anybody got any other ideas as to this intermitten problem. I've also noticed the gas mileage is down, it used to get 24-25 MPG on highway
and 16 MPG in city. Now it gets 21 MPG Highway and 14 city.
I know why and the only way to fix it is to solve the MAF 33 HV.

 

It says if the problem is MAF 33 and its intermitten which mine is to replace the MAF power relay. I already replaced it with a relay from
my Chevy Dealer. I replaced it again today and took a 20 mile trip and the problem didn't happen. Will that solve it only more time and trips will tell. When I first replaced the MAF, and 2 relays it worked for 2 weeks
about 6 trips on road. Will let you know.

 

Well its been about 2 weeks since I changed the MAF power relay since reading the GM TSB on error code 33. Changing the relay has helped
and has reduced the problem to very intermitten now. Its been out on
9 trips about 25 miles round trip to work, it has only happened twice
and was on its 6'th and 7'th trip when it happened. Usual MPH between
60-70MPH on highway, however the surge was less right before the service engine light came on. So the problem is not totally solved
but much better. Hard to say exactly what's doing it. I'll post another update in 1-2 months after longer summer trips and work trips of 25 miles and report.

 

Well its been 6 weeks and the service engine light has not come on once. It's been over 25 trips ranging from work 25 miles to 200 miles to the Jersey shore and back to watch son play summer college baseball. Replaced the ECM was the final solution. It was $160 for a new one.
Of course there was several other things done see the prior posts as it started as the light stayed on all the time and then changed to a
intermitten problem. I'll put what I think would best help others that come up with the same problem. Let me first say nothing really new
was found about the problem and that the Haynes Corvette manual
I had told me what to do in 3 steps, I just decided to ignore the 3rd step replace ECM. I just decided nah it can't be the ECM. That's why I don't write auto manuals for a living. However I did learn alot on how to use the GM OBD 1 computer tool connected to engine and have helped other locals on engine problems, the tool can read numerous things about the engine. Here's what I did:

1-Replaced MAF sensor ($160) and 2 relays one for MAF and burn off
relay.

2-Checked for any intake system air or vacuum leaks. Check and cleaned
MAF sensor connectors. Checked for bad grounds high and low.
Checked oxygen sensor.

3- Replaced MAF relay a second time. Only need to do this to make sure
relay is good. First one might have been bad even though it was new.

If problem still persists after these 3 steps check your ECM. I don't know how you can check the ECM when car runs pretty normal but has a intermitten error code 33 so I decided to replace it per Haynes manual
it was $160 for a new one. Car is back to its old self now and running great. I just hope it continues that way.

Finally I hope this info will be helpful for others who have similar problem or for others who come down with this problem in the future. Enjoy your vette this summer.

Jim in NJ