Sanctuary

Hey guys.

Wondering if you can help me out.

I have an intermittent Code 33 coming up on my '87. Usually appears within 5 minutes of driving under moderate acceleration on an incline.

As this relates to the MAF, I've replaced the MAF and both relays. That didn't help, so I replaced the TPS (I know this is related to the circuit). The TPS was set to the default value (0.54V). I've also tried a different ECM and that hasn't helped. I guess my next step is to start tracing the wiring which looks like a nightmare. Is there anything I can look at before I do that? The Code 33 is the only code coming up and obviously forces the car into "limp home mode" which is such a pain in the ****.

Any help would be hugely appreciated.
Thanks!

AGENT 86

Intermittents are a pain
Have you followed the GM flow chart etc ?
http://members.shaw.ca/corvette86/Code%2033.pdf
Good luck

jackdaroofer

Did you replace both relays at the same time? This is important!

Did you "clean" the electrical connection at the wire harness when you uinstalled the new MAF..also important

seeya

Sanctuary

Yeah, I take it that's from the Service Manual? I can't actually read that PDF for some reason. But anyway, I have the '87 Service Manual and it basically says go trace your circuits if it's an intermittent.

I also replaced both relays at the same time and used contact cleaner on the MAF connector.

Do these circuits go up the back of the transmission tunnel? I assume so. That seems to be the area where the damn things earth out. Such a beatch to get to. Oh well.

Any other suggestions before I start tracing?

rick lambert

Just curious if I read the manual diagnosis posted by our friend Agent86 correctly???? where it references the fuel
pump relay-I'm quite familiar with at least part of the function of the relay-but was I reading something into it? or did the diagnosis refer
to voltage controlled through the FP relay??? One thing is it's a $12.
part-the other is the fact that alot of us here have found the wiring
insulation at least to the FP relay shrinks back exposing bare wires-
which in my case were comming into contact with one another. Now
I need to go back and see if I read it correctly-or in correctly.

Just went back and re-read it. I think I'm correct-but will humbly accept a correction-just hope someone can explain it to me.

Morley

Try this. I got it from a web page some time ago.

There are three codes that pertain to the MAF sensor: Codes 33, 34, and 36. Code 33 (mass air signal voltage high) is set when the computer sees a high mass air reading at low engine speeds. In other words, there appears to be more airflow through the MAF sensor than the engine can physically breathe at a specific throttle and rpm.

Code 34 (mass air low) sets when the computer registers less than 5 grams of airflow at idle or 2.5 grams/second of air when the engine is first started. The mass air wire inside the MAF sensor should heat up after the car shuts off to clean any contaminants from the wire. When this fails to happen, the computer shows a Code 36 (mass air burn-off malfunction). We'll address and troubleshoot each code individually.


The mass air sensor is a simple design. It's comprised of a wire that heats up when the car is running. The air entering the engine through the MAF sensor cools the wire. The more air ingested by the engine, the more air must flow through the sensor. More air means a cooler wire. The computer knows how much air is entering the engine by the temperature of the wire inside the MAF sensor. The air/fuel mixture is dependent upon having accurate readings from the MAF sensor. If there were a problem, a Code 33 or 34 would be set. Once a Code 33 or 34 appears, the MAF sensor will shut down and the car will go into "basic strategy" mode, aka limp-home mode, until the key is turned off for at least 15 seconds. You'll know the MAF has stopped working when, in Chris' words, "You let off the throttle and it feels like an anchor just dropped."

A Code 33 can be set for several reasons. It appears if the dark green wire (circuit 998) has an open circuit or a bad sensor; if the black, burn-off control circuit wire (circuit 900) has power with the car running; or if the system has a bad ground. You must check for an open circuit on 998 at the ECM on connector B12. There are several places to check for a grounding problem. The cast-iron-headed '85s and '86s have the grounds on the rear of the driver-side head. You'll have to pull the wiper motor to check. The aluminum-headed '86s and '87s have a pack of grounds next to the oil-temp sensor and filter. On the '88 and '89 Corvettes, the ECM, dash, and sensor grounds are on a lower lefthand-side bellhousing stud.


A Code 34 could be set for several reasons as well. A bad ground, an opening in the ducting between the MAF and the throttle body, incorrect minimum idle-air setting of the throttle body, or a worn throttle body can all cause a Code 34. Typically, a worn throttle body or a bad ground will set a Code 34 intermittently (every third or fourth time). To see if the minimum idle air is causing the problem, open the throttle slightly as you try to start the car again. If it's easier to start, check the minimum idle air.

To check the throttle body, simply depress the cruise-control bellows and observe if the throttle-body linkage moves laterally instead of only rotating on the shaft. If there is noticeable movement, your throttle body is worn and the engine is getting unregistered air through the gaps.

'86-'89 Corvettes use a powered fuse link and connector that supplies power to the fuel pump and MAF relays behind the battery. The '85 Corvettes do not use relays for the MAF, but instead use a module next to the ECM. This module rarely goes bad; but to check the module on '85 Corvettes, look for 12 volts at connection E on the MAF with the engine running. Then, run the engine for at least 5 minutes, turn off the key, and check for 12 volts at connection D (MAF burn-off). The powered fuse link is a common cause of intermittent MAF codes. Check the connection and wire for a possible open circuit.
The oil-pressure switch is another possible cause of intermittent MAF codes. If the engine feels as if it has an intermittent hiccup with a fuel pump and MAF code set, the oil-pressure sensor would be the place to start. If the switch is bad, the engine tries to start because power to the pump at startup is fed through the ECM's fuel-pump-relay drive (connection A1).

The relays changed in late '87. Relay No. 14089936 operated the MAF power, burn-off, and fuel pump. Although not always accurate, the connectors for the 9936 relay are the gray connector for the burn-off circuit and the black connector for the MAF power circuit. You'll notice that the new connectors are weather pack connectors to seal out moisture.
To check that the MAF burn-off is getting power, look for 12 volts at the MAF connector position E.
If you get a Code 36, MAF burn-off malfunction, it should immediately reveal itself at startup and it won't affect performance. '85 Corvettes will never show a Code 36, only '86-'89 Corvettes. You can check to see if the ECM is calling for an MAF burn-off by running the car for at least 5 minutes, and checking the black wire at connection F (circuit 900) on the gray connector. It should be grounded for approximately 30 seconds. This is the ECM grounding the wire to actuate the relay. If there is no ground, the ECM is at fault. If you have ground, check the power to the relay. If the relay is sending power to the MAF, check that the dark blue wire (position D) on the MAF sensor connector has power.

rick lambert

Morely, that was great of you to provide all of that what I am referring to is paragraph 2 of the CODE 33 pages from the Helms-
"The oil pressure switch or the ECM, through control of the fuel pump relay,will provide 12 volts for the MAF power relay which provides the 12 volts needed by the MAF sensor."

the part where it says "through control of the fuel pump relay" that's what I'm curious about-am I reading this wrong-or could the relay-or bad wiring to it be causing a voltage problem for the MAF??

Morley

Yes, the pump relay could cause the MAF code to be set.

rick lambert

Thanks Morley-I thought I was reading it right-guess I never thought of the FP relay being a possible cause of Code 33, just goes to show the value of two things.

1. The Helms manuals
2. The Forum

Sanctuary

Thanks guys. I'll check those areas out

Snuffy

Just a thought. I had trhe same problem a few years back on my 88. The 33 code would not always set and therefore I wouldn't believe that it was the MAF since it was expensive to replace. I replaced the relays and cleaned the contacts numeorus times but still a code 33. With a new MAF from GM no more problems.
Is it possible that the MAF you bought is faulty? Did you buy it new? Just a new perspective on the problem.

Sanctuary

Yes, it was brand spanking new. And yes, I did try another just in case

Sanctuary

Just an update guys. Looks like the problem is SOLVED!

I found the bunch of grounds near the oil filter, cleaned those up and while I was at it noticed the fuel pump relay had bare wires heading into it So I cleaned the relay, re-insulated the wires and just took her for a drive. Not a problem now! So thanks guys!

AGENT 86

Merry Christams

Sanctuary

All the best, mate and thanks