MAF code 33???

[iroc343]

I have a 1987 iroc with the 350 tuned port engine. Sometimes, at just about the time I think it is going from open loop to closed loop it will hesitate and the service engine light will come on. It will still run but won't idle and the engine hesitates when accellerating. Sometimes if I shut it off and restart it the service engine light will go out and all is fine. Other times I have to power down the ECM for about 30 seconds and then it will run fine, until it goes through another cold start. If I pull the codes I get a 33 - MAF sensor. I have replaced the sensor and get the same results. I replaced the MAF power relay - same thing. The car runs great but this is driving me crazy. I have replaced about every sensor/switch, the ECM, injectors, coil/ignition module, EGR valve, and the TPS is set to .54 millivolts as spec'd. If anyone could help me I'd be forever grateful

[Morley]

Replace the MAF burnoff relay and the fuel pump relay. The MAF system is a finicky bastage and power for it is routed through some strange places.

[iroc343]

Thanks Morley. I have purchased a new burn-off relay, but I am having trouble locating it on the car, there are three relays by the brake booster, but none of them have the same part number. I think one is the fuel pump relay and one is the MAF power relay, but I don't know what the third one is. Maybe that's the burn-off relay.

[Morley]

Originally Posted by iroc343

Thanks Morley. I have purchased a new burn-off relay, but I am having trouble locating it on the car, there are three relays by the brake booster, but none of them have the same part number. I think one is the fuel pump relay and one is the MAF power relay, but I don't know what the third one is. Maybe that's the burn-off relay.

You got it. One is the power relay, one is the fuel pump relay and the other is burnoff.

[iroc343]

It's back. Replaced all three relays with ac-delco replacements and problem still exists. Runs fine sometimes, then will stumble/hesitate, service engine light comes on and engine hesitates and won't idle properly. Power down the ecm for 30 seconds and all is good for some period of time. Tried cleaning all the contacts on the relay plugs. I'm lost for ideas. Anyone have a guess? Even replaced the MAF with a new Granatelli unit.

[Morley]

Originally Posted by iroc343

Even replaced the MAF with a new Granatelli unit.

Is that one of the "adjustable" MAF's?
Other things to check, look at the female pins on the harness side of the MAF connection. Make sure none of them got "opened up" (spread apart) when you were hooking up the MAF. Also check that the TPS voltage is .54 volts +/- .017v with the key on engine off, upper 2 wires on the TPS.
And this I got a long 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.


(See Previous Caption)
Chris has spent some time adding wires, meters, test lights, and jumpers, chasing down several specific reasons why Codes 34 and 33 will appear. Here is what he found: The dark green wire from the ECM to the MAF (circuit 998) can short to ground or short together with circuit 450 (black or black/white) and cause a Code 34. To get a Code 33, circuits 998 or 450 could have an open circuit. Recently, Chris has seen a few cars for which the only cure was to run new wires for both circuits. This cured the intermittent codes immediately.
Checking The Minimum Idle Air: The first step in checking the minimum idle air is to put the car into Field Service Test Mode. Do this by putting a spade connector into the A and B positions of the ALDL connector. With the key on and the engine off, the fan will turn on and the SES light will flash. Also, the Idle Air Controller (IAC) will fully close.


on to the links http://www.corvettefever.com/techart...odes_problems/

I have a troubleshooting flow chart but I can't attach pictures. If you want them I can E mail em to you.

[iroc343]

Morley, thanks for the info. Yes, it is an adjustable MAF that was supposedly set to stock settings from the manufacturer. I've had three different MAFs on this thing and they all do the same thing. The frustrating thing is that this is a 33,000 mile original 1987 305 tpi 5 speed that I took a 350 from a 1987 tpi iroc and trans planted. Every wire is as factory and all sensors are connected. I mean this thing is done as right as it can be. The 305 didn't have any of these problems and I used the complete tpi from the 305 and now this.

[wrightz28]

Originally Posted by iroc343

I took a 350 from a 1987 tpi iroc and trans planted. .

That would have been nice to know

A) did you use a 350 knock sensor or a 305?

B) Did you make sure all the grounds are connected and clean?

C) Morley, please tell me all that was a cut and paste text

[iroc343]

The knock sensor, located at the oil pan rail, is from the 305. I assumed they were the same. Are they not? All the grounds are attached in the original position both tight and clean. I assure you that this is no hack job. As I had posted before, the tps is set to .54. I will check the female portion of the MAF plug. It's got to be something goofy as it runs perfect until it acts up.

[wrightz28]

Not saying it's a hack job, just that the fact that it's a transplant makes a pretty big difference and yes, the ,knock sensor is an issue.

[iroc343]

Wrightz28, just didn't want someone who may be able to help me give up because they thought this was a hopeless situation. I'll change the knock sensor to the one from the 350. Thanks.

[wrightz28]

If anybody will get you through your issue it would be Morley. Sorry if my initial post carried any interpreted negative tone (which rereading it, it could go either way).

I just always try to convey that it is important when asking for help from those that aren't right ther using all the same 5 senses, odn't leave anything out of the norm, out of the description of the problem. So it's kind of a bombshell gone off to find the motor had been swapped.

Also, my comment to Morley was jokingly as that was alot of typing and I've seen a similar post before.

And the knock could very well be a culpret to your problem.

[Morley]

Change the knock sensor, change the electronic spark control module, make sure you put in the calpack (PROM & carrier) from the 350's ECM.
Triple check all of the wiring connections, especially the MAF connector I mentioned above. Since you say the 305 ran fine and the 350 is having these problems... it has to be in the wiring if the 3 items above don't fix it.

[Morley]

Originally Posted by wrightz28

C) Morley, please tell me all that was a cut and paste text

Yes, that was a cut and paste, I've posted it here more than a few times for others.

[wrightz28]

Originally Posted by Morley

Yes, that was a cut and paste, I've posted it here more than a few times for others.

Thank goodness, my fingers ached just scrollling down though all that.