Engine is running rough with an occasional backfire. Analyzer is indicating Codes 33 and 36. I have replaced both the MAF Burn-Off Relay and MAF Power Relay with no improvement. Not sure what to try next. Does anyone know what the voltages going to the MAF should be?
Thanks!

 

http://www.chevythunder.com/maf_code_33.htm

 

The MAF power relay supplies 12 volts to power the MAF.

The MAF output signal going to the ECM varies from zero to 5 volts, depending on the air flow. It's directly proportional to air flow, so low voltage means low air flow.

The MAF burnoff wire has power on it when the ignition is shut off. The ECM turns on the MAF burnoff relay for about 2 seconds. If you look in the MAF you can see the sensor wire glow red during burnoff.

 

Just a thought have you checked the EGR (exhaust gas recirculation valve) is shutting fully?

If that is not shutting the car will run rough at idle? You can try removing the vacuum hose from the egr and experiment with the car running put some manifold vacuum to it and check the valve opens and closes when the vacuum is removed.

You will need to shine a flashlight under the plenum, or remove the plenum and one of the runners.

Backfiring is usually timing, confirm the timing is right also.

Also check the IAC (idle air controller) is secure, they can come loose and the idle will be rough and unstable.

You can always get some MAF cleaner and spray that to clean the sensor wires, maybe you need to find someone with a vette who will let you try his MAF.

 

OK, here's the results I have found so far. I worked through the flow chart provided by www.chevythunder.com. Voltages all checked out with the voltage between term "C" and chassis ground reading 4.99 volts. So, replaced MAF and erased both codes. However, once engine was restarted both Codes 33 and 36 came right back. Any other suggestions I should try?
Thanks,

 

This is a long shot.....well, not actually !

I have had to replace the PLUG to the MAF at least 3 times. The wires are very crowded in this small plug that's in a place that's exposed to a very harsh environment.....heat, wet etc...
The wire harness is **** on C4s anyway, and the least bit of water/dirt in the plug starts oxidation that eats its way up inside the insulation and ruins the wire AT the plug. Cross contamination, grounding, plain shorting IN the plug. TRY to separate each wire with a pick and see IF that might make ANY difference. IF SO, get a new plug pig-tail and solder it in...

You should also try wiggling the injector harness driver side where the MAF wires originate. They tend to rot and short on that side. the harness is a splice that drops out of the trunk line on the fire-wall. Not a great design. I chased a mystery misfire for MONTHS and accidentally discovered the rotten wires simply by wiggling the inj harness on the driver side valve cover...
You should also do this same wiggle/inspection of the wire to the 2 MAF relays....the burn-off is NOT critical...and has nothing to do with the engine running....the POWER relay however, IS very sensitive to rotten wire issues...swap 'em around just to be damn certain you got a good working relay In the POWER plug...never assume these come from the store GOOD....they do not !

Hope this helps

 

Error code 33 is MAF output high. That could be cause by a bad connection at the MAF or somewhere between the MAF and the ECM. If the MAF connector is not plugged in you will get this error.

Error code 36 is MAF burnoff fault. The ECM looks at the MAF output as it applies 12 volts to the MAF sensor wire. If the voltage is too high or too low it will set this error. Again, you will get this error if the MAF connector is unplugged.

It appears that the connection between the MAF and the ECM is open. That could be a corroded pin in a connector or a broken wire. Checking the MAF output with a scanner would be very helpful. That would tell you exactly what the ECM is seeing.

 

OK, here's where I am to date. I replaced the plug to the MAF with a new one and checked the wiring going to both the plug and the two relays. No noticeable problems and the two codes still come back after being erased with the scanner. I then replaced the MAF relay with a new one and still no improvement. One question: On the wiring diagram I obtained from "chevythunder" three relays are shown, i.e., burnoff relay, power relay, and sensor relay. I only find the burnoff and power relays at the location on the firewall behind the battery. Is there a third one and, if so, where should I be looking?
And thanks again for all your suggestions.