87 305 TPI: MAF Issues....

[Craig Moates]

Most respected folks,

I am in search for a resolution to my problem. I presently receive an intermittent Code 33 from my system (87 305 TPI TransAm). I've tried throwing parts at it (pigtails, relays, ECU, TPS&MAF sensors), tried adjusting it (TPS, hammer and nails;<), etcetera. 

The MAF appears to get good power supply voltage (coincident with initial firing of fuel relay and steady 12-14v after startup), and the MAF seems to give good readings (ballpark 400-700mV @ 700-1200rpm) initially. However, if I monitor the signal voltage from the MAF, a bauble occurs at seemingly random times (often very soon after start-up), where the voltage jumps up to 3.2v or so (or was it 2.8v or so, similar to voltage measured with MAF power supply cut off). This is quickly followed by a Code33 set. To eliminate, the ignition usually must be cut off for 10 seconds or so prior to restart. Once it sets, the voltage stays around the 3+v level (if I remember correctly...it may still vary with engine speed, though). I've gone 2-3 days without coding, other times it's every start-up event. Sometimes as it codes, it gets real rough and rich till the code sets, then it smooths out.

If I disconnect the MAF, the signal wire to/from the ECM reads 5v. If I pass this to ground through 100ohms, it reads about 0.5, indicating 1kohm internal resistance. Anyways, I'm at my wit's end from a tinkerer's perspective. I'm ready to start looking at it from an EFI ECM perspective. 

I have some minor modifications on the engine, and this problem seemed to crop up after head rework and header installation. I am not prepared to spend much cash (Haltech or such), but I am ready to spend attention and time to learn what can be done to better diagnose and/or alter (burn new PROM myself, perhaps?) the present system. Initially, I suppose my thoughts lean towards a home-grown Diacom-type serial interface, or a swap-over to a more versatile/supported ECU for read/write activities. 

I see this as a potential for gaining knowledge, but if anyone has a basis of experience from which they might be able to help with my present issue rather handily, it would be much appreciated. Right now, the MAF and accompanying relays have been removed to keep the car from stumbling and stalling each time the Code33 sets. I assume this throws the car into 'limp-home mode' (which is really rather smooth), but this is unacceptable for any extended period.

I'm a 30-year old engineer (PhD Chemical) with a fair electronics background, and I'm willing to listen. I hope someone here can give me guidance, or redirect if there is a more appropriate forum. I've been searching for some time now.

Sincerest thanks,
-Craig Moates

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