I believe the 90 Auto and 91 Auto ECU's are the same. The way to double check is the part number on the outside of the ECU.

I can get the part number later today as its still in my truck. I paid 30, i'll get rid of it for 30 shipped since i'll never need it.

Part number for the 90 ECU is MD146946 for those interested. I could trade it for a great working stock MAF and air filter. <img src="/forums/images/graemlins/smile.gif" alt="" />


Mindblowing. Somebody please explain this to me. The truck starting running like ass again with the intermittent cutting out/running PIG RICH and rough idle. I touch the maf plug and it goes away then comes back. This may have been my problem all along but whats even crazier is that I unplug the maf and the idle straightens up and goes straight to 750 like a champ. The black smoke stopped and the smell went from raw fuel to a clean burn.

Check engine light is on but I say "Lets see how it drives" My Galant wouldnt rev over 2500 rpm without the maf plugged in but oh no, this truck drove perfectly without it. I dont know how this is even possible unless there is some sort of a base map? Coming to a stop the idle was perfect, like BRAND NEW. Accelerating fine and no hiccups. The only down side is the CEL. Is my problem the MAF all along and if so why does the truck run perfectly without it? WEIRD.

When the engine is cold, the ECU reverts to open loop - that is, a basic preset configuration. Once engine temperature is in the operating range, the ECU switches to closed loop where input is taken from sensors and the fueling lookup tables are consulted by the ECU for fine adjustments.

I don't know if a bad MAF can switch the ECU into open loop, but maybe so. If the MAF is bad, you should have a code stored in the ECU..... but since this MAF is a multi-function sensor (barometric pressure, air temp, air flow, etc.) some MAF functions may not be code reporting. Poll the ECU for codes first.

Frank

The truck was running so rich it was ridiculous. Normally when cruising in closed loop on the highway every thing worked out even with the bad maf. Now, it's 10 times better. So basically its just running a base map thats a good idea but its not compensatating as close as it would with a good working maf. Still its a vast improvement and I'll drive it like this until I can pick up a good working one.

I think the plug connections is the problem. I think you have intermittent connectivity in that plug. Sometimes it's fine, sometimes it's not. When it's not, the engine reverts to limp home mode where it just uses the MAP sensor and TPS sensor to guestimate the air flow, and it your case it looks like it does a pretty good job of limping. Better'n Festus, anyway.

The internal guts of the MAF sensor is all solid state, and not nearly as likely to fail as the connector is to get either "almost" plugged in, or a tad corroded, or some other 'lectron gremlin that keeps the little 'lectrons from going where they're supposed to. Might possibly have some air flow crud on either the ultrasound emitter or receptor in the Karmann vortex sensor, but they are after the air filter.

My thought on too rich is a stuttering connection to the MAF that's throwing off the measured voltage that the maf sends to the ecu, in effect, telling the ecu a wrong number, in this case showing more air than actually passes to the motor, making the ecu dump a very rich mixture.

I don't remember more than a very few cases of actually bad MAF's on here.

The reason I don't think it's just a "going to open loop" problem is that it shouldn't go real rich in open loop if the maf is doin' right and you have consistent ignition and the tps and map are also ok, as is the rpm signal and the cts. Closed loop has a pretty limited amount of trim factor compared to the total afr in this kind of system.

The intake air temp sensor is also in the maf housing and uses the same connector, and may also be sending wonky for the same reason. The ecu uses the map and the iat sensors for readings for mixture corrections for varying ambient conditions. If the iat is sending an incorrect voltage over time (it's a thermistor, and varies resistance with temps to vary the voltage signal the ecu hears), you might get and incorrect air density, also throwing off the ecu calc'ed fuel amount to rich. I'd have to logic some more on that one in my head before I'd have an opinion on how much effect it would have.

Crank, I'll trade you a known good MAF for your 90 ECU. PM me if your interested.

Let me work on the truck a little bit this weekend and see if thats really the issue. I am buying new battery terminals and going through to every connectors to try and track it down. I'll let you know.

Ok I replace the battery terminal with the brass ones with the wingnut and cleaned out the maf. (had some oil from the intake puddle around it) Havent had any starting problems and runs well with the maf on now. I still dont trust it to drive to work but I am going to run errands in it around town to see how it goes. I have a feeling the intermittant (running like ass) might come back if the maf gets dirty with oil or that connector screws up again.

Alright everything is running well. The new battery terminals seemed to solve it. I kind of feel like a moron that replaced just about everything and then did those last. I wonder what all I didnt need. Eh well it was preventative maintenance and now I have spares.

The 90 ecu is still up for grabs for 30 shipped. PM me if interested. Thanks for everything guys!