ECU failure? lots of info inside

[ssjkouru]

So what it is here is my 95 gst is running very rich ever since I got it, I had a apexi turbo timer plugged in and on the a/f setting it would read a nice 14.2-14.6 while I was driving but as soon as I left off the gas or push in the clutch the a/f ratio would jump up to 20.0. surges when i'm just idling

I get backfires in my exhaust all the time when i'm driving and simply left off the gas so I have reason to believe my ecu needs a reflash (a local mechanic said it needs a reflash too). When I took it to the mechanic & the Mitsubishi dealership they both said that the ECU and MAF sensor aren't communicating to each other. I had my apexi SAFC hooked up for a while but removed it thinking this would fix the problem of it running rich, but to no avail.

Any help would be much appreciated on what might be the root cause, if you need any pics i'll gladly take some.

recap;
-surging idle
-rich gas use at idle
-ecu and maf not communicating
-turbo timer reads 20.0 at idle

 

[JusMX141]

By leaving off the gas, the ECU is sensing 0% throttle position at engine speeds higher than idle so the injectors do not open- hence the lean AFR's. This is normal.

Idle surge is not normal...good chance your ISC is bad or the FIAV is not functioning properly.

If the ECU did not read the MAS at all, you would be in limp mode and not able to accelerate above 2000-2500rpms. Either something it wrong with the MAS, or something is wrong with the way you have the AFC hooked up as the AFC's job is to alter the way the ECU reads the MAS signal. I'm not blaming the ECU just yet.

 

[ssjkouru]

Hm, well I have a few "noobie" questions for you. mainly on the abbreviations. ISC and FIAV, what are they abbreviations for?

My second point is that I had a local mechanic shop change out the Mass air flow sensor for a new one, then when the dealership said the ECU still wasn't recieving feedback from the MAS so I had it changed for a second time (in case by some chance it was a dud) but it had no effect. It seems like the ECU just isn't reading the MAS when i'm idling, is what i'm kinda taking in here. What do you think?

 

[JusMX141]

Hm, well I have a few "noobie" questions for you. mainly on the abbreviations. ISC and FIAV, what are they abbreviations for?

Idle Speed Control and Fast Idle Air Valve. Both parts are on the throttle body.

My second point is that I had a local mechanic shop change out the Mass air flow sensor for a new one, then when the dealership said the ECU still wasn't recieving feedback from the MAF so I had it changed for a second time (in case by some chance it was a dud) but it had no effect.

If the MAS has already been swapped with no change in running condition, then the problem either lies in the way the AFC is hooked up or perhaps the AFC itself is not working properly. When you say you removed the AFC, did you hook the wires back up at the ECU harness or did you just physically unplug and remove the AFC from the car, leaving the harness untouched?

If you have a good ECU to borrow that would eliminate the ECU as the possible cause.

 

[ssjkouru]

Idle Speed Control and Fast Idle Air Valve. Both parts are on the throttle body.

If the MAS has already been swapped with no change in running condition, then the problem either lies in the way the AFC is hooked up or perhaps the AFC itself is not working properly. When you say you removed the AFC, did you hook the wires back up at the ECU harness or did you just physically unplug and remove the AFC from the car, leaving the harness untouched?

If you have a good ECU to borrow that would eliminate the ECU as the possible cause.

I got under the dash and just unplugged the safc from what it was hooked up to and set it aside.I did not rehook the end that was connected to the safc, I looked everywhere and could not see where it would plug into exactly so after several hours I just gave up.

 

[JusMX141]

You can't just unplug an AFC- it's hard-wired into your ECU harness. Without the AFC in place the signal wire from the MAS is an open circuit.

 

[ssjkouru]

Could you please elaborate? I think i'm a little confused.

 

[donniekak]

Could you please elaborate? I think i'm a little confused.

The signal wire from the mas is cut for an afc install. The afc intercepts the signal, modifies it, then passes it on to the ecu. By removing the afc, and not re-connecting the wires, you leave the mas disconnected from the ecu.

 

[ssjkouru]

The signal wire from the mas is cut for an afc install. The afc intercepts the signal, modifies it, then passes it on to the ecu. By removing the afc, and not re-connecting the wires, you leave the mas disconnected from the ecu.

Ah I see. Shouldn't the afc have 2 wires then? one for recieveing the mas data and one to send the modified data to the ecu? and why wouldn't they install a afc from the factory?

 

[donniekak]

Ah I see. Shouldn't the afc have 2 wires then? one for recieveing the mas data and one to send the modified data to the ecu? and why wouldn't they install a afc from the factory?

It does have 2 wires for the mas signal, input, and output. The factory doesn't install them because it is an aftermarket tuning device.

 

[ssjkouru]

It does have 2 wires for the mas signal, input, and output. The factory doesn't install them because it is an aftermarket tuning device.

Ah, Thank you for your help then. I don't think I have many other further questions.