MAF bad or lost calibration in ECU?

[1BadSVT]

Hey guys,

I've been diagnosing an ok start up but quickly stall non idle condition on my 96 Cobra. In summary, it's not stock, KB SC 16+ psi, 42 lb injectors, exhaust, meth, Lightning 90 MAF, etc...

It does not have a chip and was programmed via SCT pro software by a previous owner.

I replaced the IAC, the TPS, and no major change. I cleaned the MAF and it ran better briefly and then returned to the stall and die condition.

I unplugged the MAF and the car magically idled....

The issue I have now is I don't know if the car lost the MAF calibration or if the MAF is bad.

I plan to measure the voltage at the MAF after work today but wanted to see if anyone had any pointers on how to rule out or confirm a bad MAF. I know this is a unique situation since it's not stock but I think the MAF is what it is. The only calibration would be made in the ECU tune.

 

[lwarrior1016]

Yeah, there isn’t really a calibration to be lost for the maf. That would imply that the tune went corrupt (which does happen but is much less common). Unplugging the maf defaults to a set fuel amount because the engine doesn’t know how much air it’s getting. I’d venture to say the maf has gone bad.

 

[1BadSVT]

Yeah, there isn’t really a calibration to be lost for the maf. That would imply that the tune went corrupt (which does happen but is much less common). Unplugging the maf defaults to a set fuel amount because the engine doesn’t know how much air it’s getting. I’d venture to say the maf has gone bad.

Maybe Im saying it wrong. From what I've gathered sorting through all the good and bad info out there is the computer actually has a table of values for the MAF. The MAF is just a sensor, but that info is sent to the ECU as a voltage out. The ECU then converts the voltage signal into an air flow based on the programmed table specific to that MAF and coupled with a few other sensors and the O2 sensors, it will adjust your short term and long term fuel to maintain or achieve target AF ratio.

Thank you; I'm leaning towards bad MAF as well.

 

[lwarrior1016]

Maybe Im saying it wrong. From what I've gathered sorting through all the good and bad info out there is the computer actually has a table of values for the MAF. The MAF is just a sensor, but that info is sent to the ECU as a voltage out. The ECU then converts the voltage signal into an air flow based on the programmed table specific to that MAF and coupled with a few other sensors and the O2 sensors, it will adjust your short term and long term fuel to maintain or achieve target AF ratio.

Thank you; I'm leaning towards bad MAF as well.

No, you’re saying it correctly and your understanding is right. But the table that is calibrated in the computer won’t lose its calibration.

 

[96blak54]

Interesting
Watching this one!

Honestly tho, if and whenever i get my car put back together, im doing away with the 30yr old ecu and harness in favor of aftermarket

 

[I_LIKE_TURTLEZ]

If it stalls with the MAF plugged in, majority of cases that's indicative of a large air leak that the ECM can't compensate for with fuel.
You need to look at a freeze-frame of your fuel trim(s) when it stalls & then how many grams of air the MAF is reading, about 1g per engine liter is what we expect to see at idle.
That's the preferred method.


To answer your meter-testing question & in the event you have MAF codes:

1. Use a diagram to verify you have 12v at the MAF connector with key-on-engine-off(KOEO), being careful not to spread the terminals.
2. Verify you have ground coming from the ECM on that same connector.
3. Verify your signal wire going back to the ECM, car needs to be running & voltage should rise smoothly as you actuate the throttle by hand.

 

[1BadSVT]

I did a little more reading and was thinking about this today and I'm going to check for coolant leaks around where the intake gasket is and spray with starter fluid with the MAF unplugged to see if that changes idle rpm. I think it's down to 3 things, MAF bad, MAF tables/program incorrect, or intake gasket leak (I've checked everywhere else for vac leaks).

I remember there is more "spray" coming out driver's side after start up but I thought this was just the running rich condition. That might actually be some coolant leaking out. It's minimal as the car wasn't running hot and the coolant reservoir looks fine and not draining but worth checking. Will keep you guys posted on what I find out tonight.

 

[ttocs]

looking at the pic of the maf above, are you 100% sure the harness is completely seated and making good contact? It looks like it is just barely sticking out, and that could cause some resistance in the connection that would throw everything off. Just out of stupidity start the car up and grab the harness and give it a good pull/tug/push/twist and see if the motor changes how it is running.

 

[1BadSVT]

I'm completely lost at this point. Getting a high idle condition and I can't really explain what it's doing.... I might have a brand new O2 sensor or sensors that are bad as well. I'm probably going to take off Friday to spend some time debugging.

 

[ttocs]

it shows your throttle position is at 20% open which i would guess would give about 1700 rpms. See if the butterfly is stuck open, if the harness is on tight(make sure there are clips on both sides) and then you can measure it with an ohm meter and a quick vid on youtube to show you how.

 

[1BadSVT]

it shows your throttle position is at 20% open which i would guess would give about 1700 rpms. See if the butterfly is stuck open, if the harness is on tight(make sure there are clips on both sides) and then you can measure it with an ohm meter and a quick vid on youtube to show you how.

That's a great catch. I completely forgot about setting the TPS to the correct voltage. It looks like I'm fighting a couple issues all at once. The oxygen sensors are new but looks like both sensors on bank one are showing zero volts. I was curious about the 20% but didn't know what the RPMs should be so I appreciate that. This is a new TPS and I for a fact I did not adjust it or check the voltage.TPS adjustment

 

[ttocs]

I mention to make sure that the harness is on tight because I had a similar problem with mine. While remote tuning and I was outside the car my tuner noticed that the TPS was going between 4-6% for no reason. When I tapped on the sensor it would get even worse so I thought that it must be a bad sensor. So I get a new one, install it and confirm that it is working correctly and then again once the motor is hot the tuner notices that it is floating around again. This time I grabbed the harness and jiggled it and it again went nuts. One of the two clips were broken and once the motor got hot, the one would not hold it tight enough. A new harness and everything was good.

 

[95opal]

That's a great catch. I completely forgot about setting the TPS to the correct voltage. It looks like I'm fighting a couple issues all at once. The oxygen sensors are new but looks like both sensors on bank one are showing zero volts. I was curious about the 20% but didn't know what the RPMs should be so I appreciate that. This is a new TPS and I for a fact I did not adjust it or check the voltage.TPS adjustment

No real need to set TPS...the ecm takes a base reading upon start up. I'd be looking for vac leaks

 

[I_LIKE_TURTLEZ]

That's a great catch. I completely forgot about setting the TPS to the correct voltage. It looks like I'm fighting a couple issues all at once. The oxygen sensors are new but looks like both sensors on bank one are showing zero volts. I was curious about the 20% but didn't know what the RPMs should be so I appreciate that. This is a new TPS and I for a fact I did not adjust it or check the voltage.TPS adjustment

It's near impossible to diagnose vehicles over the internet but my intuition tells me you have a vacuum leak/failing MAF, it explains all of your problems and is the most common cause.
A vehicle not staying running while it's cold won't be the o2 sensors, the ECM completely ignores them until closed loop is achieved at 175F.

Your o2's are right where they should be from what I can see.
Hopefully you didn't buy a store brand TPS, you're looking for 0.98v-1.0v from it.. please don't make the mistake of adjusting the throttle-body itself

If I was going to shotgun a part it would be an OEM 80mm MAF, best I can come up with on a whim without being there to test things.

 

[1BadSVT]

It's near impossible to diagnose vehicles over the internet but my intuition tells me you have a vacuum leak/failing MAF, it explains all of your problems and is the most common cause.
A vehicle not staying running while it's cold won't be the o2 sensors, the ECM completely ignores them until closed loop is achieved at 175F.

Your o2's are right where they should be from what I can see.
Hopefully you didn't buy a store brand TPS, you're looking for 0.98v-1.0v from it.. please don't make the mistake of adjusting the throttle-body itself

If I was going to shotgun a part it would be an OEM 80mm MAF, best I can come up with on a whim without being there to test things.

Absolutely understand but I appreciate everyone's thoughts for sure!

The plan is to measure/adjust the TPS and see if that resolves the hanging idle. I didn't know that about the O2 sensors so thank you. I was thinking both bank 1 sensors could be bad since they are reading zero volts. The plan is to hook up the SCT and double check the readings. I'll shake the harness and see if anything changes as well. In addition I'm going to break out my mirror and see if I can find the source of the fluid leak on the front of the engine. It appears to be coolant and is on a bracket behind the power steering reservoir. This is why I'm suspecting the intake gasket.

Unfortunately, by changing the O2 sensors and TPS I may have added variables to the issue. I need to systematically measure each of these and rule them out.

 

[I_LIKE_TURTLEZ]

Absolutely understand but I appreciate everyone's thoughts for sure!

The plan is to measure/adjust the TPS and see if that resolves the hanging idle. I didn't know that about the O2 sensors so thank you. I was thinking both bank 1 sensors could be bad since they are reading zero volts. The plan is to hook up the SCT and double check the readings. I'll shake the harness and see if anything changes as well. In addition I'm going to break out my mirror and see if I can find the source of the fluid leak on the front of the engine. It appears to be coolant and is on a bracket behind the power steering reservoir. This is why I'm suspecting the intake gasket.

Unfortunately, by changing the O2 sensors and TPS I may have added variables to the issue. I need to systematically measure each of these and rule them out.

Intake gasket would explain the idle issues and un-metered air causing a rough/stall, and yes that's the problem with shotgunning parts, it adds unknown variables and frankly most aftermarket parts are junk compared to OEM, especially these days.

Whenever I'm diagnosing my goal is to physically touch as little as possible, there's nothing worse than touching something and fixing the problem. I save the ol' wire tug for a last resort until I've narrowed down a section of harness.

Plan to work and work the plan.

 

[1BadSVT]

Checked the TPS voltage and it rang up 1.06. I put the old one back on and it's roughly 0.94. no major changes but idle did come down a little. She purrs but it's still hanging idle. The car sat a day since I last cranked it so the WD-40 had some time to settle. I sprayed the area I suspected again and can see the WD-40 getting sucked into the engine. It appears it's an intake leak. I picked up both intake gaskets and they are in route now. (IMRC and intake gaskets)

Time will tell but I'll go ahead and get everything torn down and use this time install my new gauges. Glowshift fuel, boost and wideband for the win!

 

[1BadSVT]

Well, I wanted to double check the intake leak suspicion before ripping the whole engine apart.

I'm not really sure what's going on at this point. Start the car up and it's purring but again at a higher idle. I took it for a short test drive and the idle was hanging up higher at 1900 to 2000 rpms. I took a look at the SCT logs for the startup and didn't really see anything that stuck out but I did notice the IAC numbers looked different than what I remember seeing when I first started logging the car. The driving log made it more apparent as the IAC didn't really correlate well with the other variables. I changed out the IAC for the original that was on the car and the idle magically went down to around 1,000 RPMs.

I've seen horror stories about brand new parts not doing well and this was one of them. I'm not ruling out the intake gasket leak at this point because there are other signs and the previous no idle condition mentioned at the start, but I may have magically sealed it in the short term with the WD-40 or with the other sprays I used to check it.

I unplugged the fan and put a dab of darker oil near the intake gaskets and did not see anything sucking into the intake so my first test may have just been blown around by the fan or my eyes just aren't as good as they used to be.

In the end, I have to mark this one as unsolved mystery, just get some more miles on the car and monitor.

To be continued....

 

[07GtS197]

All non oem iac valves are junk. Try cleaning the stock one. If you reset your ecu recently that’s probably why it’s idling high. You’ll need to do an idle relearn or put x amount of miles on the car before it starts idling like normal again.

 

[1BadSVT]

I replaced the MAF and it has a wider value range on the OBD log. I reset the ECU. Next, I tried to train the idle.

I cranked up the car and let it idle til it warmed up a bit, turned on the AC full blast and it struggled a bit. I let it warm another minute or so then turned AC on full blast again. The car seemed to learn the idle. I drove the car to work with no major issues. The RPMs did float a little with clutch in but would settle after stopping for a few seconds. I left early at lunch and afterwards, I tried to start the car and it wouldn't idle... Lol. I'm enjoying debugging and will need to do a writeup once I'm finished figuring this one out.

I had to hold the throttle after start for a second or two them it figured itself out. I'm not sure if I mentioned replacing the EGR purge regulator but I just noticed the hoses to the canister are in terrible shape. It may be the source of a vacuum leak. I'm also seeing P1443, which is related to this canister not functioning correctly.