Question about MAF & Baro - strange miss here & there

[cougar694u]

I have a 99gt with about 156k miles on it. It's been having intermittent misses, which I assumed were related to a coil on it's way out the door.

I did a track day and ended up blowing out the flange gasket on the exhaust and it got worse. I replaced it and it got better, but never went away.

I lost one o2 sensor about a year ago, so I replaced both (for good measure), and cleaned the MAF.

I hooked it up to my Snap-On 2500 and the Baro reading was like 140-145Hz. Here in San Antonio, we're about 800, which according to the snap-on computer, 159 is sea level, dropping 3hz every 1000 feet, so 156 should be close for us here. So my car thinks it's about 4000 feet above sea level, could account for missing (runs leaner and advances timing perhaps?)

It also showed pressure to be between 24 & 25 inches of Hg (when I checked the weather, it showed 29).

The Snap-On computer said to replace the MAF if the Baro reading was more than 7Hz off, which mine is, but I wanted to know for sure the MAF is what's bad before I replace it.

Anyone have any input?

I have a C&L 80mm MAF, C&L Inlet tube, 75mm TB, & C&L upper, and the missing is gradually getting worse.

TIA!

 

[SLN043]

Have you checked for spark, fuel, and compression? do you know which cylinders? or is it all of them? Im no master tech, but you need to test to see if you have those three things, then you can trace the problem back to the source. Or at the very least, replace the MAF and barometric sensor with a known good part, but its not like everyone has spare parts sitting around.

Atmospheric pressure at sea level is like 14.7 lbs. And as you go higher in altitude, he air gets thinner and the pressure drops. I dont think it would run lean, but more rich. Plus if it was running too lean, you might eventually overheat and damage something.

And if you have an aftermarket MAF, do you have the right specs for it?
Did you have your car tuned with the C&L MAF?

 

[joshjwc9]

Have you pulled each coil and checked it?
Is the car throwing a CEL? If so, what are the codes?
I had a similar problem except I live in FL, but the strange misfire was because of a cracked boot to one COP. Good Luck and update us with any information.

 

[cougar694u]

It's not throwing any codes at all, thus I don't know which cylinders are missing, I assume it's not the same one every time.

I changed plugs & coil boots about 6 months ago as part of general maintenance. It got better, but never fully went away, and the random miss was happening before I change the intake. Again, it got better after I changed everything, but is still there.

I have the correct metering tube in the MAF from C&L to go with the stock injectors.

The miss only happens at part throttle & low RPMs. I can be doing 30 MPH, put it in 5th and lightly throttle to accelerate and it'll miss. If I go WOT, the miss goes away and it runs 100% (was trying to force a cylinder misfire code). It's not the TPS, as it sweeps smoothly, no jumps in voltage. It passes every diag I throw at it with the Snap-On 2500, but the only thing out of place was the BARO reading 140-144 Hz.

Our cars don't have a barometric sensor, as it uses the MAF for barometric readings.

Also, when it's cold & dry outside (30's & 40's), the missing is almost nonexistent, of course the air is more dense. When it's cold & wet, it still does it (probably due to the high moisture content in the air).

Anything else?

 

[Mike97gt]

I have a 99gt with about 156k miles on it. It's been having intermittent misses, which I assumed were related to a coil on it's way out the door.

I did a track day and ended up blowing out the flange gasket on the exhaust and it got worse. I replaced it and it got better, but never went away.

I lost one o2 sensor about a year ago, so I replaced both (for good measure), and cleaned the MAF.

I hooked it up to my Snap-On 2500 and the Baro reading was like 140-145Hz. Here in San Antonio, we're about 800, which according to the snap-on computer, 159 is sea level, dropping 3hz every 1000 feet, so 156 should be close for us here. So my car thinks it's about 4000 feet above sea level, could account for missing (runs leaner and advances timing perhaps?)

It also showed pressure to be between 24 & 25 inches of Hg (when I checked the weather, it showed 29).

The Snap-On computer said to replace the MAF if the Baro reading was more than 7Hz off, which mine is, but I wanted to know for sure the MAF is what's bad before I replace it.

Anyone have any input?

I have a C&L 80mm MAF, C&L Inlet tube, 75mm TB, & C&L upper, and the missing is gradually getting worse.

TIA!

As long as you did not just clear KAM and then check the baro then yeah it's shot..

If you did clear KAM it takes a little while for baro to update. On these cars BARO is a calculation the PCM does based off of maf readings

 

[cougar694u]

My battery's starting to get weak. I tested it with my load tester and it's borderline. I've noticed that when starting, it sometimes resets the key cycle count, trip count (not trip odo), and think's it's the first time starting.

Is it possible it's resetting the KAM? I hadn't thought of that, as I figured it's too short of a voltage drop. My presets remain in my after market radio, along with the clock, but the ECU will sometimes reset the OBDII cycle count (did it when it's in the 30's and first start of the morning). I have to replace the battery anyway, so I may do that today and monitor my readings.

 

[cougar694u]

Replaced the battery Sunday and have been driving it around every day since to let it learn it's surroundings.

It searches pretty bad with the A/C on, but seems to idle better with the A/C off. It has had a few hiccoughs when leaving a stop light, but the missing seems to have improved.

I'm gonna hook it up to the Snap-On and see what my Baro reading is and post my findings.

 

[cougar694u]

BARO is holding steady at 144Hz, showing 25.2 inches of Hg, but weather show 29.8. Guess my MAF is toast?

It also showed bank 1 & 2 to be lean, probably due to the MAF.

PS,
According to the Snap-On computer, BARO readings are as follows:

Sea Level = 159Hz
1000' = 156
2000' = 153
3000' = 150
4000' = 147
5000' = 144
6000' = 141
7000' = 138
...

As stated, my elevation is probably 800'-1000'.

 

[Mike97gt]

My battery's starting to get weak. I tested it with my load tester and it's borderline. I've noticed that when starting, it sometimes resets the key cycle count, trip count (not trip odo), and think's it's the first time starting.

Is it possible it's resetting the KAM? I hadn't thought of that, as I figured it's too short of a voltage drop. My presets remain in my after market radio, along with the clock, but the ECU will sometimes reset the OBDII cycle count (did it when it's in the 30's and first start of the morning). I have to replace the battery anyway, so I may do that today and monitor my readings.

it's possible. I would replace the battery and then drive it for awhile and check it again

 

[cougar694u]

it's possible. I would replace the battery and then drive it for awhile and check it again

LOL!

Replaced the battery Sunday and have been driving it around every day since to let it learn it's surroundings.

It searches pretty bad with the A/C on, but seems to idle better with the A/C off. It has had a few hiccoughs when leaving a stop light, but the missing seems to have improved.

I'm gonna hook it up to the Snap-On and see what my Baro reading is and post my findings.

And then:

BARO is holding steady at 144Hz, showing 25.2 inches of Hg, but weather show 29.8. Guess my MAF is toast?

It also showed bank 1 & 2 to be lean, probably due to the MAF.

PS,
According to the Snap-On computer, BARO readings are as follows:

Sea Level = 159Hz
1000' = 156
2000' = 153
3000' = 150
4000' = 147
5000' = 144
6000' = 141
7000' = 138
...

As stated, my elevation is probably 800'-1000'.

 

[cougar694u]

I've replaced the MAF, and my Baro readings are the same as before.

Now that it's hot, it misses even worse.

I pulled the injectors and the tips are spotless, I was hoping they'd be gunked up and that'd be my problem, but no such luck.

I'm beginning to wonder if the injectors are restricted internally. When I had them out, I tapped the inlet on my finger and some rust colored crap came out. I do have a new fuel filter (less than 30k miles, probably closer to 15k), fuel pressure appears fine, according to the Snap-On computer.

I still cannot force a misfire code, so I'm not sure where to go from here.

Any ideas would be greatly appreciated.

 

[Sams2000GT]

Did you ever figure this out?

I'm having the same issues with misfires. I know this is an old thread, hoping someone can chime in.

 

[cougar694u]

Yup, 2 bad coils. A friend of a friend is a ford tech. I slipped him $20 and we took a test drive in my car with his computer hooked up. He was able to show me which coils were failing.

Replaced those two and runs like a champ!

 

[Sams2000GT]

Yup, 2 bad coils. A friend of a friend is a ford tech. I slipped him $20 and we took a test drive in my car with his computer hooked up. He was able to show me which coils were failing.

Replaced those two and runs like a champ!

Sorry to bump this old thread but what kind of equipment was he using? Is it something available to the public?

 

[cougar694u]

It was a device that plugged into the OBDII port, then had a laptop that plugged in to that which had an app that graphed the power of each cylinder. I don't think it's publicly available.

 

[Sams2000GT]

Thanks for the reply.