BALLSS

First no codes are thrown

This happened like a light switch. Car was running just fine until a few days ago. Started it and it idled fine. Gave it a little gas to pull away and it stumbled. Pushed in clutch and feathered gas pedal to keep it going. If I slowly pushed gas it would go but a more quicker pedal it would fall on its face. Felt like fuel starvation

So limp home and check FP expecting it to be the issue. FP gauge immediately snaps to 42 on prime and drop to about 36 after. Start car and rev. It holds and even increases slightly on gas rev. Go for a drive to see what happens under the stumble condition at WOT and it holds solid yet car fell flat on its face but did recover. Once recovered it did rev to 5500+ rpm on pulls. Fuel pressure appears to be perfect

Opti was suspect so I swapped in my good spare. Upon pulling apart the one off the car apart I find oil on sensor and evidence of oil sling marks from center out on disc. Oil was pooled under the optical sensor and on outer edge of metal plate between rotor and guts. No front timing cover seal leaks or intake manifold (front). This is a 75k mi AC Delco Opti which I had previously found oil inside but did not affect function 30k mi ago when I put on a MSD C&R. I suspected the oil contamination was affecting opti function. NOT. my good spare opti (new Delphi) same symptom

so now I move on to test other components with DVM:

MAP (with mighty vac to add vacuum)-GOOD
TPS-GOOD .65 vdc to 4.5 vdc smooth when opening blades
EGR-Swapped to my spare-No Change. Both hold vacuum
Coil-Ohm 5790 on each of the 3 pins. Matches my spare coil readings
ICM- IDK how to test but car always starts
Temp Sensor (WP)-OHM correct per water temp per shoebox chart
MAF-12v power, ground good, signal is 5vdc all good. MAF is clean
Grounds-all good on DS head
Plugs & wires changed a few hundred miles ago
Vacuum Gauge on intake manifold-16.5-17.5 (varies by 1 following idle. Have a baby cam)

Did check every vacuum line on manifold. All solid. Carb spray around intake shows no leaks

Scan:

02 fluctuate up/down like normal. rears seem to stay mostly in 700 range but did fluctuate down to 50 occasionally
STFT are high. 13-25
MAF-8.5 at idle and 21.36 at 2500 rpm. I believe idle is in spec. IDK about the rest. It increases as RPM increase

I did read about fuel trims where fuel is added (+ in my case) and it says if STFT do not drop quickly to desired level when RPM is 2500 (they actually rise) there is no vacuum leak (and I can't find any source of one). MAF and 02, plugged exhaust are possibilities

IR temp gun shows temps about 30-50 degrees hotter on rear of CATS vs front

I don't have a spare MAF but have my suspicions on it. I read the following about testing a MAF and it says the following about a bad MAF that throws no codes (and my car does all of them)

1. MAF sensor malfunction that DOES NOT light up the CHECK ENGINE LIGHT (CEL).
2. Lean and/or Rich code(s). (STFT show rich)
3. Fuel Trim code(s). (adding fuel)
4. A tremendous lack of power upon acceleration. (yes)
5. Black smoke coming from the tail-pipe. (no but black marks on my garage floor from reving)
6. BAD gas mileage. (IDK, have not driven it far enough with this crap happening)
7. Vehicle may idle rough and stall. (yes)

CAR WILL NOT IDLE AT ALL ON ITS OWN ON COLD START IN OPEN LOOP. IDLES ONCE IN CLOSED LOOP

So I am at the WTF is it place....any suggestions??

96lt1m6ta

BALLSS

I did unplug the MAF....it started, idled and rev fine. 02 and STFT settled down to WAY more normal.

Given the MAF "tested" good in terms of +12v, ground and .5 vdc signal and then scan showed 8.xx gr/S at idle and 21.xx gr/S at 2500 rpm to my understanding it was "functioning" right.....assuming the gr/S scan info shown was good.

Weird no codes were thrown and I did not know a MAF could cause such drivability issues and affect 02 and STFT so much...then I read this on a site about testing a MAF....

"MAF sensor malfunction that DOES NOT light up the CHECK ENGINE LIGHT (CEL).
Lean and/or Rich code(s).
Fuel Trim code(s).
A tremendous lack of power upon acceleration.
Black smoke coming from the tail-pipe.
BAD gas mileage.
Vehicle may idle rough and stall."

my car did all the above but no black smoke out tail pipe

I did look at my MAF and it had no broken resistor wire and was quite clean

Ordered a new MAF, I will report back once that is in.

ACE1252

We had a MAF go bad on a Nissan Maxima one time. It didn't fall flat on it's face, ran smooth in fact, but it was waaay down on power. Felt like it lost two cylinders. It would set the SES light, but would take several days to set it again after reset.

Unplug it, go for your test drive. See if it runs clean.

ACE1252

Have a look at this....

ACE1252

Here is troubleshooting a GM 3.8 MAF....looks like a Series II.

ACE1252

...and one more....

BALLSS

I did unplug it and car starts and idles but did "surge" for about a minute before it settled down, it did not stall though. Now I can rev it without issue so at least can drive it

I also saw the 3 videos you posted during my search for any info on "testing" a MAF.

I did use DVM to check +12v, ground and signal volts. All good. MY DVM does not have hz function so could not test signal using a DVM. Also scanned and watched gr/s go up/down when I reved the motor. It was 8.xx gr/s at idle (825-850 rpm) and 21.xx gr/s at sustained 2500 rpm. IDK if those values are correct and can't find a chart that shows what gr/s per rpm these MAF should be

NO CEL when the problem suddenly occurred and none even with the MAF unplugged. I would have thought at least a code with it unplugged.

So my assumption is that the MAF is bad. with it unplugged my 02's read "normal" vs high with it plugged in and my STFT are not pegged at 25. They are 0-6 now

I ordered the new Delphi style from rockauto. It has a pigtail with it to plug right into our 3 wire connector but is fitted with the 5 wire plug you could pin & run the intake temp into if you wanted since these MAF also include that sensor.

http://www.rockauto.com/en/moreinfo....035931&jsn=467

BALLSS

I put the new MAF in.....same problem.

It did start and idle fine though in open loop...in closed loop just when you give it gas it falls on its face and then catches

Put FP gage back on and hooked up scan tool and went for a drive. Did some WOT and it fell flat on its face and FP was solid, did not drop at all while car stumbled almost to the point of stalling.

FP is 34 at idle and snaps to 40 when I give it gas. With the vac line pulled from FPR it is 43.3. It is a adjustable FPR so I can increase FP but my understanding is the FP should be 43.5 with vac line off, which it is.

Should FP be more like 36-37 at idle with vac line on??

02's are pegged at 850 at idle but do fluctuate up.down when on the gas. Mostly staying in the .050-.350 range

STFT peg at 25 during acceleration. They do settle to 0-6 at idle

would bad 02's cause this kind of stumble, no power, slow rpm build??

The new MAF was the only $ I have spent and Opti was a spare so I don't want to spend any $ on parts until I find WTF is causing this

There are still no codes

I did unplug battery to clear PCM to see if that helped, nope.

Does ICM only affect starting the car?

SS RRR

iTrader: (2)

plug wire(s) arching somewhere is my guess.

BALLSS

UPDATE: It was the ICM

Had no idea it could cause so much of the STFT issue and the drivability issues. I thought they just fail and car won't start anymore.

Car always started but had progressed to not idling on its own since the problem 1st occurred last week in open loop but did once it went into closed loop (and engine was warmer)

So unless some wire magically is not a issue anymore, car runs fine. FP is solid and STFT sit at 0 now and occasionally are 1-3 on sustained WOT pulls but otherwise they stay at 0

I am returning the MAF and will eat about $12 in shipping but other than my time, and spare parts (ICM & Opti), I am out $ on a gallon of coolant and $.95 on heat sink paste for the ICM.

On the Opti I found oil inside it and on and inside the optical sensor so sooner or later that alone would have been a issue. Cleaned all that out and re-sealed that opti as my new spare as it was working fine otherwise

Gangly

Glad to see you got it figured out. How'd you identify the ICM as the culprit?

BALLSS

swapped in my spare

Hindsight is always 20/20...should have done it earlier in the check process. Without any codes it was best guess.

With that said the ICM failed opposite of usual symptoms so, after checking fuel pressure, Opti & Maf were the focus.

it always started
never cut off completely
would not idle cold/open loop
worked better hot

SS RRR

iTrader: (2)

It's rare that an ICM will cause stumbling issues. From all the complaints I remember they either give up completely or run fine for a spell and then give out, much like a coil.

BALLSS

yep, my understanding also which is why I was focused on other things as cause.

I never found any "test" procedure for ICM so since I had a spare I changed it.

Had no idea it could cause such havoc and still start the car and run, albeit the car did not want to idle in open loop. It caused PCM to dump fuel as STFT were pegged. I assumed its weak ground signal to coil caused weak spark...but yeah was weird that this was a ICM issue based on symptoms

I absolutely thought I found the "why" when I opened up opti to find oil inside. Not huge amount but optical sensor was covered in oil. Opti was still working fine but suspect the oil would have eventually killed operation

long way around the repair block for a 2 screw 5 min part swap.