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1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT

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3 years 5 months ago #44106 by olds88
1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT was created by olds88
As title states I have a 1998 Oldsmobile 88 with the N/A 3800 engine.

The issue I am having is I have a very slight stumble at idle. No codes, and I am ruling out the IAC due to it being fairly new and the pintle responds well with KOEO and retracts just the same. I notice that at idle my LTFT is sometimes as high as 13% at idle with closed loop just starting, but after driving around a while it evens out at around 6-8% at idle. My STFT numbers are always hovering 0% so no problem there.

At idle when I snap the throttle or slightly depress it, I see the LTFT numbers drop. I know this is a classic vacuum leak issue, but I'm not sure that is the only contributor to my issue. I have smoke tested the intake many times. I've fixed all the vacuum leaks, except I discovered I have a leak in the crankcase. On this engine, the side of the block that bolts to the transmission has a cover similar to a timing chain cover. I suspect the gasket on that cover has gone bad because I get smoke in the bell housing of the transmission/engine block. I know this theoretically will contribute to a lean condition due to the PCV system getting unmetered air, but I'm not sure how much this would contribute to my issue since the PCV valve is a fixed vacuum leak. I do acknowledge the leaky gasket is a problem, but as it is I cannot repair this. I did a ton of work to this car and just got it aligned after front end suspension rebuild, and did a lot of internal transmission repairs. Separating the engine from the trans to repair that gasket is just not something I plan to do as of right now.

I can live with the LTFT as it is and accept that the repair is too great for me to want to deal with at hte moment, but I am wondering if my stumble at idle may be a separate issue. I've tested o2 sensors and there is no fixed rich or lean condition, and the sensors respond appropriately to a propane test.

I wanted to get a second opinion on my MAF data though. At longer sample rates, it seems my square wave is not consistent, and the stumbles are consistent with the longer PWM pulses I see on the scope. Due to my scope being a cheaper chinese scope and not specific to automotive testing, I cannot graph frequency, but I still feel like the MAF may be bad based on the scope data. When I say the car stumbles at idle, I am talking very slightly. It idles at a fixed rpm +/- 100 or so, and it drives fine. But the stumble is there, and the drop outs in the MAF signal correspond to the stumble as stated.

Any help is greatly appreciated.

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3 years 5 months ago - 3 years 5 months ago #44107 by olds88
Replied by olds88 on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT
Sorry, the video showing my signal seems to not have embedded properly. Here is the link: MAF Signal
Last edit: 3 years 5 months ago by olds88.

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3 years 5 months ago #44110 by Chad
Replied by Chad on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT

olds88 wrote: I wanted to get a second opinion on my MAF data though. At longer sample rates, it seems my square wave is not consistent, and the stumbles are consistent with the longer PWM pulses I see on the scope.


I am not familiar with that scope, or it's capabilities. Aliasing happens when a scope's sample rate is not fast enough for the signal. This can make a signal look like it has dropouts or glitches, when it actually doesn't. That being said, that doesn't look like a good signal, to me.
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
I have learned more by being wrong, than I have by being right. :-)

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3 years 5 months ago #44111 by olds88
Replied by olds88 on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT
Thanks for the reply. Yeah, this scope claims it has a 1GS/s rate with a 100 MHz bandwidth. I think that's definitely an exaggeration, but based on reviews I found for this scope people seem to have been able to measure some very fast signals fine with it. That along with the fact that I can see the stumble when the signal drops out makes me think the signal is in fact bad.

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3 years 5 months ago #44119 by Matt T
Replied by Matt T on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT
Could try a DMM that'll read frequency on that MAF for a second opinion. If the signal really is dropping out that bad my Flukes would definitely catch it.

Might also catch it graphing MAF PID by itself. The MX+ is kinda slow on early GMs but it might show the longer dropouts, which look to be ~1 second long.

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3 years 5 months ago #44128 by olds88
Replied by olds88 on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT
I have an Extech and Milwalkee meter that bother read frequency. ScannerDanner has a video specifically on this MAF. In the video the MAF on that car had a glitch where it would spike the frequency to 100k+ but the meter was too slow to catch it, and even the scan tool wouldnt catch it unless it was on graphed frequency function. My scope doesn't have that feature. Closest it has is FFT which is kinda useful but not the same.

My MAF seems to be doing the opposite where the drop out is on the lower frequencies, but my meters definitely don't catch it. I was only able to see it even on the scope when I set the sampling rate to 2s or more. I wouldn't doubt a Fluke could pick it up though. Definitely on the list of want items.

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3 years 5 months ago #44133 by Matt T
Replied by Matt T on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT

olds88 wrote: My MAF seems to be doing the opposite where the drop out is on the lower frequencies, but my meters definitely don't catch it. I was only able to see it even on the scope when I set the sampling rate to 2s or more. I wouldn't doubt a Fluke could pick it up though. Definitely on the list of want items.


A decent Fluke would pick it up if it was really happening. And the fact neither of your DMMs would makes me question your 'scope. Some of those dropouts were long enough any half decent DMM should show a dip in frequency.

And FWIW if all you're doing is automotive you'd be better putting the money into a better 'scope than a Fluke.

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3 years 5 months ago #44135 by olds88
Replied by olds88 on topic 1998 Oldsmobile 88 3800 series 2 slight stumble at idle and high LTFT
Yeah, I went to school for auto tech but it's not the industry I'm in anymore. I just work on my own stuff or friends so its an informal thing in my life at this point. I leaned on the side of getting a general scope because I do more electrical work that isn't car related now.

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