Intermittent engine hesitation (m20b25) [now with video]

[frolf]

I don't have much experience tracking these things down, and after a few searchers of this board I couldn't find anything similar to what I am experiencing.

Half the time I drive (varying conditions) I get a strong hesitation at 1900rpms, 2600, 3100, and 3900. It will pull through them, but just feels like you are hitting pot holes (which I am not). This is in second and third gear. And the hesitation is noticeable at 3900 in 4th and 5th.

Clutch is fairly new. And revving in neutral is just fine. Idle is pretty smooth.

Any ideas on what to check/replace first?

Thanks.

[jlevie]

The first things I'd look at would be the fuel system and AFM if the engine only hesitates. However if it feels like it cuts out the cause is probably different (CPS, DME power, DME)

You might get a clue from the behavior of the tach and econometer. If the tach reading drops the DME is loosing timing data, power or ground, or is the cause. If the tach doesn't react but the econometer drives towards either extreme there is a fuel delivery problem or bad data from the AFM. Rarely one may see the DME being the cause of this.

[frolf]

Thanks Jim. Nothing crazy happens on the dash during the hesitation, and I have to be at about 50% throttle or above to induce the hesitation.

An additional symptom: when giving it enough throttle to barely maintain speed (think feathering it while you are reaching a stop sign on a residential road) in second or third gear at about 1900rpm it, at times, will buck like crazy. The econometer does lurch back and forth at that time.

[jlevie]

The 50% throttle might be significant. That is right about the point that the WOT switch in the TPS should close and change the behavior of the DME. A barely open throttle might be tickling the idle switch in the TPS. Make sure that both the idle and WOT switches in the TPS are working (test at the DME connector) and that the idle closes when the throttle is 1mm off the stop screw.

[frolf]

I haven't spent much time investigating this further, as the problem seemed to go away for the most part. However, the hesitation is back.

This time, I actually recorded a video of the tach while I am doing WOT pulls in second gear. Notice the change in pitch of the exhaust right around 3500rpms? The third pull is the most noticeable. When it switches pitch it feels like i have let off the gas, and then it somehow makes it through.




Any further advice after viewing this? Or should I go through all the components Jim listed?

Thanks to Jim for the previous advice, and thanks in advance for (hopefully) spending 30 seconds on this again for me today.

Dan

[paperplane94]

Tee in a fuel pressure gauge and have the hose long enough so you can attach the gauge to the window. Reproduce the hesitation and see what the pressure does at that moment.

[jlevie]

I can't see the video at work, but a dead spot in the AFM resistance trace could do what you describe.

[frolf]

Thanks paperplane and Jim. As you can tell, I'm hesitant to throw parts at the car given I simply don't have much time to do the work.

Jim, if you do get a chance to watch the video, I would love to hear your feedback after hearing my car more or less crack it's voice.

[Gregs///M]

Jim is correct. Certainly sounds like a bad AFM.

Here is an oscilloscope capture of a recent AFM that I tested. This is the before capture. Noticed the drops to 0 volts, while the normal resting voltage should be a minimum of 0.4 volts.


After electrical refurbish...Notice there are no drops to 0 volts.


This is probably what your AFM is experiencing. It needs to be refurbished or replaced with working model.

Notice the drops to 0 occur near the 0-10% flap open range. However, I've seen many AFM's drop to zero in all or many parts of the full range of the AFM's flap.

[frolf]

Thanks so much for the detailed response! I will actually start throwing a few parts at it now and see what I come up with. For posterity, I will update this thread with my results.

[frolf]

Update 7-14

I swapped the AFM with a known working unit. Unfortunately, the car still hesitates all over the place. And the change in pitch you can here in the video above occurs at each hesitation point.

Jim recommended looking at the tps. Any takers on what to do next? I am a little discouraged tonight, but will continue to throw parts at it to try to Get rid of this. I just want nice smooth power.

[jlevie]

Some things are best diagnosed by a parts swap, but most things can be diagnosed by test. The TPS can be tested with an ohmmeter. And you need to see what the rail fuel pressure is while driving the car. Do that by teeing in gauge and zip tie it to a windshield wiper.

Check C191 (the round connector under the intake) for corrosion. Peel back the boots and also check the back sides.

[frolf]

Thanks Jim, I will get on this stuff next weekend!

[frolf]

After a quick conversation with my old friend Blunt, I went ahead and ordered some new plug wires, cap, and rotor.

In the mean time, I replaced the spark plugs. The old plugs looked worn (gray crust forming on 4/6).

There were two strange things:
1. One of plug wires was really lose. Taking it off and putting it back on, you couldn't feel the wire sliding over the threaded stud of the spark plug
2. Two of the 4 worn plugs had a papery light black crust physically between the electrodes.

New plugs in while I wait for the plug wires to arrive, and plug wire #1 feels like it could just fall off.

However, the hesitation is GONE! I will test the car some more today, but after some driving last night I could not get it to trip through varying load requirements, nor could I get it to buck while feathering the throttle at 1900rpms.

It still feels sluggish right once I put my foot in it off idle, but I am so happy to have it pull from there to redline without any fuss.

Thanks to everyone who helped me out!

Lastly, is the off idle sluggishness something that can be fixed? Or is that the nature of the beast?

[jlevie]

The hesitation off idle is most commonly the result of intake leaks, possibly aggravated by a problem with the ICV and/or TPS. A smoke test of the intake and repair of the leaks it reveals as well as cleaning the ICV and testing the TPS are called for.