Rhythmic chirp from engine, but only under engine braking (and now also engine load) - JeepForum.com
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post #1 of 3 Old 06-08-2020, 12:28 AM Thread Starter
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1998 TJ Wrangler 
Join Date: May 2018
Posts: 2
Rhythmic chirp from engine, but only under engine braking (and now also engine load)

I have a '98 TJ, 4.0L, manual transmission, 215K miles on the odometer.

I bought it two years ago and have been steadily fixing whatever breaks, but am still pretty much a newb to Jeeps and auto repair. So there's no reason to assume I've done the obvious thing that a knowledgeable person would have done first.

Anyway, the specific problem today is this: a repeating high-pitched squeak that only appears during engine braking. Depress the clutch pedal and it instantly goes away. Or, press the throttle to maintain speed instead of engine-braking, and it goes away. The squeak is high pitched and sounds like a bird chirp, and repeats several times per second. As far as I can tell it's proportional to engine RPM rather than wheel rotation.

Occasionally, I will also get some hesitation (a silent sputtering feeling) in the engine when climbing a hill with the throttle mostly open. But that goes away after a few seconds and I can't seem to provoke it to happen again.

In the last week or so it has begun to also chirp during engine load. Let off the throttle and the chirp goes away, or at least is reduced until I can barely hear it.

The check-engine light is not lit. With an OBD-II tool I can see that there was a pending fault P0301 "Cylinder 1 Misfire Detected" but a spirited drive trying to provoke the sputtering last week was unsuccessful and that fault has now cleared.

Guessing that it might be a vacuum leak, I inspected hoses and did the "spray propane around the hoses and see if the RPM changes" test but that didn't reveal any leaky areas.

I bought a vacuum gauge and tried that today, and the vacuum at idle is at 16 in. Hg. At idle the needle vibrates steadily, maybe +/- 0.1 in Hg. Opening the throttle briefly with my hand causes the vacuum to dip down to 7 in. and then up to about 20 in. before it settles at 16 again.

Keeping the throttle partly open for a longer timer, the needle stays almost totally steady at 19.5 in Hg.

The vacuum gauge's dial says that 16 in Hg is in the "LATE IGN TIMING" range, though its idea of the top end of that range is 17 in Hg ("NORMAL" is marked as 17-22 in Hg), so I'm not sure I should believe the gauge manufacturer's opinion of what "normal" is. Another thing making me wonder if this is a red herring is the posts on this forum saying that TJs' ECUs manage ignition timing, so I'm not sure how an ECU would get confused about what timing to use or if it's even possible.

According to A Guy On The Internet (I'd post the link but my post count is too low), my results sort of match his diagnosis of worn piston rings. I'm not losing oil, which AFAIK would be happening if it were really worn rings.

I have photos and videos of the gauge and chirping but I didn't want to post a bunch of irrelevant stuff if I'm barking up the wrong tree.

So, questions:
1) Are these levels of vacuum reasonable for a TJ 4.0?
2) What should I be looking at for possible causes of this chirp?
3) What additional diagnostics can I do to narrow down the possible cause?

Thanks a million!

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post #2 of 3 Old 07-11-2020, 03:58 AM Thread Starter
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1998 TJ Wrangler 
Join Date: May 2018
Posts: 2
An update: I took it in to a professional who quickly diagnosed the chirping problem as being worn U-joints, front and rear. Replacing them fixed that problem. I guess the correlation I noticed between engine load and the chirp being present was actually a correlation between torque on the U-joint and a chirping noise coming from the U-joint.

The engine hesitation problem is still there, but based on some more reading I'm starting to wonder if it's a bad/dirty sensor, so I'll troubleshoot that next.

The behavior I see is: starting is fine, initial driving and acceleration are fine, but after it's been running for a while, accelerating from a full stop at a stoplight results in hesitation, and flooring the gas pedal makes it much worse, including backfires. Letting off of the throttle makes it run properly again, and cruising at highway speeds is no problem. The sputtering appears to be proportional to how open the throttle is.
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post #3 of 3 Old 07-11-2020, 10:07 AM
Jonny Jeep
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1997 TJ Wrangler 
Join Date: Feb 2007
Location: Manchester
Posts: 6,768
Might want to check your fuel pressure. I had similar problems before my pump failed and left me stuck at work last winter.
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