(Engine experts needed) Chirp, Vibe, Pop (What is Wrong?)

[toximus]

I've got a weird one for you guys.

A rattling chirping squealing noise (sounds like saying "chi-chi-chi" using the short "i" sound, but also kind of like a belt squeal except not continuous. The timing seems to be every revolution of something something touches.) has been occurring somewhere in or around my engine bay when the M90 supercharger goes into boost on the 4.0L while the 42RLE transmission is in overdrive — this primarily happens between 45mph and 55mph. It also seems to be happening over 60mph when running the engine foot to the floor. For either of these conditions if it happens while accelerating the noise eventually goes away once the RPM, speed, or engine load changes. My theory has been that under these conditions a bad idler pulley has been receiving extra load and making the noise, I have a replacement idler on the way, but I think I might be wrong after yesterday.

I found a few big long hills that I could keep the noise happening the entire way up and figured I'd take a video to see if I could make more sense of it. Instead I seemed to have angered the beast and made things even more confusing.

MP4 Video - 53MB.

I put the engine in this boost condition (boost, overdrive, 50mph) to induce the chirping noise and held the throttle to keep it chirping all the way up (chirping starts in the video at 1:08). As I climbed the hill, the frequency of the chirp changed as well as the tone. You can hear the progression of this change in the video at 1:17 until about 1:39 when the chirping changed more suddenly missing a few of the chirps and changed more into a higher pitched "tee-tee-tee" sound. Then the Jeep started violently vibing like driving over continuous rumble strips (difficult to see but starts around 1:45 in the video and continues until the end) the vibe stays consistent and does not change intensity. The first 2 hills I lifted off the accelerator when the vibes started and the vibe stopped. The 3rd hill I angered the beast and kept my foot where it was to see if the vibes would change and I kept my food there until a single pop/thud occurred. It sounded and felt like something thumped against the tub. I immediately lifted my foot and the vibes stopped and I pulled over. My first thought was either the transfer case cracked in half or a ujoint cracked, my second thought was wondering why there was no followup sounds if it was driveshaft related. The engine stayed running the engine time, no CELs.

I looked over the drivetrain and checked fluids etc and I couldn't find any leaks, cracks, or issues.

I drove a little to see what would happen and it felt completely normal. I put a jack under the rear axle and everything spun by hand like normal. Because I couldn't find any issues after 45 minutes of looking I went back out on the trail for 4 more hours and then drove home another 2 hours 55-65mph being careful to let off the accelerator whenever the chirping sound started. Aside from the chirp under those specific conditions, if I were buying this Jeep I'd have no concerns with the engine — it drives great — but clearly something is wrong with the drivetrain?

The maximum boost this pulley provides is 8lbs. I've checked the peak gauge and it has never gone over. If I'm hard accelerating I can go up to 8lbs of boost without the chirping noise but the boost quickly drops off again after reaching my desired speed. Is it possible that under the chirping conditions that sustaining the boost for that long is lifting the head allowing for a misfire (causing the vibes) until the boost "burps" (causing the pop) out of the head? Wouldn't that cause lasting damage that would be seen under all driving conditions?

Edit: The front has manual hubs so nothing is spinning up front. This isn't the typical "TJ vibes".

 

[Lou]

It’s always hard to troubleshoot noises over videos but from the video it sure sounds like detonation to me.

 

[Rick Flair]

Yes it does sound like detonation. If your in overdrive and the sound/vibes happen under boost, I’d be inclined to think torque converter. You might be blowing through it when it’s locked up in overdrive.

I experienced something similar with my Cummins when I had my overhead camper on and pulling my boat going up a hill. On a mild tune, it would blow through the TC.

 

[toximus]

I cross posted this to a few other forums to hopefully find someone who's had this issue before.

 

[toximus]

It’s always hard to troubleshoot noises over videos but from the video it sure sounds like detonation to me.

Yes it does sound like detonation.

Could the chirping be detonation?

 

[Rick Flair]

Could the chirping be detonation?

Honestly hard to tell through the cell phone video. I hear more rattle (detonation).

 

[Lou]

The noise I hear in the video sounds like detonation. I wouldn’t describe it as a chirp. Sounds more like a rattle. But noises over videos are VERY subjective.

 

[toximus]

If your in overdrive and the sound/vibes happen under boost, I’d be inclined to think torque converter. You might be blowing through it when it’s locked up in overdrive.

I experienced something similar with my Cummins when I had my overhead camper on and pulling my boat going up a hill. On a mild tune, it would blow through the TC.

You might be onto something. Would I have noticed a major spike in temp though? I have a sensor on the post cooler line and the max temp is 134F at 32F ambient. It's a big cooler though which may have covered up the spike.

I also realized there might be a squeal that sounds like belt slip at the moment of applying the throttle suddenly and hard. Like in the 0-60 video. My tires also squeal which covers it up. I wasn't worried about it since I never do that under normal driving conditions.

 

[Lou]

I have never looked into supercharging the 4.0, but is 8 psi commonly run on other builds with the 42rle?

 

[toximus]

I have never looked into supercharging the 4.0, but is 8 psi commonly run on other builds with the 42rle?

I don't know specifically about the 42RLE, but 8psi is the highest recommended to be safe with the stock head gasket and bolts.

The chirp can happen starting at 1psi.

 

[Rick Flair]

You might be onto something. Would I have noticed a major spike in temp though? I have a sensor on the post cooler line and the max temp is 134F at 32F ambient. It's a big cooler though which may have covered up the spike.

I also realized there might be a squeal that sounds like belt slip at the moment of applying the throttle suddenly and hard. Like in the 0-60 video. My tires also squeal which covers it up. I wasn't worried about it since I never do that under normal driving conditions.

No, not initially. If you stayed in it long enough you would, think of it as a clutch slipping and in a auto trans this will equate heat.
I’m not real familiar with your auto, but played with tuned diesels enough to know that full boost in overdrive is bad and even in lower gears with the TC locked you roll into the throttle to slow the boost as to not blow through the TC, otherwise a built trans is coming sooner than later.

Again, I’m no expert and maybe another boosted 4.0 owner will say the trans can hold the extra power fine.

 

[pagrey]

Do you hear it from under the Jeep out the window? Maybe flex plate bolts or torque converter rattle of some kind. Mine has always had a pretty obvious rattle when it locks up from time to time, it sounds similar but I put nowhere near the load on it you do so the noise might be a bit different. Mine is really obvious when I am close to a curb or wall that reflects the sound.

 

[toximus]

Thanks for the insight. You pointed me in the right direction I think. I found this video which seems to be a similar vibration/shudder:

I think you're right that I'm having a torque converter issue under those conditions.

I don't fully understand why increasing speed/rpm while still in OD stops the chirping problem since load would also increase but maybe it receives a signal to clamp the converter tighter under those conditions? Because the sound progresses and the chirping stops when the vibrations start I'm inclined to think they're directly related and not 2 different conditions. But I could be wrong and there may also be a detonation issue?

 

[Rick Flair]

Thanks for the insight. You pointed me in the right direction I think. I found this video which seems to be a similar vibration/shudder:

I think you're right that I'm having a torque converter issue under those conditions.

I don't fully understand why increasing speed/rpm while still in OD stops the chirping problem since load would also increase but maybe it receives a signal to clamp the converter tighter under those conditions? Because the sound progresses and the chirping stops when the vibrations start I'm inclined to think they're directly related and not 2 different conditions. But I could be wrong and there may also be a detonation issue?

Possible detonation as well.

FYI though, converter is either locked or not, it can’t apply more lockup as load increases.

 

[pagrey]

Torque converter can be pulsed to partially unlock, it's called PMMT or partial lockup. It's one of the "special" features of the 42RLE that makes it so great. That's what makes mine sing.

Interestingly Jeep disables PMMT when you disable overdrive. The reason they recommend this while towing is because you can burn out the torque converter clutch using partial lockup under heavy loads. PMMT also happens when the A/C cycles on to dampen the shock of the compressor engaging, high tech stuff.

 

[toximus]

Do you hear it from under the Jeep out the window? Maybe flex plate bolts or torque converter rattle of some kind. Mine has always had a pretty obvious rattle when it locks up from time to time, it sounds similar but I put nowhere near the load on it you do so the noise might be a bit different. Mine is really obvious when I am close to a curb or wall that reflects the sound.

I used to have a "vum vum vum" sound around 45mph in OD but that was back when the Jeep was stock and I think was the typical front driveshaft TJ vibes. Since my build that noise has completely stopped. Aside from that noise back then, and the current issue the transmission has been silent aside from the occasional nonconcerning low speed shift into 1st.

 

[toximus]

A knock sensor would detect detonation issues, correct?

I may pick up a KnockLink G4 so I can narrow the issue down between the engine, transmission or both. And probably wouldn't be the worst thing to have on a tuned engine anyway.

@Chris have you ever used a knock sensor?

 

[toximus]

Do you hear it from under the Jeep out the window? Maybe flex plate bolts or torque converter rattle of some kind. Mine has always had a pretty obvious rattle when it locks up from time to time, it sounds similar but I put nowhere near the load on it you do so the noise might be a bit different. Mine is really obvious when I am close to a curb or wall that reflects the sound.

Thinking over this again, I have MT/Rs and a soft top on so it's really difficult to say for sure that there's not a quiet chirp/rattle/noise that's happening under other conditions that I just can't hear.

 

[Rick Flair]

Torque converter can be pulsed to partially unlock, it's called PMMT or partial lockup. It's one of the "special" features of the 42RLE that makes it so great. That's what makes mine sing.

Interestingly Jeep disables PMMT when you disable overdrive. The reason they recommend this while towing is because you can burn out the torque converter clutch using partial lockup under heavy loads. PMMT also happens when the A/C cycles on to dampen the shock of the compressor engaging, high tech stuff.

Well that’s interesting, I don’t anything about that trans. Lol, thanks for clearing up my statement.

 

[Rick Flair]

A knock sensor would detect detonation issues, correct?

I may pick up a KnockLink G4 so I can narrow the issue down between the engine, transmission or both. And probably wouldn't be the worst thing to have on a tuned engine anyway.

@Chris have you ever used a knock sensor?

It should and cut timing but you can still create silent detonation as well, I think you are on the right track and can’t hurt to have more information!!