Slight rev when coming to stop

[Not_Invented_Here_]

This is a total shot in the dark question.

I've noticed lately that as I'm braking/coming to a stop, whether decelerating from fast highway or moderate/slow neighborhood speeds, I get what feels like a little spike in the RPM's or a verrry slight rev right before I come to a complete stop at light/stop sign. I feel it through the brake pedal and can faintly hear it.

It's over almost as soon as it happens. I'm rocking a 42RLE with 96k miles. Is this something anyone else has experienced? I can't say I've noticed any other symptoms or problems that seem like they'd be related.

 

[Steel City 06]

That's normal in some cars. If the RPMs drop quickly, the idle controller can sometimes overreact, and cause a very small spike in the RPMs.

I don't have an auto TJ, but my RPMs will sometimes drop and spike slightly when I hit the clutch between 700-900 RPM. I think it's similar.

 

[Not_Invented_Here_]

Cool, appreciate the response.

 

[Not_Invented_Here_]

Posting again because this problem seems to be persisting, on occasion seems worse than before. When decelerating anytime (from highway speeds at a stop light, in traffic when I'm at a crawl and stop/starting, or even in normal flowing traffic), two things are happening.

One, as I gradually drop to and past around 5-10 mph, I occasionally hear/feel a slight rev but it stops again almost immediately.
Two, more significant, as I come to a complete stop I hear/feel a more substantial revving of the engine and a bit of "crunchy" feedback from the break pedal. I don't think I'd describe it as grinding but it feels crunchy for sure... I've observed the RPM's spike a bit when this happens.

Looking at old threads, there's talk about IAC's and other stuff which is a rabbit hole I'm trying to avoid until necessary. Is there any other explanation for this kind of behavior and is there anything I personally can do/check to further assess what's going on?

No other symptoms or issues that I've noticed besides those two things...it's always when braking as I'm about to stop and then right when I come to a complete stop.

 

[Steel City 06]

Posting again because this problem seems to be persisting, on occasion seems worse than before. When decelerating anytime (from highway speeds at a stop light, in traffic when I'm at a crawl and stop/starting, or even in normal flowing traffic), two things are happening.

One, as I gradually drop to and past around 5-10 mph, I occasionally hear/feel a slight rev but it stops again almost immediately.
Two, more significant, as I come to a complete stop I hear/feel a more substantial revving of the engine and a bit of "crunchy" feedback from the break pedal. I don't think I'd describe it as grinding but it feels crunchy for sure... I've observed the RPM's spike a bit when this happens.

Looking at old threads, there's talk about IAC's and other stuff which is a rabbit hole I'm trying to avoid until necessary. Is there any other explanation for this kind of behavior and is there anything I personally can do/check to further assess what's going on?

No other symptoms or issues that I've noticed besides those two things...it's always when braking as I'm about to stop and then right when I come to a complete stop.

Does it only occur when braking?

If so there could be a vacuum leak in the brake booster. I've never seen it happen but if it's letting a slug of air into the vacuum lines then it could cause the engine to rev and the brake pedal to act abnormally.

@mrblaine can correct me if that's not possible

 

[toximus]

Let's pretend it's not a transmission issue for now and start simple.

1) Check all vacuum lines for leaks. Replace any that are.
2) With the engine off, manually move the throttle body open to closed and make sure it's not sticking.
3) Clean the IAC. This is easy and only takes an hour tops if you've never done it before. Search the forum, there's got to be several good tutorials.
4) While you're cleaning the IAC go ahead and wipe out the TB by removing the intake tube.

Let us know if that changes anything.

 

[mrblaine]

Does it only occur when braking?

If so there could be a vacuum leak in the brake booster. I've never seen it happen but if it's letting a slug of air into the vacuum lines then it could cause the engine to rev and the brake pedal to act abnormally.

@mrblaine can correct me if that's not possible

Very possible and sounds like the vacuum control valve in the booster is dying based on the "crunchy" part.

 

[Not_Invented_Here_]

Appreciate the tips, I will plan to look into this over the weekend and see what I can chase down.

To Toximus, yes, LET'S pretend/pray/hope/beg...no transmission issues...

 

[Not_Invented_Here_]

Does it only occur when braking?

If so there could be a vacuum leak in the brake booster. I've never seen it happen but if it's letting a slug of air into the vacuum lines then it could cause the engine to rev and the brake pedal to act abnormally.

@mrblaine can correct me if that's not possible

To answer your question, yes - I have only noticed it/been able to produce it when braking.

 

[Not_Invented_Here_]

Quick follow-up question to this... Is there anything that would cause the issue to occur intermittently? I've noticed on the hotter (75+) days it seems to pop up quicker. Could be my imagination but I'm still learning about the causes here and figured I'd check...

 

[ac_]

This happened to me last year also. I am not saying that this is your problem, but this happened to me.

When I hit the brakes the RPMS would go up. but it would happen every time I hit the brakes, not just when stopping. Also I could hear it if I didn't have the radio on. Then soon after my brakes weren't as good. I mean the pedal pushed normal but the Jeep didn't stop as fast.

These were some of the symptoms of my bad brake booster.

See if any of those match your symptoms.

 

[Not_Invented_Here_]

This happened to me last year also. I am not saying that this is your problem, but this happened to me.

When I hit the brakes the RPMS would go up. but it would happen every time I hit the brakes, not just when stopping. Also I could hear it if I didn't have the radio on. Then soon after my brakes weren't as good. I mean the pedal pushed normal but the Jeep didn't stop as fast.

These were some of the symptoms of my bad brake booster.

See if any of those match your symptoms.

Not quite the same problem but helpful input nonetheless - I'm trying to eliminate other possible issues as much as I'm trying to fix what I think the cause is.

For me, the issue has only occurred right before a stop and then when finally coming to a complete stop. It happens intermittently though...today, I have not noticed it happening at all and that seems to coincide with cooler temps, but who knows. Will have time to do more of a deep dive this weekend.

 

[ac_]

In that case it may have been mentioned earlier, but try cleaning you iac that could make your symptoms intermittent. Also are you all of a sudden running your AC now? That can screw with your coming down to idle. If it isn't any of those three things then I am just as interested as you and will have to wait to hear what actually fixes it.

 

[Not_Invented_Here_]

In that case it may have been mentioned earlier, but try cleaning you iac that could make your symptoms intermittent. Also are you all of a sudden running your AC now? That can screw with your coming down to idle. If it isn't any of those three things then I am just as interested as you and will have to wait to hear what actually fixes it.

Cleaning IAC was suggested yeah and I'm just about to hit 97k so I'm in the right mileage range for it to need a cleaning. Haven't run my AC once yet (besides checking if worked when I bought it) but still helpful to know about that.

The problem hasn't been severe enough to warrant a post until yesterday, about 80 degrees outside and I got caught in crawling traffic on the highway. That was when I really started to notice it, even after getting back on open back roads. But then I came out this morning and drove to work, nothing really to report at all.

 

[ac_]

Cleaning IAC was suggested yeah and I'm just about to hit 97k so I'm in the right mileage range for it to need a cleaning. Haven't run my AC once yet (besides checking if worked when I bought it) but still helpful to know about that.

The problem hasn't been severe enough to warrant a post until yesterday, about 80 degrees outside and I got caught in crawling traffic on the highway. That was when I really started to notice it, even after getting back on open back roads. But then I came out this morning and drove to work, nothing really to report at all.

Well keep us posted and/or post a fix if you find one. Dollars to donuts this will be googled and someone would really like to know how you fixed your issue.

Good Luck to you Sir!

 

[RoscoeJack]

I had the same problem this winter. It ended up being a problem with the vacuum lines. It was also intermittent.

 

[Not_Invented_Here_]

I had the same problem this winter. It ended up being a problem with the vacuum lines. It was also intermittent.

Good to know - any tips for tracking it down based on your experience?

 

[ErikOffroad]

To check for a vacuum leak, most shops will use a smoke tester. Unfortunately that equipment is kind of expensive. So what I tried once was I pulled off a vacuum line, then I called my buddy Jake to come over (he drives a Subaru) and I had him
Vape and blow into the line I unplugged while I went around the vehicle andchecked the rest of the lines for leaks. I ended up finding a bad elbow

 

[pagrey]

The speed you are describing is when the 42RLE shifts back into 1st. It can be rough and most of the time you can clearly hear the transmission solenoids chatter as it engages. If you have worn motor mounts it could be making it worse. It will change the idle but shouldn't have a real effect on the brakes or vacuum unless you have a problem there. You might take a good look at your motor mounts and check the transmission fluid level.

 

[Not_Invented_Here_]

To check for a vacuum leak, most shops will use a smoke tester. Unfortunately that equipment is kind of expensive. So what I tried once was I pulled off a vacuum line, then I called my buddy Jake to come over (he drives a Subaru) and I had him
Vape and blow into the line I unplugged while I went around the vehicle andchecked the rest of the lines for leaks. I ended up finding a bad elbow

Love it, great idea.