MayDay!! - Hard Accel Killed Idle

[AnotherOldJeepGuy]

On my way back to the house today I made a quick/hard acceleration followed by a quick decel into a parking lot.  This immediately messed up something, rough running through parking lot.  At parked it idled very rough then after trying to maintain for a minute or so it just stalled.  Restart and rpms slowly cycle from a bit higher than normal to very low/rough.  It managed to not stall but the cycling continued.

 

I managed to make it home by keeping the RPMs up manually at a stop.  While driving at city street speeds of 30-40 it seems to be running without issue although I didn't try any accel, just took it easy to get home.

 

Looking around I don't see any hose that came loose.  There's nothing dripping anywhere.  While I as in the parking lot I thought I could see a very tiny amount of smoke in the vicinity of the yellow area in the pic below, very faint.  And now at home I don't see this at all.

 

 

I took a short video so the sound could be heard, I'll add it if I can figure out how to do that, so far I have not had any luck getting a video file off my phone so odds are I won't get that today either.

 

What could have happened directly related to a quick accel/decel to result in killing my idle???

 

[89 MJ]

2 minutes ago, AnotherOldJeepGuy said:

I took a short video so the sound could be heard, I'll add it if I can figure out how to do that, so far I have not had any luck getting a video file off my phone so odds are I won't get that today either.

Your best bet might be uploading it to youtube and then linking the video here.

 

I would carefully inspect vacuum lines, including the one going to the brake booster. 

[AnotherOldJeepGuy]

I don't have a youtube account or any cloud storage, I believe this is the file.  Whether or not anyone can play it is another hurdle..

 

IMG_2771.HEIC

[AnotherOldJeepGuy]

3 minutes ago, 89 MJ said:

Your best bet might be uploading it to youtube and then linking the video here.

 

I would carefully inspect vacuum lines, including the one going to the brake booster. 

I have looked, and I will look again, but how would an acceleration break a hose???

[hgeranium]

How does the MAP sensor to throttle body vac line look? If you haven't done cruiser's fix to install a threaded tap then the vac connection is a bit dicey

[89 MJ]

12 minutes ago, AnotherOldJeepGuy said:

I have looked, and I will look again, but how would an acceleration break a hose???

About the only way I can think of would be a combination of an old, brittle hose and the acceleration causing the engine the engine to torque over farther and breaking the hose. Its a stretch though. 

[hgeranium]

Since the vac line just kind of sticks into the hole in the throttle body, enough vibrations and enough bad luck can cause it to come loose and leak vacuum or come out entirely. The former and the latter have both personally happened to me. Worth a check

[AnotherOldJeepGuy]

The MAF line got me when I did my new VC gasket a few months back, I have loosened it from one end to get it out of the way and forgot to put it back, and the engine was way rough, but that lesson learned was applied today as the first thing I checked and it is connected 

 

[AnotherOldJeepGuy]

7 minutes ago, 89 MJ said:

About the only way I can think of would be a combination of an old, brittle hose and the acceleration causing the engine the engine to torque over farther and breaking the hose. Its a stretch though. 

Literally....

[hgeranium]

So is the truck still running rough right now? Also how do the EGR vac lines look? You may have done this already but if it’s still running poorly, you can try to find the source with brake cleaner

[89 MJ]

25 minutes ago, AnotherOldJeepGuy said:

Literally....

To be honest, I was going more off of the smoke coming from that area than the acceleration. I've seen stuff like that come out of brake boosters when unhooking them before. I'll be honest, I haven't watched the video yet, but I will when I have better signal.

[AnotherOldJeepGuy]

28 minutes ago, hgeranium said:

So is the truck still running rough right now? Also how do the EGR vac lines look? You may have done this already but if it’s still running poorly, you can try to find the source with brake cleaner

Yes it will start and still cycles from very rough to less rough but for the moment it can manage to stay running.  I watched what I think is the EGR valve and I see the little shaft moving with the RPMs and the hose looks the same as they normally do as best I can tell (I have not removed them).  I tried spraying [edit: with water]where the manifold connects, months back I did have to tighten them, but since the engine is already cycling I can't tell if the water is doing anything, but I don't think it is.  I'm going to try checking them for tightness again after the engine cools off.

 

The elephant in the room, or the elephants, are the head and manifold gaskets.  So I am hoping it's not an elephant.

[AnotherOldJeepGuy]

9 minutes ago, 89 MJ said:

To be honest, I was going more off of the smoke coming from that area than the acceleration. I've seen stuff like that come out of brake boosters when unhooking them before. I'll be honest, I haven't watched the video yet, but I will when I have better signal.

  I have not witnessed the tiny bit of smoke again since I originally saw it in the parking lot.

[Gojira94]

Just a thought, what you asked your MJ to do is probably well outside of how you normally would drive it. WOT would ask a lot of steps of the IACV, and immediately followed by a lot of steps the other way on hard decel. If you've had any carbon buildup in its normal range of operation (fairly narrow), it may have stuck itself close to closed off on hard accel. That smoke could have been from some of the buildup breaking away and being ingested/ burned.

[AnotherOldJeepGuy]

I don't know that I hit WOT, but for sure went that direction.  And you are correct this is extremely rare.  I just tightened the top manifold bolts, they were just a bit loose.  I was surprised they had worked loose so quickly.  Just put up on stands to try and snug the lowers just in case.  Tightening the tops ones didn't fix anything, but who knows.  I did clean the IAC back in Feb, 2025 but I'd much rather check into cleaning that again before taking on major gaskets.  For what it's worth, when I cleaned it back in 2025 it did improve my stalling at that time.

[AnotherOldJeepGuy]

Well the IAC was a good thought, but it is still clean from the cleaning last Feb, as it the TB.  So that's no it

 

I also noticed as I limped it into the garage, brakes are not working well.  I know @89 MJ already mentioned looking at the hose to the brake booster, but it looks completely fine.  But what ever failed is something that is causing very bad idle and at the same time power brakes don't seem to be working either.

 

I currently have the battery disconnected in an attempt to reset the ECU.  It's a hail Mary but it's something to try.  If that doesn't work I'll probably have to trust it to one of the local shops since I'm thinking this is quickly getting beyond what my ASTM degree can handle.

[pizzaman09]

I'm very suspicious of the exhaust manifold, leak at the head due to jarring the engine hard one way then another.  Check if your bolts are tight.  The smoke is likely exhaust smoke.

[AnotherOldJeepGuy]

Yeah I've learned that can be an issue.  I got a wrench on all the ones you can get to from the top and a couple were a little loose.  I got to 2 bottom ones that I could snug a little and 2 were tight.  But there are still 2-3 that I can't get to with any tool that I have.  I'll need to find a 9/16 box end that is at least 2 feet long and I might get to the others.

[Porquer]

Could you have damaged your booster diaphragm when you slowed down? I’m not very familiar with vac boost, but it seems like that could cause a vac leak that could cause weak brakes and poor running.

[AnotherOldJeepGuy]

Beats me, but someone else might know if that is a possibility.

[Eagle_SX4]

1 hour ago, AnotherOldJeepGuy said:

Yeah I've learned that can be an issue.  I got a wrench on all the ones you can get to from the top and a couple were a little loose.  I got to 2 bottom ones that I could snug a little and 2 were tight.  But there are still 2-3 that I can't get to with any tool that I have.  I'll need to find a 9/16 box end that is at least 2 feet long and I might get to the others.

If you take the air box out you can get to all the bolts from the top side. It is a little bit of a guessing game on the center 2 on the bottom but you can get them.

[AnotherOldJeepGuy]

11 hours ago, Porquer said:

Could you have damaged your booster diaphragm when you slowed down? I’m not very familiar with vac boost, but it seems like that could cause a vac leak that could cause weak brakes and poor running.

 

11 hours ago, AnotherOldJeepGuy said:

Beats me, but someone else might know if that is a possibility.

 

To eliminate the brake booster as the source of the problem, I think I will disconnect the hose to the booster and plug it and see if the engine runs better.  Anyone see an issue with doing this?

[89 MJ]

1 hour ago, AnotherOldJeepGuy said:

 

 

To eliminate the brake booster as the source of the problem, I think I will disconnect the hose to the booster and plug it and see if the engine runs better.  Anyone see an issue with doing this?

Other than the hard brake pedal, that’s a great idea. Exactly what I’d do.