surging idle

[brucecooner]

I drove the MJ a bit this evening, and noticed on the way home at a stoplight that sitting at idle, the RPM's go up and down and up and down, on about a 2 second cycle.

It's not an extreme fluctuation, maybe 100-300 rpm up before falling.

 

This seems to be a warm engine behavior, I have not noticed this when first getting in and going, but can't 100% verify it's not a cold behavior too.

 

It's a recently acquired '88 4.0, all stock far as I know.

I took a short video watching the tach.

 

 

 

[cruiser54]

First, make sure all the intake manifold bolts are snug. The factory torque spec was too low. 

Best to remove the air cleaner to do it. Here's the sequence:

 

[brucecooner]

3 hours ago, cruiser54 said:

First, make sure all the intake manifold bolts are snug. The factory torque spec was too low. 

Best to remove the air cleaner to do it. Here's the sequence:

 

Will do. 

 

Thanks again cruiser.

[brucecooner]

I actually had a few minutes to get out the wrenches the other day. 

I checked the bolts along the top of the intake manifold, I didn't have time to pull off all that breather box and related business around it.

A couple of them did snug up, but only now do I see that I probably should have been testing them in sequence, dang it.

I'll have to get all of that...stuff off the intake manifold and see if I can access the bottom bolts, and do all that in the sequence.

 

At any rate, even after snugging those top ones, it's still got the surging idle.

Are you thinking this would be a vacuum leak somewhere?

[cruiser54]

To access the lower bolts, just remove the air cleaner assembly.

 

Easy way to find vacuum leaks. 

At idle, spray carb cleaner around suspect areas. If there's a vacuum leak, the idle will immediately change. 

 

check out the integrity of this hose.

 

[brucecooner]

I managed to get the breather box off this evening (a most delicate operation given all the hoses and lines), but unfortunately even with that off I lacked sufficiently limber limbs to get to all of the intake manifold bolts that were hidden behind the exhaust manifold.  At this point, I'm not entirely sure how people get it off without deconstructing the entire front of the vehicle.

 

I had more success checking that throttle body to MAP line.

I wiggled both ends lightly,  and they seemed pretty secure.

 

Would it be of any use to install a vacuum pressure gauge?

Or would that just confirm what the surging idle is telling me?

 

I see videos to fix "rough" idle on youtube, but I wouldn't describe this as a rough idle.  It's just uneven.

 

 

 

[DirtyComanche]

Definitely would check for vacuum leaks as Cruiser said, carb cleaner or flammable non-chlorinated brake cleaner works good.

 

Otherwise just do all of his tips, grounds and cleaning stuff especially.

[cruiser54]

Inspect the "line" to the MAP closely. Best to remove it and make sure you can blow through it.

 

Here's a good looking vacuum line I found the other day. Looked great. I removed it and couldn't blow through it because it was melted closed on the bottom.

 

My buddy was happy because he bought the Jeep for a real good price thinking the engine was bad. This is all it was.

 

 

[brucecooner]

I removed the line this afternoon and checked it out. No leaks I could find, and I could blow through it so it wasn't blocked.

 

I pulled and tugged at fittings looking for loose lines, and ran my finger along the underside of most of the lines under the hood, everything is actually very clean and smooth.

 

I have a can of non-chlorinated brake cleaner, maybe it's time to spritz it at various...fittings? 

Maybe I shoot it at the intake seam?

[cruiser54]

Absolutely.

[Heinri_k]

Any possibility this could be an O2 sensor or idle control sensor?

[AZJeff]

I gotta ask one question, OP:   you are here in the PHX area, and it's already been warm enough to use the A/C in a car.  Does your MJ have A/C?

 

I mention this because, although my MJ has the OBD-II control system and instake manifold, and thus is not exactly like yours, my idle will jump up and down slightly depending upon whether the A/C compressor is on or not.

[brucecooner]

On 5/19/2022 at 10:04 PM, Heinri_k said:

Any possibility this could be an O2 sensor or idle control sensor?

I wouldn't know, not a mechanic here.

cruiser's been helping with this in another thread, and he thought it was too early to blame the idle air control (is that the same thing?).

 

On 5/20/2022 at 7:30 AM, AZJeff said:

I gotta ask one question, OP:   you are here in the PHX area, and it's already been warm enough to use the A/C in a car.  Does your MJ have A/C?

 

I mention this because, although my MJ has the OBD-II control system and instake manifold, and thus is not exactly like yours, my idle will jump up and down slightly depending upon whether the A/C compressor is on or not.

My MJ does have AC, but I've rarely run it so far.  I do most driving in the evening away from town, and leave the AC off and the windows down.

The last person to mess with the comfort controls in the cab was my wife though. And I haven't checked them since she did.

She might have just turned the blower off and left the AC on, but no cold air's coming out of the vents so I'm pretty sure it's off.

I'll check next time I'm in the cab though.