Hello everyone, I’ve been trying to figure this one out for about 2.5 years now, with no luck. I’ve found multiple similar threads around, but nothing seems to fix the problem.

Jeep starts right up every time. Idles normally while cold. Once warm, has a really rough idle. Sounds like it is going to shut off, then revs itself back up again. And keep doing that indefinitely. Often it will settle, and run great for a few miles. However, every time (100%) I move it in gear after starting it up, it has zero power with the pedal to the floor. It will stutter and then catch up and rev up. After that initial sputter, it will intermittently keep doing that all day long. Fine for 10 min, or an hour, and then right back to struggling.

While it is struggling to idle, it sounds like the intake is sucking in air very loudly. At a stop, it runs poorly in park or neutral. However, right as I put it in gear, it levels out and idles pretty well.

On the trail, it is pretty much fine while under power. Sometimes struggles in technical spots, and isn’t smooth. Any time it’s not under power, (stops, anything downhill) it really struggles to keep running. Most of the time going downhill on trails I have to be in neutral revving it up so that it doesn’t die. It has died multiple times on the trail, but it always fires back up.
Last time I was out, I pulled off a vacuum cap on the throttle body and it helped it run better with a vacuum leak which is confusing.

It is very unpredictable, and sometimes runs completely fine for hours. I’ve been just dealing

didn’t get under water, but liquid splashed up more than ever. It was my daily at the time, and I’ve avoided water at all costs since. I’m not saying that is what caused this, but it’s the only thing I can point to.

Context: Stock 4.0, tons, 40s, cage, triple fans for radiator, dove nose (battery relocation to trunk, gas tank in the trunk, AC delete). Rarely on the street, 100% trail rig. Battery on a trickle charger all the time.

What I’ve done so far: (from buddies recommendations, and forums)

Replaced MAP sensor, Cleaned then replaced IAC, replaced TPS, replaced CPS, cleaned throttle body.
Cleaned, then replaced air filter.
Tested battery and it is great.
Checked all fuses and relays. Replaced starter relay.
Replaced exhaust headers (found cracks during all of this)
Replaced entire exhaust- both 02 sensors, muffler, cat. (I was running a straight pipe without downstream 02 for years)
just did rear main seal/oil pan gasket. Oil looked good. No obvious problems
Replaced distributor cap, ignition coil, spark plugs, and spark plug wires.
Replaced radiator (I know a few of these don’t affect how it runs)
Just spent 4 hours trying to find a vacuum leak. Can’t locate one- I used carb cleaner on all vacuum lines and it never revved up.

Any help is much appreciated. Starting to get frustrated!
Thanks!

 

My first thought was intake manifold bolts might be lose causeing a vacumn leak.

You can mist starting fluid every but the intake track to track down vacumn leaks.

When i got my jeep it would barely run. Because of this i got for next to nothing. Nearly all the intake manifold bolts where lose.
3 minutes to tighten them fixed it. That was 8ishyears ago.

 

Great suggestion, I just replaced the exhaust headers, and cleaned up the intake manifold as well. So I assume I got them tight, but I’ll check that first thing this week.

Thanks- I’ll update

 

You’re saying mist everywhere except the intake track?

Or are you saying to mist the intake track to find leaks?

thanks

 

Everything exept where air is suposed to go like intabe tube from filter

 

How is everything going?


 

Tried that and can’t find a leak anywhere. Checked the bolts and tightened them to spec again as well. Nice and tight.

Where to next? Compression test?

 

Have you checked your ECT coolant temp sensor and wiring? It sounds to me that your ECT sensor is not telling the computer that the engine is warm. The symptoms you state; starts fine when cold, idles rough when warmed up and runs better with a vacuum leak, sounds like your engine is running too rich.. Are you running a colder than stock thermostat or none at all?

 

interesting. Stock thermostat. I’ll check that out and let you know. Sorry for the late response, I actually took it to a shop, and they’ve had it for a month. They’re just so busy right now they haven’t had much time to look at it. I’m gonna give them one more week and then take it back. Getting tired of waiting. Thanks!

 

Hello everyone, ended up fixing the issue today. Turned out to be the ECU. Swapped it out with a new one and it ran like a champ. Put about 20 miles on it today, with ZERO popping, hesitation, or sucking air. Felt really good to drive it confidently again, it’s been a long time coming. Thanks to everyone who gave me ideas, I really appreciate your time and help.

I’ll post again if it comes back! Hoping that was it!

 

Did you reset the new ECU as well?