4.0 Sounds Different After Sitting All Winter

[SquareFace]

Hey everyone — new here and figured I’d introduce myself and ask for some guidance.

I bought my Jeep back in May and had it shipped out from California in June. When it arrived it ran beautifully — smooth, steady idle, classic 4.0 sound that just felt right.

Since then it’s spent most of the winter in shops getting work done (full repaint and re-gearing), so it hasn’t been driven much. I finally got it back recently and now something feels off.

The idle RPM itself doesn’t really fluctuate, but the engine note sounds different — almost airy or “puffy,” like the tone changes instead of that steady consistent sound I remember. It’s subtle, but noticeable enough that it doesn’t feel the same.

Before I start chasing random things, I wanted to ask:

• Has anyone experienced this after a Jeep sits for a while or after major work?
• What would you check first on a 4.0 that sounds like this but holds a steady idle?

Appreciate any advice — I’m still learning and trying to do things the right way.

 

[Swedenboundanddown]

Does it sound "puffy" all the time or just under load? Currently having the same problem, thats why im curious!

 

[SquareFace]

Does it sound "puffy" all the time or just under load? Currently having the same problem, thats why im curious!

I notice it most at idle

 

[SquareFace]

Wussy Cali Jeep can't handle the Illinois cold.
p.s. Drive it for a while and see if things change. "Puffy" isn't a word I'd ever use to describe any Jeep symptoms, unless baby oil was involved.
p.p.s. Check for exhaust and vacuum leaks.

Ha! Yea. Terrible way to describe it. It sounds like the idle dips, but when I look at the dial, there is no change. I guess another way to describe it is a sputtering sound. But again, the idle on the gauge stays steady.

I’ve done barely any driving in the winter. Maybe 20 miles. Just wondering why anything would change with minimal usage.

Damn you P.Diddy! Ruined my post!

 

[Steel City 06]

Could even just be the gasoline in the tank. Gasoline does degrade over time. See if it persists after a fresh tank of gas

 

[GoldenGorilla]

Perhaps something along the way pushed or pulled against the exhaust and the exhaust is not sealing perfectly.

 

[SquareFace]

Could even just be the gasoline in the tank. Gasoline does degrade over time. See if it persists after a fresh tank of gas

Good point.

 

[Moglocker]

Good point.

If it’s not throwing a code,I’d send it,got a video?

 

[KennethS]

. Just wondering why anything would change with minimal usage.

Not driving can result in all sorts of issues. Seals can dry out, oxidation on electrical contacts, rust on metal parts, etc.

 

[Moglocker]

Not driving can result in all sorts of issues. Seals can dry out, oxidation on electrical contacts, rust on metal parts, etc.

In the car business we called it(LOT ROT)

 

[KennethS]

Yeah, Lot Rot. My classic stays in the garage through the brutal summer, then comes out in the winter. This year the turn signals and headlights didn’t work until I cycled both switches a thousand times and apparently scrubbed the oxidation from the contacts. And the utility trailer running lights didn’t work until I removed the bulbs and polished the contact surfaces with steel wool.

 

[Camdenst]

Maybe a fuel additive and an Italian tune up...

The guys at my local Napa like the Chevron project.

 

[SquareFace]

Yeah, Lot Rot. My classic stays in the garage through the brutal summer, then comes out in the winter. This year the turn signals and headlights didn’t work until I cycled both switches a thousand times and apparently scrubbed the oxidation from the contacts. And the utility trailer running lights didn’t work until I removed the bulbs and polished the contact surfaces with steel wool.

Ok. This is somehow reassuring. Lesson of the day is don’t baby a jeep. They shouldn’t hibernate.

 

[KennethS]

an Italian tune up...

That reminded me of a story my sister told me back in the 80s. One of her college friends had a Ferrari that was running rough. He brought it to the dealership and the mechanic asked him if he had been driving it hard, to which he responded absolutely not, he would never do that. The mechanic told him to drive it hard and rev the snot out of it as that is what it is designed for, and him babying it was causing the issues.

 

[SquareFace]

Maybe a fuel additive and an Italian tune up...

The guys at my local Napa like the Chevron project.

View attachment 673581

I’ll definitely give this a try. Can’t hurt! Thank you.

 

[SquareFace]

That reminded me of a story my sister told me back in the 80s. One of her college friends had a Ferrari that was running rough. He brought it to the dealership and the mechanic asked him if he had been driving it hard, to which he responded absolutely not, he would never do that. The mechanic told him to drive it hard and rev the snot out of it as that is what it is designed for, and him babying it was causing the issues.

Just looked up what an Italian Tune Up is. Awesome.

 

[SquareFace]

Quick update since I got a lot of helpful responses here.

First thing I did after posting was address something I had already planned. I had Kolak set me up with a full exhaust system and got that installed. 2.5 inch setup with a Flowmaster 50 Series Delta Flow muffler, Magnaflow high flow converter, and a mandrel bent 2.5 inch tailpipe. The Jeep immediately sounded more like a proper 4.0 IMO.

For context, this Jeep also just went through a Revolution regear to 4.56 earlier this year, so between that and the exhaust, a lot had changed from when I first got it.


After that, I decided to stop guessing and just reset everything I could control. I went through almost the entire input and ignition side in one session and forced the ECU to relearn from scratch.


Here’s what went into it:
Mopar ignition coil
Deka 9A34 battery
NOS Mopar MAP sensor
Mopar coolant temp sensor
Taylor 8.2 mm Thundervolt wires
Autolite Iridium XP985 plugs
Bosch 4 hole injectors from CS Performance
Mopar fuel pressure regulator
Hesco IAC motor
Mopar TPS
BBK 62 mm throttle body
We did it all at once.

Initial startup took a few cranks to get fuel back through the rail, then it fired and settled. First drive was interesting. It started smooth, then got choppy off idle for a stretch. Felt like it wanted more throttle to get moving. Some light popping out of the exhaust. Once it was moving it was fine. Idle was fine the whole time.

After about 15 to 20 minutes it cleaned itself up and has been smooth since. Let it sit for a few hours, drove it again, still smooth. Cold start the next morning fired right up and the off idle issue didn’t come back.

While I was in there I found something that probably mattered. The crankcase vent tube going into the elbow had clearly been glued at some point by the previous owner. When I moved it out of the way it split right along that old repair. I wrapped it with high temp silicone stretch and seal tape just to get it sealed since I don’t have the replacement yet.

After sealing that, the engine smoothed out even more. The airy or “puffy” sound I originally described is basically gone. The slight dipping I used to notice is still technically there if I really focus on it, but it’s minimal now and doesn’t affect how it drives. Throttle response is clean and it just feels consistent.

Where I landed on all of this is it was probably a combination of things. Sitting for a long time, airflow and fueling not being quite right, and that vent line not being sealed. Resetting everything and letting the ECU relearn with a sealed system seems to have brought it back.

I’ll replace that vent line properly once I track down the right piece, but for now it’s running the best it has since I got it. Attached is a pic of the torn vent tube and the temporary repair.

Appreciate all the input.

 

[SquareFace]

Quick note:

This is a 1992 YJ, not a TJ.

I know where I am. I ended up here because the level of knowledge on this forum is hard to ignore, especially when it comes to the 4.0. A lot of what I’m working through translates across both. I hope everyone is ok with this.

 

[tm dean]

Before winter sets in, do yourself a favor and install gasoline stabilizer. Stabill is the one i recommend. I put it in my boat, tractor, and lawn mowers. It has a guarantee that if installed correctly. The gas will be fresh for up to two years. Another thing that will help before storing your equipment is to fill up with non ethanol gas.

 

[SquareFace]

Before winter sets in, do yourself a favor and install gasoline stabilizer. Stabill is the one i recommend. I put it in my boat, tractor, and lawn mowers. It has a guarantee that if installed correctly. The gas will be fresh for up to two years. Another thing that will help before storing your equipment is to fill up with non ethanol gas.

Yes. I will definitely be using a fuel stabilizer for next season. Non ethanol gas is impossible to find close by. But I may take the road trip if it’s worth it. Wish I could run non ethanol all the time.