Engine problems: sputtering power above 3200 RPM

"Just Jeepin'" (macintux)
10/19/2018 at 15:35 • Filed to: None

Pic for your time. Nearby bakery promoting its wares with death imagery.

I have just discovered that, in certain gears (doesn’t seem to be all of them, primarily 1st and 2nd) my engine starts sputtering at high RPMs. It’s not really a problem per se, but it is disturbing, and combined with synchros that started failing a few months after a transmission rebuild and the lifter noise that started several months ago, sometimes make me wish I had never stumbled into my Jeep addiction.

But setting aside any mental health issues, what in particular should I be investigating? I had severe stalling problems up until about a year ago, when I traced them to problems with the fuel injectors (the engine gets pretty hot, fuel was igniting prematurely and clogging the injectors, so replaced them and wrapped everything with heat shielding).

2005 Jeep Wrangler Unlimited, 4.0L.

DISCUSSION (20)

benjrblant > Just Jeepin'
10/19/2018 at 15:43

Does it run fine otherwise? Have you pulled any spark plugs?

Just Jeepin' > benjrblant
10/19/2018 at 15:46

Runs fine otherwise, just noticed it a few days ago so haven’t tried anything yet.

I replaced the plugs almost exactly a year ago during the battle with my stalling problems. Champion 7034 double platinum.

benjrblant > Just Jeepin'
10/19/2018 at 15:49

Just guessing here, but I’d be leaning towards something restricting fuel or airflow. Fuel filter, air filter, maybe a dirty throttle body? I dunno- no mechanic here but thats my guess.

DipodomysDeserti > Just Jeepin'
10/19/2018 at 15:50

I’ve been tuning my motorcycle all week, and my gut reaction was, “check your float levels”.

It’s obviously not that.

Just Jeepin' > benjrblant
10/19/2018 at 15:51

Unfortunately my knowledge is effectively zero, I just rely on smart people around me , but fuel flow does seem likely.

Tristan > Just Jeepin'
10/19/2018 at 16:04

Jeep 4.0s like cheap copper plugs. Have you replaced boots on the coil rail? Might be a good idea, though arcing is usually more prevalent at low RPM/high load.

ItalianJobR53 - now with added 'MERICA and unreliability > Just Jeepin'
10/19/2018 at 16:06

Random thought: If the Joop has a wideband O2 sensor you should be able to look at the A/F ratio using a cheapo OBD adapter(mine is for 10 bucks ) . Maybe it’ll run lean at higher revs?

I haven’t tried this to be clear, my car has a narrow band.

Future next gen S2000 owner > Just Jeepin'
10/19/2018 at 16:07

Fuel filter and clean the injectors. See if that fixes anything. If not look for a vacuum leak.

Just Jeepin' > ItalianJobR53 - now with added 'MERICA and unreliability
10/19/2018 at 16:07

Thanks. No idea but will dig.

I stashed my OBD adapter in a ziploc to protect against water...and haven’t been able to find it since.

ItalianJobR53 - now with added 'MERICA and unreliability > Just Jeepin'
10/19/2018 at 16:08

LOL. Pop over to Columbus, we could try with mine.

Akio Ohtori - RIP Oppo > Just Jeepin'
10/19/2018 at 16:10

I saw that skull for sale at Target yesterday and I was really bummed it wasn’t metal.

Anyway... as for the Jeep I think checking your air filter and for air leaks is a good place to start. If you were running lean before, an air leak can also do that, though it sounds like replacing the injectors fixed that.

I’ve also had a company car that would do something similar. Newish car, but at very specific RPMs it would stutter. It got worse and worse and eventually was determined to be a bad spark plug wire. For whatever reason at specific RPMs it would short to the block and make the engine run REALLY rough. Towards the end it was getting difficult to power through it, but it wouldn’t present any symptoms at lower RPMs and drove just fine in town.

diplodicus > Just Jeepin'
10/19/2018 at 16:10

Look at your plugs and see what they tell you.

Just Jeepin' > diplodicus
10/19/2018 at 16:11

Mine always tell me the same story: dude, you’re not a mechanic.

promoted by the color red > Just Jeepin'
10/19/2018 at 16:12

Bad fuel pump? I noticed my Volvo choked at the exact same rpm and sometimes needed a bootful to stay running. Te sts indicated the in-tank pump was done .

Just Jeepin' > Tristan
10/19/2018 at 16:13

Got entirely an new coil rail during the epic stalling troubleshooting saga. I replaced every sensor and effectively (I think) everything under the hood related to fuel distribution over about 8 months.

Just Jeepin' > ItalianJobR53 - now with added 'MERICA and unreliability
10/19/2018 at 16:13

Next Saturday, right? I still haven’t gotten any firm commitments, though.

Just Jeepin' > promoted by the color red
10/19/2018 at 16:14

Interesting. I’ve had a bad sensor there for almost 2 years. Have to track my mileage carefully.

diplodicus > Just Jeepin'
10/19/2018 at 16:18

Kinja'd!!!

At the very least looking at the plugs should tell you which cylinders are having issues.

Check the gap too.

IDK about jeep 4.0 but I’d try copper plugs like tristan suggests. I prefer copper plugs in my M30 and M20. I’m pretty sure all the marketing crap about platinum and iridium plugs having better spark is marketing bullshit. Since copper is a better conductor than both. Copper plugs just don’t have as long as a service life.

ItalianJobR53 - now with added 'MERICA and unreliability > Just Jeepin'
10/19/2018 at 16:18

Yep.

I’m trying to get rid of a steering vibration before that(replacing the outer b all joints ). If it dosent go away, I may not take the Mini for an on-road drive. If we end up going somewhere on-road, I’ll come with you or some of my non-oppo friends may join us and I’ll hitch a ride with one of them .

Jason Spears > diplodicus
10/24/2018 at 11:03

On plugs, definitely run what Jeep says to run. I know from Saturn SL2 ownership that running a “ fancy” plug was a source of trouble for many people who didn’t know better, and had their problems solved by switching back to the NGK coppers.