For 'Knowlegable' 4.0 Jeep Engine Techs Only!

[jbett98]

PVC hoses are generally a big vacuum leak and easy to check.

 

[drose672]

I had very similar issues with mine recently.i didn't have the air flow noise, and it didn't throw any codes. Mine is a 95, I searched for vacuum leaks and found one. Still ran the same. Then I checked the throttle position sensor with ohm meter. 2 out of 3 times it tested bad, swapped in a new one and runs perfect again. Gas mileage also went back up considerably

 

[RVTECH]

One thing that keeps popping up in my head is throttle body spacer?

Well I checked the four bolts that secure the throttle body and while not 'loose' per se they did take a litlle torquing but were not loose enough to leak

 

[RVTECH]

Ok I may be 'on' to something with this. Cleaned the 02 sensor connections and still runs the same HOWEVER I finally connected a vacuum guage. OK starts and runs smooth and holds vacuum BUT as soon as it warms up it starts 'stumbling' and the vacuum drops dramatically and the needle starts to 'fluctuate' with the running of the engine. Lets say its more of a rough idle than a stumble but it definitely changed when warm and the vacuum drops. All ideas will be considered!

 

[Jamie6.5]

Ok I may be 'on' to something with this. Cleaned the 02 sensor connections and still runs the same HOWEVER I finally connected a vacuum guage. OK starts and runs smooth and holds vacuum BUT as soon as it warms up it starts 'stumbling' and the vacuum drops dramatically and the needle starts to 'fluctuate' with the running of the engine. Lets say its more of a rough idle than a stumble but it definitely changed when warm and the vacuum drops. All ideas will be considered!

How much vacuum does it have at idle before it warms up? At the altitude where you are it should be around 17-18" Hg at idle.
Before the engine warms up, the FI is in enrichment mode, and may hide a vacuum leak. When it goes into closed loop after warm-up. a vacuum leak always gets worse.

Did you ever try the Carb Cleaner trick? And if you did, what were the results?

 

[jbett98]

Read this story on Jeep's Unlimited.
He was told to tightened up the exhaust bolts, replace all the hoses, etc. Then he did this.

"Tonight I replaced as many vacuum hoses as possible, and even secured many of them with zip ties. After doing this, I tried running the jeep again, but STILL had high idle. So I decided trying one more thing that I read on another forum. The FSM says not to adjust the factory set screw on the throttle body since idle is controlled by the computer. However, someone else with the exact same symptoms I have (idle gets higher as Jeep warms up) ended up taking off the throttle body and adjusting this screw. So, I removed the throttle body, took a look at the screw, and noticed that the screw had rust only about half way up the threaded part (almost as if it was farther in at some point). So I adjusted the screw so only the rusted part was showing, reinstalled the throttle body, and IT WORKED. Idle is now ~500rpm's when fully warmed up, and it runs great. No idea why, but the previous owner must have turned up the idle by adjusting this screw at some point."

 

[v0lcom13sn0w]

a needle that fluctuates from 15-20hg rapidly would indicate a valve issue

 

[RVTECH]

Did you ever try the Carb Cleaner trick?

Well my vacuum gauge was most likely used to test the first Model T off the Ford assembly line and its accuracy is in question but it was showing about 15 " but it was steady. No must have missed that post - please restate the procedure

 

[RVTECH]

the fluctuation after warm up and drop is not that dramatic. the vacuum drops then the needle just fluctuates slightly with the run of the engine - almost in time with the rough idle

 

[Jamie6.5]

Well my vacuum gauge was most likely used to test the first Model T off the Ford assembly line and its accuracy is in question but it was showing about 15 " but it was steady. No must have missed that post - please restate the procedure

Spray some carb cleaner around the base of the throttle body with the engine warmed up and idling.
If the idle speeds up and/or smooths out when you do that, the base/mounting gasket under the throttle body is leaking.
But the carb cleaner has to be the flammable type. It doesn't work with non-flammable.

 

[Member 5605]

Spray some carb cleaner around the base of the throttle body with the engine warmed up and idling.
If the idle speeds up and/or smooths out when you do that, the base/mounting gasket under the throttle body is leaking.
But the carb cleaner has to be the flammable type. It doesn't work with non-flammable.

Works with the non-flammable type too! Just doesn't rev the engine. Stalls it out. Maybe it's not the best choice to use the non flammable type.

 

[RVTECH]

Can the MAP be cleaned on the 4.0 or is this a non-serviceable part?

 

[notazombie]

Non serviceable.

 

[RVTECH]

Well final word (most likely out of frustration and due to the massive personal issues I am dealing with) I was told to consider getting the PCM checked before doing anything else as they can sometimes be 'bad' but still function to run the vehicle. SO I called the local Jeep dealer and talked to the Shop Foreman who asked me a what was going on and what I had done. I told him and long story short he interrupted me and said "I take it when it is cold it is running fine and as soon as it warms up it starts to idle rough and you lose vacuum right"? Well he was spot on and he said he sees this a lot and it is because the system is not going into 'closed loop' and he said this is very likely an 02 sensor and NOT a failed PCM but they need to test th PCM IN the vehicle as there is no bench test for the Jeep PCM. He sounded very confidant this was the problem as he mentioned this is a common issue with the Jeep 4.0 SO I am crossing my fingers and toes on this with an appointment made!

 

[jbett98]

I would replace the O2 sensor myself. They are only around $35.00 and rather easy to replace.

 

[RVTECH]

Followup from my appointment last week - a bad upstream 02 sensor and a cracked exhaust manifold (according to the mechanic but I can't see the crack - or hear an exhaust leak) Changed the 02 sensor and it has never run better - didn't skip a beat in about 20 miles of driving yesterday!

 

[Mikej]

Doesn't that feel good?

A few years ago my '84 GMC pick-up, had it since new and use it to tow the boat, ran poorly. I futzed with it for weeks. I finally bit the bullet and took it in to a recommended shop and spent in the neighborhood of $350.00 for a complete rebuild of the Rochester "Quadratoilet" sitting on top of the 350 CI motor. Felt/ran real good after that.

 

[jbett98]

Years ago, I gave my old 80 Chevy G20 van a tune up.
Afterwards it ran fine for awhile, but then it started to misfire and lose power.
I decided to pull the motor and install a new GM crated long block.
What started out as simple engine swap turned into a case of the (Might as Wells) and proceeded to rebuild the tranny and replace the starter, water pump, carb, radiator, etc.
Days later, I fire up the new motor and come to find out it runs as bad as the old motor.
I took it to a genius of a mechanic friend of mine and after listening to it for a minute he turns around to me and says, "Jeff, you're an idiot."
He's not one to spare anyone's feelings, and he proceeds to pull off the HEI distributor cap and proves to me that I forgot to swap the grounding strap from the old cap to the new one.
A new grounding strap costs all of $2.49 and it fixed my very costly mistake.

 

[Stomper]

Damned O2 sensors.... my '04 Suburban has a total of four, two on each side of the exhaust system, one pre-cat and one post-cat. It started to throw a code and it was the #1 O2 (driver side pre-cat), but IMHO if you need to replace one, do them all so they're all the same "vintage". Not the most accessible components to get to w/o a car-lift!

 

[RVTECH]

Not the most accessible components to get to w/o a car-lift!

Fortunately VERY accessible on the Cherocar - and a good idea - I'll do the rear next.