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uglefisk 09-02-2011 03:26 PM

High idle when rolling, normal when standing still
Hi, this is my first post here, I bought a 1991 YJ this july, it was in pretty bad shape (air filter soaked in oily goo, spark plugs with barely anything left, leaking fuel filter, broken/rusty rear shock mounts, VERY worn and rusty brakes, fluids missing all over +++).

I shopped parts for about $1400 including shipping to Norway, totalling about $1700 including Norwegian taxes delivered to my door.

I have changed oil, oil filter, air filter, spark plugs, spark plug wires, ignition coil, serpent belt, coolant. I have also cleaned the air intake, the IAC was covered in a hard, oily substance. I also cleaned the throttle body as it was full of the same substance as the IAC.

The engine is much easier to start now, and it idles stable at about 6-800 rpm's untill I start to drive. (Although it idled pretty stable before the IAC cleaning, amazingly. It's almost a wonder it even started before the overhaul, as the air filter seemed almost clogged).

The strange thing is, when I start to drive, then let of the throttle and press in the clutch, the idle rpm is about 1200 rpm UNTIL the vehicle is stopped. When the vehicle stops completely, the idle drops back to the normal 6-800 rpm.
When I start driving again, and repeat, it does the same thing, it must stop completely for the idle to drop from 1200 to 6-800.

Is this normal?

I have searched all over to try and find a solution to this - maybe the O2 sensor could be the problem? I can't find any vacuum leaks, although I'm not a pro at cars, this is the first car I have ever been working on (and I love it).

Garyk 09-02-2011 06:02 PM

You might want to check the TPS.

uglefisk 09-03-2011 03:54 AM

1 Attachment(s)
Ok, but do the TPS (throttle position sensor) know if the car is moving or not?

The idle returns to 6-800 rpm when the car is stopped. I will check if the idle is increasing when I roll forward without putting it in gear

Btw here is what the old filter looked like

Attachment 66103

Csechols38 09-04-2011 09:30 PM

My yj does the same thing.. 95 yj 2.5. Its supposed to idle around ~1500 when rolling . It makes for cleaner shifts... thier is a sensor on my tranny this is most likely what its for.. my Ford ranger does the same thing as well. I was told it keeps the rpms a little higher so when you shift it doesn't jerk all hard since it would be at a higher rpm

Csechols38 09-04-2011 09:32 PM

The oil in the air filter is from your clogged pcv that comes off the top of the valve cover and into your airbox

uglefisk 09-05-2011 09:46 AM

Ok, so it's normal that when the car is moving, the idle increases about 4-500 rpm's above normal idle?

I googled the PCV, and found a statement:


The 4.0 engine has a CCV tube at the back of the valve cover that performs the function of a PCV. There is no actual valve as on other engines. The front hose is the fresh air inlet from the filtered side of the air cleaner.

Read more: Where is the PCV valve in a 1991 4.0 jeep wrangler and how do you replace it
So maybe the hose could be clogged?

Csechols38 09-05-2011 11:09 PM

Yea its normall. You'll notice if you were to coast down a hill it would be idling normall. It idles up after you take it out of gear because when shifting from one gear to another it makes it way more smooth.. my ranger had a broken iac valve and when I would shift the rpms would drop down to idle at it was so annoying cuz I would have to hit the throttle in-between shifts so they were smooth.. and yea the ccv is a copper fitting that screws into your valve cover and it has a tiny hole to let air be sucked through .. sometimes they get clogged with oil or sludge.

uglefisk 09-07-2011 10:25 AM

Ok, thank you for the information, It's really useful!

As a side note, I checked the hoses from the airbox to the valve cover, and they broke when I tried to move them, I guess they are as old as the engine (20 years). They were also full of black goo. Have to get them replaced :)

I also found 4 of the valve cover bolts to be loose, two was even so loose I could use my fingers to turn them!

Luckily I had a "upper valve grind" gasket set laying around, so I'll replace the gasket tonight. Hope the engine gets as good as new when I'm done with it :)

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