Hey All Jeepers,

For the past 5 months I have been trying to solve my Jeep's rough idle
problem, and recently, I finally solved it! Since the problem is
solved, I decided to document how I fixed it. Hopefully this will
save some of you a lot of time, headaches, and maybe some money..

This article is geared more toward fuel injected Jeeps. If your Jeep
has a carburetor, there are plenty of articles out there for solving
rough idle related to the carburetor. Some of the info here will
still apply though.

My Jeep:
- '89 Jeep Wrangler YJ 2.5L TBI.

Symptoms:
- Idles rough between 1000 and 1400 RPM's.
- Possibly runs rich at idle.
- Consistent miss at idle.

Steps to fix the rough idle:

1)If your brakes are weak and your engine idles rough, the power
booster for your brakes is probably bad! Replace the power booster
immediately! This is what I ultimately did to fix my rough idle
problem, which is why I listed this fix first.

2)Check your vacuum lines and hoses. If you have an older Jeep like
me, a lot of your vacuum hoses are cracked and worn out. If this is
the case, replace them all; it is cheap and easy. Make sure every
vacuum line and hose is properly connected. There are vacuum diagrams
in the Hayes Manual to help you out.

3)Check your PCV valve. Take it out and shake it. If it doesn't
rattle, it is bad.

4)Check your EGR valve. Mine was rusted closed, so I replaced it.

5)If it has been a while, replace your spark plugs, wires, and rotor.

6)Do a compression test on your engine.

7)Clean your fuel system. If your Jeep has or had a steal gas tank,
you need to take some action here. First, if you have an old steal
gas tank, replace it with a polypropylene tank. The rust from the
steal tank will clog the fuel filter and injector. Replace the fuel
filter if you haven't done so in a while. Use some fuel injector
cleaner in your tank (I did this two times). Finally, take your fuel
injector out and clean it with some throttle body cleaner. I blasted
my injector with the cleaner. I then sprayed the cleaner into a small
cup and put the injector in it. After soaking it for about 20
minutes, I had a lot of rust particles at the bottom of the cup. The
injector worked much better after this.

8)Check your sensors and replace the bad ones. All you need is a
voltmeter. I summarized some info I found at:
http://www.wagoneers.com/XJ/tech/Eng...-problems.html
This info should apply to most Jeeps.
- O2 SENSOR TEST:
Should be between 5 and 7 ohms. If infinity reading, replace sensor.
- COOLANT SENSOR TEST:
Should range from 100,700 (100.7K) Ohms at –40°F to 185 Ohms at
212°F.
- MAT SENSOR TEST:
Should be less than 1000 ohms with warm engine.
- MAP SENSOR TEST:
A is ground, B is output voltage and C is 5 volts. With ignition
switch on, engine off, A+B should equal 4-5 volts. Start engine and
run till warm/hot. The voltage should drop to 1.5-2.1 volts with hot
neutral idle. Also test A+C, should be 5 volts +/- .5 Volts.
- CRANK POSITION SENSOR TEST:
With engine off, A+B should equal 200 +/- 75 ohms with hot engine.
Also, when measuring voltage, volts should spike when engine is
cranked or is running. If no reading, CPS is bad. Oops, disconnect
sensor when doing ohm test...
- KNOCK SENSOR CHECK:
Make sure the knock sensor is snug and all the wiring is intact.
- THROTTLE POSITION SENSOR TEST:
I don't have the ohm values for this. It shouldn't cause rough idle
unless it is causing your engine to idle too low. People with a bad
TPS usually have idling at high ranges and the engine stalls a lot.

9)Check the purge valve on your charcoal canister. Look for some
posts by Mike Romain for more info on this.

10) Give up and replace the engine with a Chevy 350.

Hope this info is useful.

-Brent Bogrees
'89 Jeep Wrangler YJ 2.5L TBI – 33x10.5 BFG AT Tires
'94 Jeep Cherokee Sport 4.0L – 31x10.5 BFG AT Tires
http://www.chrr.ohio-state.edu/~bogrees/html/Jeep.html

 

Brent Bogrees did pass the time by typing:
> Hey All Jeepers,

Good list.

One more item..

If you can move the rotor from side to side then
replace the distributor.
(acutally if you can see oil building up in the
base of the distributor it's also time to replace
it cause the bushing is going.)

There is a writeup on my site on that.


--
-- DougW -- 93 ZJ 4.0 http://members.***.net/wilsond
HESCO Supercharger - 300W IASCA Stereo - Edelbrock IAS Shocks
Gibson Exhaust - rear DCpower - custom gauge install - Stillen Rotors

 

Brent Bogrees did pass the time by typing:
> Hey All Jeepers,

Good list.

One more item..

If you can move the rotor from side to side then
replace the distributor.
(acutally if you can see oil building up in the
base of the distributor it's also time to replace
it cause the bushing is going.)

There is a writeup on my site on that.


--
-- DougW -- 93 ZJ 4.0 http://members.***.net/wilsond
HESCO Supercharger - 300W IASCA Stereo - Edelbrock IAS Shocks
Gibson Exhaust - rear DCpower - custom gauge install - Stillen Rotors

 

Bad booster? I want to know more about this. Is this the same for all 4.0's?
KH

"DougW" <post.replies@invalid.address> wrote in message
news:VWmeb.1262$Gb.1181@news2.central.***.net...
> Brent Bogrees did pass the time by typing:
> > Hey All Jeepers,
>
> Good list.
>
> One more item..
>
> If you can move the rotor from side to side then
> replace the distributor.
> (acutally if you can see oil building up in the
> base of the distributor it's also time to replace
> it cause the bushing is going.)
>
> There is a writeup on my site on that.
>
>
> --
> -- DougW -- 93 ZJ 4.0 http://members.***.net/wilsond
> HESCO Supercharger - 300W IASCA Stereo - Edelbrock IAS Shocks
> Gibson Exhaust - rear DCpower - custom gauge install - Stillen Rotors
>
>

 

Bad booster? I want to know more about this. Is this the same for all 4.0's?
KH

"DougW" <post.replies@invalid.address> wrote in message
news:VWmeb.1262$Gb.1181@news2.central.***.net...
> Brent Bogrees did pass the time by typing:
> > Hey All Jeepers,
>
> Good list.
>
> One more item..
>
> If you can move the rotor from side to side then
> replace the distributor.
> (acutally if you can see oil building up in the
> base of the distributor it's also time to replace
> it cause the bushing is going.)
>
> There is a writeup on my site on that.
>
>
> --
> -- DougW -- 93 ZJ 4.0 http://members.***.net/wilsond
> HESCO Supercharger - 300W IASCA Stereo - Edelbrock IAS Shocks
> Gibson Exhaust - rear DCpower - custom gauge install - Stillen Rotors
>
>

 

Kevin in San Diego did pass the time by typing:
> Bad booster? I want to know more about this. Is this the same for all 4.0's?

A bad booster creates one hell of a vacuum leak.
It's the same for all vehicles with vac assist brakes.

--
DougW

 

Kevin in San Diego did pass the time by typing:
> Bad booster? I want to know more about this. Is this the same for all 4.0's?

A bad booster creates one hell of a vacuum leak.
It's the same for all vehicles with vac assist brakes.

--
DougW

 

Fortunately that's very rare. My 25 year old Bronco began to leak
about ten years ago and all I did was remove was left of it's filter and
squirt in WD-40, I thought would temporarily fix it, and forgot about
it.
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

DougW wrote:
>
> Kevin in San Diego did pass the time by typing:
> > Bad booster? I want to know more about this. Is this the same for all 4.0's?
>
> A bad booster creates one hell of a vacuum leak.
> It's the same for all vehicles with vac assist brakes.
>
> --
> DougW

 

Fortunately that's very rare. My 25 year old Bronco began to leak
about ten years ago and all I did was remove was left of it's filter and
squirt in WD-40, I thought would temporarily fix it, and forgot about
it.
God Bless America, ßill O|||||||O
mailto:-------------------- http://www.----------.com/

DougW wrote:
>
> Kevin in San Diego did pass the time by typing:
> > Bad booster? I want to know more about this. Is this the same for all 4.0's?
>
> A bad booster creates one hell of a vacuum leak.
> It's the same for all vehicles with vac assist brakes.
>
> --
> DougW

 

The booster vacuum leak problem might be rare, but I think the
boosters in Jeeps might not be the best made. Along with my '89
Wrangler, I had to replace the one in my '94 Cherokee.

One thing I wanted to add to my original post.. Before I replaced the
booster in the Wrangler, I replaced every part of my brake system
(brake lines, front disk components, and rear drum components), and
the brakes were still weak. Make sure your brake system is in working
order before you replace the booster. I did the booster test in the
Hayes Manual, and the booster passed; however, it was obviously bad,
so don't put 100% confidence into the test.

-Brent

'89 Jeep Wrangler YJ 2.5L TBI – 33x10.5 BFG AT Tires
'94 Jeep Cherokee Sport 4.0L – 31x10.5 BFG AT Tires
http://www.chrr.ohio-state.edu/~bogrees/html/Jeep.html


L.W.(ßill) ------ III <----------@***.net> wrote in message news:<3F7A1BC8.6AC92617@***.net>...
> Fortunately that's very rare. My 25 year old Bronco began to leak
> about ten years ago and all I did was remove was left of it's filter and
> squirt in WD-40, I thought would temporarily fix it, and forgot about
> it.
> God Bless America, ßill O|||||||O
> mailto:-------------------- http://www.----------.com/
>
> DougW wrote:
> >
> > Kevin in San Diego did pass the time by typing:
> > > Bad booster? I want to know more about this. Is this the same for all 4.0's?
> >
> > A bad booster creates one hell of a vacuum leak.
> > It's the same for all vehicles with vac assist brakes.
> >
> > --
> > DougW