'89 4.0L VERY high idle problem

I have recently acquired an 89 Jeep Cherokee with an inline 6 cyl which is
fuel injected. It quite suddenly developed a dangerous problem with the
idle speed. At idle it sits at anywhere between 2300 to 3300 rpm so you
almost have to have 2 feet on the brakes at a stop sign. When I pull the
vacuum hose off the map sensor it drops down to about 1100 with some white
smoke out the tailpipe. I pulled the tps out to see if it was maybe stuck
but it seemed okay and moved freely. I am not sure where to go from here
to fix this thing. Anyone else with a similar problem?

Disconnecting your Manifold Absolute Pressure sensor tells me the
problem is there or in the way your computer is interpreting that
information. Sounds like the most common idle step motor problem is
working properly. Might find a way to diagnose it via:
http://www.billhughes.com/temp/89JeepFI.pdf I once gave up, and just
closed off the step motor with a beer can, and used the throttle stop
screw.
God Bless America, Bill O|||||||O


scmadhatter wrote:
>
> I have recently acquired an 89 Jeep Cherokee with an inline 6 cyl which is
> fuel injected. It quite suddenly developed a dangerous problem with the
> idle speed. At idle it sits at anywhere between 2300 to 3300 rpm so you
> almost have to have 2 feet on the brakes at a stop sign. When I pull the
> vacuum hose off the map sensor it drops down to about 1100 with some white
> smoke out the tailpipe. I pulled the tps out to see if it was maybe stuck
> but it seemed okay and moved freely. I am not sure where to go from here
> to fix this thing. Anyone else with a similar problem?

(sigh) Is there anything that beer can't do?

>I once gave up, and just closed off the step motor with a beer can, and used the throttle stop screw.

On Fri, 14 Oct 2005 21:00:15 UTC "scmadhatter" >
wrote:

> I have recently acquired an 89 Jeep Cherokee with an inline 6 cyl which is
> fuel injected. It quite suddenly developed a dangerous problem with the
> idle speed. At idle it sits at anywhere between 2300 to 3300 rpm so you
> almost have to have 2 feet on the brakes at a stop sign. When I pull the
> vacuum hose off the map sensor it drops down to about 1100 with some white
> smoke out the tailpipe. I pulled the tps out to see if it was maybe stuck
> but it seemed okay and moved freely. I am not sure where to go from here
> to fix this thing. Anyone else with a similar problem?

First off. pul the connector on the tps with contact cleaner (or WD40
in a pinch). after you plug it back in, take a volt meter and check
the voltages. I can't remember which pins are which, but with the key
on (don't start it) probe the plug from the wire side. One pin should
read 5 volts, one about .9 volts, the third zero - that's ground.
probe the one that reads about .9 (or somewhere between about .5 and
4.9 some-odd) as you move the throttle cable - it should vary smoothly
with no breaks. If you can't find a pin that does vary from just
under 1 volt to something close to 5 volts, you have a shot tps. If
the tps checks out then what you describe could well be the MAP
sensor. Probe it the same way, but you really need a vacuum pump
(hand style) or good lungs to see if the reading changes as pressure
changes. AIR, the MAP sensor is relatively cheap - $20 range.

The dirty TPS connector is a common problem on the old Renix systems.

--
Will Honea

THANKS! I spent the evening reading that fuel systems manual. I liked the
fact that it explained how each component worked rather than just stating
what it did and assuming that the reader was savvy enough to figure it
out! Sort of like fuel systems for dummies! I will start testing
tommorow. I was wondering though, could the stepper motor plunger get
carboned up and stick enough to cause a high idle condition? Thanks again
for the info.

Does all the time. Similar:
http://members.cox.net/wilsond/tb/tb.html
God Bless America, Bill O|||||||O
http://www.billhughes.com/

scmadhatter wrote:
>
> THANKS! I spent the evening reading that fuel systems manual. I liked the
> fact that it explained how each component worked rather than just stating
> what it did and assuming that the reader was savvy enough to figure it
> out! Sort of like fuel systems for dummies! I will start testing
> tommorow. I was wondering though, could the stepper motor plunger get
> carboned up and stick enough to cause a high idle condition? Thanks again
> for the info.

If all the other fixes mentioned don't work, then I would be looking
hard for the vacuum line that fell off.....

If a vacuum line has fallen off, you will get your symptoms. One check
is to see if it will go into 4x4. If it does, then you likely don't
have a leak and it is a sensor or sensor connection like on the TPS
issue.

If it doesn't go into 4x4 with the dash light on, then it is likely a
vacuum leak.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's

scmadhatter wrote:
>
> I have recently acquired an 89 Jeep Cherokee with an inline 6 cyl which is
> fuel injected. It quite suddenly developed a dangerous problem with the
> idle speed. At idle it sits at anywhere between 2300 to 3300 rpm so you
> almost have to have 2 feet on the brakes at a stop sign. When I pull the
> vacuum hose off the map sensor it drops down to about 1100 with some white
> smoke out the tailpipe. I pulled the tps out to see if it was maybe stuck
> but it seemed okay and moved freely. I am not sure where to go from here
> to fix this thing. Anyone else with a similar problem?

I think that might have done the trick. It was really caked with grime so
I took the degreaser to both the stepper motor and the hole it mounted in
and cleaned it all up. When I re-installed it I started it up with the
intake elbow off and nothing had changed. Then I put my finger over the
odd shaped hole which ports down to the stepper which is on top of the
intake flange just to see what would happen and it immediately dropped
down to a normal idle!?! Remounted the intake hose and took it for a test
drive and it ran just fine. I dunno! Hope it stays that way. The stepper
motor was really caked and I don't think it could have moved the plunger
at all in the condition it was in. Thanks for the info, but don't go too
far away as I am posting another problem after I eat dinner. Steve in OKC