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ROLYAK 12-18-2008 12:46 PM

Idle problem and MAP sensor
'97 4.0L 5 spd.-158,000 mi+

IAC and TPS replaced with new OE parts. Linkage checks OK

Problem: Slow to return to idle, sometimes hangs around 1200-1500 then drops to 750-950 Idle may be OK when first started, then starts to act up.

MAP sensor appears to be original plus scanner shows 12" HG while actual engine vacuum is 17" HG.

Any ideas, y'all?

Jerry Bransford 12-18-2008 01:07 PM

Did you clean the throttle body oriface (hole) the IAC plunger fits into? Normally the IAC causes that kind of problem by its plunger not being able to move freely in/out of the hole in the throtte body. By the way, it is rare that the IAC actually needs replacing. It can normally be restored back to good functionality by just cleaning all the gunk off its solenoid actuated plunger and the hole it fit into.

ROLYAK 12-18-2008 08:19 PM

Thanks Jerry
I did clean both the passage and the TB but maybe I better R and R the TB to get a more thorough cleaning.:cool:

Still curious why the scanner (Auto X-Ray) shows less manifold vacuum than the manual gauge.:confused:

I'll post back after cleaning the TB.:)

rrich 12-18-2008 08:56 PM

Check the line going to the MAP. Even a small leak will lower the vac at the MAP.
Maybe even put your gauge right at the MAP sensor and compare.

And - is the gauge accurate? Try another one to compare.

Charging system voltage - check to make sure it's correct too. Sounds odd, but a charging voltage too high or too low can manifest itself in lots of different strange ways.

ROLYAK 12-19-2008 06:26 PM

R and R'd the TB, was not real dirty but is now immaculate! :cool:

Mity-Vac'd the MAP direct and via the TB passage, held 21"HG and inspected the rubber elbow that connects the two.:cool:

Charging circuit-14V at idle.:cool:

Professional vacuum gauge indicates 16.5" HG at idle.:cool:

Returned a suspect IAC today and he mentioned MAP sensor? :confused:

Might be time to spring for a new MAP even though I've get no codes. I've read that a MAP sensor can be out of specs but still not throw a code.

Problem is not extreme but going downhill I have very little engine braking. After a few seconds it will settle into a normal idle.

Thanks for your help!

ROLYAK 12-22-2008 09:30 PM

Update-12/22/08 '97 4.0L
Swapped in a known good MAP sensor - No change!
Also swapped in a known good canister purge solenoid -No change!
Cleaned all ground cables for PCM.
Inspected PCM connections for corrosion- looked good.
No codes!

Seems to idle OK when in open loop but after being driven (in closed loop) begins to hang at various rpm's 950-1500+. Hot restart will usually give a normal idle (750) until I start driving.:banghead:

Occasionally during a shift it hangs at 2000 for a fraction of a second before dropping to 1000-1200 or 1500.:banghead:

Could the speed sensor have anything to do with this problem, how about a PCM problem?:confused:

Getting close to a trip to the dealer!:(

rrich 12-23-2008 01:30 PM

Probably not it - but --

I had a Camaro or something come in with symptoms similar. Turned out the vacuum line to the MAP had been replaced - but with normal rubber vacuum line rather than the stiff plastic. The rubber line was "sucked closed" from the high vacuum under deceleration, then it "trapped" the vacuum to the MAP.
That was a "hair puller" till we found it.

No exhaust leaks near the engine? The exhaust pulses don't just leak exhaust out, they can pull O2 in and affect the O2 sensors.
A fairly easy way to find a small inaudible exhaust leak (usually works) - a very dark garage, small but strong pencil beamed flashlight, rev it up and down while looking at the pencil beam light. You can usually see the beam of light distort from the heated air and pollution. Blowing cigarette smoke down there while doing it also helps.

mknight759 12-23-2008 03:07 PM

i have the exact same problem in a 95 nissan pickup

ROLYAK 12-25-2008 10:35 PM

Thanks RRich
The MAP sensor bolts directly to the intake with a tiny L-shaped rubber elbow, tried one off my wife's G-Cherokee.(no difference) Elbow is solid, not deteriorated.

Exhaust system is tight, new manifold, cat, muffler,tailpipe only the headpipe is original. I did have several leaks with the original cracked manifold but not sure if I had the problem at that time, thought the TB was gummed up.:)

Ran a road test with a professional vacuum gauge, seemed pretty normal, 16.5" idle, 22" under decel and near zero under WOT.:)

Here's the real kicker, two days ago, took a 60 mile trip (in the rain) and it appeared to be almost normal dropping to 750 rpm's when completely stopped, as long as it was moving it would only drop to about 1000-1100.
(maybe the PCM is programmed that way?):confused:

Today the problem returned, dropping to 1500-1600 (clutch depressed) but still rolling, once completely stopped it drops to 1000-1200.:banghead:

I'm going to take a look at the speed sensor and any possible connections going to the PCM.:zap:


ROLYAK 12-27-2008 07:24 PM

Possible breakthrough on the high idle??!!
Removed the connector for the speed sensor and discovered oil (from the transfer case) in the cavity. Sprayed brake cleaner into both the male and female connector then dried it with compressed air.:cool:

Drove about 80 miles today and it returned to idle (750) each time once the vehicle was at a complete stop.:D

My thoughts are that since the signal from the speed sensor is such low voltage that oil could somehow ground or short that signal.:confused:

May still need a new speed sensor since it has an internal leak.:zap:

I searched the archives and had found another Jeep owner with a similar problem however no solution was ever posted or found.

rrich 12-28-2008 12:43 AM

Good job!

zinger1974 09-07-2013 12:34 PM

High Idle FIX!!!
Well after reading numerous forums and 3 months of frustration:pullinghair: it is fixed.
My specific problem was idle slow to return to normal when stopping, hard to restart once warm, running really rich, and terrible gas mileage.
93 Wrangler 4.0
In order of replacement:
1. Throttle body clean - no Change
2. IAC - no change
3. new vacuum lines - no change
4. Fuel injectors (four hole upgrade) - no change
5. Speed Sensor clean re-install - no change
6. TPS - no change
7. MAP - FIXED Running perfect!!!!


Hope this helps someone someday!

Notoriously_rob 09-07-2013 01:45 PM

I had a bad dle when i was coming to stop lights- it would bog down but didnt cut off. I just changed my map sensor (new iac, tps beforehand). Just drove it around the block and it idles a lil lower than it used to, maybe like 100 or so rpms, just below the line, and it didnt really bob up and down when i pushed in the clutch to go to neutral and slow down for a light.
We shall see...

Notoriously_rob 11-07-2013 04:19 PM

It was my rear o2 sensor- changed it yesterday and drove to work- no bobbing throttle anymore

bodad 04-05-2014 08:14 PM

Hey guys. I'm new to this site and a jeep owner only for the second time in my 31 years of living. I have a 93 2.5 wrangler with bad idling and sputtering problems. It idles up and down in neutral and then when I wind whatever gear I'm in out, it stalls and sputters when the rpms are high until I change gears and lower the rpms. I have changed maps sensor, tps sensor, plugs, etc. and can't figure out the problem. Can anybody point me in the right direction? Thanks so much for your time.

04wranglernoob 05-23-2014 04:36 PM

2004 TJ 4.0 Manual.

I've been toying with this for about a year now. The first post described the symptoms perfectly. For me it is more noticeable when it is cold and/or wet outside.

So far I've cleaned the TB, replaced the TPS, replaced the IAC, replaced O2 sensors (all 4) with no luck. I'll check out my speed sensor connection and MAP sensor next.

I'm glad I've learned a bit about the fuel system trying to troubleshoot this issue but I'm ready to have it fixed. I'm worried that if I let the problem go, the high engine speed will wear my clutch much quicker. Anyone else notice the issue more in wet/cold conditions?

I'll check back in after looking at that MAP and speed sensor.

Thanks to everyone who posted their findings. This site is a great resource for guys like me who are a bit new to this stuff.


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