Topic: Idle Issue

[Desert Bob]

I did check all of the vacuum connectors but not the quick disconnect.  Will do that today.  The jeep runs well and idles smooth when in the morning when cold.  When it warms up it idles rough and kind of surges in a very rhythmic cycle.

[Desert Bob]

Here is what I have done so far:

My 95 2.5l starts fine in the morning and idles smooth.  Once underway it idles rough (like a miss) and rises/falls rhythmically (+/-100rpms) at stop lights.  It runs fine and accelerates smoothly once off the line.  When I start it when warm it idles at about 1,500rpm for about 30 seconds and then goes back down to the rough idle.  I drove the Jeep to work this morning and based on a humming sound it sounds like the Fuel Pump is cycling with the rough idle. What controls this? I would have thought the fuel pump would run in a steady fashion or does it cycle based on the ECU or MAP determined by engine load.

I have done the following:

Fixed the #4 exhaust stud and replaced the intake/exhaust gasket

Replaced all vacuum hoses

Cleaned the throttle body and IAC motor and passage

Replaced the plugs (Old ones were hardly worn and were a nice light brown)

Replaced the Cap and Rotor

Replaced the Plug Wires

Replaced the Fuel Filter

Sea Foamed the Intake and fuel system

Replaced the Valve Lifters

Compression: 1st=157 2nd=155 3rd=162 4th=165

Should I go for the O2 Sensor and TPS?  How do you test the fuel pump? I think that is all I have left.  What next?

[Jeffy]

Have you checked the PCM for stored codes?

[Desert Bob]

Yes.  Nothing out of the normal.  I think they were 12, 33 and 55.

[Jeffy]

Hmm, can you still hear the fuel pump after it primes?  Also have you taken a fuel pressure reading?

[Desert Bob]

I think so.  I hear a humming that cycles with the idle. 

[Jeffy]

I think so.  I hear a humming that cycles with the idle. 

Hmm, mine goes quiet after it primes.

[Desert Bob]

How do I pressure test my fuel system?  I thought it either worked or didn't.  It starts fine and seems to run OK.  My last 2 Jeeps were 4.0 HO motors so I can't compare power delivery.

Hmmmm.  I am beginning to think I should have stayed with Baja Bugs.  Not much to trouble shooting an aircooled motor.

[Jeffy]

How do I pressure test my fuel system?  I thought it either worked or didn't.  It starts fine and seems to run OK.  My last 2 Jeeps were 4.0 HO motors so I can't compare power delivery.

Hmmmm.  I am beginning to think I should have stayed with Baja Bugs.  Not much to trouble shooting an aircooled motor.

There's a tool for it but you could make one.

What RPM is it idling at when 'rough'?  Have you checked the timing?  You won't be able to adjust the timing but it could help diagnose the problem.  Is this a manual or auto? What year is this again?

[Desert Bob]

Jeffy,

Thanks for all the help.  The engine when cold idles at around 1,000rpms.  It then drops down to about 750 at the end of the block.  In the morning when the air is cool it feels like it is going to stall but does not - then it idles just above 500rpms.  When the engine is warm it idles at about 600rpm and misses hard then goes smooth, pulses a few times and then misses hard again.  I can feel the miss through the entire chassis.  It is not a completely predictable cycle like I thought before.  I pulled each injector plug while the engine was running and noticed no difference except for the obvious miss fire on that cylinder. 

Thank,

Bob

[Desert Bob]

Oh and it is a 1995 YJ with the AX5 transmission.  No AC.  Just a basic Jeep.

[jetcracker82]

My 1995 YJ 2.5L has issues at idle. It runs well but it idles rough (kind of pulses) and when started when it is warm it idles high (1500rpms). As soon as I drive off the idle drops down to normal (750-800 rpms). When it is cold it acts like it is going to stall out (but never does). I cleaned the throttle body, IAC and passage and ran some injector cleaner (sea foam) through it but is still acts exaclty the same. Is this the TPS or IAC that controls this circut? The Jeep has 163,000 miles and to my knowledge neither of these have been changed.

I had the same problem in my 90 yj.  It turned out the EGR was slightly open because of carbon.  Had it cleaned and it runs like a champ aside from the cracked exhaust manny, but the shop that replaced it the first time is going to replace it for free so I'm not that butt hurt.

[Desert Bob]

No EGR on a 95.

[Desert Bob]

More data.  I checked the timing and it is 14* btdc which seem like way too much advance at idle.  I would have expected more like 10*.  Also checked my vacuum with a gauge and it is fluctuating between 15-20in./hg at idle which according to my Chilton guide is either a sticking valve or ignition miss.  Right now I am leaning towards the ignition miss as my compression test checked out pretty well.  Would this be the ECU or Distributor?  I did notice that there was quite a bit of play in the distributor shaft when I put on the rotor last weekend.  Hmmmm.  I thought this was OK as I did not see any damage in the old cap/rotor. But this might be the culprit.  Any way to test this as I don't want to keep buying parts hoping to find the right one.  Thanks.

[Jeffy]

More data.  I checked the timing and it is 14* btdc which seem like way too much advance at idle.  I would have expected more like 10*.  Also checked my vacuum with a gauge and it is fluctuating between 15-20in./hg at idle which according to my Chilton guide is either a sticking valve or ignition miss.  Right now I am leaning towards the ignition miss as my compression test checked out pretty well.  Would this be the ECU or Distributor?  I did notice that there was quite a bit of play in the distributor shaft when I put on the rotor last weekend.  Hmmmm.  I thought this was OK as I did not see any damage in the old cap/rotor. But this might be the culprit.  Any way to test this as I don't want to keep buying parts hoping to find the right one.  Thanks.

Well the distributor can't be moved.  What controls timing is the CPS which would be cheaper then replacing the PCM.  Worth testing I think.