High altitude rough idle

[jeepme]

SO lately when I go to the mountains (5-7000 feet) for a wheelin day, after I shut the engine off upon re start the idle drops to about 500 rpms and sputters real bad. Once I give it some gas and hold it at about 1000 rpms for about 15 seconds it smooths out and runs fine. Any ideas what could be causing this. I was thinking MAP sensor but not sure. It only happens at high altitude, once I am back home (300ft) it runs fine.

Thanks

 

[willyb]

Deffinitly sounds like it's related to your MAP... The weird thing is that the MAP sensor sould compensate for the increase in altitude without any hiccup's. I have only heard one other case of where a MAP was 'set' (like your describing)... Maybe someone has had similar circumstances occur??

Will.

 

[XJXJ]

'Sorry, my damn crystal ball just ain't seeing the YEAR of your XJ?

 

[jeepme]

Sorry, its a 1995 4.0 133,000 miles auto with no major mods just exhaust and K&N.

 

[XJXJ]

Sorry, its a 1995 4.0 133,000 miles auto with no major mods just exhaust and K&N.

I think with a '95 you can pull codes with the right ignition on/off sequence but I dunno the exact procedure. If so, try that the next time this happens.

For a couple guesses, have you cleaned the TB (throttle body) & IAC lately? EGR valve sealing tight? Intake manifold bolts secure? Fuel pressure regulator check okay? Injector connectors good? 'Gotta be something!

 

[XJXJ]

Sorry, its a 1995 4.0 133,000 miles auto with no major mods just exhaust and K&N.

I think with a '95 you can pull codes with the right ignition on/off sequence but I dunno the exact procedure. If so, try that the next time this happens.

For a couple guesses, have you cleaned the TB (throttle body) & IAC lately? EGR valve sealing tight? Intake manifold bolts secure? Fuel pressure regulator check okay? Injector connectors good? 'Gotta be something!

 

[jeepme]

TB and Iac are all clean just did it not to long ago.Could be the EGR I just dont know why it would only happen at high altitude. Maybe the TPS??? I dont know I am totaly stumped. :dunno:

 

[XJCasper]

I have a similar problem, but mine has hawked up a "Check Engine" light. Ran a computer check and it noted that the "First Bank is running Lean"

Any ideas?

Thanks
Jason

 

[saltilloxj]

I live in the mountains in Mexico now (6000ft+/- great trails here!) and I have the same problem low idle and rough idle. I have noticed that I think it has only gotten worse lately. I cleaned the TB, cleaned or replaced the valve cover and breather lines etc. and replaced the plug wires, does anyone have any idea what I should do next? My friends computer pulled that it was running lean as well. By the way its a 90' - np242 - 30/35c

 

[MACH90XJ]

Have you changed your CPS lately ? There is a "high altitude" version.Maybe ?

 

[RichP]

Have you changed your CPS lately ? There is a "high altitude" version.Maybe ?

I was going to say that too. They do have a HA version of the cps that times the motor a bit differently. The puzzling part is after the restart and holding the RPM high for a few seconds it smooths right out. Try pulling the CPS connector apart and cleaning it out see if that helps any.

 

[ChuckD]

It's the MAP. Mine did the exact same thing at high altitude,5000 ft.

My second guess would be the CPS, but there is no High-alt CPS for the HO's.

 

[RichP]

It's the MAP. Mine did the exact same thing at high altitude,5000 ft.

My second guess would be the CPS, but there is no High-alt CPS for the HO's.

Don't quote me on this but the high altitude CPS is not listed as a HA, it's listed as a kit and my understanding is they are standard on Jeeps that are sold in colorado. I'm relying on a distant memory here, also the 'kit' was cheaper than the normal CPS by quite a few bucks. I just remember this from some postings a couple of years ago where this got discussed in quite a few threads.

 

[saltilloxj]

This may be a stupid question but here it goes anyway what is CPS??? Also where is the MAP and can it be fixed or must it be replaced?

 

[RichP]

This may be a stupid question but here it goes anyway what is CPS??? Also where is the MAP and can it be fixed or must it be replaced?

The cps pretty much times the engine, it is a hall effect device, there are magnetic chips on the flywheel that tell the motor when the #1 piston is at TDC and this starts the injectors and spark plugs working. Also in back of my mind is that you should be using a lower octane fuel at high altitudes, 85 or 87 octane. If you are putting in a higher octane in the belief that you are doing your engine a favor try running lower octane and see if that helps.
The map sensor is attached to the throttle body as is the throttle position sensor. The cps is on the bellhousing at about the 1 O'clock position as you look from the front to the back, 11 O'clock position as you are sitting in the drivers seat. You can try cleaning the map, just be careful as it is plastic and most cleaners are not plastic friendly. we just pulled the TJ's throttle body and did a complete workbench cleaning yesterday and also replaced the TPS as it was throwing a TPS code.

 

[ChuckD]

CPS

The Map

Green connector