2001 Cherokee Engine Surge

flippant07 · #1 02-23-2007, 10:36 AM

I recently bought a '01 Cherokee Limited XJ (4.0 ltr straight 6). It only has 70k miles on it and seems to be in good shape except for this very strange engine surge. I've read a few other posts and the suggestion is usually replace the PCV valve & hose and O2 sensors. Here's the twist: my engine only surges (from about 750 to 1250 rpm) when I have the vent/heat/fan on. I have not tried it with the AC on, but I'm assuming it will do the same thing. When this happens and I turn the vent/fan/heat off it immediately stops. With the vent/fan/heat on, it surges about every 10 seconds. Any ideas or guidance would be greatly appreciated! Thanks all.

1996Chreokee · #2 02-23-2007, 11:24 AM

is it your air conditioning compressor cycling on and off? If it cycles on that quick your a/c is probably a bit low on refrigerant...

let us know

flippant07 · #3 02-23-2007, 11:34 AM

I thought about the AC compressor, but I'm not running the AC. The compressor shouldn't come on when I just have the heat or vent on should it? I'm positive I am not running the AC, unless the selection dials are messed-up. Even if I was running the AC, you wouldn't expect a surge like that would you? This is a true engine surge...when I'm sitting in traffic the car actually moves forward a little bit if I only have the brakes half-applied.

1996Chreokee · #4 02-23-2007, 11:39 AM

Actually yes the A/C compressor does still cycle on and off even with out the a/c on.... if you notice when your a/c cycles your secondary fan on your radiator kicks on too... lift the hood and watch... if your a/c keeps kicking on and off quickly thats usually a tall tale sign of it being low on refrigerant...

I am not so sure that it would make the vehicle lunge forward but it does sound like the engine speed picks up

flippant07 · #5 02-23-2007, 12:17 PM

Thanks for the feedback! I'd be a very happy person if that turns out to be the problem!

hundahunta · #6 02-24-2007, 05:31 PM

you possibly have a vacuum leak. The hvac doors in this vehicle are vaccum operated. check your vacuum hoses, espically the one near the side of your battery. When you are driving up a hill for a extended time does the doors move by itself

citizen@large · #7 02-25-2007, 02:19 AM

Quote:

Originally Posted by hundahunta

you possibly have a vacuum leak. The hvac doors in this vehicle are vaccum operated. check your vacuum hoses, espically the one near the side of your battery. When you are driving up a hill for a extended time does the doors move by itself

WWTTD? You must be having a piss, b/c your #'s are defying logic here...

You drive a bug don't you. OH!

A honda from Hawaii, Ua mau ke ea o ka aina I ka pono, bro...

hundahunta · #8 02-25-2007, 04:39 PM

Quote:

Originally Posted by citizen@large

WWTTD? You must be having a piss, b/c your #'s are defying logic here...

You drive a bug don't you. OH!

A honda from Hawaii, Ua mau ke ea o ka aina I ka pono, bro...

Defying logic??

sector95 · #9 02-28-2007, 10:15 PM

I'm not quite sure what's going on with the last two postings but the vacuum leak is what I'd be looking for. The HVAC system is controlled by vacuum drawn off the intake maifold. The HVAC system uses vacuum motors which operate the various diverter valves to control the air flow. If one of them is leaking or there is a control valve leaking this can cause the lean surge you're experiencing. With the engine running and the HVAC system working in vent, listen for any hissing noises coming from under the dash or near the control panel.

Mike
00XJ

AlohaBra · #10 02-28-2007, 10:22 PM

I will agree with Sector...what say you flippant??

Oh yeah...aloha to hunda...eh bra...!!

flippant07 · #11 03-01-2007, 11:41 AM

Thanks for the input Sector. Here's another twist I've discovered: this surge only happens when I have the vent on (as I stated earlier), and when the controls are set to window defog or the window defog/feet settings. If I set the vent to feet only or cabin vent - no surge. And again, with the vent completely off - no surge. Somehow this is all tied to the window defog setting. Do you still think it has something to do with the vacuum intake? I'm taking the Jeep in Sat morning for new shocks. If I have them run engine diagnostics, will they be able to pinpoint the problem? I'm afraid this will be one of those things where it won't happen as soon as I get the car to a shop. Thanks again for the feedback.

sector95 · #12 03-01-2007, 05:30 PM

Quote:

Originally Posted by flippant07

Thanks for the input Sector. Here's another twist I've discovered: this surge only happens when I have the vent on (as I stated earlier), and when the controls are set to window defog or the window defog/feet settings. If I set the vent to feet only or cabin vent - no surge. And again, with the vent completely off - no surge. Somehow this is all tied to the window defog setting. Do you still think it has something to do with the vacuum intake? I'm taking the Jeep in Sat morning for new shocks. If I have them run engine diagnostics, will they be able to pinpoint the problem? I'm afraid this will be one of those things where it won't happen as soon as I get the car to a shop. Thanks again for the feedback.

It could be the vacuum motor that operates when the setup you've described is enabled....it could also be the control valve that directs the vaccum from its main input off the intake manifold; this is typically a "busy" valve with many opportunities to spring a leak...and since it is used often, it could simply be worn out. Engine diagnostics will probably show you have a lean condition but not pinpoint what is causing it....it will simply confirm what you already know about the lean condition and your HVAC control settings. I think, in your case, the best solution would be to bring your Jeep to either a shop very familiar with your vehicle or to a dealership..... there may be some factory service bulletins regarding your condition that can help diagnose the problem. I would not put much faith in the guys down at the local Big Brand tire store or muffler and brake shop.

Good luck...

Mike
00XJ

AlohaBra · #13 03-01-2007, 11:15 PM

This is n not an engine diagnostic problem.

A rubber hose has probably fallen off. If you don't do work on cars, this Jeep could be a problem for you. They are kind of primative and need "old fashioned" hands on kind of work to keep the stuff going.

Or you could buy one of new "High tech" Mercedes-Benz designed ones...the 2007 Wrangler...with electronic stuff ...
Then...the dealers can get to work for you....for a price of course. Just MHO. Actually, I like the new Rubi..

hundahunta · #14 03-02-2007, 03:09 AM

the easiest way to check is to remove the hvac control head, remove the vaccum connector from the rear and with a vaccum pump apply vaccum to the lines and check with one does not hold pressure, possible a actuator leaks, a pin hole in a line the are plastic, or the hvac control head itself is leaking, you can also smoke test the lines if you have a eeld or some other type of smoke machine.

flippant07 · #15 03-02-2007, 12:44 PM

Thanks again everyone. I've done some research this morning and discovered the HVAC system is problematic on the XJ's. This makes even more sense now...another symptom I failed to mention was a whistling sound behind the dash on the right side with the control set at defrost and the fan set at '2'. I'll check the hose by the battery. If that's not it, I guess I'll have to take it to the dealership - which I was trying to avoid. It still seems strange to me that this would cause an engine surge.

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