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Unread 11-10-2011, 01:39 PM   #1
Double0Jeep
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Rough idle, Low Power, Idle Vacuum Steady 15 inHg

Hello,

I need some advice with a mysterious rough idle / low power condition on a '00 Cherokee 4.0L, 140K. It is bad enough that if I am idling with my foot on the brake, I can feel the engine power surging. I have also noticed on a couple occasions (warm day, AC on, stopping at a stop light) that I can smell exhaust in the cab. Lastly, when the engine is warming up, there seems to be a lot of moisture coming out of the exhaust. It is even spitting out of the muffler drain hole. It does not smell sweet and I am not losing coolant. This stops once the engine is warm. (Maybe this is normal and I just never paid attention to it before.)

One other note: for about a year, the car was primary used for very short trips where the engine probably did not have a chance to warm to normal operating temp. (Hence, I first suspected carbon build up and dirty injectors)

What I have looked at so far:
  • No CEL, no codes stored
  • New Champion #438 / RC12ECC Copper Plus plugs, gaped to .035
    -old plugs were dry with minimum carbon buildup. Only very slight yellowing on a few of the plugs. No major differences between plugs.
  • Fuel injectors cleaned and balanced
  • Fuel pressure tested good
  • exhaust/cat checked by muffler shop for blockage/leaks
  • I pulled each injector cable with the engine idling. There was no noticeable difference between cylinders. i.e. it does not appear that one particular cylinder is the problem. (Jeep has the coil pack rail so I could not pull individual spark plug wires)
  • MAP sensor tested good (~4.7V at 0inHg, ~.9V at 19inHg, ~1.9V at 15inHg)
  • checked for vacuum leaks (spraying carb cleaner method)
    .
  • Vacuum test results
    - idle: steady at about 15inHg (near sea level)
    - quick rev: fall to about 3 then jump to about 19
    - 2000rpm: steady 19inHg

I think the smoking gun is the low vacuum. I'm going to run a compression test over the weekend to look for worn rings.

Is there anything else I should look for?

Thanks for your help!

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Unread 11-10-2011, 02:20 PM   #2
CJ7-Tim
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The 2000-01 Cherokee have the 0331 head that is slightly more prone to cracking than average, and you should carefully monitor the coolant level. Google Jeep 0331 head for more info. The cracking is topic of more Internet threads than actual occurrences.

Moisture dripping is normal. Carbon build-up is certainly possible, the 4.0L head can sometimes have sticky valves from carbon build-up.

Clean your Idle Air Controller and see if that helps.

Cleaning the Idle Air Controller (IAC) :

Purchase sensor safe Throttle Body cleaner spray.


1. Remove the Throttle body from the intake manifold.
2. Remove the IAC with a TORX 15 driver (2 bolts)
3. Gently wiggle out the IAC from the throttle body
4. Clean the IAC with Throttle Body Cleaner (not carburetor cleaner). Use cleaner, a rag, and a toothbrush. Be gentle; don’t twist or pull on the pintle as it is fragile and can be damaged easily
5. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
6. While you have the Throttle Body off, give it a good cleaning also.
7. Reinstall IAC, the Throttle body, and check idle quality.

.
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Unread 11-10-2011, 06:30 PM   #3
Double0Jeep
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Thanks for the info! I cleaned the throttle body once but can't remember if I checked the IAC. I'll try cleaning it this weekend.

Could an IAC malfunction also effect power? An IAC problem should also cause a lower idle rpm, correct?

I hadn't heard about the 0331 head issues. It looks like coolant loss is the primary symptom. My coolant level is currently just below the Full marker. I'll watch it closely but I don't think I'm loosing any coolant (knocking on wood!)
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Unread 11-11-2011, 03:08 PM   #4
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As far as smelling exhaust in the cab, can anyone tell me where that would come from? (that is, assuming there is a problem with the IAC or something like that, and not an exhaust leak.) Would it be blown back through the intake somehow? (perhaps during hard acceleration when the vacuum approaches 0?)
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Unread 11-11-2011, 03:20 PM   #5
CJ7-Tim
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An exhaust leak should be the first thing to suspect.
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Unread 11-11-2011, 03:28 PM   #6
Double0Jeep
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Quote:
Originally Posted by CJ7-Tim View Post
An exhaust leak should be the first thing to suspect.
I agree but the first thing I did was take the Jeep to a muffler shop. They "said" they checked the entire exhaust system and did not find any leaks or blockage. It sounds like I should get a second opinion...
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Unread 11-11-2011, 07:17 PM   #7
CJ7-Tim
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So are you smelling your exhaust, or the exhaust from another vehicle ? If there are no exhaust leaks, the exhaust can't just escape or blow back somewhere.
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Unread 11-12-2011, 01:38 AM   #8
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Yeah, it does not make sense to me. It is definitely coming from my Jeep. For example, the last time I noticed it was at a gas station. Rolled in and came to a stop, and got a good whiff of exhaust before I shut the engine off.

If there is no other way to get exhaust blow back, then there are only three things I can think of:
  • There is excessive amount of exhaust blowing out of the muffler drain hole - enough to send a plum forward into the fresh air vents
  • The muffler shop I went to completely missed some giant crack or hole... which is sounding more and more likely...
  • There is a leak that a muffler shop might have trouble detecting - I'm thinking a bad manifold gasket or loose bolt

I'm leaning toward the last one since the intake and exhaust manifold share the same gasket. That could explain the smell and the low vacuum. (And its easier to fix the worn rings!)
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Unread 11-12-2011, 09:18 AM   #9
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Are you smelling exhaust, or raw gasoline ?
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Unread 11-12-2011, 12:06 PM   #10
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Just exhaust. Definitely not gasoline.
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Unread 11-14-2011, 02:44 PM   #11
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I don't appear to be losing any coolant so I think it is a head issue. IAC nice and clean now. Still have rough idle / low power and a loud puff-puff out of the muffler drain hole.

Since I have an idle vacuum of only 15inHg and what must be an exhaust leak, I suspect the problem must lie with my intake/exhaust manifold gasket. However, I haven't been able to find the intake leak by spraying carb cleaner. I'm not sure if I want to try to replace the gasket with the tools I have. Looks like I might have to admit defeat on this one and take it in.

Thanks for you help!
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Unread 11-14-2011, 05:08 PM   #12
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Regardless of what the muffler shop says, I'm suspicious of a clogged converter. How they could "see" that it's not clogged without removing it is beyond me. Check out the instruction sheet that came with your vacuum gauge and find the test for clogged exhaust.
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Unread 11-14-2011, 05:28 PM   #13
Double0Jeep
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Quote:
Originally Posted by cruiser54 View Post
Regardless of what the muffler shop says, I'm suspicious of a clogged converter. How they could "see" that it's not clogged without removing it is beyond me. Check out the instruction sheet that came with your vacuum gauge and find the test for clogged exhaust.
That's a good thought but I tried that already. Held the engine at 2000rpm for about a minute but the vacuum stayed steady at around 19inHg. I think it should gradually go down if there is any exhaust blockage. Do you know of any other tests?
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Unread 11-14-2011, 05:39 PM   #14
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Quote:
Originally Posted by Double0Jeep View Post
That's a good thought but I tried that already. Held the engine at 2000rpm for about a minute but the vacuum stayed steady at around 19inHg. I think it should gradually go down if there is any exhaust blockage. Do you know of any other tests?
That's not the right test. Hold RPM at 2000, check vac reading. Let throttle snap shut. Vac should rise only 3 to 5 inches above that reading and then immediately return to the initial reading you saw at 2000 RPM. If the needle SLOWLY returns to the original reading, in jerky motions, you have an exhaust restriction.
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Unread 11-14-2011, 06:47 PM   #15
Double0Jeep
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Quote:
Originally Posted by cruiser54 View Post
That's not the right test. Hold RPM at 2000, check vac reading. Let throttle snap shut. Vac should rise only 3 to 5 inches above that reading and then immediately return to the initial reading you saw at 2000 RPM. If the needle SLOWLY returns to the original reading, in jerky motions, you have an exhaust restriction.
Thanks for the clarification. So it holds steady at around 18inHg at 2000rpm. When I snap the throttle shut, it jumps to about 20inHg and then drifts down to 15inHg in about 2 seconds. The motion is pretty smooth but I think I can see some jerkiness (however, if its there, its very slight). Is 2 seconds too slow?
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