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Unread 12-28-2008, 07:12 AM   #1
alhefner
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1990 XJ Cherokee 
 
Join Date: Nov 2008
Location: Reno, NV
Posts: 190
no vac leaks - backfire, rough idle, fast idle, slow response

I thought all my problems were vacuum related but for the life of me, I can't find any vacuum leaks! Tried all the tricks to find vac leaks but still no go on that front.

Symptoms:
cold engine - runs well except a lack of throttle response until reaching 3000 RPM then it is like a rocket! Idle on cold engine is at around 750 to 800 RPM.

Engine at operating temp - fast idle (up to 1500RPM at times!), still slow to respond to throttle until about 3000 RPM. Also, if I have been driving a couple of hours, engine will backfire through the throttle body if I punch the gas even a little until the RPM reaches about 2500.

I experimented with the IAC (idle air control) tonight. I allowed the engine to warm up and when the idle started to rise, I unplugged the IAC. There was no change in idle quality at all. I have cleaned the IAC wit throttle body cleaner.

The backfire has me thinking ignition timing but not sure the rest of the symptoms fit. Perhaps it is multiple problems?

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Unread 12-28-2008, 07:28 AM   #2
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
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Just brainstorming here....but for these difficult problems, testing may be the best way to isolate the root cause(s).

How about a vacuum "smoke" test? Sometimes valuable for tough to find vacuum leaks. Artificial smoke is introduced into the system-under-test under slight pressure and any leaks are indicated by the escaping smoke.

Also, backfiring (from the intake) may be indicative of a "lean" fuel burn condition. Backfiring out of the exhaust can mean too rich. Exhaust gas analysis could help confirm?? Another test that I would definitely perform would be fuel pressure testing. Takes only a few minutes and is a good data point. Long shot but when was the last time your fuel filter was changed?

Good luck and keep us posted!
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Unread 12-28-2008, 07:33 AM   #3
alhefner
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Location: Reno, NV
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hmmm...smoke test eh? Hey, I am a smoker so getting the real thing isn't a problem! I like the idea for a visual indicator! I'll give that a try this afternoon before I head in to work.
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Unread 12-28-2008, 08:16 AM   #4
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
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Quote:
Originally Posted by alhefner View Post
hmmm...smoke test eh? Hey, I am a smoker so getting the real thing isn't a problem! I like the idea for a visual indicator! I'll give that a try this afternoon before I head in to work.
You need specialized equipment to perform the smoke test effectively. But you can do simple vacuum testing on your own if you have a vacuum gauge. Have you tried that?
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Unread 12-28-2008, 08:20 AM   #5
alhefner
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1990 XJ Cherokee 
 
Join Date: Nov 2008
Location: Reno, NV
Posts: 190
No vac gauge but I tried a quick "blowing smoke" test just a few minutes ago. Didn't see any smoke anywhere but it sure was pretty easy to blow into that vacuum line! I could feel some resistance and it was kind of like blowing up a stiff balloon. I think there should have been much more resistance. It was the main one going to the vacuum tank mounted in the bumper.

Next step for me is to pinch shut or plug lines, one at a time, I think to see what that does to the system.
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Unread 12-28-2008, 10:06 PM   #6
alhefner
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1990 XJ Cherokee 
 
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Location: Reno, NV
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OK, so I isolated (plugged or capped) each of the vacuum lies to no effect. I have come to the conclusion that I need to replace the fuel pump anyway so that is my next thing. My conclusion about the fuel pump is from other discussions on here and I have many classic symptoms of a very marginal fuel pump. Might also replace the pressure regulator on the fuel rail while I am at it.
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Unread 12-30-2008, 11:00 PM   #7
alhefner
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1990 XJ Cherokee 
 
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Location: Reno, NV
Posts: 190
OK, so the backfire, slow response, lack of power were all caused by the fuel pump! A lack of fuel from the pump acts just like a vacuum leak and has the same symptoms. Still have a slightly high idle (1000 RPM) and some rough idle so I'll work on those now.

BTW: Fuel pump change/ fuel tank removal and install is a major P.I.T.A when working alone and limited lifting methods! Think I'll make a post with lessons learned and suggestions to make the job just a little easier for those doing it for the first time.
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Unread 09-16-2009, 01:58 AM   #8
perryferrell
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1992 XJ Cherokee 
 
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ive been reading your problems but mine doesnt fall into the same categories as yours do maybe you can help me ????? my 92 starts and idles fine but when i give it gas it stumbles real bad...... when im driving down the road if your on the gas it runs normal but when your speed levels out it with the your peddle it runs terrible..
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