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Unread 11-29-2008, 12:21 AM   #1
94gashog
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1994 FSJ Cherokee 
 
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Engine performance lags, won't rev above 2500-3000 RPM.

This is in park or neutral.
In driving, it has a hard time revving above even 2000. At that point it acts like it is going to stall out, like it is fuel or air starved.
Engine idles real smooth.
Vehicle accelerates very slowly because of engine problems.
The previous owner was replacing the heater bypass valve and broke the back hose on the CCV system, and the CCV hose was replaced by me.
She said this problem started right after that she broke the hose... she doesn't know anything else about what could have happened...

It is like something is holding the engine RPM down.
What I have checked or replaced -
CCV hose.
spark plugs.
distributor.
fuel filter.
fuel pressure and regulator.
Timing not adjustable but it IS advancing with RPMs.
Engine codes point to lean fuel mixture, TPS code, and MAP sensor code.
MAP and TPS are both good.
compression reads 120
Fuel injector resistance measures 14.9 ohms each.
Engine does not seem to be consuming any anti-freeze or oil, no weird color smoke out the tailpipe.
Trans fluid good and up to level.
Gasoline of a good grade.

The vacuum pressure holds steady at 14 PSI which is a couple pounds lower than it should be. This is a sign of a leaking manifold but even after the engine shuts off, the vacuum holds for a while. How bad could it really be leaking if at all?

OK so the last thing I can think of is maybe the catalytic converter is partially clogged but have not disconnected it...
And with not knowing for sure how it ran before the CCV mishap...

So what else could possibly cause this problem? How would a catalytic converter get clogged to begin with?
Truck has 201,000 miles and was not maintained the best.

4.0 engine, 2WD only, auto,

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Unread 11-29-2008, 12:40 AM   #2
2500AK
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Sounds like a clogged cat, see if the pipe before the cat is hotter than the pipe after the cat, they will probably be hot so don't burn yourself. They tend to fail when they get old, if the engine was running to rich, and a number of other reasons.


Those symptoms are usually indicative of either a clogged cat or lack of fuel.
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Unread 11-29-2008, 06:15 AM   #3
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by 94gashog View Post
Engine codes point to lean fuel mixture, TPS code, and MAP sensor code. MAP and TPS are both good.
If you currently have a check engine light or codes set, the best strategy is to concentrate on eliminating them first. The odds of the OBD codes you are seeing being related to your symptoms is very, very high.

How did you verify that both the MAP and TPS are good? Did you test them and compare your results to Jeep recommended values? Also remember that the code means that either the MAP or the TPS "circuit" has a problem. The circuit consists not only of the sensor, but the connectors/wiring associated with that sensor. Use a fine tooth comb on the wiring/connectors as they are often overlooked and can be the cause of snaky problems.
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Unread 11-29-2008, 10:13 PM   #4
94gashog
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Well I used the test methods in the Haynes manual to test the MAP of course when it is hooked up and it came out good. it is the same test for other vehicles but I did the specifics of the Jeep...
The TPS, I replaced it with a brand new one and that did nothing.
I took the new TPS back for refund after that...

I just figured if the CAT was clogged, the engine could not breath at higher RPM's.
What I am going to do is see if it is a lot hotter before and after the CAT. That is a good idea and no I won't burn myself...

I might also simulate this problem on my other car and see if the same symptom happens...

And yeah I might have to find my fine tooth comb in my tool kit, boy won't THAT be a fun way to spend a weekend?

Thanks folks!
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Unread 11-29-2008, 10:28 PM   #5
Gsmyth75
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Quote:
Originally Posted by 94gashog View Post
Well I used the test methods in the Haynes manual to test the MAP of course when it is hooked up and it came out good. it is the same test for other vehicles but I did the specifics of the Jeep...
The TPS, I replaced it with a brand new one and that did nothing.
I took the new TPS back for refund after that...

I just figured if the CAT was clogged, the engine could not breath at higher RPM's.
What I am going to do is see if it is a lot hotter before and after the CAT. That is a good idea and no I won't burn myself...

I might also simulate this problem on my other car and see if the same symptom happens...

And yeah I might have to find my fine tooth comb in my tool kit, boy won't THAT be a fun way to spend a weekend?

Thanks folks!
If it helps I have the same kind of problem, and uh.. I know it's not my "cat" (or lack of a cat). I'm thinking it's more of a fuel pressure / coil thing.
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Unread 11-29-2008, 11:40 PM   #6
BrunoS
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lack of power or engine bog (when engine runs smoothly despite lacking power) is lack of fuel volume or pressure.

you've replaced fuel filter....which should have helped greatly...but since all else was done, you really need to check fuel pressure at the fuel rail shraeder valve connector.

borrow/buy a fuel pressure gauge, should be 39 (methinks) psi when running. i truly believe that is going to be your major indicator of a problem.

A clogged cat will typically glow cherry red at idle at night shortly after starting it and letting it idle, with a huge drop in manifold vaccum like us old timers used to look for after a "blip" on the throttle.

since all else is ok...the bad news is you most likely just needs a new $80 fuel pump (it does have 200k + miles! on it)

the good news is in this jeep you don't have to drop the fuel tank to replace the pump!
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Unread 11-30-2008, 06:48 AM   #7
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by BrunoS View Post
you've replaced fuel filter....which should have helped greatly...but since all else was done, you really need to check fuel pressure at the fuel rail shraeder valve connector. borrow/buy a fuel pressure gauge, should be 39 (methinks) psi when running.
I agree with Bruno here....fuel pressure testing is very easy to perform and it should be next on your "process of elimination" checklist.
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Unread 11-30-2008, 04:48 PM   #8
94gashog
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the fuel p[ressure is 31 normally and jumps to 39 when the pressure regulator is disconnected. These are what they should be.
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Unread 01-02-2009, 12:03 PM   #9
94gashog
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Working in a dark garage I was not able to see everything under the truck.
So what was the problem?
The exhaust pipe between the manifold and the cat. was bent right where it connect to the manifold.

So now the thing runs but still needs exhaust work, not sure just how much.

I just cannot believe I was passing people like they were standing still, on the highway. Normally that thing would not do above 35.
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