Knock Sensor, Fuel Filter, clogged injectors, something worse?

2000JGCLTD

MEMBER
1980 JEEP GRAND CHEROKEE
V8
AWD
AUTOMATIC
116,000 MILES

I have a 2000 Jeep Grand Cherokee limited ed. With a 4.7 Liter 287ci v8. Recently my camshaft sensor failed so it was replaced along with my crankshaft sensor (mis-diagnosis) The Jeep ran perfect for about 500 miles after which, I started to notice a loud knocking sound similar to a back fire coming from the engine compartment. This is ONLY audible at speeds greater than 60mph during accelleration. At first I thought I had picked up some bad gasoline. I flushed the system with additive and tried using premium gasoline. No change. I found the spark plugs to be extremely worn so I replaced them with.40 gap autolite premiums. Acceleration through lower RPMs is drastically improved but no change on the knock/back fire sound at speeds over 60mph during acceleration. I have read diagnosis regarding the replacement of a "knock sensor." Can anyone provide advise on this fix based on the symptoms I have described?

5 Replies

ERNEST CLARK

MECHANIC
1,730 POSTS

First of all, are you getting a check engine light? Because if your knock sensor has failed, you should get one.

Also, check your fuel pressure, fuel pressure regulator for leaks, and clogged filter, injectors (also leaking injectors). These won't show up as engine codes, but will cause the problems you described.

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Wednesday, November 24th, 2010 AT 6:53 PM

2000JGCLTD

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My check engine light is not on. Can anyone explain why the symptoms only occur during acceleration past 60mph? Gas mileage isn't being affected (according to the computer). Acceleration through gears 1-3 up to 60mph run strong and clean without symptoms.

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Wednesday, November 24th, 2010 AT 7:01 PM

ERNEST CLARK

MECHANIC
1,730 POSTS

It's either an ignition system failure, sensor failure, or fuel system failure.

When you have an ignition system or sensor failure, the computer usually picks up on it, after all, that's what it's designed to do.

With the fuel system, the computer will only detect a "multi cylinder misfire", but won't know what's causing it. This is why I suggested you check the fuel system components.

Having said that, an intermittent failure with the ignition system or sensors won't always set a computer code. But they're easily checked with a capable scanner during a live data test.

Fixing your problem will be a process of elimination, and unfortunately, I can only point you in certain directions based on the information you provide.

Remember this, instead of replacing parts in hopes of fixing your problem, take a logical approach and test each component. This will save you money and time.

Start with the fuel system.

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Wednesday, November 24th, 2010 AT 7:21 PM

2000JGCLTD

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Thanks for the advice Ernest.
Its sounds like I have a fun Thanksgiving Holiday project.
Happy Holidays,
Ryan

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Wednesday, November 24th, 2010 AT 7:31 PM

ERNEST CLARK

MECHANIC
1,730 POSTS

No problem. If you run into a particular procedure that you need help with, let me know and I'll try to help. The more specific you can be, the more I can help.