Mechanics

ENGINE IDLES, BUT BACKFIRES

1998 Jeep Cherokee

Engine Mechanical problem
1998 Jeep Cherokee 6 cyl Four Wheel Drive Automatic 166000 miles

I replace the engine in this jeep with one from a 1997 year model. The old engine had a catastrophic failure.

The replacement engine has to crank a while, but does start. It idles just great, but when any throttle is imput it sputters and backfires. Any thoughts?
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TPA70
November 18, 2008.




HI: Can you tell if the backfire is through the intake or exhaust? Also, do you know how many miles were on it? My concern's are timing or burnt valves. Do you have a timing light you could check the ignition timing with?

Joe

I can and will check timing. As far as where it backfires, sometimes out intake. Sometimes out exhaust. If you open the throttle rapidly, out the intake. If you open the throttle slowly and try to force the engine to accelerate, it runs very rough, almost like a misfire, then out the exhaust.
Thanks for your response.
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Tiny
TPA70
Nov 19, 2008.
Hi: Also recheck the plug wires. Make sure you have them on right.
Joe

Timing is about 6 to 8 before at idle and goes up to around 14 or so when accelerated to about 1800 to 2000 rpm. This doesnt sound so far advanced as to make it backfire, I am thinking a vacuum leak, so this is the direction I am going. Got anymore ideas?

I appreciate the input about the plug wires, but if they were wrong, it would run crappy or not at all. This thing idles beautiful. Just cant push the gas.

Tiny
TPA70
Nov 19, 2008.
Hi: I'm going to speak with a Jeep specialest. I will get back with you or he will.

Joe

These are what to inspect first althow my manuals say timing is not ajustable it is ajusted through the pcm let us know what you have also check the position of #1 cylender put #1 at top dead center on compression pull distibutor cap see if rotor is at #1 position also check the cam and crank sensors they tell pcm when to inject fuel and when to spark also how high is rpm just after start with foot off peddle
1 Inspect Ignition Timing Incorrectly adjusted engine timing.
2 Inspect Timing Set Slipped timing chain or worn timing gear(s).
3 Inspect Distributor Cap Loose or cracked distributor cap
4 Inspect Rotor - Ignition Damaged, worn, or corroded ignition rotor.
5 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor.
6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s).
7 Inspect Spark Plug Fouled, damaged or broken spark plug(s).
8 Inspect Firing Order Spark plug wire(s) not connected in correct firing order sequence

Jnovack
Nov 20, 2008.
Thanks for all the great advice. I worked on it a few hours this evening and this is what I found.

First. Went through timing again, all correct, on TDC rotor is at #1 dead on.

Second. The engine is a 97 and the truck is a 98. I compared the distributor that came with the engine (used engine) to the 98 out of the vehicle. The cam pick up that rotates is not clocked the same. So I put the 97 distributor in the vehicle. But this made no difference in how it ran.

Third. Timing at idle is about 8 degrees before TDC. Sound pretty correct. This did not change with either distributor, stays the same.

Fourth. Found forward O2 sensor connector very dirty, cob webbs and all sorts of stuff. Cleaned this out and THE INTAKE BACKFIRE IS GONE. Punch the gas and no backfire.

Now. At about fifteen hundred RPM it gets very rough will popping out the exhaust. Sounds like very rich mixture? Why at fifteen hundred RPM and not before?

Have any advice on what would make it pop out the exhaust and run very rough above 1500 RPM?

Tiny
TPA70
Nov 20, 2008.
If 02 sensor looked that bad how do the plugs dist. Cap rotor and wires look like

Jnovack
Nov 21, 2008.
New plugs, rotor and dist cap.

Tiny
TPA70
Nov 21, 2008.
After some more troubleshooting today.

It does not backfire out the intake at all.
It runs great until about 1400 - 1500 RPM, then it begins to miss, keep going and you get unburnt fuel popping out the exhaust.

Cant tell if the miss is from ignition or too much fuel. I think ignition. I changed coil and plug wires to some known good from a running vehicle. No change. Changed all plugs, no change.

I paid you guys ten bucks and so far, you have had suggestions that are all obvious to any garage mechanic. So. Do you have anything to add?

Tiny
TPA70
Nov 21, 2008.

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