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Moderately Burned Exhaust Valve?

streetxj

NAXJA Forum User
I think I might have a moderately burnt exhaust valve. I keep having a slight miss fire at idle, even though the ECU has no codes and all the sensors are good. It smooths out a bit when the engine warms-up. The engine runs strong with zero backfires. The previous owner drove it quite a bit with a bad fan clutch. It always ran right before the overheat (red line) mark if he drove it in stop and go traffic. A dumb mechanic told him that it was just a miss-read of the temp sensor. Well, after changing the fan clutch, it never ran hot again. I also already installed a new fuel filter and changed all the ignition components. The engine has 160,000 miles.

If I do have a bad valve,there is a newly rebuilt 7120 head on ebay for 269 bucks and free shipping. Will that work on my 96' engine without any issues?
Mine has the two coolant temp sensors. One for the ECU and one for the gauge.
 
You should do more diagnosis before throwing parts at it, especially parts that important and expensive.

Do a compression test and a leakdown test.
 
"Slight misfire at idle"--constant?

A mechanical issue such as a burned valve would tend to present consistently. Is your miss consistent or intermittent?

I agree with Kastein--compression test, leakdown test, vacuum gauge test--all will tell you more than what you now know.

You may have as simple a problem as carbon buildup on the valves, and a can of MCCC--Mopar Combustion Chamber Cleaner or Seafoam would improve things.
 
The miss is intermittent. But revs very smoothly and you can rev hard and not have any popping noises from the exhaust. I have a high flow cat and dynomax superturbo catback. It used to have a vacuum leak due to the evap canister. I bypassed it temporarily and it smoothed out alot. But, still has that minor miss at idle which is worse when it's cold. I would like to hear another 96' 4.0 idle though. Maybe it's a normal thing and I'm mistaking it for a minor intermittent miss. The rpm gauge stays steady on 700 rpms at idle. Due 4.0's tend to idle a bit on the rough side?
 
Bad or poor condition dizzy Cap, rotor, wires, spark plugs, dirty injector(s), carbon in the head, loose / poor contacts at the injector, can all cause your problem. Does not sound anything like a burned valve to me.
 
Bad or poor condition dizzy Cap, rotor, wires, spark plugs, dirty injector(s), carbon in the head, loose / poor contacts at the injector, can all cause your problem. Does not sound anything like a burned valve to me.

I'm going to put di-electric grease in the fuel injector plugs. The cap, rotor, ignition wires (autolite pro-series), coil (mopar replacement), spark plugs (champion coppers .035 gap), and new precision auto fuel injectors. I've ran sea-foam before and not even a puff of smoke came out and the previous owner used fuel treatments at every fill-up. Being that it has 160,000 miles on the clock, it might have some oil build-up in the intake ports and probably some stubborn carbon-build up on the exhaust valves. I'll check it out when I take off the intake manifold. Thanks for your help everyone!
 
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