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Diesling Problem

tgoff

NAXJA Forum User
Location
Eureka, CA
I have an 86 XJ with the 2.5L (TBI) and I have been struggling to solve my dieseling problem. It used to be almost everytime but now its only once in a while. I have replaced the coolant temp sensor since it didn't spec out, new 02 sensor, and new plugs, plug wires, distributor cap, coil, and idle speed actuator. I also used seafoam. I have checked the MAP and MAT and they spec out. Any suggestions on where to go from here would be greatly appreciated. Thanks.
 
Is this at idle, under a load... If you get a pinging, or Diesel sounds upon acceleration, or under a load, bump up the octane grade of your fuel. You might want to check your timing also, if it is advanced it can pre detonate, with lower grade fuels. If it is performing like this at idle, then you may have some deposits on your valves or piston face. If you run an engine for long periods at idle with a rich mixture, deposits can form. Also oil burning (bad valve seal) can create deposits. I hope this helps. Scott.
 
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I have been running premium for the last appox 30000 miles (It has 204000 miles). It pings a little bit but not as bad since I just seafoamed it, changed plugs and wires but it still diesels or runs on when I turn the ignition off, I also get a slight backfire sometimes as well. Again this doesn't happen everytime and it seems random.
 
Generally speaking, at least on non-computer controlled engines, diesling is a symptom of ignition timing that is too far advanced (running too hot a plug can also cause it).

I'm assuming you're talking about when you shut the engine off it acts like it tries to continue turning over, but very erratically, barley catching several times before finally shutting off?

Basically it's the result of the unburnt fuel in the combustion chamber being lit off at top dead center on the compression stroke becasue the temps in the chamber are too high, either the result of the ignition timing being too advanced which causes hot spots, or the plugs being too high a heat range, or both.

Gonna quit rambling now...
 
Thanks 2001XJeep but on RENIX controlled engines the ignition timing is computer controlled. I am looking as to where to go from here as to which sensors would cause this problem. Also is 1,000 rpm normal hot idle speed for a 2.5L as this seems a little high to me.
 
no it's not a normal idle speed.
I'd start looking for a vaccum leak or loose intake. That would also cause the motor to run lean, which would raise chamber temps and cause detonation.
 
Thats odd real odd because you have Fuel injection and they don't diesel because with the keys off the fuels off. Carburetor engines will deliver fuel as long as the engines turning.
Is there a solenoid throttle stop thats not working.
Ok you have a ISA motor. This opens the throttle to adjust the idle speed, it also closes the throttle with the key off, yours don't work.
Or you have a injector that is leaking fuel when its off. It can't diesel without fuel and air.
 
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I'll start looking for vacuum leaks. I thought that might be the cause of the idle problem. I just replaced the ISA motor and it works so other than a vacuum leak would the MAT or MAP sensor cause this problem as well? How do you diagnose a leaking injector? Thanks for the replies.
 
tgoff said:
I'll start looking for vacuum leaks. I thought that might be the cause of the idle problem. I just replaced the ISA motor and it works so other than a vacuum leak would the MAT or MAP sensor cause this problem as well? How do you diagnose a leaking injector? Thanks for the replies.
You can see fuel comming out after the keys off. The MAP can't cause your problem, It all comes back to how is it still getting fuel with the key off.
 
I'm with Langer, probably an injector leaking, possible the Charcoal canister is full of fuel and/or the purge solenoid is hanging open.
I've heard before of the charcoal canister, getting pretty soggy with fuel on some of the early XJ's. Don't really know the cause or cure, just remember a couple of posts about the problem. Worth a look if the injectors are OK.
Something else that could be a possiblilty, is the ignition switch hanging. Not likely, as it would probably either run or not run. But it is likely possible, the switch is passing just enough current in the off position, to confuse the things.
 
Thanks for all the responses guys. I have just a couple of questions. What is the spec cold and hot idle for a 2.5L TBI XJ? What is the best way to test for a leaking injector since there is no way to tell without taking the injector out? Do you just un-mount it and have it spray into a gas can then kill the ignition and see if fuel comes out after? Any suggestion and guidance would be much appreciated, Thanks.
 
I'm not real familiar with the four banger TBI, but I'm guessing it's fairly generic. I use a piece of paper towel or something white, and hold it on the bottom end of the sprayer noozle (injector). You can see if it's wet, you can also often smell fuel. The TB will generally hold pressure (when the motor is off), it will deffinately pressure up with the key on (at least for a few seconds) and then supply constant pressure when it's running.
I was thinking, it's also possible the inside of your intake is dirty enough to soak up some fuel.
Motors typically have a coating of carbon in the crown of the head, on the valves and/or on the top of the piston, that can hold a spark for awhile. As long as the motor is still getting fuel, from a leaky injector (TB), fumes from the intake or possibly out of the charcoal canister, it can diesel for while. Pretty common with carburated motors.
You may have to jump the fuel pump, to a hot wire, so it is constant on (without the motor running). It may leak down rather quickly and it may be hard to spot the leaky injector. The pressure may leak off, faster than you can test for a little extra fuel dripping out after the motor is stopped. Or you may need somebody to turn the key on and off a few times, to get the fuel pump to cycle for a few seconds (typical prime cycle), to keep enough pressure on to test for a seep.
A 1000 RPM seems a little high (most motors are set between 600 and 750). But not really high enough for a serious vacuum leak. I'd unhook the purge line from the charcoal canister to the intake manifold and plug it, just as a test.
 
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Thanks for the information. When I have the chance to have the XJ out of commission for a couple of days I will send my injector here http://www.witchhunter.com/. My roommate has used them for his Toyota Pickup with very positive results. They clean the injector with an ultrasonic machine, for you multi port guys they flow match, replace o rings filters pintle caps, and show you before and after flow data. They got my roommates Toyota 22re injectors to 99.8% flow matched opposed to 95% when they went in. They will also repair if possible and at $15 an injector I think it’s worth it.
 
If i remeber right on the 2.5 TBI you can see the injector spraying down into the engine and a visual may be all you need to confirm if fuel is still flowing after the engine is shut down
 
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