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98 XJ continual pre-ignition / spark knock

C

crossmax03

NAXJA Forum User
Location
Boulder, CO
98 XJ sport, auto tranny, 4L straight 6

Immediately after shift into O/D i get spark knock. every time. guaranteed.

Here are the things i've tried:
93 Octane for 1 mo. +
new plugs & wires
new distributor cap & rotor

I thought maybe the cps, but it seems like a super hassle to change that.

I wanna lift my jeep & get some off roading, but seems like the engine is gonna die. maybe not worth it?
122K

any experiences with bad spark knock like this?
 
Next thing i'm gonna do is unplug the front O2 sensor and see if it changes anything.

After that, change the fuel screens/filters (in the tank on the pump/sending unit)
 
Would carbon buildup cause something like this...??? I remember reading somewhere that excessive carbon buildup can cause hot spots in the cylinders which can lead to preignition and knock.

maybe some professional fuel system cleaner (not just fuel injection cleaner) is needed...
 
thebrick said:
Would carbon buildup cause something like this...??? I remember reading somewhere that excessive carbon buildup can cause hot spots in the cylinders which can lead to preignition and knock.

maybe some professional fuel system cleaner (not just fuel injection cleaner) is needed...
Click to expand...
Yes it could.
 
I've had that problem with my 95 a couple of times. Carbon is a big contributor, and so are plugs. Since you just did the plugs, it might be worthwhile to consider a carbon treatment. "GM Top Engine Cleaner" worked nicely on mine. The procedure is basically to run some through the intake, then dump enough in to stall the engine, and let it sit for a while. When you start it up, there will be huge (I mean prodigious, frightening, neighbors calling-911) cloud of smoke, which goes away and when you're done, if you're lucky, the knock goes away too. My only problem with this treatment was that I couldn't get the trusty old 4.0 to stall, so I settled for serious bogging-down and shut it off.
 
Matthew Currie said:
I've had that problem with my 95 a couple of times. Carbon is a big contributor, and so are plugs. Since you just did the plugs, it might be worthwhile to consider a carbon treatment. "GM Top Engine Cleaner" worked nicely on mine. The procedure is basically to run some through the intake, then dump enough in to stall the engine, and let it sit for a while. When you start it up, there will be huge (I mean prodigious, frightening, neighbors calling-911) cloud of smoke, which goes away and when you're done, if you're lucky, the knock goes away too. My only problem with this treatment was that I couldn't get the trusty old 4.0 to stall, so I settled for serious bogging-down and shut it off.
Click to expand...

So I will pick up some GM top engine cleaner and try this, after I find a suitable location away from my apartments to do it.

I've read about changing the plugs after running these cleaners? good idea?
 
Had the same issue on my 98. Several combustion chamber cleanings did not help it even a little bit. Never did get it fixed, ended up running 93 all the time.....
 
crossmax03 said:
So I will pick up some GM top engine cleaner and try this, after I find a suitable location away from my apartments to do it.

I've read about changing the plugs after running these cleaners? good idea?
Click to expand...

I wouldn't automatically expect to change them, but it would be a good idea to check them.
 
BG44K works well for cleaning as well.

Before you get too bogged down in this, take your rig and have a tail pipe CO test done. You may be running too lean, causing the ping. On a Renix, the most common ting causing this is clogged injectors (use BG44k), the MAP, followed by the O2 sensor.
 
Wondering if it could be the fuel filter in the tank, either that or the injectors may be clogged, i presume this is a similar effect ...

I am probably going to run an engine cleaner through just before my next oil change ~2000 or 3000 miles from now

and then i'll check the plugs, and then if that doesn't fix I may drop the tank and check the filters. I've got a couple (yes there are 2 parts) new ones I had picked up a while back ...
 
unplugged the cat, ran it for 5-10 miles (MIL came on)
symptoms were slightly worse

plugged back in, MIL went off.

next up: fuel filters this week ...
 
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