So 01 xj. Nasty detonation. Drove it for a couple years and it would have slight detonation once in a while. Then it got so bad with 87 octane I had to run 89. That was good for a while and now it's detonating pretty bad on that during any half throttle or steep hills. Now running 92 and it just barely does it on heavy engine loads. If I pour in a bottle of Lucas complete fuel system cleaner it stops. Have tried a few other fuel system cleaners with no luck. I do have a slight vac leak in the brake booster. Not sure if that's enough to cause a lean situation. No CEL. Not sure where to start. Any suggestions or anyone experienced this same problem and resolved it?
Was thinking of starting with plugs since it has begun to idle just ever so slightly rougher than when we got it. Or maybe I should stop putting off changing the booster. Was also thing maybe hooking up to live data reader to see if o2 sensors may be out if spec. Hopefully not a bad cat either. Just don't know where to start. Hoping someone may have been through this already. Can't afford to start throwing money at it unnecessarily. Also can't afford to keep running 92 octane.

 

Also if it helps:
Has about 195,000, had 168,000 when purchased
Second owner and it was well cared for
Haven't checked which head is on it
Does run slightly above 210 sometimes
Does have the heat shield on injector
Last time I took it on kind of a far drive it did start with heat soak several times and it wasn't a very warm day. Usually only get heat soak issues in mid summer and stop and go traffic. Only had an issue a handful of times though in 3 1/2 years.
Maybe it's a combo of engine running slightly hot and something else?

 

Does the check engine icon bulb work?


It would be a good idea to check for set codes or pending codes using a code reader or scanner.


What spark plugs are in there now?


NGK ZFR5N plugs are recommended. Gapped to .035".

 

CEL worked a few months ago when it went off during a heat soak start issue. I reset it with my code reader and it hasn't been back on. Not sure what plugs. They were in it when we got it. Lol. Could old plugs cause it? Definitely planning a tune up soon. I'll plug my scanner in just to make sure nothing is pending. Back when it went on for the heat soak it was a random misfire code. Only time it set a code out of the half dozen or so times it's done it.

 

Use BG 44K. 1 can to a full tank of gas. Drive til 1/2 tank. Refill with gas. Run that tank down to 1/8.

 

OP,


Wrong plugs or carbon build up may cause detonation, if that is what is really happening.


Pull the plugs as soon as you can to examine them. Have a set of NGK ZFR5N plugs ready to install.


Do what Cruise54 advised above.


Edit: Check the CEL bulb to make sure it's working. With detonation I would think you would have a CEL blinking.

 

Definitely could benefit from a plug change anyway I'm sure. Was thinking about trying some BG just wasn't sure if it could have been something besides carbon buildup. I have used several products that claim to remove carbon buildup to no avail. As far as I know it's definitely detonation. I also wasn't so sure carbon buildup would cause detonation but preignition instead.

 

With detonation the fuel doesn't burn, it explodes. In a lean bog motor like the 4.0 it won't go on for years.

 

Exactly why I'm trying to figure this out. I'm as careful as I can be when driving it. Just sucks not being as to accelerate quickly and having to drive up hills slower. Going to pick up plugs this week. If I'm going to pull the coils off I might as well change the plugs. Or should I do the BG 44 k before I change the plugs. Sounds like it would make more sense that way.

 

Here's some "pinging" reading for you.


http://dodgeram.org/tech/gas/Trouble/ping.htm

 

44K ASAP. Doesn't matter which plugs are in it when you do it.

You could also "water" it in the meantime.

 

Some spark plug data:


http://dodgeram.info/Engine-Gas/Spar...ook.html#Signs

 

Actually, the RC12LYC plugs referenced in this article are outdated for the '99-'01 4.0 but does illustrate proper spark plug importance. See the spark plug application chart below.


The Champion heat range "12" is the equivalent to NGK "5" heat range.

 

Thanks for all the great input. Will try some water tonight if I can't find the 44k locally. One other question is how will all this decarboning affect the cat?

 

So I tried some internal steam cleaning Tuesday night. May have helped a little but hard to say as I still have 92 octane in the tank. Pulled the plugs Thursday night and gaps were greater than .035. At least two of them were greater than .055. Closed the gaps up to .035 and reinstalled for temporary. Plugs had no carbon or signs of detonation. Were slightly lean and a little worn. Tops of pistons were very black and carbon buildup fairly thick. I probably didn't spray as much water into it as I should have. Just doesn't feel right spraying water into intake. Been having trouble finding a fuel treatment containing polyether amine locally and haven't had a chance to leave town to purchase elsewhere. Did some more research on products other than bg44k that contain PEA. had to read a bunch of msds and I think the local napa has a product there containing it. Will run some of that through a few times and change plugs then try running some 87 and report back.