My brother has a 98 F150 4.2 auto. It recently began detonating at very low cruise rpms. At full or half throttle it has no problems with this. However, crack the throttle just a bit and cruise like that and it detonates.

We have cleaned the iac, throttle body, mass air meter, new air filter, new plugs, etc.
We put new O2's after cat and it got worse, we put new O2's pre cats and it got a little better, not much.

Anyone with any experience in fixing this?
Thanks.

 

Well, the new O2's didn't do anything, still detonating. The check engine light is still on for catalyst/O2 . I would think that if the cat is clogged, it would cause pinging at a higher rpm, or under load at least?

 

Did you clear the codes by disconnecting the batery for a few minutes?

 

Yes, why do you ask?

 

Just curious,if the codes are left in the computer, the light won't go off, and it can still affect how it will drive. With the codes aside, mine detonates in the same way. I have found, that certain brands of gas have an effect on this.You could also try running one step colder plugs, but no more than one step, and check them after a few hundred miles to be sure they are burning clean.

 

I would rather fix the problem than use band-aid fixes like higher octane, colder plugs, etc.

 

Mine Has Done That Since It Was New. I Don't Have A Light On, But It Does It Like You Mentioned With A Light Throttle. The Engine Has Compression In The 9's I Believe And 91 Octane Has Eliminated The Problem For Me.

 

The check engine light has gone off after driving around on the new O2's. Using 93 octane has lessened the detonation, but not totally eliminated it.

 

Did you gap the plugs correctly? If the gap is not within specs it can change the timing....

 

I know this is apples to oranges, but my wife's brand new '04 Mitsubishi does the same thing. We made note of it with the dealer, but I don't expect any satisfaction from them. I will let it continue until the warranty runs out, then chase it down and fix it myself.

 

Yes, but how would that affect timing??

 

Maybe, the larger the gap, the longer it will take to jump the gap with the spark. I know it's fast, but maybe it's tuned that closely.

 

But that wouldn't addect the timing. It would effect spark intensity and length.

 

True. Another thing, is that if it would affect timing, then every time your gap grew larger due to electrode wear, you would have to adjust timing.I know that the ecm adjusts timing according to many parameters, but I doubt that spark gap is one of them.

 

I know this thread is 1 year old, but I need the fix for this exact problem. The only difference is my truck is a 99.

I've heard detonation can cause major damage and I'm ready to fix it myself if I can.

Thanks, Joe