Yet another "knocking" question (already searched topics)

My sister just gave up on her 4.0 HO XJ. After replacing the head (due to her lack of overheating knowledge) she drove it for a while and now it knocks, like a spark knock or loose rocker . I took off the valve cover to take a look and all the valves seem to be moving fine when running, all tight etc. I was thinking about the timing maybe , like I said it has a serious lack of power stinking exhaust and the "spark knock" before I get too far I am looking for some help and maybe an easier fix than just tearing stuff apart till I find the cause. Thaks ahead for the help.

HO 4.0's are pretty notorious for piston slap, and there's a possibility this was made worse when it overheated - it might even have a collapsed piston* as a result. Can you determine whether this noise gets better or worse when it heats up? Piston slap tends not to vary a lot with engine speed and tends to improve as the engine heats up and the pistons expand.

I'd think a good start would be to check timing, including timing when it cranks ( badly indexed distributor can cause problems even though the engine computer will correct the timing when it runs). It might also be a good idea to check compression.

I would also consider the possibility of a collapsed valve lifter, perhaps badly carboned from overheating, damage to the catalytic converter or the O2 sensor, and carboned-up injectors.

*if you're unfamiliar with "collapsed pistons," this is when the piston skirt is actually pushed inward at the bottom, often accompanied by cracking. It can happen from serious overheating. I had a Ford F150 with collapsed pistons, which ran great for a long time despite fierce piston slap, until one day one of those cracked piston skirts fell off. It made a very 'expensive noise' as I drove it home.

sorry for the possible wrong info, the XJ I have out back in the weeds is an HO , this is a 4.0 in a 93 if that makes a difference . I havent messed with the dist at all yet or anything else except for removing the valve cover to take a look. the knocking isnt there at idle , only as the rpm's get about 1500 and above, also alot of spitting and sputtering, no change with higher engine temp either. also the exhaust smells aweful, like rich maybe. I havent worked on any vehicle since my 86 olds with a big block, carbed . so I am kinda lost as far as this motor goes (injected,cat, etc.) She did say that someone replaced a fouled plug if that helps at all.


GREAT HELP THOUGH AND APPRECIATED.

jb660r said:

sorry for the possible wrong info, the XJ I have out back in the weeds is an HO , this is a 4.0 in a 93 if that makes a difference . I havent messed with the dist at all yet or anything else except for removing the valve cover to take a look. the knocking isnt there at idle , only as the rpm's get about 1500 and above, also alot of spitting and sputtering, no change with higher engine temp either. also the exhaust smells aweful, like rich maybe. I havent worked on any vehicle since my 86 olds with a big block, carbed . so I am kinda lost as far as this motor goes (injected,cat, etc.) She did say that someone replaced a fouled plug if that helps at all.


GREAT HELP THOUGH AND APPRECIATED.

Click to expand...

OK, the 93 is an HO too, no problem, but this does not sound like a collapsed piston or piston slap. Piston slap sounds kind of diesel-y at idle and settles down under load, and a collapsed or skirtless piston will rap at idle.

I'm assuming you've checked for engine codes - if not, try ignition on/off/on/off/on and then count flashes of the check engine light. Each code is two digits. "55" is normal, no codes. "12" is battery recently disconnected. Anything else is a code.

I'm wondering if you have a bad injector or injectors, but you should also check the MAP sensor. If you have a 4.0 of any vintage from 87 to 95 in the weeds, swap it out with that one and see if it makes a difference. A good dose of injector cleaner might just be worthwhile to try, too. Might not help but like chicken soup, it couldn't hurt!

Another diagnostic, woefully neglected in this digital age, is a vacuum gauge. It might be worthwhile to hook one up and see how the gauge reacts when the misbehavior starts. When I was having trouble with the injector wiring harness on a 93 I found that a failed injector will not show on a vacuum gauge. Ignition or internal engine problems, however, will. This might be a good way to see if you can narrow the problem down to ignition or injection, or mechanical. A vacuum gauge should detect a bad compression situation as well. If you don't have one I really suggest you go out and buy a new one. They're still cheap. It will probably come with a simple set of diagnostic instructions - if not, most Chiltons and other such books will contain a good rundown.

Finally, don't forget to check the really obvious - plugs and distributor cap! I was experiencing a high-speed miss in my 95, and hunting all around for solutions until I suddenly realized I hadn't looked at the plugs for over a hundred thousand miles! It started fine, so why worry, right? I pulled the plugs, and found, of course, that the plugs were worn to about 3 times the normal gap. The XJ ignition is so good that it will fire across the room, but it doesn't help performance.