Burnt Piston

[JeepFamily]

We are having trouble now with two sets of burnt pistons. Both times is has been #6 the first after about 40 minutes and the second at about 300 miles. We are running 9.8:1 ICON 944 pistons, Comp 232 cam. Took the spark plugs out this time and found them to look ok with with black on bottom of the plug and the ground electrode clean to gray. No apparent vacuum leaks but not a definite. Any ideas would be very much appreciated.

Rob

[JeepFamily]

We are having trouble now with two sets of burnt pistons. Both times is has been #6 the first after about 40 minutes and the second at about 300 miles. We are running 9.8:1 ICON 944 pistons, Comp 232 cam. Took the spark plugs out this time and found them to look ok with with black on bottom of the plug and the ground electrode clean to gray. No apparent vacuum leaks but not a definite. Any ideas would be very much appreciated.

Rob

[dwg86]

What year engine? What injectors?

[CobraMarty]

Sounds like a bad #6 injector.

[JeepFamily]

After the first time we replaced all injectors with new ones. The engine ran without a missfire as well.

[JeepFamily]

I re-read my original post and maybe it's not clear.

By burnt i mean the piston pin begins to stick caused by overheating. I haven't taken it appart this time but the first time this occured the piston pin began to stick causing skirts to strike the cylinder wall. THe piston also began to burn just below the oil ring on the thrust side. THis rattling is not your normal forged rattle. It is very significant.

In talking to my Machinest he states the problem results from overheating the piston. This can happen due to a vacuum leak or other causes for lean operation? Any body seen this? What did you find? Actually any ideas would be greatly appreciated.

Rob

[SilverXJ]

Check the wiring to the #6 injector possibly.

[racing89]

shouldnt be a vacuum leak. If the injector is to blame its more likly to be caused by the #6' s position on the rail ( lowest, remember crap always flows downhill) This is common result of fuel contamination but is usually asociated with a miss-fire which you dont have. Engine oiling?

[Cheromaniac]

By burnt i mean the piston pin begins to stick caused by overheating. I haven't taken it apart this time but the first time this occurred the piston pin began to stick causing skirts to strike the cylinder wall. The piston also began to burn just below the oil ring on the thrust side.

The balance pads on the small ends of the 4.0L rods need to be ground down so that there's enough clearance for the IC944 pistons to rock on the piston pins. If the small end wasn't ground down far enough, the result could be exactly what you described so you might want to check that out.

[SilverXJ]

Thats a good point. If that happened you should see contact marks on the underside of the piston. Not that much needs to be taken off.

[dwg86]

Good call Cheromaniac, I forgot about the upper balance pads. This could bite a lot of people running the IC pistons.

[Cheromaniac]

Good call Cheromaniac, I forgot about the upper balance pads. This could bite a lot of people running the IC pistons.

Yeah, the only way to be sure that the balance pads have been ground sufficiently is to check that the piston rocks all the way in each direction on the rod before you insert the piston/rod assembly into the cylinder. You should also make sure that there's some clearance between the rod small end and the underside of the piston crown because when both get hot and expand, that clearance could disappear and the piston could bind inside the cylinder especially at BDC where the rod angle is highest.

[JeepFamily]

We did find slight binding during the build but verified with each piston that the rod could move the maximum amount without contact. Cheromaniac you raise and excelent point that things change when it get hot! I will check for evidence when we get it a part.

The first time we lost #6 we did see evidence that it was hot. It was a darker brown from the underside when compared to the other pistons. Also #1 showed no sign of thrust side wear while #4 and #5 showed progressively more wear, thrust side only. #6 had the most wear with some initial burning just below the oil ring.

Again we have not tore into it this time yet, it just acts and sounds the same. I wanted to get everyone's input here before we pull it out in case there is some testing that needs to be done with the engine in.

More data. at idle I can remove each injector plug individually and do not notice much change in the knocking. If I rev the engine slightly maybe to 1500 rpm and do the same test, removing #6 the knock completely goes away? I've pulled the dip stick and found clean oil as well.

[W_A_Watson_II]

Dang, so running the IC944 Pistons with the 4.0 rods and 4.2 crank require rod end modifications?

Glad I found this out now, just before assembling the rods and pistons.

What other tricks do I need to know before putting my engine together?

Will

[Cheromaniac]

What other tricks do I need to know before putting my engine together?

None other than this one:

The balance pads on the small ends of the 4.0L rods need to be ground down so that there's enough clearance for the IC944 pistons to rock on the piston pins.