Burnt Piston

[W_A_Watson_II]

Cheromaniac,

Thanks, The was going to do the pin/rod installation Monday or Tuesday, this was very timely information/help.

Will

[JeepFamily]

Ok, here is the list of areas to check compiled so far;

1. Verify there are no vacuum leaks including Intake manifold to head and attachments like power brakes, cruise control etc. Almost all the connections are on or near #6 runner. A large enough leak would allow air to enter bypassing the MAP sensor therefor leaning out that cylinder.
2. Poor performing injector or dirt in the fuel rail. Again causing lean operation and over heating.
3. Injector wiring. Again causing lean operation and over heating.
4. O2 sensor wiring. This issue was brought up at the local jeep shop where the O2 sensor wires had been melted by the exhaust manifold. Although this leaned out the entire engine, #6 is always hotter and resulted in two engines with #6 piston burnt both times.
5. Connecting rod contacting with KB 944 piston causing piston to rattle. Was measured cold and verified not to contact but may have tighter tolerances at running temperature.

Also after reviewing the thread again I don't think I answered dwg86's question back on September 27. It's a 97 block (0.040 over zero decked), 94 (7120, 58 cc) head and 83 Scrambler 4 weight crank (index ground to correct timing and stroke, 3.912" stroke).

[JeepFamily]

The injectors a 27# flow balanced Bosch Design III 4 hole discharge. Part #19824je from Injector Connection.

[Retlaw01XJ]

Is it possible to install the head gasket wrong?
My Victor gasket has water passage openings in the rear only. If gasket is installed wrong (flipped front-to-back) there will be little coolant flow around cylinder #6.
On most engines water flows from front of block to rear of block, then into the head at the rear and forward to the radiator.
Excessive silicon sealant can block passages too (if you used any at all).

If you have or can borrow an IR temp gun, check around for hot areas on that cylinder. Checking temp of the exhaust manifold for each cylinder (if you can get to them) may give you some indication if #6 is running lean/hot. When I fired my 401 Buick a few years ago, The rear cylinder header was hotter than the rest. Culprit was a vacuum leak due to a missing seal in the power brake booster.
I'm betting it's the rod/piston interference issue already mentioned.

[JeepFamily]

Thank you Retlaw01XJ. We did note the differences in the head gasket and I'm pretty sure it's installed correctly but not positive. A will add this to the list. we did not use rtv in this area so shouldn't be a problem. I did use an IR gun and the readings on exhaust runners 1-5 were all within 20 F. Unfortunately runner #6 is hidden by the intake manifold and I could not get a good reading. Will post results of tests as I complete them. Thanks again for you great input.

[JeepFamily]

Ok, here is the list of areas to check compiled so far;

1. Verify there are no vacuum leaks including Intake manifold to head and attachments like power brakes, cruise control etc. Almost all the connections are on or near #6 runner. A large enough leak would allow air to enter bypassing the MAP sensor therefor leaning out that cylinder.
2. Poor performing injector or dirt in the fuel rail. Again causing lean operation and over heating.
3. Injector wiring. Again causing lean operation and over heating.
4. O2 sensor wiring. This issue was brought up at the local jeep shop where the O2 sensor wires had been melted by the exhaust manifold. Although this leaned out the entire engine, #6 is always hotter and resulted in two engines with #6 piston burnt both times.
5. Connecting rod contacting with KB 944 piston causing piston to rattle. Was measured cold and verified not to contact but may have tighter tolerances at running temperature.

Also after reviewing the thread again I don't think I answered dwg86's question back on September 27. It's a 97 block (0.040 over zero decked), 94 (7120, 58 cc) head and 83 Scrambler 4 weight crank (index ground to correct timing and stroke, 3.912" stroke).

Here's the latest I've found.
1. No vacuum leaks found from break booster or the utility line that serves the cruise control and heat control actuators.
5. No sign of contact between the connecting rod and piston.

Viewed the underside of the pistons for the first build and found #6 to have black or burnt spots on the underside of the dome. Strong evidence that this piston got hot from the combustion chamber in the form of lean operation. #4 and #5 showed some evidence as well. In additional to the above mentioned causes in terms of a vacuum leak, timing could also have an impact. Things that affect time would be camshaft degree, worn distributor bearings, computer, sensor, etc. I do know our distributor has a loose bearing on top. A timing light should tell us a lot. Another thing we discussed was fuel pressure. Fuel pressure checked out about a year ago but we will have it checked again. Anyway that's what I know as of today. Has anyone seen this?? More thoughts?