Rod knock or pre-ignition in 4.0 ?

[DaveB]

I have transplanted a 99 4.0l jeep tj motor into my 84 cj8. It runs great when it is cold but once it warms up it pings or knocks on acceleration. It will do it without any load on the engine but worse under load. I have swapped out or tested the MAP, MAT, and the TPS sensors, checked fuel rail pressure, and timing. I have an XJ and an MJ to compare test measurements against and everything looks normal. The engine came out of a wreck that rolled badly. The engine oil pan was hit by the front axle and shoved the pan up enough to bend the long stiffner that bolts on below the crank. One of the rods did lightly graze the stiffner but it didn't look bad enough to me to damage a rod or bearing. Could this rod have a damaged bearing that only makes noise once the engine is warm and the oil thinned out or should I keep on looking at pre-ignition as the source of my noise ?????
Thanks for any ideas,
Dave

 

[Matthew Currie]

You might first try this relatively simple rod knock test, of shorting or pulling the plug wire for each cylinder (you might need some tool to pull the wires and avoid shock, or pre-arrange to short or pull one at a time, stopping the engine each time). If you have a bad bearing often the knock will go away or at least audibly change when you kill the right one. Remember too that a rod knock will usually be pretty regular in frequency, whereas preignition will more often sound like marbles in a can.

It is pretty common for a slightly damaged bearing to wait until it warms up before knocking, so it's definitely worth trying to pin down. ON the other hand it's also pretty common for preignition to wait until it warms up too. Have you tried a tank of super-premium gas? If that changes it, you know it's ordinary knock.

 

[DaveB]

You might first try this relatively simple rod knock test, of shorting or pulling the plug wire for each cylinder (you might need some tool to pull the wires and avoid shock, or pre-arrange to short or pull one at a time, stopping the engine each time). If you have a bad bearing often the knock will go away or at least audibly change when you kill the right one. Remember too that a rod knock will usually be pretty regular in frequency, whereas preignition will more often sound like marbles in a can.

It is pretty common for a slightly damaged bearing to wait until it warms up before knocking, so it's definitely worth trying to pin down. ON the other hand it's also pretty common for preignition to wait until it warms up too. Have you tried a tank of super-premium gas? If that changes it, you know it's ordinary knock.

Tonight I tried several tests, I pulled the first three plug wires off but there was no change in the knock. I'll try the other three. I swapped the only sensor that hasn't been checked (exhaust sensor) with no change. I hooked up a cold coolant sensor after the engine was warm and it didn't run well at all and the knock was still there. I re-checked the timing and it looks good. I used a mechanics stethoscope and listened to the noise which appears to be near the top of the engine along the whole length. I am using regular gas to which I added a bottle of octane booster as a test. The booster may have helped a bit. There were a few gallons of old gas in the tank but I added 15 gallons of new gas. I'll have to try some of the premium gas next. I wish I could figure this one out so I could go wheeling.
The engine is very quiet and smooth at idle and cruising speeds. It knocks when I lug it down or hit the gas really quick. If I accelerate slowly it doesn't knock.

 

[old_man]

That description is ping due to low octane or carbon buildup.

 

[DaveB]

That description is ping due to low octane or carbon buildup.

I'm hoping it is ping and not a bearing. The engine only has 19K miles on it so I doubt carbon build up. I'll have to try a tank of the expensive stuff. Maybe the octane booster doesn't really work. I drove it in to work today and it ran well, pinged if I lugged it down, but if I accelerated slowly and shifted it down going up steep hills so that it didn't shift into 4th it was fine.

 

[old_man]

From what you have said, you can pretty well rule out rod knock. You may have an injector that is partially clogged. Run a can of BG44k through the gas.

 

[DaveB]

You may have an injector that is partially clogged. Run a can of BG44k through the gas.

I had the injectors professionally cleaned but afterwards the jeep sat for over a year waiting on paint and other stuff, maybe one or more got gummed up. I need to find where to buy the BG44k stuff. I looked at the manufacturers web site and they don't have a local distributor here in Utah. I'll have to check some of the car dealers.
Thanks,
Dave

 

[old_man]

If it sat for a year, you can just about guarantee you have a sticky injector.

 

[dizzymac]

Did you use the same the gas that had been sitting in it for a year?

 

[DaveB]

Did you use the same the gas that had been sitting in it for a year?

Well there was probably a few gallons of it still in the tank then I added a bunch of new stuff. I did go out and start it every so often and it always ran fine at idle. I found some of the BG44K at a NAPA store and put it in after I topped off the tank with Chevron Premium. I did end up overfilling the tank when I tried to add the 44K and spilled some on the driveway :rattle: Should not have filled it up so far..... Hopefully I got enough in to do the job.
The more I drive it the better it seems to be getting. I took it out on the freeway last night and it actually ran well, still pinged a bit on the hills but not as bad as before. Hopefully after I burn this tank off it will be running even better.
Thanks for all the help.
Dave

 

[DaveB]

We took it in for inspection and emissions testing and came back with interresting results. It passed every test but the numbers from the emissions test were too good to be true. They spent a long time in the emissions bay because I think they had trouble believing the numbers, I know I do.

The test showed:
HC at 1 with an allowed max of 338
CO at .01 with an allowed max of 2.43
NO at 1 with an allowed max of 2288

I think it is running lean but I thought that if it was running lean the HC numbers would be low but CO would go up. It was tested using 84 numbers not the 99 numbers that match the motor and catalytic converter. I don't get it.