Valve contact with pistons

i feel i am very capable to diagnose most issues, i have torn engines down to the last piece and put them back together. i feel that i have been rolling this issue in my head for a year and possibly over diagnosing the problem:shiver:.

So...

I want to start from square one and get some insight. all while not giving away too much at once. start with the fundamentals.

what causes valve tap in the manner of valve to piston?

The only time piston to valve contact occurs is when the cam and crank go out of time and a valve is open when a piston reaches TDC. This cannot happen on a 4.0 as it is a non-interference engine.

Valve tapping occurs when there is too much clearance between the rocker and the valve tip. This can be caused from things like a bad lifter, bent push rod, worn valve, rocker or rocker pivot, flat cam lobe....

Your not having piston to valve interference.

A little off topic, but you can have water/coolant on top of the piston that contacts the valve and/or water/coolant that flashes to steam as it is trying to compress.

I had a head gasket leak into number six, that dished the valve face pretty severely (not a little, a lot, it looked like an upside mushroom). The funny part was it didn't really affect my compression enough to notice. the valve still sealed.

You can look down some of the sparkplug holes and see most of the the piston top when the piston is down. Using a mirror you can see down the hard to get to pistons, somewhat.

I'm guessing it may also be possible to get the correct valve stem and valve diameter but the wrong length. But this would likely keep the valve from closing completely and mess with your compression.

As hard as I've tried to not screw it up, I've never put one together with the wrong valve timing. So I'm not sure if it will contact or not. But I doubt it will contact even then.

Talyn said:

Your not having piston to valve interference.

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Ditto. Not an interference engine.

The only way a valve could contact the piston while both are installed in the engine is if the valve breaks and falls into the combustion chamber.

Are you sure you are not hearing piston slap?

Why so secretive?

MountainRhino said:

what causes valve tap in the manner of valve to piston?

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To answer this question in more detail: When a piston hits a valve the valve usually loses and bends (best case). This prevents the valve from fully seating which in turns alters the valve tip height. If the bend is great enough it can lower the valve tip enough to remove the preload from the lifter creating lash. The lash is what causes the tapping sound. A bent valve that isn't seating fully will also show up in a leak down test and compression test.

peterjford said:

The only way a valve could contact the piston while both are installed in the engine is if the valve breaks and falls into the combustion chamber.

Are you sure you are not hearing piston slap?

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I was thinking kind of the same thing, the spring failed and/or the spring retainer keeper failed and the valve fell onto the piston. Sure would make heck of a racket.

Sorry its been a while.
as for the tapping noise, i noticed it while driving under load. hard accel, trailers, and uphill. while driving back from Smith mountain Lake I had to stop and get midgrade gas due to the station being out of 87oct. the tapping went away instantly during all conditions. obviously i continued to use 92oct form this point on.
months went by before it started again, even then it only happened towards the end of the tank of fuel, going quiet with a fill up.
now its back to doing it again, even under normal driving conditions.

I have a check engine light stating multiple misfires, leading to a coil in my best diagnosis. but I cant figure its related to the tapping as it was making this noise before any light came on.

so, Im to assume that the sound comes from the top of the valves and not the bottoms. so, what do you guys recommend as a short term fix? I stay on top of my oil changes, should I add seafoam or something?
perhaps an engine flushing solvent before my next oil change?

as far as a dropped valve, I highly doubt it. the jeep runs splendid, best I have ever had.

and the noise isnt from one valve, it seems rather similar across the board

That would be spark knock, aka detonation.

Digger87xj said:

That would be spark knock, aka detonation.

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This. Check for vac leaks, fuel pressure, o2 sensors, tune up items, etc. Valves bent.. lol

MountainRhino said:

while driving back from Smith mountain Lake I had to stop and get midgrade gas due to the station being out of 87oct. the tapping went away instantly during all conditions. obviously i continued to use 92oct form this point on....

now its back to doing it again, even under normal driving conditions....

I have a check engine light stating multiple misfires, leading to a coil in my best diagnosis. but I cant figure its related to the tapping as it was making this noise before any light came on...

so, what do you guys recommend as a short term fix?

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Fooking ouch!

I'm betting it misfires because the plugs are fouled. I'm betting the plugs are fouled because you have been running the wrong grade fuel. Time to pull the plugs and have a look.

The "miracle in a can" approach might clear up most of your problems. Make sure to use a product that is labeled "safe for catalytic converters." I prefer the less harsh solvents to clean the fuel system and combustion chambers.

The Renix Jeeps have a knock sensor that will adjust the timing if a knock is detected. If yours is renix, check that also, and your timing. Digger87xj is correct.

Profile says 2001 so no renix. Always give year in post as well as engine/trans combination.
If knocking goes away on mid-grade, you obviously have pre-ignition. Possible causes: bad gas, too high compression from carbon build up, too much spark advance (which is basically not adjustable with our ignitions). You don't tell us how many miles on vehicle either.

You shouldn't need anything other than regular gas.

Check out some of the recommended additives to clear out the carbon. What do the spark plugs look like. Ashen white or darker?

Thanks guys, Im on christmas break from work and Im getting a new coil and plugs. going to the correct fuel will be nice at $1.99/gal. I will be sure to leave feedback of the results.
I appreciate all the help.

Pull the plugs and do a plug reading. If you are not familiar with this, take a pix of each plug and post them up. This will tell you more about your engine than just about anything else. Either way, I would run a can of BG44K through the fuel. It will clean the injectors and get rid of carbon buildup. I run a can every 10K miles in each of my vehicles and have for decades.