Jump to content
Moses Ludel

The exchange below is relevant to engine noises and separating valvetrain issues from actual piston slap.  A footnote to the piston issue is that these later Jeep 4.0L inline six engines have a tendency to break piston skirts.  This problem has a two-fold cause:  1) short and narrow skirts on the 4.0L pistons and 2) close piston-to-wall clearance (0.0013" to 0.0015") to compensate for the short piston skirts and meet EPA/CAFE fuel consumption goals.  Lighter oil (5W-30 and 10W-30) recommended for these engines compensates for the narrow piston-to-wall clearances and helps achieve EPA/CAFE fuel ratings...There is much more in the exchanges below.  Take time to read through the material. 

Message added by Moses Ludel

Recommended Posts

Hello all, 

Three years ago my engine got hydrolocked. After that I replaced piston on cylinder no. 6, because connecting rod was bent a bit. Even after that, there is some knock while engine is cold, that lasts about 2 minutes when engine is cold. After warm up knock is gone. In three years things were not getting worse, even knock is still there. I can't detect knock origin for sure, it sounds like its on the side of the engine where the distributor is, but I can't be sure if it is in the upper or lower part of the engine. After 1300-1400 rpm even when its cold its very hard to detect (but I'm not sure if it is not masked by the engine or fan noise).  Last time I checked compression it was in upper factory range. Rod bearings and crankshaft were replaced year before hydrolock.

I always thought that this is valve related because lower part of the engine appeared fine, but I've recently heard about piston slap in 4.0 engines. I guess that piston skirt is not broken since noise disappears after warm up. My questions are:
1. How can I detect what is causing this noise without disassembling engine? I've read that if I remove spark plug wire from cylinder and listen for knock, if it gets quieter then its piston slap, if not then rod bearings or valves

2. If it is piston slap, will it do any damage to cylinder walls? Do I need to replace anything else than entire piston?

Share this post


Link to post
Share on other sites

zidodcigalah...Was the noise there before the hydrolock?  My 1999 XJ Cherokee's 4.0L engine has had a subtle noise since we bought the Jeep at 94K miles.  It sounds exactly as you describe.  The sound could easily be mistaken for a valvetrain noise but goes away as the engine warms.  A rod bearing or valvetrain noise will not go away.  Hydraulic lifters bleeding down will likely continue after the engine warms up.  A lean fuel mixture during warm-up phase can sometimes mimic a piston slap or pinging.

Pulling the spark leads one at a time with the engine cold and idling, you should be able to pinpoint which cylinder makes noise.  Removing the plug lead takes the combustion pressure off the piston and reduces the tendency for the piston to rock or "slap" at its skirt.  (Slap is too much piston clearance causing noise as the piston rocks on its wrist pin and rattles against the cylinder wall) Without the combustion pressure and load, the piston is just going along for the ride and will quiet down.

Note: If the wrist pin is loose, you get a double-knock metallic noise as you open the throttle.  Wrist pin noise will likely not disappear when you remove the plug wire and combustion load. Wrist pin noise will usually occur by lightly cycling the throttle up and down. A rod bearing knock is at a steady fast idle or higher speed, a rapid metallic rapping. 

Pulling the spark lead will also reduce a rod knock noise, but if your engine has been driven in this condition for a some time, you would most likely have tossed a rod by now.  Excess bearing clearance rapidly leads to engine failure.  Minor piston slap can go on for some time.

A noise also common to many 4.0L engines, allegedly most often in mid-'nineties blocks, is piston slap from cylinder core shift.  With core shift, the cylinder jacket castings are not uniform thickness, and the walls can be thin on one side.  

The core shifted block casting should always be sonic tested for wall thickness—before and after boring.  When boring, the increased bore size will further thin out a shifted cylinder casting.  This can create a weak, unstable bore.  Thin sections impact cooling and distort the bore, causing piston fit to deviate at different temperatures.  This can cause noisy pistons, damaged skirts and even lead to piston seizure,  

When I rebuild my '99 4.0L, even though it is a "better" block, I will stick with 0.030" overbore and sonic test the cylinders after boring.  Sonic testing and the right piston-to-wall clearance are especially important for a 4.6L stroker engine.

If you follow through with the spark lead removal test (using a safe plug insulator pliers), trying one cylinder at a time with the engine cold and idling, let us know if this helps you pinpoint your knocking noise.  Piston slap and wrist pin noise will be heard below the cylinder head deck at the side of the block.  

Separate oil pump noise from piston noise when listening at the middle cylinders.  If you don't have a stethoscope or sound testing tool, a simple piece of copper or PVC tubing can act as a sound isolator and amplifier.

Moses

Share this post


Link to post
Share on other sites

Thank you for extensive response. Here is some kind of history of this engine:

  1. I've bought this Jeep with rod knock, so crankshaft and rod bearings were replaced. No more rod knock. Compression was checked - it was in the upper factory range
  2. After some time engine developed random tick (it would start randomly at idle and disappear randomly - sounded like it comes from distributor) and it sounded lot more metallic than this knock. But this occurred rarely so i guess that this could be random lifter tick
  3. Engine got hydrolocked, and piston connecting rod was replaced. Compression was checked after replacement - it was in the upper factory range. I'm not sure if knock developed immediately after hydrolock or after replacing piston. But since then it is there every time on cold start and it disappears after warm up.

As far as I understood, there are two options after I detect origin cylinder of this knock:

  1. If it is wrist pin, i should check cylinder walls and if there is no damage - only that piston should be replaced without boring anything?
  2. If it is cylinder core shift, engine block should be bored and all piston rings replaced with adequate size?

Because of compression test results I guess that rings are not worn on pistons. 

 

 

 

Share this post


Link to post
Share on other sites

I was unable to pinpoint which cylinder could be the culprit but I've found the video of exactly the same sound. This is an 4.0L cherokee, but on 1:51 when he gets underneath oil pan, its that same hammering sound. This car has also exhaust leak but there is that one hammering sound that can be best heard underneath, like in my situation.

 

Share this post


Link to post
Share on other sites

zidodcigalah...My bet for the video engine is piston slap or excessive piston skirt to wall clearance.  A rod bearing would be constantly knocking, not intermittently.  A rod knock would increase with engine rpm and become a rapid rattle as speed increases.  The noise in the video is low enough to not be valvetrain noise transmitting through the engine.

 Did you try pulling the plug wires one at a time to eliminate the noise with the engine idling cold?  As far as a wrist pin noise, the video did not sound like a wrist pin, and skirt slap is more common on these engines.  If a single wrist pin were bad, without cylinder wall damage or out-of-round, the pin and piston could be replaced.  This is an unlikely prospect.  Wear is usually more uniform.  The need for a wrist pin would likely call for a short block rebuild.

If your noise has not increased much over time, and if the noise stops consistently when the engine warms, this would likely be excess piston-to-wall clearance or "slap".  The engine could run a very long time like this, certainly long enough to plan a rebuild or 4.6L stroker build on your timeline.  However, when an engine has a wrist pin rattle, there would be risk of piston failure or cylinder wall damage.

The ticking does sound much like a lifter or rocker arm clearance issue.  However, the noise disappears when the engine warms, more like a piston symptom than a lifter(s).  Lifter noise does not go away unless there is a lifter bleed-down or lifter bore problem that goes away with parts expansion.  Parts expansion is less likely with an iron engine block. An oil pump or distributor noise would be more constant and rhythmic.  Pump or distributor noise would not "disappear" when the engine warms. 

Try separating issues...There are two possible upper valvetrain noises:  1) inadequate oil flow to the rocker arms and 2) noise caused by lifter bases that are wearing, which will increase the valve clearance. The rocker pivots, arms and rocker arm tips can starve for oil if the lifters are not flowing oil upward through the pushrods.  On a Jeep inline six with its normally high oil pressure, this is unlikely.  What is the oil pressure at an idle?

To troubleshoot valvetrain noise without pulling the engine apart, try removing the valve cover and idle the engine cold to hear the noise better.  You can avoid an oil spray by using valve rocker oil clips:  https://www.nationaltoolwarehouse.com/Rocker-Arm-Oil-Deflector-Clips-on-Holder-16pc-P172921.aspx?gclid=Cj0KCQjwn6DMBRC0ARIsAHZtCeMDylgLmrXB0DcVKxUX8c0F1gaBA0dbIBVOOPeKt3OyzMgFUoEkkHUaAr3hEALw_wcB.

To get the engine to idle with the valve cover removed, you may need to seal the crankcase hoses.  This is a huge air leak that the IAC will try to offset.  ONce the engine is idling, put gloved finger pressure on each rocker arm at the pushrod side and the valve stem side.  Do this one rocker arm at a time while the engine is cold and noisy.  See if an arm or arms stop clacking.  This is a sure test of valvetrain and lifter oiling and also the valve clearances.  You're checking for oil flow and valve clearance.  The hydraulic lifters are preloaded and should show no tappet clearance while running.  If there is lifter/rocker arm clearance, you have either 1) lifter bleed-down caused by worn lifters or 2) excessive lifter clearance resulting from worn camshaft lobes and/or worn lifter bases. 

If the noise still exists and is not the valvetrain, eliminate oil pump/distributor and crankshaft bearing possibilities.  If you decide to drop the oil pan for closer inspection, you can run the pistons down in their bores and check the piston to wall clearance at the piston skirts.  Inspect the visible portion of the piston skirts.  You're looking for a piston(s) with loose fit and excessive wall clearance.

You can replace just one piston if that's actually the case but only if the cylinder bore is round and within specification.  If the bore is out-of-round or tapered, reboring and oversized pistons would be next, and this would involve all six of the cylinders.

Note that core shift can cause the thin cylinders to warp or distort (no longer 90-degrees from the crankshaft) once the engine has been in service and heat cycled repeatedly.  The block casting can distort excessively as it seasons.  Core shift can reduce the thickness of the cylinder jacket(s), which poses a challenge when re-boring is necessary.  Re-boring would realign the cylinder bores with the crankshaft's centerline, but reboring the cylinders would also make the cylinder walls even thinner.  The concern would be wall thickness and strength of the cylinder walls.  That's why I recommend sonic testing before and after reboring to make sure the cylinder jackets are strong enough to prevent distortion and resist local overheating.

Moses

Share this post


Link to post
Share on other sites

Thank you a lot. I was unable to detect anything by removing spark plug cables. Maybe I have more than one slapping piston. I guess that I will drop oil pan on next oil change, and check for loose pistons. If I don't find any I will proceed with valvetrain troubleshooting. I'll be back here with the results.

Share this post


Link to post
Share on other sites

Hello,

I was recently replacing water pump and because I had to wait for Jeep to idle and burp air out of cooling system I've noticed some things:

  1. The ticking I've described doesn't begin immediately after engine starts. When I first noticed this I thought that it happens because engine starts on a bit higher rpm  and that is why it could not be heard. But later I realized that it corresponds to oil pressure - as soon as oil pressure rises to maximum ( it never goes past 60-65 psi) the ticking fades in. I've tested this on warm day when revs would drop to 800 rpm soon, and there was no ticking (or it was too silent to be detectable) until oil pressure got to maximum.
  2. I left my Jeep idle to burp any air from cooling system, and even engine got to operating temp and idled for 20 minutes, ticking wasn't gone. But after driving it for a minute or so, oil pressure was a bit under 40 psi and there was no more ticking.

Would those two things be the symptoms of piston slap? I guess that slapping would start as soon as engine starts (or at least drops down rpm).

Share this post


Link to post
Share on other sites

zidodcigalah...Very interesting observation!!!  At ambient noise levels, the noise on my engine does seem to come from the distributor/oil pump section of the block.  The oil pumps on these engines are noted for wearing out over time, and if your observation is correct, you may be listening to an oil pump knock and not a piston noise.  Interesting!

I have a Steelman noise amplifying diagnostic tool like this one: (http://www.tooltopia.com/steelman-6600.aspx?utm_source=google&utm_medium=cpc&utm_campaign=paid_search_google_pla&adpos=1o1&scid=scplpSTE6600&sc_intid=STE6600&gclid=EAIaIQobChMIgOCelZ361QIVhYNpCh0stw3fEAQYASABEgKND_D_BwE.  When I have the time, I'll hook up and listen to my '99 4.0L engine closely.  The noise originated with idling cold and seemed to go away once the engine warmed.  Now it's more regular and unaffected by engine temperature.  Oil pressure related?  We'll see.

I have not pursued this noise more seriously because a complete engine rebuild is planned for the near future.  I'm at 170K miles with the original engine.  The noise is not "life threatening" and does not sound like it will cause greater damage or impact the rebuild.  We drive the Jeep all the time, including interstate trips, and it just keeps ticking!  Most of the time, the engine is quiet, though recently this symptom has become more noticeable.

Ironically, my first consideration two years ago was that the power steering pump had issues, as the noise mimicks that kind of trouble.  The Steelman tester will isolate the noise and help pinpoint the source.

Let's keep the dialogue going with our findings...

Moses

Share this post


Link to post
Share on other sites

Finally found a solution to this problem - lifters at #3 and #5 cylinders were bad. I've replaced all 12 of them and no more ticking. Should I have replaced timing chain and camshaft? 

Share this post


Link to post
Share on other sites

Watch for timing cover noise if the chain stretches that far.  This does affect camshaft/valve timing and camshaft position versus crankshaft position.  If out of phase far enough, the camshaft position sensor will let you know.  Spark timing can be affected, too, as the index point is retarded by a loose chain.

If the engine runs okay and there's no grating of the chain against the timing cover, you're getting by for now...

Share this post


Link to post
Share on other sites

Well, it is definitely stretched a bit and slaps when cold (its pretty silent when warm). I will replace it however very soon.
For now, idling is much smoother than before, but I guess that it'd be even better with chain being replaced.

Share this post


Link to post
Share on other sites

zidodcigalah...A fresh chain will raise the idle manifold vacuum and improve low speed engine performance...This in turn should help the fuel mileage.

If you're curious how much stretch there is in the chain, with the key off and coil wire removed at the distributor cap, rotate the crankshaft until #1 piston is at TDC.  Remove the distributor cap.  Watch the rotor carefully as you very slowly turn the crankshaft backward.  At the point that the rotor just begins to move, stop turning the crankshaft.  Note the number of crankshaft degrees that the crankshaft has moved before the rotor turns.  This is the approximate play in the timing chain.  I like to see a maximum of 5/8" movement at the crankshaft pulley surface.

Moses

Share this post


Link to post
Share on other sites

×
×
  • Create New...