Piston slap... questions for those who have or had it.
 

My jeep has unknown mileage. Its a 97 with what I would guess has under 200k on it. It has had piston slap since I bout it... est 20k ago. Recently it has gotten much louder. When I first start the jeep cold (about 70-80 deg out) it starts fine/ normal idle sits around 1,000 rpm for a sec or two with no abnormal noise then idle drops to around 750rpm and the slap begins. If I rev the engine lightly it goes away bit comes right back when it idles again. I start off on my drive and get about a min down the road to the first stop with no abnormal noise on the way and when I stop at the light the no piston slap is heard.

My jeep uses no oil between oil changes, I use delo 15/40, runs good, gets decent gas mileage.... nothing out of the ordinary other than the fairly loud piston slap/ knock when cold at idle.

I've been researching and found a lot of people saying that they had piston slap for a while then one day a piston skirt broke. I'd kinda like to get a jump on this before it gets to that point and given how loud it has gotten o er the past couple of weeks I fear a broken skirt is right around the corner.

I've read where a bunch of people say they just threw pistons in and everything was fine. The only mileage report I could find was where one guy said he had 22,000 miles since he broke a piston skirt and replaced all his pistons. He reported the engine still had no slap.

Now on to my questions...

For those of you that have piston slap how long have you had it (miles please) and how loud is it?


For those of you that have fixed it, what have you done? If pistons only were replaced how many miles since the repair? Also did you check the size of the bore with a bore gauge or piston to cylinder wall clearance?




The worst part of all of this is I sold my spare engine last week to a friend of mine. It had 215k on it but had no slap and ran great. He wanted it as a spare and I needed the room. I could get it back from him if I really needed it but to be honest I'd rather have a fresh honed engine with new pistons over a engine with 215k on it.


Thanks for any input guys.

How loud is it? Post a recording?

The 4.0's slap a bit from the factory. Iron block and aluminum pistons do that until the pistons warm up and expand a bit. A certain amount is normal and unavoidable. Certain GM motors are even worse.
Chrysler even tried adding teflon? to the piston skirts to quiet the noise a bit, in '96 I believe.
So the long and short is- some cold engine piston slap is normal, and harmless.

If you have an excessive amount it can stress the skirts. The question is, how to tell what is "excessive"?
If the noise is going away once the engine comes up to operating temp- I'd say yours is not excessive, the pistons are still fitting properly once they warm up. If the noise stays even with a warm engine, it's excessive.
FWIW, letting the motor warm a bit before loading it and staying out of high-RPM operation makes life easier on the piston skirts, if you are worrying about them.


ETA: you say it's recently gotten louder- disappears when you rev the motor- then comes back at idle. Are you certain that's piston noise? That is more typical of valvetrain/lifter noise that relies on oil pressure..

I've had piston slap since i got my Jeep 3-4 years ago.
In fact, i made a video. You can hear the slap best when i have the camera on the side of the jeep, still faint due to my exhaust noise.
You can kinda hear lifter/valve tap at the beginning.

I honestly don't have plans to fix mine. I might if it gets worse.

Name its piston slap. Its a dull knocking noise.

I'll try and get a recording in the morning before I go to work. Not sure how good the cell phone will pick it up.


Best I can describe volume of the noise is that I can start the jeep in the parking lot at work and my buddy can be in the isle over talking to someone and the guy he was talking to mentioned how bad my jeep sounded. And that point my jeep had been running for at least a minute.

My valve train doesn't sound like yours but I think my knock is spot on with yours. Mine might be a bit louder but its hard tell from a recording. Defiantly the same knock.

I know putting new pistons in isn't a huge deal so I was wondering if I could stop this noise by putting some pistons in.

Your video also mentions a possible bad wrist pin.... I was wondering the same with mine cause it seems to have gotten louder with a new oil change.

I can make a better video, now that i have a glasspack and tailpipe on. Also it's cold now (Made that vid in the summer on a 90 degree day) so it will be louder.

Chrysler was sued over this issue some years back, it's an interesting read with a good discussion of the different types of piston slap and what causes it.

http://www.romingerlegal.com/illinoi..._2/1994079.htm

OK real quick I'm going to try and post the videos I took this morning.. I'm posting these from my phone on tapatalk app so hopefully it works.


First is cold start.
https://www.youtube.com/watch?v=jpbq...e_gdata_player

Slap with slight rev
https://www.youtube.com/watch?v=H62T...e_gdata_player

Warmed up.... after about a 2-3 mile drive to work.
https://www.youtube.com/watch?v=2-Nc...e_gdata_player

i searched a bit last night and came across a bunch of people saying that they sea foamed their engines and the knock went away. when i came to work this morning i asked our older drivability tech if he had ever come across my symptoms and if he's ever heard of a induction service fixing it. he said yes and that there was a tsb about it a long time ago. he found the tsb for me and here it is:

NO: 09-21-93
SUBJECT: Carbon Buildup On Top of Piston
DATE: Dec. 31, 1993
SYMPTOM/CONDITION:
Symptoms vary from light ticking, to a ticking/hammering to a hammering/knocking noise as the carbon buildup increases with mileage and time. The noise is constant and more pronounced when the engine is cold. However, it may disappear when the engine warms up. The noise is most noticeable during idle, but will get louder as engine speed increases.

DISCUSSION:
Carbon buildup is a relatively new condition that occurs during the combustion process and results from certain compounds in gasoline. These compounds form deposits on the top of the piston and cause contact between the cylinder head and the piston. The condition affects all gasoline engines and is industry wide.

The brand of fuel and the type of detergents used by the gasoline supplier significantly contribute to the condition. Also, vehicles used in city traffic or light duty cycle operation are more prone to incurring the condition.

DIAGNOSIS:
Make sure that all vehicle systems are operating as designed.
Using the Mopar Diagnostic System or the Scan Tool (DRB II) with appropriate diagnostic procedure manual, verify that all engine systems are functioning as designed. If Diagnostic Trouble Codes (DTC s) are present, record them on the repair order for future reference, and repair as necessary.

If all engine systems are functioning as designed, ask the customer about the fuel that is being used in the vehicle. If there is a question about the quality of the fuel, suggest that only fuels which are "warranted" by the gasoline company against the build up of deposits be used.
REPAIR PROCEDURE:
The use of Mopar Fuel Injector Cleaner has shown reasonable results in removing carbon buildup deposits. Add one bottle of Mopar Fuel Injector Cleaner (P/N 4318007) to a full tank of gas.

CAUTION: UNDER NO CIRCUMSTANCES SHOULD THIS FUEL INJECTOR CLEANER BE ADDED TO A PARTIALLY FILLED FUEL TANK. ALSO, LIQUID CLEANERS ADDED THROUGH THE ENGINE INTAKE SYSTEM COULD RESULT IN BENT CONNECTING RODS DUE TO HYDROSTATIC LOCK.

Mopar fuel injector cleaner is not intended for continuous use. Owners should be advised of the cause of the noise problem and a recommendation should be made to try other brands of gasoline.

POLICY:
Information Only



so i did a induction service this morning for the hell of it, it smoked like crazy when reved and looked like this at idle:
https://lh6.googleusercontent.com/-z...018_083645.jpg

there was definatly a bunch of carbon built up.

i tried to let the engine cool a bit but i wanted to pull the jeep into the shop so i could put a stethoscope on it to figure out exactly where the noise is coming from. the engine is still warm but with the scope i can't hear a damn thin abnormal. i have it in the shop with a fan on it now. i'll give it a shot again after lunch and see if it still makes noise and if it does where it's coming from exactly.

The piston slap thing was more associated with first gen XJs especially around the 92-93 model years. Jeep and its dealers were well aware of this diesel-like noise. At first, they offered no fix and considered it "normal" for the 4.0. However, Chrysler supposedly did add some stiffening to the engine eventually to address this issue.

The noise in your recording does sound mechanical but be sure it is not an exhaust manifold leak...it can sound very similar and it goes away as the engine warms up and the manifold expands.

a new exhaust manifold was just installed a couple weeks ago.

https://lh3.googleusercontent.com/-m...915_114321.jpg


edit: on a side note maybe this jeep had its engine replaced at some point. how do i tell what gen motor it has... mainly the block?

i work with a hd mechanic that worked at a chry dealer for over 20yrs. we were talking the other day and he actually brought this particular issue up. he asked me 'have you had any piston slap?'
i said no.
he said 'oh thats good, i had to fix a lot of jeeps that had bad slap on cold start up. we had to install new pistons and fit them to the bores, they were all under warranty'...word for word

i'll ask him when he comes in tommorow for particulars on years, which cylinders were always the worst and how a guy should go about fixing this issue himself and report back. hope this will help you out.

thanks i appreciate it.

i actually am a chrysler tech of 11 years and am still at my dealer now. i never worked on jeeps tho and the techs that are here now that work on jeeps weren't around for all the 4.0 knock issues.

this is the TSB where i'm sure your buddy was replacing all the pistons:

NO: 09-08-94
SUBJECT: Piston Knock at Idle
DATE: Apr. 22, 1994
NOTE: THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 2.5L OR 4.0L ENGINE.

SYMPTOM/CONDITION:
The engine may produce an objectionable piston knock sound at idle (either cold or at operating temperature).

DISCUSSION:
The piston knock at idle condition may be due to one or more of the following:

Incorrectly classified cylinder bore(s).
Negative piston skirt taper which may cause wear at the top of the piston skirt near the oil control ring.
Flaking of the piston skirt coating on a small number of pistons.
ITEMS OF SPECIAL INTEREST:

The new style piston-connecting rod assembly can be used to service engines having pistons with the old style tin-plated skirts.
The old style tin plated piston cannot be used as a replacement for the new piston-connecting rod assembly.
NOTE: WHEN PERFORMING THIS BULLETIN, USE ONLY THE NEW STYLE PISTON-CONNECTING ROD ASSEMBLY AS THE SERVICE REPLACEMENT PART.

If it is determined that a piston requires replacement due to piston knock at idIe, all four/six pistons must be replaced with the new piston-connecting rod assembly.
The coating on the new style piston-connecting rod assembly is black in color. The coating material is applied to the piston skirt after the final piston machining process. The coating may vary in thickness. Therefore, the Service Manual procedure for measuring the outside diameter of the piston to determine size or piston-to-bore clearance is not recommended.
NOTE: MEASURING THE INSIDE DIAMETER OF THE CYLINDER BORE WITH A DIAL BORE GAUGE IS MANDATORY. TO CORRECTLY SELECT THE PROPER SIZE PISTON, A CYLINDER BORE GAUGE CAPABLE OF READINGS IN .0001" INCREMENTS IS REQUIRED.

Cylinder bore measurement should be made 58.74mm (2 5/16 in.) from the top of the bore. Start perpendicular (across or at 90 degrees) to the axis of the crankshaft and then take two additional, equally spaced (120 degrees), bore readings. Use the minimum bore diameter when selecting the correct piston size. (Figure 1)
Order the new style piston-connecting rod assembly to match the cylinder bore dimension.

CYLINDER BORE
PISTON LETTER

3.8759 to 3.8763"
B


3.8763 to 3.8767"
C


3.8767 to 3.8771"
D


3.8771 to 3.8775"
E


3.8775 to 3.8779"
F


3.8779 to 3.8783"
G


3.8783 to 3.8787"
H




If a scuff condition exists, the replacement of the short block assembly is required.
PARTS REQUIRED:
Quantity
Part No.
Description

4 or 6
4773157
"B" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4773158
"C" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4773159
"D" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4773456
"E" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4773568
"F" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4778842
"G" Size - Piston, Piston Pin, Connecting Rod Assembly

4 or 6
4778843
"H" Size - Piston, Piston Pin, Connecting Rod Assembly

1

4773160
2.5L Short Block Assembly

1

4778882
4.0L Short Block Assembly


NOTE: THE ABOVE LIST OF PISTON ASSEMBLIES INCLUDES THE STANDARD SERVICE REPLACEMENT PISTONS COVERED IN THE RESPECTIVE SERVICE MANUAL.

REPAIR PROCEDURE:
When replacing pistons, follow the procedures listed in the Service Manual except for the following:
Piston Fitting: Do not measure the outside diameter of the piston. Select the proper piston size from the information developed from item 5 in the above discussion. Use a .0001" increment dial bore gauge.
NOTE: DO NOT RELY ON THE LETTER CODE STAMPED ON THE FLAT BOSS AT THE TOP OF EACH CYLINDER TO DETERMINE PISTON SIZE. A .0001" INCREMENT DIAL BORE GAUGE MUST BE USED.

Piston Pin: Removal, inspection, and installation procedures are not required. The piston, piston pin and connecting rod are available as an assembly.
As covered in the respective Service Manual, use guide dowels when installing the oil pan seal and oil pan.
POLICY:
Information Only


i'd like to hear his take on the subject tho. if it fixes the problem i'll probably pull the oil pan and head off the jeep today and measure the bores and get the pistons ordered. i still want to check it with my scope tho so i can know for sure where it's coming from.

huh thats pretty much what i had in mind when we were talking about it. maybe a rebuild is in the cards for you?