questionable timing chain

[89base]

89 MJ with 2.5l engine 5 spd. 208K miles Told it was not the original engiine ( don't know this for sure). I'm checking the distributor alignment ( indexing ). Engine starts and idles smooth

Indexing looks to be where it should be but: During the crankshaft rotation  to TDC, I found this.  Take the crankshaft to TDC then backed it up and noticed that the rotor did not move right away so I checked position of the timing marks this time. In this back and forth movement of the crankshaft I found a crankshaft rotation of 10 to 12 degrees before the rotor would follow movement. Does that sound to be within limits or is it a sign of a worn timing chain that needs to be changed ?

If this is a worn timing chain, what driveabilitly issues would I notice ?

Thanks.

[Eagle]

The 2.5L uses the same timing chain as the 4.0L, but the deck height is lower on the 2.5L and that results in slack in the timing chain. To compensate for this, the 2.5L uses a timing chain tensioner that's not used on the 4.0L. Usually the tensioner goes long before the timing chain. My guess is that you need a timing chain tensioner.

[Jeep Driver]

I agree with you, sounds like a worn chain. 

 

That's a lot of mile on a 2.5.

[mjeff87]

I agree with Eagle, the tensioner is probably shot.  It is self-adjusting, to compensate for wear, but has a finite lifespan.  Once it reaches the end of its adjustment range it's not providing the takeup in the chain that it's supposed to.   

 

You can replace the tensioner itself, but since you're in that far to replace it, it just makes sense to put a new timing set in as well.  Especially with the miles that you have on yours.

[HOrnbrod]

I don't see any reason why you couldn't install the 2.5 timing chain tensioner assembly on the 4.0 engine since  They share the same chain and timing cover. It seems like all you would have to do is drill and tap two holes in the 4.0 block on the passenger side of the timing chain matching the chain tensioner mounting hole locations on the 2.5. This could help to increase the rev capabilities of the 4.0, especially on a stroker. And of course keep a slack timing chain tighter longer. I can't see a chain tensioner on the 4.0 being detrimental for any reason. Any know of a good reason(s) why not?

[Tex06]

Because Chrysler didn't design it that way!

 

In all honesty, as long as it doesn't over tighten it should be an improvement. Too tight and it will accelerate wear on the whole timing set. Where that happy medium is is anyone's guess though.

 

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[big66440]

I don't see any reason why you couldn't install the 2.5 timing chain tensioner assembly on the 4.0 engine since  They share the same chain and timing cover. It seems like all you would have to do is drill and tap two holes in the 4.0 block on the passenger side of the timing chain matching the chain tensioner mounting hole locations on the 2.5. This could help to increase the rev capabilities of the 4.0, especially on a stroker. And of course keep a slack timing chain tighter longer. I can't see a chain tensioner on the 4.0 being detrimental for any reason. Any know of a good reason(s) why not?

Agreed, early small block mopars ( LA ) didn't have a "tensioner" for the timing chain but when Mopar introduced the Magnum small blocks they came equipped with a "tensioner" which a lot of people install on their LA small blocks as an upgrade.

[Minuit]

Of course, a tensioner would be quite valuable even with a brand new timing chain these days because most aftermarket timing chains are looser than the old ones they replace...

[89base]

As a follow up: The timing chain and sprockets along with the tensioner were replaced. The tensioner being the worst for wear. It was at the end of travel. It operates be advancing to a tension position then locking in that position by engaging small grooves in it's plastic body. On mine these engaging grooves were worn smooth not allowing it to lock into position but sliding back and forth. The chain had appreceable wear also. Now the engine performs as I'd expect a 4 cyl.

In case anyone is interested in the symptoms : Engine would start and idle normal. But going down the road in 5th, it would take a lot of gas pedal to go 55 mph. The gas mileage was poor also ( 10-12 mpg). No timing chain noise up front. My final clue was the degree of crankshaft movement I had while checking  distributor indexing ( see my initial question ).

[mjeff87]

Good deal, glad you got it fixed.

 

For those that haven't seen what the 2.5 tensioner looks like, or don't know how it functions, here's a shot of mine when I replaced it way back when.  Like 89 says above, it self-adjusts via the little block underneath the face by the spring that pushes on it.  The little block and the opposite face of the tensioner have teeth cut into them, which holds the block in the tensioned position.  As the tensioner wears (and/or the chain starts to slacken) the block travels downward to keep tension on the chain.  Once it reaches the end, that's it....no more tension can be applied.  In my case, the block hit the end of travel, then the chain started digging a groove in the face of the tensioner (mine made a noise for quite awhile until I diagnosed and fixed it).

 

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