Chick or egg: Timing chain clatter and blow-by

You can be pretty sure that if you are having blowby feeding back into the intake, the rings are worn or the cylinders are damaged. A worn timing chain might support the suspicion that this engine is getting pretty 'long in the tooth'.

I would hesitate pay the mechanic $800 to replace the timing chain alone, as the blowby is almost certain to continue and eventually worsen. Of course, if the chain breaks or slips, you have lost your ride for a while.

You could probably find a reasonable junker engine for $200-450, install a fresh chain, and slip it in the car. If you can put it in yourself, you can get by pretty cheaply.

In a leak down test they pump air into the cylinder and record how long it takes to leak out. I can leak out past rings, valves, or gaskets.

A handheld vaccum guage is cheap and usefull for this kind if diagnosis. Look under Cars on my website for a writeup on vacuum guage diagnosis to see if you want to try it.

You might take off the valve cover and look on the underside. I was able to find a problem by the carbon deposits on the underside of the valve cover. In that case all it required was some engine cleaning chemical and an adustment to the valve clearances to get the compression back in line.

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------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage:

What is the 8th digit of the VIN? If it's an "R" engine code you have the old style set up where the cam gear is pressed onto the camshaft and $800 isn't so far out of line. If it's a "U" engine code, the timing gear set-up is more conventional and $800 is a bit steep. FWIW, the amount that you paid for the vehicle has little bearing on what the repair costs will be...

Related? Only WRT the engine maintanance being neglected.

Have you checked to see if the vacuum passage for the PCV circuit is plugged with gunk?

True enough, a leak down test would give better information WRT cylinder sealing.

More thorough diagnosis.

Sorry, but I missed the 2.5 litre identification on the first reading.

I assume this is the 'Iron Duke'.

If so, I had an earlier version of this engine in an 84 Fiero, and it started blowing by and rattling at something over 50,000 miles.

I had heard the horror stories about the blocks cracking and went to GM with it, but they assured me that my engine was not subject to this failure, blah, blah.

When I pulled it down, the block had cracked horizontally just above the lifter gallery.

I went through 4-5 junkyard blocks of the same year and casting number, finding them ALL cracked. Finally I found one that apparently was off spec enough that it survived. Rebuilt it, and it was fine.

Now, I don't know how long this cracking problem persisted, but it damn sure was in effect AFTER GM claimed it was cured.

Anybody know the true answer??

The blow-by and timing chain clatter aren't likely directly related, but quite likely stem from the same root cause, poor maintenance and lack of regular oil changes causing increased engine wear. 70K seems early for those sort of problems..