Snapped timing chain 4.0 SOHC | Ford Explorer Forums - Serious Explorations

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Jack Regan

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March 16, 2019
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2006 Ford Mustang
I don't normally search for help on internet threads, but I'm getting dangerously close to scrapping this project.
I recently replaced the timing chain cassette and tensioner on my 4.0 SOHC with 150,000 miles. I bought Ford's OTC 6488 for the timing, and it ran great for about 4 hours. As soon as I hit 50 mph something snapped and the engine rumbled then died, it turned over but wouldn't start. I towed it back to the house and removed the valve covers to find the driver side timing chain I had just replaced had snapped. I honestly have no idea how this could have happened. It ran with no issues until I hit 50 mph.

My next step was to find out If I had bent any valves, but without a timing chain on the driver's side I wasn't sure how to do a compression test. Instead, I pulled the Schrader valve out of my compression tester and used a T connector with the pressure gauge and an air compressor to see if the cylinders would hold any pressure. I'm not sure how well this worked, but all cylinders seemed to be holding 90 psi with minimal leakage.

My questions are: What could have caused the timing chain to snap? obviously, it has to be something with the camshaft or valves, otherwise, both sides would have snapped. Could It be that I got the timing wrong the first time? Is there any chance that my janky leak down test was wrong?

At this point, I'm going to pull off the timing chain cover. I'll let you know If I find anything.

Thanks for reading, I'm hoping someone has an idea of what is going on.


So here's the deal,
I'm convinced that somehow none of the valves are bent. It seems that the pistons don't make contact with the valves when the cam is its neutral position (the position it naturally falls in when the timing chain is not connected).

I'm going to assume that I either got a faulty chain from Cloyes or there is an issue with the camshaft, like excessive friction (though it does rotate by hand). There is a possibility a piece of debris got stuck in the chain, but there doesn't seem to be any sort of damage on sprockets, whereas the chain is damaged in several places (broke in one, and bent and smashed in several other places).

Here are a couple of pictures of the damage, I found the timing chain guide has some very deep grooves cut in it. I'm not convinced that this is normal wear.




The groove is from how the chain broke, looks like debris made contact with the side of the chain.

See if your cam journals are scored up. Same thing happens on 4.6 dohc's when oil flow is restricted. Cam starts to lock up and chain is the weakest link.

Pull the cam, remover the followers (rocker arms) and reinstall the cam. It should spin nice and smoothly. Also it should move very easily. Report back with what you find.