There are a lot of causes of poor compression, but there are only two
general areas for them.
The rings can be worn or otherwise inadequate, or the cylinder walls can be
damaged. This is primarily a condition of high mileage; if you have had
regular oil changes and the engine is under 200K miles you are not a
candidate for that. It is also possible the top compression rings have been
broken by detonation ("pinging"), but that's what knock sensors are designed
That brings us to the second cause of poor compression - the valves. There
are really only two causes of valve trouble of that magnitude also:
sticking/burned valves and valves bent by timing belt trouble. Some engines
are subject to the valves sticking if the rpms are not run up once in a
while; I'll let the Ford experts weigh in if they have heard of such a thing
in your engine. Sticking can lead to burnt valves and can cause an engine to
fail to start. The other (and more common cause) of valve trouble is timing
belt failure. The valves no longer are out of the way when the piston comes
up and they get bent. It can do an amazing amount of damage. However, if the
tensioner allowed the belt to jump it is also possible the valve timing (and
ignition timing) is so far off the engine is not working right. The Gates
website indicates the Zetec is not an interference engine, so the scary part
may not apply to you. A non-interference engine is not damaged when the
timing belt fails or jumps.
If the shop is saying all the cylinders are very low in compression, I'd
expect to find the timing belt has jumped badly and may have teeth missing.
If some were decent compression and others were very low, I'd look at the
I'm not familiar with prices for this engine in the Focus, but typically you
can expect to pay in the neighborhood of $500 US for a timing belt
replacement... call around because prices vary widely. Check your owner's
manual - I bet a belt change is overdue if you haven't had it done yet.
The Zetec isn't "supposed to be " an interference engine, but it is. If the
just snaps, you are usually OK.
If the belt becomes loose for a time before it fails due to worn tensioner or
pulleys, then valve damage almost always results, usually before the belt
breaks. (The valves hitting the pistons due to retarded valve timing causes
the old belt, causing it to break) This scenario happens within in a few
There are usually warning signs, like a slapping or rattling sound at the front
engine timing cover as the belt gets loose.
Thanks for the clarification. I don't have a lot of confidence in the Gates
site, which shows the venerable Volvo "red block" engines as interference
although all versions (including the performance K cam version) are
I fear your scenario is the most likely. Is a crashed Zetec engine worth
repairing or is a replacement the better bet?
problem is usually just a couple of tweaked valves. If it were mine I would pull
head and inspect it, then have refurbished or swap in another head. It's not
it's easier than doing the whole engine.
IMO- If the valves are capable of hitting the pistons in any position
with the timing belt/chain removed, it should be classified as an
interference design. Why are some manufacturers apparently playing word
games with this subject?
Aside from a near total lack of information, we are left wondering why
anyone would focus on the timing belt tensioner and not the timing belt
Rule of thumb with Ford products.... interference engines get chains -
non-interference engines get belts. This is not to say that some really
talented people can't get bad things to happen any which way.
First step is to check cam timing... this is a relatively simple process if
your shop of choice has the documentation (read that as manuals - factory,
Mitchel on Demand or AllData). Without these basics (and a healthy dose of
experience) your shop is one to avoid.
From the sound of it, this should not be a difficult concern to diagnose...
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