dave c 83

hi all....what are the symptoms of low compression? I have a 04sti...it drives fine,uses no oil or water but ive just had a compression test done and it came back as low on 1 and 3?....please understand this is a well known Subaru specialist so im not going to mention the name....thanks.

Blue by You

iTrader: (23)

There are two factors you need to consider, absolute values as in individual cylinder pressure, and comparitive across all cylinders.
If you have one or more cylinders showing significantly lower than the others then that would indicate a problem usually.
If all cylinders are 'low' but comparitively equal then you don't necessarily have an issue.

Assuming your specialist is a capable mechanic then your result could indicate something like a head gasket failure between cylinders, or perhaps a cam timing fault on that bank. Further investigation will show the problem up.

jameswrx

iTrader: (4)

Did they do a leak down test?

If it's a specialist, surely they gave you some advice as to the afflicted area? Leak down would highlight the area of the low comp, ie valves, piston area. Usually if you've got a significant low compression due to valve seating issues the engine will be shakey on cold starts, the colder the ambient temp, the worse it'll be shakey.

It's a relatively common area on new age Sti heads to have valve seating issues.

Blue by You

iTrader: (23)

Interesting comment and I've not heard this before. What reason, mileage, abuse, poor design?

jameswrx

iTrader: (4)

Just my opinion formed from owning them and viewing them to buy (with exhaust valve seat issues on cold starts) And going through the repair myself, researching it and buying used cylinder heads (getting pictures of used heads, specifically exhaust valves and seats). Also there's been a lot of "my Sti runs rough from cold start" type of threads over the years where you get the usual "I've tried coolant sensor/O2 sensor/maf" and I've recommended people get a leak down test done on some.

Blue by You

iTrader: (23)

Interesting stuff.
My heads were overhauled by Paul Finch for my forged build at 45k miles and he didn't say anything about poor sealing of the valves. However he did fit new guides which would have required re-seating the valves anyway.
As you say, a leakdown test would give a more accurate picture of what problems exist, but I don't think many scooby specialists would offer that as a first check on engine condition. Comp tests are fairly quick and easy and just says 'Houston we have a problem'

Bob Rawle

the company that did the test should not be embarrassed by you telling us that they did that for you?
What were the figures for all four cylinders
was the test carried out hot or cold, was the throttle kept fully open during each test, were all the spark plugs removed for the testing?

All these things affect the results

cheers

bob