Compression Help

[Chart505]

75 280. Engine seemed like it was running a little weak. Did the cyl by cyl pull the plug and injector test to isolate. #2 cylinder seemed to be the issue. Did compression test, that was about 90 PSI in #2 while rest were in 150 +/- range. Car does use some oil (blue smoke) when under hard revving, but not under normal driving. Does not leak. Temp runs fine. Oil press is fine. I was under impression if you had bad rings or valves the compresssion would be zero, not just down about a third. Anyways, would seem to explain why the engine seemed to have a little less power that before. Has 110K on it, very regular maint, oil changes, etc. I have owned several of these cars and 110K is usually not alot of miles.

My questions:

1)Does this sound like it's time for a valve job, ring job or both?

2) Are there other tests I can do to further ID the problem. Thanks in advance.

[Arne]

Possibly sticky rings in that cylinder. Run the compression test in that hole again, but pour in a few cc's of engine oil first. If that brings the compression in that cylinder up, the rings are leaking. If it's still the same it's either a valve not sealing or the head gasket.

If the oil does bring the numbers up, you might try soaking that cylinder with ATF to try to de-gunk the rings. (Don't forget to change the oil after doing that.)

[stevef1972z]

A compression test will tell you if your engine has good compression. An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily.

As a rule, most engines should have no more than 10% difference between any of the cylinders.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

HOW TO CHECK COMPRESSION

To check compression manually with a gauge, all the spark plugs must be removed. The ignition coil must then disabled or the high tension lead grounded. If the engine has a distributorless ignition, the ignition coils must be disabled to prevent them from firing. The throttle must also be held open.

The engine is then cranked for a few seconds using a remote starter switch or a helper while a compression gauge is held in a spark plug hole.

The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders.

The individual cylinder readings are then compared to see if the results are within specifications listed in the FSM I dont have it handy now.

IS IT THE RINGS OR THE VALVES?

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve. Check the valve adjustment per the FSM and repeat the test. If it fails again, then it is time for the valve job.

Good luck!

[Chart505]

Thanks I will try the oil test tonight and post results. I was wondering if 150 is a decent compression number for the 1975 motor. I believe the motor was lower compression in 75 for emissions reasons.

[Arne]

150 is probably fine. Too many variables to say for certain. Things like the accuracy of the gauge itself, whether the engine was warm or cold (should be warm ideally), and whether or not you remembered to hold the throttle wide open while cranking. All of these things (and probably others as well) can have an effect on the final numbers.

But for now, to compare for this one cylinder, try to wet test it in as close to the same condition and method as you did before.

[bigoak]

Silly question...

What does holding the throttle open accomplish?

[Chart505]

Did the oil test. went up from 90 to 100. Did it twice just to check. Not much of an increase, is this conclusive? So valve or rings?

[ChrisB77]

Opening the throttle wide open allows the engine to draw in air unrestricted and develop max compression. If no air can be drawn in, no compression can build in the cylinder.

[Chart505]

So what's the verdict on the compression going from 90 to 100 with the "oil squirt" test? Bad rings in the cyl or think it's a valve? Rather do a head than an entire rebuild.

[Chart505]

Ive done some research on this- most tech forums indicate that if the rings are worn, the compression should come up significantly (30 to 40 psi) with the oil test. Mine only came up about 10 psi, so I am assuming I have a leaky valve, although could also be the head gasket, although there is no oil or water leaking.

[Chart505]

So I would still like an opinion from those with experience with these issue - sound like a valve or rings? Based on what I have read, if the comp only goes up 10psi with the oil test that does not seem conclusive for bad rings.

[JoelH]

Sounds like my engine results from last month. Mine had low compression in one cylinder, blew blue smoke on heavy revving as well. Additionally, mine was using about 3/4 quart every 700-800 miles and had numerous leaks.

My choice was a complete rebuild which is happening now but yours appears to need work done as well. I would look at the cost of a rebuild versus a partial rebuild but I looked at it as a "might as well" moment. I'm having it rebuilt with a new oil pump, water pump, thermostat, timing chain and new clutch. They all worked fine but while it's out we're replacing them.

My mechanic also found the front transmission seal bad and leaking and a club member had a newly rebuilt 4 speed for sale cheap so guess what? rebuilt tranny.

I'm sure you want help answering your question so you may not have to pay a fortune but if you must do a rebuild (which I'm guessing you may have to do at least a partial) you might be better off doing it once.

Good luck in any event!

Edited July 15, 2009 by JoelH