Low compression on all four cyls

[robb4100]

Hi all,

I have a 65 elan S2 that I use only on the street. It has been running rich (the jets and aircorrectors were way off) but I am getting it closer to where it should be. One thing found when testing was that the compression was 125 on all of the cylanders. I know that this is low. Will I damage anything running with the low compression? What could be causeing it? PO had new pistons put in about 3000 miles ago.

Robb

[type26owner]

Robb,
Let's cover the basics first. The engine has to be up to operating temperature and the throttlies have to be wide open in order to do the compression test. If you didn't do either or both then start over and do the test again. Don't panic, yet!

Heed Ed's wise advice and don't add much oil to check for leaking rings. Oil will detonate under the right conditions. It can explode the bourdon tube in the compression gauge and shrapnel it.

[robb4100]

Keith,

The bad news is that the engine was hot and the throttles were wide open.

Robb

[type26owner]

Robb,
That sucks. It would be best if you do a leakdown test now. That will isolate the problem. Are the rings Hepolite ones? Their compression rings are chromed and a mother to break-in and get them to bed if it's not done correctly.

[M100]

What oil have you been using?

If its not been run in with a conventional (non synthetic) oil that is maybe one cause of the problem.

[marcfuller]

What is your elevation? If it is "high" be sure to factor your elevation into your compression tests.

[robb4100]

Hi all,

The engine was broken in with non-synthetic oil and we are at sea level.

Keith
I am not sure how to do the leakdown test or what it will tell me.


Also, what are the risks associated with running with the low compression (I gather performance will be poorer) for a while. Assuming that it will take a major repair I would rather not get into that expense this year - unless I risk serious damage to the motor (I can handle the embarassment of not going really fast)

Thanks as always for the advice and help

Robb

[type26owner]

The risk is little to none if it's glazed cylinders. There is a large lack of power and getting the engine to idle will be more difficult. Oil consumption will be quite high and the blowby will dilute the oil at a faster rate so changing it more frequently will be prudent. Mileage will be worse too.

A leakdown test measures the rate of leakage by applying air at a steady rate and measuring the pressure drop due to the leak. Finding the leak is easy. One just listens in the exhaust or intake or oil filler for the sound of air rushing by. Once you know were it is leaking then a corrective plan of action can be applied. This should be done by a professional mechanic. The cost will be modest since it can be done in about 30 minutes.

I accidently glazed my cylinders the first go around. BTDT. After about 25K miles all of a sudden out of the blue it corrected itself. Glazing is the burning of the oil into the textured surface of the cylinder wall. All the experts say the only way to fix this is to hone the cylinders again. If you've ever used ChemTool to remove the hardened carbon deposits in the combustion area before you'd know there's hope afterall. I've never heard of anyone doing this but I suspect that stuff would dissolve the glazed oil off the cylinder walls if it's applied correctly without taking the engine apart. Are you game if you find you glazed your engine? There is no risk trying this stuff but possibly wasting a day's effort.

ChemTool does it's thing on burnt carbon really well because it's a brew of highly toxic solvents. Carefully read and follow those instructions!

[robb4100]

Keith,

The relatively low power (quicker than my Grand Cherokee, but slower than my wifes BMW 3 series) and the poor gas mileage (20 mpg max-usually 15 or so) supports the glazed cylander theory.

On the other hand the plugs are dry and sootty, not wet and the car does not burn any oil? Would this seem to suggest something other than glazed cylanders? If I lean it out enough I can get tan plugs.

I will have it in the shop this winter so will have the leakdown test done and see what they have to say. Does this sound like a situation where one should get the leakdown test done earlier rather than later?

Thanks

Robb

[type26owner]

Robb,
Unless you have an Air/Fuel Meter you don't have a clue as to the actual mixture via reading the spark plugs. It just does not work that way. Another myth which won't die.

With my glazed cylinders if the car idled for at least one minute it would emit a HUGE plume of blue smoke when one pulled away. That's the clue it's the rings leaking.

Take your time getting the leakdown test done. Well worth knowing exactly what's wrong so you don't waste time and effort chasing your own tail if you know what I mean.

[type26owner]

Robb,
The oil smoke was from both the glazed cylinders AND the valve guides leaking. When the rings finally bedded in the amount of smoke reduced by about half. It reduced the amount of smoke so I couldn't see anymore the constant trail that I left behind me in the rear view mirror. Anyone following me would tell me my car was smoking though. That's a BIG no-no here in California. I had to install teflon valve stem seals to eliminate the oil smoke entirely.

If you don't have it smokin then maybe there is some other problem which explains the low compression readings. Bourdon tube gauges are delicate and prone to develop errors. Perhaps your compression gauge is toast.

[robb4100]

Keith,

I do not have the flume of smoke from the engine and the compression was tested with two different gauges (ok so I am a chemist geek). I'll get the leakdown test done later this fall and keep you posted on what is going on.

Growing up in CA in the 1960's and 70's and not bein able to breathe for 3 hours after water polo practice I understand why they are so finicky about smoke from the cars. I am now out on the east coast where the car gets to rest in the winter

Robb

[type26owner]

Robb,
I'm guessing if you've got long duration cams installed with a big overlap those compression values could be ok if the stock type pistons are fitted too. Also the speed the engine is turning over affects the test. Expect lower values if the Lucas Bendix starter is spinning it over rather slowly.