Recently I learned that a compression test does not measure the quality of coolant seals. Not sure how that tidbit slipped by me - I wish I knew that when I evaluated my car before buying it.

Therefore: if you have roughly equal, but low compression numbers, which seals are at fault?

Can coolant seals be going bad on an engine with excellent compression numbers?

Is there any good explanation out there for the various engine critical seals, their modes of failure, and how this shows on direct tests?

Dave

 

another way of testing for compression

take out a sparkplug on one rotor at a time, disable the other plugs, if you want disable the fuel pump too. you don't want the car to start or flood.

get a partner to turn the motor over, put your hand by the plug hole and feel for a putt, putt, putt, putt... etc.
if you get a putt, swoosh, you've got bad compression and a broken apex.

 

Right, and if you were looking at the numbers you'd see:
normal-low-low since one broken apex changes the compression of 2 of the chambers. Apex seals are easy to diagnose.

My questions are how to tell when it's the coolant seals, and which seal is responsible for when all three chambers are reading low.

Dave

 

If it's the coolant seals you will see white smoke in your exhaust and there will be a sweet smell that's the easiest way to tell.

 

How would you be able to tell if the coolant seals were going bad from any kind of compression check?
Usually the coolant is restricting the exhaust gasses from exiting the combustion chamber and the little “champaign” bubbles occur as the exhaust gasses escape through the cooling system and out the filler neck or ast.

The coolant seals would have to be far gone by the time you would be able to see any appreciable pressure drop from a compression check wouldn’t you?

and like brocken09 stated you would also see coolent being vaporized in the exaust as well by that time.

 

The cooling system operates at 13psi, and chamber compression reaches 120psi. I don't see why a leak between the two wouldn't be nearly as clear as any other.

Dave

 

I know what your saying, but think when there is a small leak in a balloon, it doesn’t instantly equalize and reach atmospheric pressure.
There is a very small area in which pressure has to escape, and I would think that it would not be very noticeable until there was a very large breach in the coolant seals, wouldn’t you agree with that logic?

 

My understanding is
Compression check = apex seal damage / warped rotor housings etc.
Champagne buble test = O ring failure

You can have one problem without the other

 

the chamber pressure when running can surpass 2500psi....

 

u just said one way............

presurize your coolant system. Will it hold 13psi?
If not that's a problem.

low compression on the chambers could be anythign from sticking seals to worn out housings.

 

I just did a bubble test by:
1) open filler cap
2) start engine (real cold)

There were no bubbles, but after 30s the level started to rise a touch, and so I slapped the cap back on before it made a mess. What does that mean?

I'm thinking of getting one of those vacuum+pressure testers. Since they're just expensive enough to not be throwaway, can anyone recommend a cheap but decent kit?

Dave

 

So coolant seal leaks don't manifest unless there is 2500psi differential?

Dave

 

no, you're trying to be technical about something that doesn't need to be

if you have a coolant seal failure, a compression check can show it, possibly, but most people do not need one, your car will burn coolant and smoke like crazy...if it isn't doing that, your coolant seals are okay

HOWEVER, you always run the risk of losing one, it's inherient to the design of the motor

so if you have good compression and no coolant loss, no test is going to say WHEN your coolant seals will go

however, if you have lowered compression and no coolant loss or smoke, it's worn apex/side seals and/or rotor housings, which is totally seperate from your coolant system

 

Yeah that was a dumb question.

Thanks for the answer. There are 1000 posts saying to get a compression test done, but sadly very few that explain more than that. I've got the PITA tb hose to replace shortly, and after I'm done I'll do a pressure test.

Dave

 

Just a quick note on the Champagne test...wrap a towel around the filler cap, because bubles will start coming out when the thermostat opens.

I agree in principle with 93BlackFD but I would like to add that if you just have a slight O ring failure those symptoms he describes might not show up, untill things get worse.

Hence, if you are worried try and do the champagne test correctly...step by step is detailed in Scuderia Scirianis site.

Hope this helps