Midget Overheating Problem - Continued

[Monark192]

Following on from this post (I know, I move slowly) I have removed and checked the thermostat which seemed ok. I replaced with a new one (after checking that in hot water first) anyway and found that it fixed nothing. The car will still pump water out of the overflow tank after a short drive even though the temp guage never gets above N.

I borrowed a compression guage this weekend and got the following numbers (from front to rear) 170, 155, 145, 130.

What do these mean to me and how bad is it??

 

[RickB]

Anything bad looking / smelling in your coolant?
If you have gasses being pumped into your coolant then you will have trouble.
Just a WAG.

 

[jvandyke]

Did you have the car on jumpers? I think I read that as the battery weakens compression numbers can decline as the engine turns over more slowly, so the first readings are better than the last. Did you pull the plugs out for the cylinders not being tested? I've only done compression testing twice so that's why I'm asking, there's a method to it. Hate tear apart on engine based on an erroneous test. Not that I would know. Did you add some oil in each and retest? Are you suspecting head gasket leak pressurizing the coolant? Bubbles if you look into the rad when starting up? I think that's one test for that.

 

[spritenut]

Too much coolant in the radiator and overflow tank when cold will expand and spit out what doesn't fit until it seeks the proper level.
I would try driving it again with OUT topping it up and see if it overflows.
It may spit a few drops until it seeks the correct level when hot.

 

[58Custom]

<span style="font-weight: bold">Did you have the car on jumpers?
Did you pull the plugs out for the cylinders not being tested? Did you add some oil in each and retest? </span>

Also, did you block the gas pedal down or otherwise make sure the throttle was open during the compression test? The numbers should be within 10% of each other.

 

[Monark192]

Thanks for the help. The car pumps out approx the top 20% of the coolant in the radiator - after cooling you can remove the cap and see how much has gone. This is something that it did all of a sudden - I was driving it to work, parked, and noticed the coolant coming out. Now it will do it consistently.

The compression test was done with the throttle wide open but the plugs were in the non tested cylinders and I was using the cars battery (it is a large one and in good health).

I do not see any noticeable bubbles in the radiator when starting up the car. There are no signs of any contaminants in the oil - at least the oil on the end of the dipstick.

 

[nomad]

Sure you have the right radiator cap? The early cars at least [ones I'm familiar with] required a cap with a longer reach to the seal. If you have a later cap on a early rad there is nothing preventing the overflow.
KA.

 

[RickB]

I was also wondering about the cap. Almost posted the same thing!

 

[58Custom]

Sure you have the right radiator cap? The early cars at least [ones I'm familiar with] required a cap with a longer reach to the seal. If you have a later cap on a early rad there is nothing preventing the overflow.
KA.

Ooh. That's good info. Anyone know what years?

 

[Monark192]

It is a 74 Midget - the radiator cap was replaced with one from Moss when I purchased the car in 06.

 

[Bayless]

Throttle position won't make much difference with SU or ZS carbs unless you also prop that piston up someway. But you really should remove all the plugs first. I was not aware of the rad cap problem on a Spridget but I recently had that exact same problem with a XK140. The owner had installed a new cap but it didn't reach the seat in the neck. With just mild warming, it started puking coolant. Don't depend on the vendors. Actually measure the depth yourself.