Another overheating problem

[ErikN]

After some plumbing work on our M25, we refilled the coolant. We then went through the burping procedure, including hand-pumping coolant through the hot water heater. We also pressure-tested the system with the loaner tool from the auto store. Now the engine consistently overheats, but only at idle speed. If we increase the engine speed, the temperature stays around 160F, and continuously produces a stream of bubbles in the expansion tank. This has been going on long enough that I don't think it is just working an air bubble out of the system. The only other new symptom is an occasional screech, possibly the belt slipping (maybe we need to tighten it a bit, but it doesn't seem too loose) or perhaps a bearing in the coolant pump?

Does this sound like a coolant pump that is going bad and struggling to maintain sufficient flow at low rpm, or is there something else to check? Any thoughts would be appreciated--thanks in advance.
Erik

[waughoo]

The steady stream of bubbles into the expansion tank is concerning.  The cooling system is a closed loop (or should be) so any "extra" gas in the system would lead me to investigate the head gasket.  This might be a bit of an over reaction, but all the same, I think the gas bubbles are a sign of something else not right.

[Ron Hill]

Erik : As Alex mentioned - you're getting air into your closed coolant system.  considering that you've already pressure tested the system - my guess is it might be the reservoir cap ("radiator cap")?

A thought

[KWKloeber]

After some plumbing work on our M25?????

It's always helpful to post complete information.  Make it easy to help you.

Even if it isn't a cause of a problem, it can help rule out some other things.

WHEN does it screech?  Not all the time -- When accelerating RPM?  When the Alt is under more load?  Just haphazardly "occasionally"?

[ErikN]

Sorry for the terse description—it's a long story and I'm typing on a small screen. Based on a compression test, it is probably the head gasket.
Erik

[waughoo]

Erik,

Before you pull the head wholesale for a head gasket replacement, I would suggest re-torquing the head.  Use the manual for your engine's procedure and torque specs.  This won't always solve the problem, but it is certainly a good idea to start with the inexpensive solutions first.

[KWKloeber]

Sorry for the terse description—it's a long story and I'm typing on a small screen. Based on a compression test, it is probably the head gasket.
Erik

Your choice.  Without providing a complete picture you are asking for a cause and solution -- which invariably leads to unnecessary, willy-nilly replacing of parts, and chasing suggestions down rabbit holes.

[KWKloeber]

Based on a compression test, it is probably the head gasket.

How was that conclusion arrived at? 

If you pressure tested and the closed coolant system held pressure, then those results point to there NOT being a leak somewhere in the system (as in a head gasket leak.)  Did you you verify that it is not due to bad rings? 

Presumably, both tests were done under the same (cool) engine condition so the indications would show the same result -- a leak between a cylinder and the coolant jacket, or no leak between the cylinder and jacket. 

To be absolutely certain run and monitor the pressure test during the compression test.

While in life two thigs can be true at the same time, these two things cannot be true at the same time/condition: a leak and no leak.

[ErikN]

I will give that a try. For my own edification—is it possible that the gasket can handle the 15psi coolant pressure test but not the 400psi cylinder pressure? I don't know what really happens when a head gasket fails.

Is it also worth trying a combustion leak test? The stream of bubbles in the expansion tank could be either exhaust or coolant boiling somewhere in the engine. Assuming I got all the air out (I'm fairly confident, but I suppose there could still be a bit lodged somewhere), are there other sources of a bubble stream that I should look into?

Erik