[Roadsters] Overheating Question Part II

[James Fogg]

> Helo everybody!  I have another question about my little
> overheating problem. First of all, thanks to everyone who
> sent replies to my last email about this. I really appreciate
> all of the ideas!


When I repaired cars for a living (long time ago) I saw a few water
pumps that would be fine cold, but when the impeller heated up it would
slip on its shaft. I wouldn't trust the pump from your test. The usual,
but imperfect, test is to run the engine to full temp with the radiator
tank cap open and watch the flow into the top tank. It confirms pump and
thermostat functions. I've seen some clear plastic pipes inserted in
radiator lines for this purpose too (Prestone made such items once).

Otherwise, I'd still consider the head gasket. Compression doesn't tell
you much for a leak, you need to perform a leakdown test. Buy or fashion
an adapter that will screw into a plug port and allow you to connect an
air line and gauge. Include a shutoff valve at the air hose connection.
Place your chosen piston at or near bottom dead center and pump the
cylinder up with air pressure equal to 75% of your compression test
numbers (sometimes the filling will cause the piston to move to bdc).
Close the valve and watch the air pressure for a while. It shouldn't
lose much in a minute's time. If your piston isn't near bdc it can alter
the indicated bleed rate (since there's a lot less air to work with in
the cylinder) or open a valve.

Another concern is that a cylinder head or exhaust passage has corroded
or cracked and is leaking into the water jacket. An exhaust gas analyzer
(emissions tester) placed in the coolant fill hole will show HC and CO
gas in the coolant system if this has happened. Any gasses in the water
jacket can cause problems. A cracked/leaking head may not show up in a
leakdown test since the crack may be tiny or close when the head cools.
When testing try to put the engine under some load (don't slip and drive
through the back of the shop wall!).

Good luck!