All, I’m a newbie Corvette and C2 owner (’64 Coupe, 327/ 365hp no A/C) who purchased a very clean, garage kept, seemingly mechanically sound car from Ohio and had it transported to Colorado 6 weeks ago. After driving it about 200 miles over 3 weeks I took it to a racing garage (well recommended by a long-time C1-C6 Corvette owner) to have the Holley carb adjusted for the altitude. Just prior to taking it in I noticed that it started to run hot (220-240deg) after several short drives (~20 min).

After taking it in for the adjustment (and running it on a dyno for several runs) it ran hot on the way home (90deg day) and subsequently puked some coolant after parking it in the garage. I went on the forum for some advice but found everything from the thermostat to the water pump to the radiator. So I decided to start with the easiest/ least expensive fix and swapped out the thermostat & housing with a 180deg from Ecklers and replaced the expansion tank cap with a 14# one and the coolant with Zerex G-05. This helped only minimally (10 min test drive drove the temp to 220deg) so I took it back to the same shop where we have subsequently replaced the water pump (OEM replacement) and radiator (DeWitts aluminum OEM). On today’s test drive the temp gauge jumped all over and the car vapor-locked and needed to sit for 20min before it would restart. The temp at the housing is a cool 184deg so everything seems to be working properly except at the gauge.

After pouring through the extensive maintenance records for the past 20 years I found that the water pump and thermostat were 16 years old and the radiator had never been replaced. The car has also only been driven 2,000 miles in the past 9 years which I suspect may be contributing to the issue. However, even with all new coolant system components the gauge stills seems to jump all over the place from correct reading to full-hot incorrect. I would appreciate any thoughts/ ideas to help fix this issue.

 

before throwing any more parts at it get a IR gun and confirm your gauge reading

 

Wherein Colorado are you?

 

Welcome to the CF. You have covered most parts except the vacuum advance can on the distributor. Yes, it does play a part in the combustion/cooling connection. Do a search again about the "vacuum advance" it will tell you how to check the unit. Also, all temp senders do not sense the temp the same. Yours might be one of them. The gauge may be going bad. Dennis

 

Welcome to CF. The advise above (not so much below) is spot on and each item is important.

1. Buy an IR gun and verify your reading
2. Possible bad gauge, loose wire, or replacement sending unit
3. Check vacuum can function (Typically only effects idle and slow speed overheating)

Do those and you will find out something

 

All, thanks for the suggestions including the Vacuum Advance piece. I also failed to mention that the Fan Clutch wasn't always fully engaging so that was also replaced (also 20 years old). We used the IR gun and confirmed that the actual temp at the thermostat is where it should be (~184deg) and doesn't match the Temp gauge all the time. The gauge does seem to be a likely suspect so that is next on the trouble-shooting list to see if it is shorting-out or loosing connectivity.

Warrenmj, I'm in your neighborhood.......Colorado Springs.

 

Can you hear coolant rumbling before it overflows?
Coolant flow may be hindered from rust scale and sludge. May need to remove freeze plugs and block drain and power flush to clear the coolant passages

 

It would seem a lot of your problems are a bad gage but then you also mentioned the car appeared to vapor lock one day. What happened to make you think that it vapor locked? Was it just simply hard to start? Old carburetor equipped cars have to be started in a different manner than modern, fuel injected cars.

Are you a do-it-yourself car guy? It's a bit of a pain to remove the temperature gage because of it's location. The seat should be pulled. The steering column loosened up and dropped down (or just removed). The parking brake assembly loosened up at the firewall and dash brace and dropped. Then using 4" sections of all-thread to replace the main dash cluster screws, slide the instrument panel out enough to where the little screws securing the gage can be removed and the gage unit removed. The speedo cable and oil line to the oil pressure gage may need to be removed to pull the dash out far enough. I would send the gage off to be rebuilt as new gages don't match the face of the other gages very well and if unrestored won't match the existing patina.

Hope you can get the car sorted out.

 

I had a similar problem - gauge would jump around. Turned out that the connection at the intake manifold was the problem.. Secured that and all was well.

Regards,
Greg

 

These oem temp gauges Are not accurate. It’s partly the fault of the sender but between the two it’s hard to get truly accurate readings. You may want to get a mechanical gauge and mount it in a temporary place to so it can get accurate information. Mechanical gauges are cheap and fairly easy to install. Failing to start when the engine is hot is probably another problem.

 

Steve, PM sent — Mark

 

Mine was hard to cool with a 383 stroker motor in 93 degree weather. I removed the thermostat , put a restrictor plate in to control the right amount & a big block radiator not de wiitts & it stay 175 to 180 now

 

FYI. As a new owner, the coolant reservoir should only be filled about 1/2.

 

All, thanks again for the continued advice. I started out doing the trouble-shooting myself but it quickly advanced beyond my current ability so it is at a trustworthy shop where they are looking at the sending unit-temp gauge piece.

In terms of the vapor lock, that occurred at the shop while they were on a test drive trouble-shooting the coolant system, not at start-up. They have scoped the manifold and the gasket looks proper so not sure if this was an unrelated or one-off thing or not. Has not happened to me while driving it.

 

Have you put a pressure test on the system?

 

Could you post a few pictures of your radiator? Perhaps some of the seals are missing. I would also check to see if there is a spring in the lower radiator hose. Jerry

 

All,

Thanks again for the continuing suggestions. The following has been done to date. Replaced the thermostat, temp sending unit, water pump, new aluminum DeWitts radiator., fan clutch and expansion tank cap (14#). The temp still climbs quickly to 230deg after only a few minutes at idle and moderate speeds. The shop has verified the temp climb with both an IR gun and an off-the-shelf temp gauge (so temp gauge is reading correctly).

The old radiator (brass/ copper one that was at least 20 years old) had a lot of sediment/ corrosion in it when they took it out so they are considering removing the heads to see if the water jacket passages might be clogged thus restricting the coolant flow. Do not know if they have pressure checked the system or not.

 

All, forgot to mention that they also checked the vacuum advance and the timing.

 

You need to find a competent mechanic to help you, throwing parts at the problem cost you money, and puts in his pocket. now that’s covered let’s fix your problem.



If I were working on your car, I would first verify that the gauge and temp sending unit are functioning correctly, next vacuum advance pod is getting full manifold vacuum at idle, and that it is working.

Next, I would do a full timing map to verify all timing functions.

Next a full coolant flush, as mentioned with removing block drains, and radiator hoses, followed by a pressure test of the coolant jacket/system.

You have new radiator so let’s assume that is good. The “replacement” fan clutches are all emissions designed to engage at 210*, not the 180-190 like stock. Make sure they put the radiator fan on the right way as well. It’s easy to install it backwards.

 

All,

A quick update to close out this thread. First, thanks for all the inputs. In the end, the problem ended up being two fold. First, a low point had developed on the head between the #2&3 pistons which was allowing hot gas to escape under the gasket and enter the #3 piston which caused the chronic overheating (this cause took dozens of phone calls around the country to experienced C2 Corvette shops to figure out). We then ordered new aluminum heads but unfortunately when they went to torque down the new heads the 55 year old block had become too brittle and wouldn't hold the new bolts even with helicoils installed. So, I now have to source a new block.