C2 Water Pump Flow Rate.

[Patrick F. McInerney]

I have a'64 SHP roadster with 400 miles since complete frame off restoration and component rebuilds. On extended idle the temperature rises over 200 on to 240 and will lift the overflow valve.

I've read all the threads on this problem that I could find and completed all the checks, adjustments ( distributer, base timing and advance)and replacements (DeWitt radiator with tank and cap, high performance thermostat, VC-1810 vac pot, hoses and belts).

The water pump is a past replacement which, because it appeared to be Ok, I reinstalled. Being in doubt I bought a length of 1-1/2" nylon thread reinforced clear plastic hose and installed it in place of the upper hose. On start-up a trickle of coolant showed in the hose. As the thermostat opened the hose partially filled. At idle there is little or no apparent flow and the hose remains only partially filled. Rapid acceleration filled most of the hose but still with very little apparent flow.

As I recall, my 30's and up cars, running with the radiator cap off there was cosiderable turbulence in the coolant indicating a stronger flow then I'm currently getting.

Does anyone have detail on the capacity rating of the water pump and is the correct SHP pump stronger than the pump used on the base engine?

Thanks once again for your help.

Pat McInerney, #37445

[Duke W.]

Re: C2 Water Pump Flow Rate.

A flow rate spec at 2000 revs or so is listed in the AMA specs. Flow is lower at idle, but is usually dictated by heater core requirements in cold weather, so it is more than required for idle cooling.

Most SHP configurations drive the pump slower than base configurations because of their higher idle to peak revs rev range. Again, the details are in the AMA specs.

The two most common causes of hot running or overheating at idle and low speed operation are the spark advance map and fan clutch. You should set the idle speed at about 900, which is somewhat greater than Chevrolet's recommendation, and manifold vacuum should be about 10" Hg. This will pull the VC1810 diaphragm to the limit and total idle timing should be in the range of 28-32 with 10-14 initial. This total idle timing includes about two degrees of centrifugal since it starts at about 700.

Check that the fan clutch tightens. Rotate the fan with the engine cold, then let it get hot - at least 200 degrees, shut off the engine and rotate the fan again by hand. It should require noticeably more torque to rotate. If not, the fan clutch is not tightening and lack of idle air flow at high coolant temperature is the most likely source of the idle overheating.

You can also test the fan by revving the engine. When cold or cool the fan will not exceed about 1500. When the hot air off the radiator tightens the clutch it should rev to about 3500 before slipping. If you rev the engine to 3500 when cool and hot, you should definitely feel a difference in fan flow and hear a difference in fan noise.

Duke

[Mike McKown]

Re: C2 Water Pump Flow Rate.

"At idle there is little or no apparent flow and the hose remains only partially filled. Rapid acceleration filled most of the hose but still with very little apparent flow."

This doesn't sound at all right. Maybe your water pump impeller is slipping on the shaft or is eaten away. Any chance your radiator is plugged with a mouse nest?

[Patrick F. McInerney]

Re: C2 Water Pump Flow Rate.

Duke and Mike,

Thanks for your comments. I should have mentioned that the fan and clutch are new GM replacement parts and when I installed the new DeWitt radiator I removed the block plugs to drain as much of the coolant as possible. Refilling took 16 quarts. However, I bought the radiator at Kissimme in January and DeWitt did not protect the openings so a mouse nest is possible.

I hope to be able to review AMA water pump specs on Monday. I believe they will indicate that my problem is in the pump.

Pat McInerney

[Joe C.]

Re: C2 Water Pump Flow Rate.

Patrick:

What are the conditions under which the radiator boils over? How long does it take for this to happen? During the extended idling periods, is the car stationary, hood open?

I am not certain whether there should be much volume flowing @ idle......probably not, because the "turbulence in the radiator" doesn't start until well off-idle. If you TEMPORARILY remove the thermostat, and recheck the flow, that will eliminate the tstat as the culprit..........if there is one. I find it very hard to believe that there is a problem with the pump impeller, though.

Part of the problem might have to do with engine break-in. My freshly rebuilt 1965 327/365 engine would boil over with extended, stationary idling, after about 50 minutes, in ambient temps of about 90*. I haven't performed this TEST in three years now, so cannot say whether it would happen anymore. The temp gauge, under all driving conditions, ALWAYS reads a steady 180*, which corresponds to a measured thermostat housing temp of 180*. I recalibrated my temp gauge to correspond to ACTUAL temperatures, by using a t******* resistor.

The point of this long post, is that you may be worrying for nothing. Enjoy your Corvette.

Joe

[Duke W.]

Re: C2 Water Pump Flow Rate.

Even a "new" GM fan clutch should be checked as I outlined. IIRC storing the fan clutch flat can cause problems, so if the part was in inventory for a long time that could be a problem. Also, the replacement fan clutches are calibrated for 195 degree thermostats, so they tighten at a higher radiator exit air temp. than the OE clutches, which will increase engine temp. at sustained idle and low speed driving.

There should be no air in the upper radiator hose. Running the engine at 2000 revs for ten minutes AFTER the thermostat opens with the cap on loose should be sufficient for self-bleeding (and add coolant as required to maintain the tank about half-full), but it's best to drive the car for a few miles at low speed to promote self bleeding. The air bleed path is the small hose from the upper RH corner of the radiator to the expansion tank, BUT the system will not self bleed until the thermostat opens.

It's possible that the impeller is damaged due to corrosion. Later pumps (and who knows about rebuilds) have stamped steel rather than cast iron impellers, and the stamped steel impellers are known to suffer corrosion or cavitation damage, however, I recommend you verify that the fan clutch is operating properly and give the system a chance to self bleed before you tear into the coolant pump.

Duke

[Patrick F. McInerney]

Re: C2 Water Pump Flow Rate.

Duke,

Took the car out for a 15 minute spin. Since we have a beautiful 80 degree afternoon and there was little idling temperature rose to 190. With engine idling the plastic hose was full so, thanks to your suggestion, I can rule out the pump. By the way, the $6 plastic hose from Home Depot permits a very good viual of the coolant flow.

Also checked the fan clutch as you suggested. There was little difference in effort to turn the fan from cool to hot so out it comes for further checking.

Last Saturday I lined up with 149 other Corvettes to show the car at Corvettes at St. Armands Circle, Sarasota, FL and, thanks to a balky ramp (for driving over the curb)and the C-5 hung up on it I had to idle for about 20 minutes in low 80 degrees. The gauge pegged out and I dropped out of line to let it cool. Did make the show though. The problem occurs at idle and extended low speed driving.

Pat McInerney