View Full Version : Hornet Cooling issues?

06-12-2012, 10:52 PM
My 72 Hornet has the original Chris Craft 350 CID 320hp engine. When starting it cold, it runs and cools fine. When I take it out for a run at 30 mph and higher, it cools just fine. When I come off plane, the heat starts to climbe to 200 degrees according to the gauge, and I think it is fairly accurate. I replaced the impeller, all looks good there, tested the thermostat and it seems to be working properly as well. It has two springs located in the exhaust elbows that I am wondering if that may be the problem. I am wondering if the springs are getting too weak to properly maintain cooling equalizing. Any thoughts or ideas on if these springs are available? I am wondering if anyone has experience with this cooling system that may be able to direct me in the right direction...

06-13-2012, 12:09 AM
Dunno about the springs, will leave that to others. (Though I'm curious to see/hear more about how exactly they are set up and what they do in the cooling system.)

Two questions:

I take it that you have observed that the boat didn't do this before, and now it has started?
Does anything overheat, or do you just see a temporary spike that goes away in a minute? If so, how high does it go?

My inline sixes both did that. I never sweated it because it only went up to the green/red split on the gauges, and always went right back down within 1-2 minutes.

06-13-2012, 09:16 AM
Like Ghost says what you are experiencing is "False" over heat, it is normal in raw cooled engines with stock block mounted circ pumps. When the RPM's drop after a run the flow within the block slows way down which allows the hotter water to rise. Boats with constant running electric circ pumps show no deviation, key words don't sweat it!!

06-20-2012, 11:22 PM
The only difference is that my boat did not cool down. It was like it developed steam pockets and did not allow the engine to properly cool back down. When I pulled the waterpump and impellor apart, it all looked fine. One thing that I noticed was that the graphite bearing on the waterpump cover that the pump shaft rides in was a little worn. It did not look worn enough to cause it not to coll back down. When I removed the pressure relief springs on the exhaust risers, they seemed to be pretty weak. I thought I read that those springs should hold 5-6 pounds. After looking at this, it seems that this is the only part of the cooling system that I have not seen. Would anyone have access to these pressure relief springs that are NOS? All thoughts and comments are appreciated...

joseph m. hahnl
06-21-2012, 06:38 PM
200* is not that hot. Did you check the thermostat? what is the open degree on your thermostat? What Rpm is it idling at? Does it cool back down when you idle up? ?Fresh or salt? closed or raw water?

See if this thread is applicable to you problem


06-21-2012, 07:23 PM
I boat in fresh water and this issue has been getting progressively worse in the last year. We have a 25 mph speed limit after dark, so at that speed, the boat will overheat and not cool down. When I tested the thermostat, it worked properly and opened at 140 degrees. I believe Chris Craft called this a Thermocon cooling system. It was meant to have even cooling on the engine. It would seem that once it gets at a certain heat, (hot) it will not cool down at high speed. If the gauge it correct, it gets over 200 degrees and I can tell it is plenty warm, so I think my gauge is correct. The manifolds are too hot to touch and steam is everywhere and coming off the exhaust tips. I am afraid to do any damage to the engine, by running it. ( I know when a boat is overheating as I have been around boats all of my life).
When I launched it, I warmed it up at the dock, watched the water start to circulate and then start dumping out the exhaust. I ran out to the bouy, (300 feet) then planed off and raced 5 miles across the lake. Temp reading at 3000- 4000 rpm was 160 degrees. When I came off plane to the dock, it immediately climbed to 200 degrees and was climbing while I was idling in to the dock. Approximately 300 feet of distance. I let the boat cool for an hour and then reversed my trip at idle and it immediately got hot (200 degrees and climbing) as I could not go fast due to the night speed limit law and lake patrol was watching me. Dual transom exhaust does elicit their attention! LOL
Ahh, the joys of older boats! Any ideas or experience with this type of heating system from Chris Craft?
I appreciate all of the thoughts and ideas...

06-21-2012, 07:23 PM
Have you ever taken off the manifolds and cleaned them out-I assume you have the factory square manifolds.My 67 Chris Craft 283 has the same manifolds and they do have a tendency to scale up inside.

06-21-2012, 07:24 PM
I believe Chris~Craft , as well as other very early marine V8 engines such as Graymarine, & Cal-Connel Crusaider Cadillac Marine,, used a combination of thermostats & pressure bypass relief valves, that were designed to aid the engine during it's initial warm-up phase when the thermostats were still closed.

Those sping-like poppet valves you describe are the springs only of the pressure relief bypass valves.
There should also be a twin pair of poppet style valve heads & related seats behind those springs.
The springs are Stainless Steel~~~ I believe?
Are the pressure bypass valve heads & seats themselves still there ?
Or have they rusted & rotted away ?
Possibly only the Stainless Steel springs remain ?
The early versions were metal~~Later production valves & seats were Nylon or some other plastic material.

A symptom of failed bypass valves , or their companion thermostats, is the very overheating situation that you describe.

If the valves do not seat & seal properly much of the engine's coolng water will just remain in the engine block & heads causing the engine to overheat as you describe.

The bypass relief valves ultimately feed the cooling water to the exhaust elbows.

I believe I have an old Chris~Craft V-8 engine service manual that shows a great engine water circulation circuit chart & describes the bypass valve's proper operation.
It also gives various Chris~Craft replacment part numbers #

The complete valve assemblys I believe are still available as reproduction items sold by various suppliers that sell vintage reproduction Chris~Craft parts on the web.
There are still thousands of these early Chris~Craft V-8 engines still in operation.

I will go up in my very hot 100* F attic early tomorrow morning & try to dig-out the old Chris~Craft V-8 service manual for you & try to post the engine cooling water circulation chart & it's description aswell as the valves & thermostat part numbers #.

I once owned a Chris~Craft Lancer with this very Chevy based V-8engine !

06-21-2012, 07:46 PM
Look over the hose routing you will see the the RYE (what you are calling PR valves) are for over heat protection. They are there to prevent over heating from exhaust restriction. IMO pull the risers I think some rust flaked of and is plugging them. I have a lot of time servicing the Chris's


06-22-2012, 12:05 AM
Phil/MOP may be correct that rust scale may be blocking-up the exhaust manifolds & elbows .

These older Chris~Craft engines usually should run at a thermostat temp of 160*F , or 140*F if you are running it in salt water.

After a hard run the engine temp may spike upward a bit; but should cool down fairly quickly after idleing for only a few min.

06-22-2012, 07:47 AM

had the same issue with the 71 cig with the same motor. as Mop said pull the risers. we found that over the years as fresh water boat quite a bit of rust accumulated in the block and risers. We removed the old CC risers in favor of new SS marine risers with a crossover system. The CC risers were heavy and had a ton of plumbing on them . I know how you feel about originality but it may be worth a look at updating the exhaust system. The original risers are in storage now just incase someone wants to take the cig back to 1971.

there are a few advantages performance weight savings , a clean look ( i know all original old motors look cool) and ease of work on the back of the motor

06-27-2012, 11:26 AM
Thanks for all of the advice. The PR valves are brass and are very much in tact. I am thinking that the spring pressure on the releif valves may be the culprit. The exhaust risers seem to be clean as it really pumps plenty of water out of the exhaust. I am currently in contact with an old Chris Craft employee that has experience with this system and slowly working through the probable issues. I will keep you all posted as to my progress and appreciate all of the help!\