overheating, but not with the thermostat out?

[matt k]

1987 comp. Put it back in the water this year, and the temp climbed up to about 200 degrees. I pulled it out, replaced the impeller (old one looked fine) and the thermostat (with a 143* tstat from skidim). Started it up on the trailer, at idle the temp steadily climbed up to 200ish again. Shut it down, tested both the old and new tstats in a pot of water on the stove, they both appear to work correctly. Tried again, 200. Tried running it without the tstat in place, and the temp stays down in the 130* range (inlet water from the bucket is probably around 50*).

So basically, if I put the thermostat in, it overheats. if i leave it out, it doesn't. But both the new and old thermostat appear to work fine.

I pulled the belts and checked the circulation pump for wobble and end play, and it feels fine.

Any ideas? I'll replace the circulation pump if I have to, but I'd rather not drop $120 if that's not the problem...


Is it possible for air to get into the cooling system, and need to be bled out, like in a normal pressurized car coolant loop? I sort of assumed not, since the system isn't closed...


edit: on the trailer, i had all the belts off and spun the circulator pump by hand, it feels fine. A couple bubbles came out of the thermostat hole (had that open at the time). I put it back together, and now it seems to hold steady at 160*, where it's supposed to (at roughly idle speed, anyway). also, i seem to get a lot more cooling water flow through the right exhaust than the left. possibly related? I can't figure it out.

[michael hunter]

Have you checked the strainer , trans cooler and raw water pump for blockage? How is the impeller? Any missing fins off the impeller can be blocking flow.

[Supra-in-steamboat]

Michael has you on the right track. I have had grass in the trans cooler screen before that caused the temp to bounce around. If you have super-trapps, pull them off. If you have fried impeller pieces back there, you most likely have them inside the motor as well. I had some pieces inside the right exhaust riser. We pulled the plugs on the back of the exhaust risers, drained them, repeated 4 or 6 times until we had a chunk release. You could tell the difference in temperature between the risers prior to the chunks release. Good luck.

[matt k]

strainer and trans cooler are all clear. No obvious chunks missing from the old impeller, but I can definitely tell a difference in the riser temps, so maybe I will try draining them a couple of times and see if I can get a chunk to come out of the hot one...

No super trapps, just flappers on the exhaust outlets.

Thanks for the ideas!

[matt k]

hm. got a bunch of scale out of the exhaust risers, but no major chunks of anything else, and it didn't seem to help as far as the overheating or exhaust riser temp difference.

[docdrs]

make sure all your clamps are tight before the impeller so you are not sucking any air into the system

[rideevol]

Something you said in your post about water in the coolant, How old are your riser gaskets? Although they don't generally wear out they could cause air in the coolant. Any rust seepage on the outside of the riser joint signals time for replacement as you do not want water in the engines exhaust passages.

[Seabeejamie]

Hate to ask the stupid question...but is it possible you have it in upside down?