running HOT, then not, then hot again

[Davelobi]

got my blocked cooling passage on my cobra outdrive gearcase issue solved (88 fourwinns, 4.3 v-6).
ran on muffs/hose for a half hour in driveway and it stayed at 160 degrees. took it to the lake...
after a short while it got up to 200+ degrees.
decided to let it idle for a while to watch it cool back down. temp didn't seem to be dropping.
tried a little higher idle (up from 7-800 to around 1200rpm) , now the temp started dropping.
when it got back down around 160 I took off and ran it some more and temp went back up.

Impeller is quite new and looks fine.
Thermostat is new and factory 160 degree.

Help me with a few ideas.
Possible actual water pump (engine mounted one) failure
Plugged/ partially blocked cooling lines? (any way to flush them chemically?)

Since the slight increase in idle rpm was enough to cool engine back down, was that because of speed of impeller compensating for poor water pump performance?

also, maybe related issue or totally seperate, WOT is no faster than about two thirds throttle. It actually seems to bog down a little when I go more than two thirds throttle. sounds like it is just getting more gas than it needs and running rich/bogging down a little.

 

[JSMoore]

Re: running HOT, then not, then hot again

The three things I'd check are: #1 - I'd Check the flow from the impeller. There's a procedure to measure the amount of water that it pumps. #2 - I'd check the water pump on the front of the motor to make sure it's not worn out. #3 - I'd check the manifolds & risers to make sure the passages aren't corroded partially shut. Problems with any of these three items could cause the conditions you describe.

 

[Jacko1971]

Re: running HOT, then not, then hot again

got my blocked cooling passage on my cobra outdrive gearcase issue solved (88 fourwinns, 4.3 v-6).
ran on muffs/hose for a half hour in driveway and it stayed at 160 degrees. took it to the lake...
after a short while it got up to 200+ degrees.
decided to let it idle for a while to watch it cool back down. temp didn't seem to be dropping.
tried a little higher idle (up from 7-800 to around 1200rpm) , now the temp started dropping.
when it got back down around 160 I took off and ran it some more and temp went back up.

Impeller is quite new and looks fine.
Thermostat is new and factory 160 degree.

Help me with a few ideas.
Possible actual water pump (engine mounted one) failure
Plugged/ partially blocked cooling lines? (any way to flush them chemically?)

Since the slight increase in idle rpm was enough to cool engine back down, was that because of speed of impeller compensating for poor water pump performance?

also, maybe related issue or totally seperate, WOT is no faster than about two thirds throttle. It actually seems to bog down a little when I go more than two thirds throttle. sounds like it is just getting more gas than it needs and running rich/bogging down a little.

Hi mate,

I had a similar problem to what your describing & my engine was not even running hot at all ( after checking with IR Temp Gun), I replaced the sender & fixed a bad earth to the motor block, turned out to be a bad in-line fuse under dash from ignotion dropping off 1.5V & causing temp gauge to go heywire. Might not be your problem but worthy of a check... Jacko...