So I am helping a friend with his rig and haven't been able to figure out this issue. He has a Mark IV engine that has about 30 minutes of run time. It has edelbrock aluminum heads, roller rockers, mild cam, flat top pistons, 9.3:1 compression, arizona speed and marine tuned port intake, aluminum water pump, 185 degree thermostat, aluminum radiator, dual electric fans, multiport injection, and coated headers.

It runs okay and takes a normal amount of time to warm up. There are two temperature sensors on it. One is on the front of the intake manifold and is used for the efi computer. The other is on the driverside head, and is used for the gauge. The temperature to the computer is always solid, between 185 and 195. The temperature to the gauge warms up at the same rate and the computer sensor, but it keeps going up after warmup. It makes it all the way to 240-260 before it momentarily goes back down to to 200-220. It shoots up and down in 10-20 seconds. If I turn the engine off, wait for a few seconds, and turn it back on, the gauge temperature goes back down to 190 or so, before jumping back up. When the temp gets too high, you can feel the coolant start to boil a little. If you hit the throttle, it sometimes bring the temperature down some.

It seems there is a pretty big coolant distribution problem. An edelbrock guy said there might be a big air bubble in it, but no matter what I do, nothing changes. I tried bleed it, jacking the front end up, taking the thermostat out, it's always the same.

Possibilities I could think of
-Faulty new water pump, low flow can't push air out of the system and get enough flow to the heads.
-Blocked passages, but all gaskets were verified to fit and not block anything, and the engine is fresh so blockage shouldn't be a problem.
-Bad head gasket is allowing hot gases into that part of the engine, but the efi sensor would overheat too, and there are no signs of coolant in the exhaust, no signs of contaminants in the coolant, and the oil is clean.

Anybody ever see anything like this?

 

http://www.chevelles.com/forums/showthread.php?t=291251

 

Thanks for the link, that's great information. I think that it's referring to a slightly different problem though. The guys on the forum say that the difference in gaskets cause an engine to rotter hotter all the time, but this one fluctuates pretty rapidly. Still not ruling out a gasket or head issue though.

 

Air purged from system?
At what temp do the fans come on--190* area?

Is it getting " hot enough " to allow the iron // aluminum heat cycles to cause some mis-alignment issue with the head gasket?
Would MLS head gaskets be an option?
Asking only because the few ppl I know with aluminum heads use a MLS gasket. Maybe it's all hype, but I'm learning too so please forgive my ignorance

 

It was the frigg'n water pump! Went to the parts store and got a $15 dollar pump. It's a pain in the *** to replace, but so cheap, I might as well give it a shot. I was leaning toward something else, like tearing the engine down. Now it stays at 185 without budging and both sending units match at all times.

 

congrats on figuring it out. I was going to suggest that you needed to purge the air out of the radiator/coolant system as T-top havoc had mentioned. That will cause the exact same issues you just mentioned because superheated air-bubbles will get trapped in the system and spike the temp when they go past the thermostats/sensors.

 

Thanks. There was probably air in the heads, and I'm guessing the weak pump couldn't evacuate it all. Edelbrock recommended jacking the front end up and starting the engine, but that didn't help. The new pump worked great after top the fluid after the engine took up some.