Underheating

[6T9QDA]

Yeah, underheating. Roller LA318, very mild cam, small 4bbl, manifolds with duals. Three-row Champion radiator, stock fan with clutch and shroud. Only about 300 miles on the engine, and has run at a solid 190* until now. I had partially drained the coolant to replace lifters, and the engine seemed to be fixed and ran at normal temp for about 25 miles. The next day went to 190 and then started to fall during the drive, gauge showing about 110, so I brought it home. First thought was air in the system and the temp sender was trying to read air temp instead of coolant. Let it sit overnight.

Today I partially drained it again, then removed the cap at the manifold heater hose fitting as a vent. Added coolant until it appeared there, capped it, and filled the system. Drove it around the block a few times, and the temp gauge just barely comes off the cold peg. Let it idle for about 20 minutes and it stays there. The upper rad hose and the bypass are warm, but I can hold them comfortably and they feel about the same. I can keep my fingers on the intake manifold near the temp sender. Nothing seems to be getting hot. Meanwhile, the engine seems perfectly happy idling away. Where is the heat going? If the thermostat was stuck closed, the bypass hose would get really hot, right? Maybe just give it a few heat cycles?

 

[66fyssh]

I would pick up a temperature gun from Horror Freight and check the temperature.

I have this one and it works well. I even used it when our hime AC was giving us trouble.

12:1 Infrared Laser Thermometer

 

[clementine]

Not a huge help here but i like those infra red guns that measure temp to verify …..

Treed

 

[6T9QDA]

Yes, I'll get one of those. The temp gauge is an electric VDO that I bought early in the build, so it's old in years but new in usage. I hope it hasn't croaked because I don't think I can get a matching replacement.

 

[niceolddart]

T-Stat stuck open?

 

[6T9QDA]

You know, maybe it’s stuck partially open. I can see some slight movement of water through the filler neck, but not full flow. The hose is warm but not hot. You could be right.

 

[A56]

Air bound?

 

[RustyRatRod]

T stat stuck open as previously mentioned.

 

[wanda]

Yeah, underheating. Roller LA318, very mild cam, small 4bbl, manifolds with duals. Three-row Champion radiator, stock fan with clutch and shroud. Only about 300 miles on the engine, and has run at a solid 190* until now. I had partially drained the coolant to replace lifters, and the engine seemed to be fixed and ran at normal temp for about 25 miles. The next day went to 190 and then started to fall during the drive, gauge showing about 110, so I brought it home. First thought was air in the system and the temp sender was trying to read air temp instead of coolant. Let it sit overnight.

Today I partially drained it again, then removed the cap at the manifold heater hose fitting as a vent. Added coolant until it appeared there, capped it, and filled the system. Drove it around the block a few times, and the temp gauge just barely comes off the cold peg. Let it idle for about 20 minutes and it stays there. The upper rad hose and the bypass are warm, but I can hold them comfortably and they feel about the same. I can keep my fingers on the intake manifold near the temp sender. Nothing seems to be getting hot. Meanwhile, the engine seems perfectly happy idling away. Where is the heat going? If the thermostat was stuck closed, the bypass hose would get really hot, right? Maybe just give it a few heat cycles?

Could still be air in the system or a stuck open thermostat. Give it a few more heat cycles and check for bubbles. If no change, maybe swap the thermostat.

 

[6T9QDA]

Thought I'd follow up. The 180* thermostat was firmly closed when I took it out, but I replaced it with a new 180 anyway. I filled the system from the radiator to just below the level of the thermostat before installing it, so I'm pretty sure there's little, if any, air in the system. The engine has had a good number of heat cycles since then, and it's behaving the same. I can drive the car twenty miles and the gauge shows about 160*, and when I get home an IR thermometer near the temp sender agrees to within a few degrees. No weird bubbly, steamy noises. We haven't had any hot weather yet, and all this has been at 60-70* ambient. The engine just runs at 160. Should I be concerned? We've had an unseasonably cool spring, and I'll watch it as the weather heats up.

 

[Demonracer]

I wouldn't worry about the 160* temp, there are some who would kill to keep their cars in that range.

 

[6T9QDA]

Oh, I know. It just seems odd that it won't come up to the thermostat termperature. Summer will tell.

 

[Bewy]

No, restricting the bypass will take the engine longer to warm up, not faster.

Make sure the stat has no 'bypass' holes it.

Another cause of slow warm up, & poor temp control, rarely thought of: how much slop/movement the stat has in it's recess. If the recess is machined deep, &/or a thick gasket is used, the stat can move up/down & act like a pump [ even when closed ] & let coolant past it. To stop this: put three ' blobs' of silicon around the perimeter of the stat to seal it & stop it moving.