I picked up my 29 a couple months ago. It has a Chev 350. When I first start the car and get it on the road the temp gauge will hit about 230 quick. Then after maybe a minute of hitting 230 it will go from 230 down it 180 in less than 30 seconds. It will stay at 180ish the remainder of the drive. The old owner said it started doing that after he put on the aluminum heads. I noticed the temp sender is located on the side of the head. I usually install them in the intake manifold. Any thoughts? Also, the electric fan is wired to always be on. So its not the fan turning on and cooling it down. If it was a stuck thermostat, I would think it would stick more than just the first 5 min of drive.
Bad ground…temp sensors work on the ground side and either the head has no ground or they installed the sensor with some type of sealant that is blocking the ground…
or an air pocket in the cooling system. Some guys drill a little hole in the thermostat to help prevent them.
I was thinking maybe a ground issue. But couldn't think of why it would not be grounded. Sealant blocking it makes sense.
I was thought an air pocket would cause temp issues through out entire trip. But I been wrong before.
If the bypass opening on the water pump/front of engine is plugged, it will heat fast, then when the thermostat opens cool down.
^^^ above is good advice about grounds. Aluminum is an excellent conductor of heat. This could be a factor. I make sure the system is burped or has no air pocket. ( Squirrel beat me too it). Also look at radiator layout relative to where the engine is. Is the top tank higher than the engine? It should be. Compare radiator location to a Stock model A relative to engine location. Sometimes custom applications are not ideal. Aluminum will expand and contact differently than iron so it may be a good idea check the torque. Problems sometimes happen like this in flatheads. The thermostats seal tight....The remedy is a small hole 1/8 drilled in the thermostat. Sometimes this works. I know this is kind of a shotgun of possibilities. If every thing checks out....maybe it's no big deal.
Could it just be the thermostat opening late? I just had a similar problem on a tractor. The 180 didn’t actually start opening until it hit a little over 210. You can check it’s opening temp. in a pot of enough water to cover the stat on the stove with a good thermometer. I use an analog one for making candy. It’s even a good idea to check new ones, they can surprise you.
You also might try an infrared thermometer just to make sure the gauge is correct. Couple of months ago I chased what I thought was a potential overheating problem. The infrared thermometer showed me I needed a new gauge. I should have bought one long ago. They're not very expensive and then you know for sure.-------well Blowby as I posted I saw ya beat me by 3 minutes! sorry
My 250 did the same thing, the sender was between #2 and #3 cylinders. I found the water in the head heated up quicker then the water under the thermostat. I moved the sender under the thermostat and the problem went away. Before moving the sender, the gauge showed 195-210 all the time, after moving it, it shows 180 to 185 and the spike at start up is gone. The engine has never shown signs of over heating, even when the sender was in the head.
I was thinking the same thing but his drops back to normal after a bit. Never ran aluminum heads with the sender in the head so I don’t know if it could drop back to normal temperature after a while.
I see quite a fluctuation of temps between the sending unit for the gauge, installed on an aluminum head, and the Coolant Temp Sensor for EFI installed on the water cross over on an aluminum SBC intake in my '55. Sometimes the gauge will read 240+ while the EFI handheld is steady at 180-degrees. I chalk it up to the aluminum head as many have mentioned as well as the close proximity to the exhaust. Move the sensor and see what you find.
I forgot to mention, mine would drop back down in a few minutes of driving, but it was always 15 degrees hotter then it should have been. The sender was picking up heat from the exhaust ports.
I've had aluminum heads, and the same sensor location, but never had this issue. It is the hottest location to sense, but that shouldn't cause the jump, and then cooling down issue. It sure sounds like a sticky thermostat to me. Also the easiest thing to replace, so I'd start there.
My SW electric water temp gauge , sending unit in the manifold , has done the same thing in 2 different engines with 3 different manifolds , for 20+ years , hasn't seemed to affect anything . Just keeps going !
Had the exact same problem with an OT build with aluminum heads. Then I noticed it didn’t happen with the heater turned on. Added a bypass hose from the wp and the problem disappeared. Drilling the vent hole didn’t move enough water to allow circulation with everything closed, and the heads showed heat at the sender before the thermostat got enough heat to open. If you’ve got a heater, give it a shot, fix or ignore. edit. FWIW, I’ve had good luck with the flex-a-lite adjustable fan controllers using the probe in the radiator. Bit OT here.
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