MGB MGB slooowly overheating

[Coastalman]

I'm still sorting out items on the 1977 MGB that I bought awhile ago. The latest is a heating problem at 55+mph. Background. The motor has been rebuilt with the block and head cleaned. The radiator was repaired and refurbished. All new hoses. I can drive the car at 45 to 50 mph with it running a normal 180 to 190. When I go on a short trip of 10 miles at 55+ the temperature climbs slowly up to 210 after about five miles. I haven't let it get any hotter because I slow down in order for the temperature to drop down. The radiator has not boiled over and the car is running fine, just the temperature keeps going up. I replaced the temperature sending unit with an original out of an extra motor. Then I installed an aftermarket gauge that has the exact temperature indicated. I have replaced the thermostat from a 180 to a 160. Nothing changed on the heating. I took the thermostat out and put the brass sleeve in which has small holes in the side of it and is, basically, a straight through tube. Same heating. Some suggestions are a new distributor or an oil cooler. I haven't been able to check the timing yet, but will soon. I use non ethanol high test gas.
If someone has had this problem, please let me know how you fixed it.
Thanks,
Charlie

 

[YakkoWarner]

Timing is a good (and cost-free) place to start. I never had this exact problem; but would also look at:

Water pump or airflow restriction? At 55+ there should be enough air moving across the radiator to keep it cool even with no fan (drove one for a couple months that way once when I was too broke to repair it).

An overly lean fuel mixture can cause hotter running as well (and potentially burn a valve or piston). Are you running the original Zenith carb or has it been switched over something else?

Does it feel like it starts to lose power as it builds up heat? Constricted exhaust could come into play, but usually that also results in poor running.

 

[Coastalman]

I have a Weber carb on the car with headers and a new muffler. I'm thinking the timing will show something. I sure hope so because I'm running out of things to check. The water pump is new.
Charlie

 

[YakkoWarner]

I have a Weber carb on the car with headers and a new muffler. I'm thinking the timing will show something. I sure hope so because I'm running out of things to check. The water pump is new.
Charlie

Header and new muffler means probably no constriction in the exhaust. If the timing checks out correctly, pull the plugs and see what they tell you. If they are really bleached-white then that would indicate running overly lean - if they are sooty black then overly rich.

 

[Coastalman]

The plugs are a light brown

 

[mezy]

Is the radiator getting hot, top to bottom when it is over heating, not just the top but the whole thing.
I have had an impeller in a water pump come detached before now, on a new pump.
If just the top of the rad is hot but not the bottom i would suspect the pump not pushing the coolent around

 

[Coastalman]

The top of the radiator was 175 degrees when I got home and checked with a laser temperature gauge.

 

[mezy]

The top of the radiator was 175 degrees when I got home and checked with a laser temperature gauge.

what about the bottom of the rad, if that is still cold and the gauge is showing over heating then i would check the pump

 

[Coastalman]

I did not check the bottom of the radiator. Thanks, that's a good tip!

Charlie

 

[YakkoWarner]

The plugs are a light brown

Light brown seems normal so probably not a fuel mixture issue. Did your timing check reveal anything unusual?

 

[bobhustead]

Put a CO sniffer on the radiator. Your symptom is consistent with mild head gasket leak.
Bob

 

[Coastalman]

The top and bottom of the radiator are just a few degrees different. I can't detect a head gasket leak, but I don't have access to a CO sniffer.

With the onset of very cold weather, for us, I will not be checking this problem for a week or two.

Have a great Thanksgiving all!