You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re troubleshooting an engine that’s running too hot.


Q: I have a 1955 Chevy two-door sedan. I have switched engines in the car a number of times, and it currently has a 327 cubic-inch, 300-horsepower small block that was last in the car about four years ago. The engine ran fine before I took it out, but after I put it back in, it began to run hot—up to 260 degree after about 20 miles of highway driving. I had the radiator cleaned and rodded, then installed a new water pump and thermostat, but it didn’t help.

I swapped the 327 for a built 350 (.030 overbore, 4-bolt mains, double hump heads, Lunati cam, Edelbrock Performer intake, Quadrajet carburetor), and replaced the original 4-speed transmission with a TH-350 automatic. I replaced the radiator with a 5-core radiator, and used a FlowKooler water pump and a 160-degree thermostat. I have a 14-inch electric fan and air conditioning.

Even with those changes, the car still runs hot—up to 220 degrees depending on the outside temperature. I have the timing set at 38 degrees at 3,000 rpm. I’ve tried timing from 0 to 12 degrees at idle with no change, and tried running with the vacuum advance disconnected. Nothing seems to help.

I try to drive the car to shows, but that is hard to do with the engine running so hot. Any ideas?

1955 Chevy Bel Air Front Low

A: We have a couple of things for you to look at. First, did you add a transmission cooler when you installed the TH-350? If you didn’t, we would add one. It will help take some of the heat out.

Second, you didn’t say what compression ratio the engine has. Higher-compression engines tend to run hotter. Even with premium fuel, a high-compression engine won’t run right without octane boost. Air conditioning will also raise engine temperature slightly.

Another thing to look at is fuel mixture. Your carburetor might be rich on idle circuit, but very lean on the top end where you seem to be having the cooling problem. You can check this by running the car at a steady 3,500 to 4,500 rpm, then pull over to check the spark plugs. If the plugs are white, the engine is running too lean. Don’t do a plug check after around-town driving, as you’ll get a reading on the carburetor’s idle circuit—not the top end.