Mailbag: Ideas for Cooling Down a Mysteriously Hot-Running Engine


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?

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.

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  1. leon riege says:

    There is almost always Latin restriction insert for head gaskets if it is not in leplace or sized incorrectly you could be in a serious holding pattern

  2. Try pulling the tstat and replace with a moroso washer 5/8 hole this slows the water enough to cool. Could not see the fan from the front so I’m guessing it’s set up as a puller as it should be. Fan shroud?

    • Rev. Charles Deaton says:

      One 14 inch fan is not enough to cool that car!! Put another fan in front of the radiator to help cool it, any how put the biggest fan you can out front of the radiator, let’s move as much air through that radiator and a\c condenser as possible and it will be fixed !

  3. It might be as simple as a bad temperature gauge,,I have personally seen it happen on a built 383 |Charger that the owner rarely drove,for fear of of overheating.He finally replaced his
    temperature gauge,and suddenly,everything was ok,..sometimes the simplest answers are the problem.

  4. Phil Foster says:

    Observe the location of the radiator fill cap it should be higher than say the thermostat housing, if it’s lower it will leave a void of air on top of the engine thus creating a hot spot were coolant can’t get to. Any way just a thought seeings how this is a reoccurring problem with two different engines. Secondly the electric fan will need a shroud to fully cover the radiator so it can pull air across all the cooling fins to get the full benefit. Just another suggestion, good luck.

  5. Zeppo Jaworski says:

    Being a betting man and knowing this problem has followed more than one engine and now exists on an engine that didn’t have the problem in the past, I would bet the farm on your temp gauge. With a stock radiator you should be able to run that 327/300 with no problem, and by the looks of the pic you have a more than adequate cooling system.

  6. Is the electric fan running the right way. I had a friend hook one up backwards (pushing from behind rad). Car was fine sitting but at speed the fan was blowing against the air flow into rad and car would run hot. He reversed wires and problem solved..

  7. Ty Gross. aka:: MOTORHEAD TY says:

    I’ve been building engines for years, I always use a 195° thermostat. I don’t use electric fan’s or recommend. I have had this problem before,,, Just recently I rebuilt a 1987 Dodge (318) B250 Van,{my own vehicle} I modified the engine with a Edelbrock 650 performance 4BBL carb,
    A Wieland Action+ intake, H.E.I Type Distributor,mild can and Headers. I left the factory Stock Dash Gauges hooked up,and the stock 2 row Radiator, new stock water pump & Clutch Fan. All the Emissions,Black Box(ECU),etc……Is gone! I reused the old clutch fan,,,it was only a couple of months old.Driving down the road my dash gauge would read just a little bit above 1/2 way,but in traffic or idle it would climb up to 7/8’s.
    I checked out the clutch fan {cold} and it had very little “Drag”, so I replaced it,,,,Same problem. I went online to Summit Racing and bought the Summit Brand Nylon Flex fan,,,That solved it! Now when I’m cruising the highway it read’s @ 1/2 way,,,@ idle it reads @ 3/4’s[depending outside temperature, it’s summer] I also recommend an auxiliary Tranny Cooler with a remote Oil Filter bracket incorporated into the return oil cooling line.All acquired thru Summit Racing!{and yes….this Van gets up and goes!}

  8. Rev. Charles Deaton says:

    All racers know to use a 160 degree thermostat. Why wait till its almost ready to over heat to open the thermostat. Get it open 30+ degree’s sooner will help a whole lot more! Never use a stock two row radiator in an engine if its putting out more power ! More power needs bigger cooling system to help keep that horse power cool!¡

  9. Adam Phillips says:

    Header tape and a 160 degree thermostat should help.

  10. John Miller says:

    I always make sure to run a lower hose with the spring in it because it can suck closed when driving and create a over heating condition that can be hard to find. John Miller

  11. I had a stocker ’56 Bel Air with the original 265 in it in high school in the 70s. Used to overheat constantly. Went thru numerous fixes until I bought a $5 recovery kit. Solved instantly. Wish I had that car now!

  12. I had the exact same problem with my S10/ 350 SB. The mechanics had put a regular water pump on a serpentine system. I put on a reverse water pump, and 2-10″ Derale fans from Summit. The fans put out 4,000 CFM, together. That fixed everything before I changed the water pump. If you decide to get a new radiator, get a 2 row, not a 4 row. The 2 row has larger tubes. The Derale Fans still cool the 388 Stroker I have in it now.

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