Cooling System Questions

[Michael A.]

The recent test drives of my 66 327/3000 roadster have gone well and the many suggestions and help from everyone on the TDB are greatly appreciated.

The engine temp gauge occassionally climbs up to 250+ in certain circumstances which alarmed me. As background I have done the following to the cooling system:

• flushed cooling system and replace coolant withe Zerex G05 (50/50 mix)
• replaced lower radiator hose
• replaced temperature sender, wire and insulation
• checked coolant after the high temp situations. There appears to be no overflow coming out of the system. The level remains constant.

I seem to recall reading in the past that the temp senders are notoriously inaccurate so I bought a simple infrared thermometer and checked a number of spots to get a sense of what is going on. Infrared readings I've recorded while the engine is idling after a "high temp" run when the engine temp gauge is still showing 240-250:

• By the temp sender:
  • About 1" to the right of the sender (towards the carb) - 210F
  • About 1" to the left of the sender (towards the front of engine - 190F
• Upper radiator hose at the radiator inlet - 190F
• Lower radiator hose at the radiator outlet - 170F

My questions for you all are as follows:

• The readings by the temp sender appear to indicate that the temp gauge readings are at least 40 - 50F high. Would you agree?
• Is a 20F temp drop across the radiator at idle a healthy sign?
• Any other suggestions as to what I should do to diagnose?

Thanks in advance for the help.

Cheers,

[Joe L.]

Re: Cooling System Questions

The recent test drives of my 66 327/3000 roadster have gone well and the many suggestions and help from everyone on the TDB are greatly appreciated.

The engine temp gauge occassionally climbs up to 250+ in certain circumstances which alarmed me. As background I have done the following to the cooling system:

• flushed cooling system and replace coolant withe Zerex G05 (50/50 mix)
• replaced lower radiator hose
• replaced temperature sender, wire and insulation
• checked coolant after the high temp situations. There appears to be no overflow coming out of the system. The level remains constant.

I seem to recall reading in the past that the temp senders are notoriously inaccurate so I bought a simple infrared thermometer and checked a number of spots to get a sense of what is going on. Infrared readings I've recorded while the engine is idling after a "high temp" run when the engine temp gauge is still showing 240-250:

• By the temp sender:
  • About 1" to the right of the sender (towards the carb) - 210F
  • About 1" to the left of the sender (towards the front of engine - 190F
• Upper radiator hose at the radiator inlet - 190F
• Lower radiator hose at the radiator outlet - 170F

My questions for you all are as follows:

• The readings by the temp sender appear to indicate that the temp gauge readings are at least 40 - 50F high. Would you agree?
• Is a 20F temp drop across the radiator at idle a healthy sign?
• Any other suggestions as to what I should do to diagnose?

Thanks in advance for the help.

Cheers,

Michael-----


I'd say you've done the right things to determine if it's a temp sender problem and, based upon what you've found, I think it probably is.

For a next step, I think I'd try to locate an original temp sender and try it in place of what you have. If what you have is a GM #12334869, the replacement sold for the last 15-20 years, you can almost be assured it's inaccurate when used with your original in-dash gauge.

[Michael A.]

Re: Cooling System Questions

Thanks Joe for the validation that I'm doing the right diagnosis. The temp sender is one I bought a few years back from Corvette Central but only recently installed. I don't believe it was in a GM box.

I think what I'll do for this summer's drive is carry the handy little IR temp gun with me and check things out from time to time. We will be running Rte 66 and crossing Death Valley so Lord knows the cooling system needs to work. I'll look to find an old GM unit at a swap meet some time. I only wish there was a Pomona meet the weekend I'm in LA.

Cheers,

[Gerard F.]

Re: Cooling System Questions

Mike,

I think your infrared readings are correct and your sender/gage is off. I went through the same thing when I replaced the gage with a repro (for cosmetic reasons) and tried at least 3 different senders (also after replacing the sender wire).

I finally matched the sender with a gage (I think the one from Lectric Limited). My test gage was a meat thermometer taped to the upper radiator hose. I keep it there (except for judging) and check it against the dash gage once in a while, they are usually right on the money.

Within the last year, I had my original radiator on my 67 327/300 reverse flushed, refilled with G-05, and put a new correct water pump on (first time off on the original). I now cruise at well below 190, maybe get to 200 on a hot day with the Vintage Air blasting. At a hot idle after a run it will sometimes get to 210, but then go back down after you get to speed.

Also make sure you are timed correctly, carb adjusted with no vacuum leaks.

Hope this helps,

[Michael A.]

Re: Cooling System Questions

Jerry
Thanks for the reminder on the timing and the vacuum leaks. I'll check those but am pretty confident it's in good shape.

I'm about to add the "California or Bust" sticker to my shopping list.
1 month to go to departure!

Cheers,