Fluctuating temperature. Solution?

**Buick2** · 07-03-2013, 07:28 PM

OK Team Buick. One more try on this problem.

I have a '67 340, recently rebuilt. It runs great with one problem, the temperature fluctuates within a range of as many as 30º!

Description: After starting the engine cold, it warms up to 180º and then continues to ca. 190-200º for a few moments and then begins to cool down, sometimes to as low as 165-170º. This all takes place in around 45 seconds to a minute, idling or driving around town. At 60mph in overdrive it heats up to nearly 210º.

Engine is in a hot-rod without power steering or AC. New Walker radiator. My old 300 ran fine in this vehicle.
Here's what I have tried.

1. I have installed three different thermostats one of which was a 160º. No difference.
2. Installed a new temp. gauge sending unit. No difference.
3. Lower radiator hose has anti-collapse wire coil.
4. Flushed radiator and block. No difference.
5. Fan is on correctly. Stock 17" 4 blade.

Any thing I have forgotten? I'm stumped and nervous about getting too far from home.

Buick2

**65specialconvert** · 07-03-2013, 08:54 PM

Originally Posted by Buick2

OK Team Buick. One more try on this problem.

I have a '67 340, recently rebuilt. It runs great with one problem, the temperature fluctuates within a range of as many as 30º!

Description: After starting the engine cold, it warms up to 180º and then continues to ca. 190-200º for a few moments and then begins to cool down, sometimes to as low as 165-170º. This all takes place in around 45 seconds to a minute, idling or driving around town. At 60mph in overdrive it heats up to nearly 210º.

Engine is in a hot-rod without power steering or AC. New Walker radiator. My old 300 ran fine in this vehicle.
Here's what I have tried.

1. I have installed three different thermostats one of which was a 160º. No difference.
2. Installed a new temp. gauge sending unit. No difference.
3. Lower radiator hose has anti-collapse wire coil.
4. Flushed radiator and block. No difference.
5. Fan is on correctly. Stock 17" 4 blade.

Any thing I have forgotten? I'm stumped and nervous about getting too far from home.

Buick2

Do you have a fan shroud? Your temps aren't that bad,real close to my mild 340.

**Buick2** · 07-03-2013, 10:41 PM

Originally Posted by 65specialconvert

Do you have a fan shroud? Your temps aren't that bad,real close to my mild 340.

No fan shroud. As you can imagine, there is no dearth of air circulating around the (1934 Ford style) radiator.The puzzle part of it is that the 300 ran fine and stayed at a steady temperature. I even ran without a fan as long as I wasn't anticipating any traffic complications. Maybe I'll just grit my teeth and run it for a while and hope that it settles down. I guess I could get used to it.

Thanks for the suggestion

Buick2

**Bob** · 07-04-2013, 07:07 AM

For what it's worth, I don't think the initial temperature fluctuation is of great concern, but the high temperature when cruising could be. The old combination may have been satisfactory, but the 340 may build more heat.

I expect the motor is not an old junker, and may even have some performance improvements? Horsepower does have a relationship with heat, regardless of cubic inches.

**Robert Hare** · 07-10-2013, 01:13 PM

where is the temp sender located, is it a mechenical or electric device???

**Buick2** · 07-10-2013, 03:26 PM

Originally Posted by Robert Hare

where is the temp sender located, is it a mechenical or electric device???

Robert,

Temp. sender is located at the front of the intake manifold just above Cylinder #2. This location is slightly different than on a '64 300, but not enough to indicate the change in temperature behavior. It is an aftermarket electrical sender and gauge (Auto Meter) and worked fine on the previous engine. I replaced the sender thinking that it might be faulty, but no difference. What did you have in mind?

I'm thinking that the bypass hose might be plugged or collapsing.

Buick2 in Oregon

**Robert Hare** · 07-17-2013, 08:48 AM

Sounds like there is no circulation until the thermostat opens then it sends hot coolant to the radiator at that time cool coolant is pulled into the engine dropping the temperature. The by-pass hose allows the engine to warm up as the heat builds under the thermostat the heater hose connections do the same job.
If the by-pass hose is missing or cloged You will have this problem.
Try adding a hose to the heater connections from the rear of the intake manifold to the water pump.
I have found that drilling a 1/8 hole in the thermostat and orenting it to the top fo the intake manifold aids in purging air and aids in warm-up of the engine as it introduces a small amount of coolant to the radiator.
I had this problem in a 50 ford with a 425 nailhead in it the heater hose with a restrictor in it solved the problem, that was 35 years ago.
Good luck, keep us posted on your progress

Bob

**Buick2** · 07-17-2013, 01:44 PM

Bob,

Thanks for the reply. I have everything apart including the removal of the intake manifold. I have checked as thoroughly as possible all water passages, confirmed that the by-pass hose is clear and still rigid enough to avoid collapse and even went so far as to thoroughly check out the water pump and all that. I had drilled holes in the previous thermostat flanges that I had on the engine and there was no appreciable difference, with or without.

Someone in an earlier exchange, and it may have been you, suggested connecting a hose to the 2 unused heater hose connections. I'll do that when I get it reassembled. First I'm going to install a mechanical heat gauge in the rear heater hose connection location and see if the readings, when it is running, are significantly different than those indicated by the electric temp. sensor in the stock location.

A 35 year old solution may be the key. That's pretty recent history when it comes to hot-rods and such.

Thanks again for your thoughts. I'll let you know what happens next.

Rich (Buick2) in Oregon

**Bob** · 07-17-2013, 04:38 PM

I have a digital temperature gauge with a wire that runs to a remote sensor. I use it for air conditioning. If you had something like that, you could attach it to the lower area of the radiator to see if the water coming back into the engine was cool when the engine was running hot. If the water was not cooled enough, it could mean your radiator was too small.

You haven't described the hot rod, or what radiator you are running, but if it is the original vertical flow from say a 40's car which only had 150 hp and you are now trying to cool a 340 with may 300+ this could be related to the problem.

Sometimes it is also a good idea to pull the block drain plugs and make sure there is not excessive rust build up, although I don't suspect that is your problem.

**Buick2** · 07-17-2013, 04:50 PM

Bob,

I'm running a new Walker radiator and an 18" 4-blade fan. It cooled the previous engine (300 ci Buick) just fine. That is why this is such a problem. Changing to the 340, a few more cubic inches, not much more horsepower, and no aluminum, doesn't predict fluctuating temperature. The 340 block was thoroughly cleaned up, washed, soaked, blown out, and everything that a good machine shop would do, before it was rebuilt.

I'll check various temp. readings when it is running. Just need to get it all back together now.

Rich