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Old 05-28-2009, 03:51 PM
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Weird Thermostat problem on SBC

Just got the new 350/330 HO crate engine in my 29 RPU and running and now have a thermostat problem I've never encountered.

When the car is cold (first start of the day or after total cool down), the temp gauge goes to 250 and stays there a short while before the thermostat opens, and then it slowly oscillates between 225-250 and 185 where it belongs.

A little more info...

Pulled out the old 305 which had no heat problems, even used the thermostat from that engine. When the first startup did the overheat, got a new thermostat and tested both in boiling water with thermometer and both were working fine.

New thermostat goes in and same problem.

All help/suggestions appreciated.

Paul

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Old 05-28-2009, 03:57 PM
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what kind of temp gauge?
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Old 05-28-2009, 04:01 PM
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Agree could be the gauge is off. Cole
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Old 05-28-2009, 04:24 PM
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Sounds like a gauge problem not thermostat. Where is the probe installed?
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Old 05-28-2009, 04:30 PM
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Quote:
Originally Posted by Ziggy1805
Just got the new 350/330 HO crate engine in my 29 RPU and running and now have a thermostat problem I've never encountered.

When the car is cold (first start of the day or after total cool down), the temp gauge goes to 250 and stays there a short while before the thermostat opens, and then it slowly oscillates between 225-250 and 185 where it belongs.

A little more info...

Pulled out the old 305 which had no heat problems, even used the thermostat from that engine. When the first startup did the overheat, got a new thermostat and tested both in boiling water with thermometer and both were working fine.

New thermostat goes in and same problem.

All help/suggestions appreciated.

Paul
Check to be sure the system is really purged of air, drill a small hole in the thermostat to help it burp. The oscillation in temp makes me think this has more trapped air in there than you might think.

Is there a coolant bypass on this engine? Many guys, especially the Roadster guys, tend to eliminate this for architectural reasons, but the SBC can be cranky without it.

Bogie
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Old 05-28-2009, 05:26 PM
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Quote:
Originally Posted by oldbogie
Check to be sure the system is really purged of air, drill a small hole in the thermostat to help it burp. The oscillation in temp makes me think this has more trapped air in there than you might think.

Is there a coolant bypass on this engine? Many guys, especially the Roadster guys, tend to eliminate this for architectural reasons, but the SBC can be cranky without it.

Bogie
I have that same engine and had the same issue. Replaced the stock thermostat with a Milodon high flow and the issue went away. The cooling system circulating pressure holds the thermostat closed. I had wild swings from 140 to 250.
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Old 05-29-2009, 07:44 AM
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To answer the many useful replies:

VDO gauges (same gauges and sender unit as in old engine, which worked fine),

Thermostat has been drilled.

Not running a bypass as the drilled thermostat is "supposed" to act as a bypass.

I will look into the "Milodon high flow".

One thing I failed to mention, changed the water pump to Edelbrock aluminum high capacity. That is the only difference from the original setup.

Thanks
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Old 05-29-2009, 09:30 AM
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Quote:
Originally Posted by Ziggy1805
To answer the many useful replies:

VDO gauges (same gauges and sender unit as in old engine, which worked fine),

Thermostat has been drilled.

Not running a bypass as the drilled thermostat is "supposed" to act as a bypass.

I will look into the "Milodon high flow".

One thing I failed to mention, changed the water pump to Edelbrock aluminum high capacity. That is the only difference from the original setup.

Thanks
I use the Edlebrock pump on most projects and have had nothing but excellent performance from it.

The drilled thermostat does act as a by-pass. The problems are not much of a bypass as the holes are pretty small and the thru the radiator flow increases warm up time.

Typically when you get these oscillating temps, it's a pretty good sign that there is trapped air in the system. I would think with a drilled thermostat that with careful filling that wouldn't be a problem. But I'm always open to surprises.

An other option to consider is that radiator can't handle the flow and the system is surging.


Bogie
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Old 05-29-2009, 12:47 PM
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Quote:
Originally Posted by oldbogie
I use the Edlebrock pump on most projects and have had nothing but excellent performance from it.

The drilled thermostat does act as a by-pass. The problems are not much of a bypass as the holes are pretty small and the thru the radiator flow increases warm up time.

Typically when you get these oscillating temps, it's a pretty good sign that there is trapped air in the system. I would think with a drilled thermostat that with careful filling that wouldn't be a problem. But I'm always open to surprises.

An other option to consider is that radiator can't handle the flow and the system is surging.


Bogie
I agree with your logic except the radiator flow. *ONCE* the temp stabilizes after 5-10 mins, there is no problem at all and the temp stays right at 185 even sitting still on a hot day. I would think that would eliminate the radiator as a problem source.

Paul
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Old 06-04-2009, 08:39 AM
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Quote:
Originally Posted by T-bucket23
I have that same engine and had the same issue. Replaced the stock thermostat with a Milodon high flow and the issue went away. The cooling system circulating pressure holds the thermostat closed. I had wild swings from 140 to 250.
T-bucket23, you're my new hero!!!!!

Just finished installing the Milodan thermostat and it *DID* solve the problem. Also with that thermostat there is no need to drill it as it has been modified to bypass a bit.

Thank you mucho!
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