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Hello All,
Im running into an overheating issue so I wanted to run some things past you folks. The car is a 69Z restored to original and has been completed for about 1.5 years. I don't have too many miles, sadly, on it since its been done (~250 miles). I have a console with the gauge pack and it used to run in the middle initially, a few months back it would creep up to 3/4's of the gauge then drop back to the middle and run there. Recently I noticed my water pump was leaking so I replaced it with a remanufactured one from NAPA and shortly after I noticed that the car will overheat - i.e. it would go just up before the hot section of the factory gauge then I would shut it down.

As I mentioned since I replaced the water pump, I thought that this could be bad out of the box but I noticed that the heater hoses and box get hot when running. So I assumed that the reman pump was circulating. Am I incorrect in assuming this? Do they really go bad (impeller) out of the box?

I then thought that either the thermostat was bad or I had air in the block that would not allow the water to contact the thermostat. So I removed the thermostat and placed it, as well as a new one, in boiling water to check it. Both worked fine. So before I put the new thermostat (195F) in I put coolant directly into the intake manifold where the thermostat goes until it was at the opening or just below it. My thought was that this would ensure that there was no air in the block. Once I got it back together, the temp went all the way up to just before the hot section again. During the run I checked the hoses and the upper and lower radiator hoses were not hot at all. But the heater hoses and box still got hot - again assuming I had circulation. So I put an IR gun on the thermostat housing/manifold area and it seemed that the manifold was still below 195F but its hard to tell since the fan was blowing on it. It was probably reading at 180 - 185F but by this time the temp gauge with up there.

So Im at the point where it seems that it could be that my temperature sensor could be bad (however this was replaced when the engine was done a few years back). Is there a way to check it with a volt meter? Or there could be a blockage in the radiator or hose lower hose - note: I checked the upper hose (internally for any obstruction or damage) and not allowing the coolant to circulate. These are new with the engine as well. Or am I wrong in assuming that the newly replaced water pump is good given the heater hoses getting hot.

Any thoughts or insights?

Thanks in advance,
Pete
 

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Possibly your gauge is bad.
195F is too hot for that car.
I run a 160, and my heater gets plenty hot.
180 is what it had from the factory.

Here is what I am betting. Your gauge is a little off, and the 195F just makes it look even worse.

You say it is restored to original. Does that include the 11:1 compression? If so, the 160 would help combat any detonation.
You can put a temporary aftermarket gauge on it to see what it is really doing. I have a cheap RAC (aftermarket) mechanical gauge I use for things like this. If you want to borrow it, I can mail it to you and you can mail it back when you are done. Priority mail would only be about 6 bucks each way. If you are lucky you have a buddy close who will let you borrow one.
 

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Its possible the impellor could have gotten loose but its not normal. They are pressed on, I never like the newer or even rebuilt units as they all have open impellors compared long ago when comparing to a GM pump that had cast closed impellors, think if it as cavitation . I started putting on what they call a FlowCooler disc that rivets to the back of the impellor , of course you have to take the backing plate off. Plates are sold or were sold thru Summit or jegs. But this is just a trick to help move water and not allow cavitation in the pump.

Now I am not sure why you want to run a 195° t-stat but you can get down to 170 or so.
I always drill a couple of 1/8" holes on each side of the t-stat flat plate, this allow water or air to move out , before the t-stat opens.
To keep from handling hot water, I would pull off top radiator hose and fill with a hose or the sort. I would take off the heater hose coming from the heater box to the intake, do the same . Once you feel you got all the air out, then start motor , leave radiator cap off until you see water moving across the radiator, once that's done you can put cap back on.

I think the movement in your temp gauge you see is actually the t-stat trying to open as you see a rise and then a fall of the needle, but if there is air in that manifold, the t-stat will not respond until hot water hits it.
 

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If the stat is opening it really doesn't matter if you run a 160, 180 or 195 if the cooling system is allowing the temp to go higher than the stat rating. 195 is not too hot for a DZ302 or any SBC for that matter. I personally like mine to run 180-185 but 210 will not damage the engine. If the factory temp gauge is accurate straight up center is 210, the 1/4 mark is 180. Also 195 stat was standard in most old '69 Chevys from the factory...

Your first described symptom; going to 3/4 on the gauge and dropping back is probably a stat opening late, it opens and the cooling system drop the temp as the fluid flows through the core.

When your upper and lower hoses are not getting hot with the engine then your stat is probably not opening. To test if it's the stat or a bad pump pull the stat and run without it for a test run. If it still gets hot and the hoses don't warm up then it's the pump. If the engine doesn't over heat and the hoses heat up from the flow it's a bad stat. Make sure you are installing the stat the right way and get a Robert Shaw (sold as Mr Gasket) T-stat...
 

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Just thinking out loud... Something I always do is leave the radiator cap off while the engine is running and your topping it off after work has been done. I kind of got use to doing that when I was in my Pontiac phase because those engines always seem to trap air in the block. That's how I bought my Judge from a guy who was fed up with it over heating all the time. When the rad is filled up and the cap is off does it want to overflow any while the engine is running?
 

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PJ - So technically you are not overheating - puking coolant out onto the garage floor? You are just worried that the gauge seems to be reading higher then it did in the past?

If your pump is flowing the correct factory rate, the thermostat is opening and closing the fan and the shroud are in working order, and the radiator is not full of gunk or blocked air flow the cooling system should be running okay.

So I would test the sender for Ohms readings to determine if it is in working order.
 

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Hey Brian my temp gauge always stays at 1/4 and I was not sure if it was working correctly or not, but now it seems like it is by the sound of this thread. It goes right down to zero, all the way to the left when the key is off. Then it moves up to a 1/4 and never goes higher. I was suspecting that it was not reading correctly and should be reading higher. Good to know that 1/4 is correct.
 

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Discussion Starter #9
Thanks to everyone for their suggestions and insight. So, I did what DjD suggested and pulled the thermostat, then ran it. Shortly after I fired it up the upper and lower radiator hoses heated up so I did have circulation. The test made sense and worked like a champ. I figured the water pump was good given that the heater hoses where getting hot but this was a great way to confirm it.

Now that I've checked my thermostat, circulation and water pump, the last thing on the list is the temperature sending unit. With that being said, Im actually heading out the door now to pick it up. Hopefully that's the problem.

Thanks again,
Pete
 

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My 230 L6 temp light comes on when idling in traffic on warm days. I'm about to replace the thermostat as a preventative measure. Climate is moderate here, should I also go with the 160 or go with original temp? Kicking myself for not replacing it when I replaced the hoses just last month and back-flushed the radiator, but I was being conservative (ain't broke, don't fix it).

Additionally, I'd read about a process some use and swear by on their 80 year old cars involving running warm with what I remember to be muriatic acid for 20 minutes to get out calcium deposits. The mechanic who back-flushed the radiator didn't recommend flushing the engine in the event it broke any sort of sludge or debris loose to cause havoc. It seems this falls in the same type of category. Any thoughts on that?
 

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Mark - IMHO you should use the factory spec as the design of the cooling system accounted for that thermostat. But you can test it on the stove. I would also consider the temp light sender maybe failing and that can be tested as well with a multi meter and take measurement of the Ohm's at a hot state.
 

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I am going to agree with the mechanic, no muriatic acid , this is what the radiator shops use to clean a radiator when off the car and tanks are off and rodded thru ( flat blades that fit the cooling tubes and open up any restricted tubes) , if any leaks show up after the treatment, tanks are soldered back up, hopefully the cooling tubes are not too thin .

I would just flush back wards thru the bottom hose , removing the t-stat to allow a free flow .
There are flush products on the market that are not as strong as muriatic acid. They are added and allowed to flow thru the system , then dump. Replace with a good corrosion inhibitor and coolant mix.
 

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My 230 L6 temp light comes on when idling in traffic on warm days. I'm about to replace the thermostat as a preventative measure. Climate is moderate here, should I also go with the 160 or go with original temp? Kicking myself for not replacing it when I replaced the hoses just last month and back-flushed the radiator, but I was being conservative (ain't broke, don't fix it).

Additionally, I'd read about a process some use and swear by on their 80 year old cars involving running warm with what I remember to be muriatic acid for 20 minutes to get out calcium deposits. The mechanic who back-flushed the radiator didn't recommend flushing the engine in the event it broke any sort of sludge or debris loose to cause havoc. It seems this falls in the same type of category. Any thoughts on that?
As Dennis noted, once the t-stat opens up, it doesn't matter if it is a 160 or a 195.

If you are overheating while sitting still, it usually means poor air flow.
 

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Do not forget the radiator cap. If this is bad, and they do go bad,
the engine will overheat repeatedly. Be sure to purchase the correct pressure rating. If you are not boiling over you are not overheating. As previously stated the gauges and senders do go bad.
 

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Thanks for the tips. Already has a new cap, old one bad. The fan doesn't look to have a clutch on it. But there is no shroud. Should there be one for the standard L6? If so, pointers on where to get one? Hopefully pulling thermostat next weekend.

thanks,
Mark
 

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Thanks for the tips. Already has a new cap, old one bad. The fan doesn't look to have a clutch on it. But there is no shroud. Should there be one for the standard L6? If so, pointers on where to get one? Hopefully pulling thermostat next weekend.

thanks,
Mark
L6? I thought we were discussing a 302? If it is a 302, should have fan and clutch as well as fan shroud.
 

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The talk was 302, until the thread was hijacked...
one small question leads to another and another and before you know it, hijack! I've got so many questions, I should make a list and post them once a week for the next year. The cars not going anywhere.
 
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