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Any big block Chevy experts?

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the Unforgiven
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PostPosted: Sat Jul 06, 2013 9:11 am    Post subject: Any big block Chevy experts? Reply to specific post Reply with quote

I've got a pump engine that is giving me fits. It runs real nice, might go twenty minutes, might go four hours, runs nice and cool but then the temperature gauge swings way up, then way down, then right back up and hits the peg to kill the engine. The needle on the gauge moves fast too, swings up and down like the needle on a tach. Start it right back up, hold the safety switch for about ten seconds and it's cooling off, throw the clutch back in and pull the throttle out and the temp goes right back down, but it will do this trick again, just a matter of how long. 454 on lp, drilled a hole in the thermostat, new big radiator, water pump is moving coolant like it should, it isn't throwing anything out of the overflow after the initial burp if you fill it to the brim when it's cold. The only time it will keep running is if I put in a gutted thermostat. Tried two different gauges and thermostats, absolutly no difference. Starting to wonder about a headgasket leaking a little but I can't understand how it could build enough of an air pocket near the thermostat and release enough of it all at once to act crazy. Been fooling with it for a week now, only have a couple handfulls of hair left to pull. Any suggestions?
 
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wijim
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PostPosted: Sat Jul 06, 2013 9:35 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

If you hadn"t added all the info I would have said head gasket. Jim
 
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jon f mn
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PostPosted: Sat Jul 06, 2013 9:38 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Check you water pump, I've seen them with the fins worn down to a point where they won't pump good but the seals are still good.
 
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1970-1655
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PostPosted: Sat Jul 06, 2013 9:40 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Possible the lower radiator hose sucking closed? Sometimes the outside looks good but the inner wall might have separated a little and higher flow pulls it shut some.
 
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Brad Buchanan
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PostPosted: Sat Jul 06, 2013 9:49 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

I don't know if it is helpful but I worked on some of those beasts back in the 80's and they were prone to carbon buildup if not revved up occasionally.

This would lead to some performance problems and in some cases they would start to knock.

GM used to sell a product called carbonx that worked pretty good. You poured it into the carb until the motor stalled, waitrd for an hour and then ran the heck out of it and the symptoms cleared up.

Brad
 
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WAYNE-N-TX
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PostPosted: Sat Jul 06, 2013 9:58 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

If it's head gasket, take radiator cap off and watch for bubbles while running.
 
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the Unforgiven
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PostPosted: Sat Jul 06, 2013 10:05 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

The water pump is fairly new and it is moving a lot of coolant. New hoses, they both have the spring inside so I don't think they are collapsing. I really don't think it is carboned up. What I can't get a handle on is why it heats so fast, we are going from 165 to about 210 and back to 165 and back up to 220 in less than 5 seconds, I don't think there is any way it could build heat that fast, it seems to be a bubble or flow problem, like the gauge is in a cool spot and is suddenly exposed to real hot coolant. And like I said it may take twenty minutes, may take four hours, same load. And it is not pulling much load, it's on a short pivot with a six inch well.
 
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the Unforgiven
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PostPosted: Sat Jul 06, 2013 10:07 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

And I might add, at least when the engine is cool enough to take the cap off and look in there are no bubbles.
 
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Janicholson
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PostPosted: Sat Jul 06, 2013 10:28 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Water pump impellers can be loose on the shaft!!
Use a infrared laser aimed hand held thermometer to make sure you are looking at reality. Sending unit? Jim
 
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the Unforgiven
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PostPosted: Sat Jul 06, 2013 10:42 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

I'm taking a temp gun out with me in just a few miutes. Two new sending units, same result. I will admit that I have not inspected the pump, but it always seems to flow real good. I just can't see it being the pump, if you reached out with your pocketknife and cut the fan belt clear off I don't think it could build heat that fast, we are talking from 165 to 220+ faster than you could close your knife and put it back in you pocket. Top of the radiator is hot, bottom tank is cooler.
 
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Dean
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PostPosted: Sat Jul 06, 2013 10:45 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Electric gage?

You have changed the sending unit?

Dean
 
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Gambles
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PostPosted: Sat Jul 06, 2013 10:46 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

I think you already have some good suggestions here posted by others that need to be checked out. I'm not a Chevy expert by any means (Ford blue in my blood), but I remember seeing a BB Chebby one time that had dual thermostats. I guess Chevy put that in there as a safety factor in case one of them would have a thermostat that stuck shut. Maybe some of them have had that problem before.
I know it's been replaced, but I'm leaning toward the thermostat or a related area, especially when it doesn't overheat if it's been taken out.
If it were mine, I'd use a portable thermal laser heat sensor and watch the hoses real close. I also had a Jeep that gave similar symptoms and it happened to be a cracked head.
 
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the Unforgiven
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PostPosted: Sat Jul 06, 2013 10:48 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Non-electric gauge, changed the gauge and sender twice, no change at all.
 
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Dean
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PostPosted: Sat Jul 06, 2013 11:15 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

Stationaly engine. Viscous fan drive?

Had it off lately? If so did you stand it on the drive hub or, as recommended, on the radiator end? If the former, delta T may have forced some or all of the silicone oil past the seal.

Verify that the thermocoil spring is in place, undamaged and that the shaft is not sticking in the housing.

If all OK, VERIFY proper operation of the centrifugal spark advance.

Dean
 
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Owen Aaland
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PostPosted: Sat Jul 06, 2013 11:26 am    Post subject: Re: Any big block Chevy experts? Reply to specific post Reply with quote

I'm guessing that will find that the temperature of the cylinder head is not changing anywhere nearly as fast as the gauge reading. The only way the temperature of the gauge can change that fast is you are getting an air bubble around the sender. You have a compression leak somewhere. I'd be will to bet that you will find combustion gasses in the radiator.
 
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