Any big block Chevy experts?

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?
 
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.
 
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.
 
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
 
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.
 
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
 
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.
 
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.
 
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
 
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.
 
I wouldnt call myself an expert but I know BBCs quite well.

Every suggestion so far is pretty good.

If I understand you correctly, when you come up to temp you start getting extreme needle swings on the mech temp gauge. Fluid does not change temp that quick. You almost have to have air entering the system somewhere. Assume you have Murphy switch.

At temp, if you have a compression leak (gasket) going to the coolant you should see bubbles in the radiator.

Is your temp gauge in the intake or mid head mounted? in the head is a more consistent reading.

A cavitating water pump impeller can create havoc on your coolant, especially at high rpm. But it can still do it in a low rpm application like your irrigation pump. I would go there first, especially when you say it runs fine with a hollowed out stat.

Have you been running this for a while and this just started happening?

I spent most of my time drag racing BBCs, but they havent changed in a long time. Only difference with drag racing the boom is usually louder and more catastrophic when the engine lets loose.

Rick
 
Have seen GM(Buick V8) but not BB chevy where a worn water pump seal would leak air INTO the system and airlock the heater. Changed the pump and all was well.
 
The lower tank on the rad should be cooler. However if the top is 220 or more the lower will be only maybe 30 less than the top if there is proper circulation. One thing I didn't see suggested is, the water pump running the proper direction for the pump and the engine. You say the temp goes up and down really fast, that has to be poor circulation or air pockets in the system getting to the sender area. Hope you get it fixed!
 
I suspect, since you changed senders and gauges,that your problem is a bad ground, as someone else also suggested. The temp will not fluctuate so rapidly as you describe; but a bad ground can cause a rapid change. I would look for loose or corroded connections, being that this seems to be an intermitant problem. Also if it is overheating, it would boil out the overflow I would think. I also would shoot it with the infra red; they are pretty accurate, I have shot around the sender before and got almost identical readings. You may need to add an extra grd or ohm out your grd from block to gauge/shut down and repair it. Let us know. Mark
 
no expert by any means but you said fairly new water pump. maybe wrong pump. Don't vortec 454 water pumps turn opposite direction from older 454s. just a suggestion.
 
I will still say headgasket, even after all the info. Combustion blows just enough air in to air lock circulation and that air is super heated combustion gases. Seen axact same thing happen on 3.4 GM v6. Exact same description. We changed gaskets and never a problem since.
 
Bad head gasket will certainly cause it to overheat, and build pressure in the cooling system. Thermostat cools the engine by closing and allowing the fan blades to cool what is in the radiator until gets hot enough in the engine, opens the thermostat, lets cool antifreeze into the block, sends hot into the radiator, thermostat closes, fans begin the cooling process in the radiator again. Not having a thermostat, or having a bad thermostat will cause overheating as well. Overall, thermostat is perhaps the cheapest piece in the cooling system, and the easiest piece to change, and should be the first item changed. I just did one in my 4520 Deere a couple of months ago. Was out plowing all day, and then out of nowhere, overheated. Changed the thermostat, not a problem since. Thermostats go bad. Running without one, one that is stuck open or closed, or a weak one is not a good idea.

Good luck.

Mark
 
I apologize. I just went back and reread your post and see that you changed the thermostat twice. But you also said that it onlt runs with the thermostat that you drilled a hole in too. That's not good. Are you sure that the thermostats that you tried are good? 195 degree thermostats? 160 degree do not allow cooling down properly, especially in the higher temperature electronic ignition egines. You could still have a bad head gasket or a crack somewhere, like a head, BUT that will cause overheating, and you WILL see built up pressure in the cooling system. If it is NOT really overheating, you could have a monitoring/metering problem that is causing some sort of safety to shut down. I guess that you really need to figure out if it really is overheating or not. Not losing coolant like blowing it out the exhaust? Not overheating because not leaking, losing, or blowing coolant into the overflow?

The thing that gets me, is that you say it will only run if you insert the thermostat that you drilled a hole in as a bypass. That's not a metering problem. That would suggest to me a need to relieve pressure and force cool coolant into the block, and I would expect to see physical overheating beyond the meter telling you that its hot. It also suggests a bad thermostat that is not opening, and now uses the hole you drilled in it to flow coolant at the same time it doesn't allow the radiator and fan blades to do their jobs. Those two thermostats you tried, you are sure that they are both good 195 degree thermostats? Try flushing the system, another new radiator, and if it still OVERHEATS beyond just saying that its running hot, go after the head gaskets IF you are sure that the water pump is good, and it sounds as though it is.

Good luck.

Mark
 
"...try...another new radiator...".

Sorry, try another new thermostat (not radiator) after flushing, and then if it overheats, you got something bigger going on.
 
I'm not an expert by any measure, but my I'd look for a poor connection, broken wire, bad ground, etc. first. As fast as that needle's moving, my best guess is an electrical problem.

Incorrect timing or plugged exhaust can cause overheating also. Had an old Dodge big block truck years ago, don't recall what I was doing to it but I severely retarded the timing. Ran smooth, but was gutless as all getout and exhaust manifolds were cherry red.

Had another one that would overheat intermittently, assumed it was a cooling system problem, went over that part several times but found no issues. Finally discovered that the flapper valve in the driver's side exhaust manifold(the one with the thermostatic clockspring just above Y-pipe connection)was occasionally sticking shut.

Good luck!

Anthony
 
Certainly no expert on anything, but I have tinkered with engines for many years.

I think I would try a mechanical temp gauge to see if coolant is actually heating up that fast. Sometimes electrical gauges go screwy.

I also think there is a test kit you can get at NAPA or other large auto stores to test coolant for combustion gasses. Using such a test would be a whole lot easier than removing the heads to test for bad head gaskets. If there is no evidence of combustion gasses in the coolant, I would bet that the head gaskets are OK.

It has been a long time since I touched a Chevy big block and I don"t remember if they have problems with air becoming trapped in the block. Some engines have a plug or plugs that you need to remove to get all the air out after you disturb the cooling system.

I also would wonder if somehow the LP carb was leaning out the air/fuel mixture which would make the engine run hot. No experience with LP systems.

Is there a computer involved? Or is it a simple electric shutdown switch if the temp reading gets too hot? Checking the wiring is a good idea, but breaks or partial breaks are not always apparent. Oil OK? I know that oil seems to last forever in an LP engine.

As someone else posted, I have had an impeller come loose on a water pump shaft, but once it came loose, I had no coolant flow, not intermittent heat readings. It got hot within a mile or so. I really doubt that is your problem.

So I would suggest verifying that the engine is REALLY getting overly hot and if it is I would check for combustion gasses in the coolant. Hope you can find the problem and it isn"t too expensive or hard to fix. Good luck!
 

Are you using the correct thermostat(s)?

Some industrial-type engines require a thermostat with a plate at the bottom that closes a
"recirculate" or "bypass" passage as the main part of the thermostat opens.

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http://www.stant.com/consumer-products/abcs-of-thermostats
 

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