Sporadic BBC overheating issue question.

I'm having a cooling issue with my motor. It's acting like I have a sticking thermostat, but I don't think I do. The temp gauge will shoot up 250. I'll shut the motor down and start it back up and it instantly drops to 190. It will even run at 190 for a while then shoot back up again.

This problem started toward the end of last year. My truck would rarely run over 220. I was driving down the freeway and the temp went up to 260. I stopped and pulled the thermostat and drove the rest of the way home no problem.

I put a new thermostat in and tried driving the truck into town. That's when it started the get hot and I would shut it down and then refire it and it would cool down instantly. I figured it was just an air lock and it was the end of the season so I didn't mess with it.

Now fast forward to this year. Over the winter I moved the radiator to the bed. I'm still having the same exact issue. Again I thought maybe an air lock because of the long lines. I took it down the road today and 2 times I had to shut it down and restart it. It also ran right at 190 for a mile or 2 and then back up to 250.

I've put the thermostat on the stove and it works perfectly. To me it seems like it's not circulating the coolant. I'll have 215 at the sensor and the top hose is 150 and at the radiator 100.

I don't get it, had any body else had an issue like this? Is it possible I have a bad water pump?

 

Is it possible the belt is slipping at highway speed ? Would it cool down if you just pull over and let it Idle ? Seems strange it cools down when refired, maybe impeller slipping on shaft.

 

No it won't cool down just pulling over. You have to shut it off. The impeller slipping makes the most sense to me. I'm going to try grabbing a new water pump today.

Anybody else have any input?

 

Have you looked into the radiator while its running (t-stat open) to see how the coolant is circulating? It should be moving fairly fast. The water pump is one of the least expensive components to replace, if it's been on for a while, give that a try.

Bob

 

That can't happen in the real world.

I'm guessing you have an electric gauge that is malfunctioning. I'd suspect the sending unit and replace that first.

If you broke a belt, it over heated and you fixed it, it would take a few minutes for the system to come back down to the normal operating temp as the cooling system removed the excess heat. You are taking heat from the engine and transferring it into the air. That takes some time and is not instantaneous.

 

Sounds like you have air in the system, big blocks tend to trap air in the upper rear corners of the heads, could be one issue. I think you have a bad head gasket pumping air into the cooling system. Wheres you temp sensor, up in front of the manifold, or in the center of the cylinder head? Easy way to check head gasket, take thermostat housing off, leave motor full of water, disconnect water pump belt and start motor. You can see bubbles coming up, and from which side.

 

Drilling a 1/8" hole in the thermostat aids in removing trapped air and solves quite a few cooling problems.

 

No I haven't but checking the temps at different points around the system. It doesn't seem like it is circulating. The water pump is a new Edelbrock thats's why I was thinking thermostat first. But really think it might be the problem.

It's not really instantaneous. I would say it instantly starts cooling down but it takes a minute to come back down to 190. I don't think it's sender because I've checked temps around the system. Pointing the gun ay the sensor it's at 215 and the top hose was 150. Which is just a few inches away.

I've heard that BBC's can do that. Any tips for getting it out? I also thought maybe I have a bad head gasket. It doesn't have any other symptoms of a bad gasket though. I haven't tried a compression test yet. But the oil looks good and I pulled the plugs and don't see any signs of coolant in the cylinders. Plus it's not down on power at all.

I've never heard of pulling the thermostat housing and checking for bubbles I'll give that a try.

Temp sensor is in the manifold by the thermostat.

 

I've got a couple holes drilled all ready.

 

If it can go from 250* to 190* in a minute and then you can drive it,I'd look at the gauge and sender.

 

They usually get HOTTER when you first turn them off. My vote is something gauge related too.

Bob

 

I think I have the answer to your problem. I used to have a 427 in a 3w years ago and had this exact same problem...drove me nuts until an old drag racer looked at my motor and said "You need to put the water pump bypass hose back on it".

It's the 90 degree rubber hose that goes from the pump to the intake manifold. The guy I bought the car from left it off for asthetic reasons...I reinstalled it and had no more problems...hope this helps.

 

Do you still have the bypass between the water crossover under the thermostat and the water pump?

If you have removed it, or in the case of some pumps like Stewart, they don't have them. You need some sort of bypass to keep the coolant circulating to prevent hotspots and steam pockets on BBC's, I know!

My twin turbo BBC did almost exactly what you describe. The issue didn't present until I changed my waterpump and drive system to the Vintage Air front runner. They described the 1/8" hole in the thermostat and that did not prevent the steam pockets. I did a de-gas bottle similar to a late model Mustang (others use it too, like VW/Audi) and it killed the temp swing and steam pocket problem dead!



The line in the top hose is a bleed line to purge air from the radiator, the other line is coming from a port just under the Moon temp gauge. The heater return is also plumbed into the bottom of the degas. The feed line for the heater just above your lower hose that would normally be the heater return is also plumbed to the bottom of the tank.

The way this works is the steam pocket can travel through the -6 hose from the water crossover directly into the degas bottle and condense inside without having to go through the thermostat. This keeps the radiator completely full and only circulates coolant through when needed.

 

Check to see if the lower hose is collapsing. When it overheats, stop and look at the lower hose before you shut it off. Cheap and easy thing to check and eliminate as a cause.

 

3w larry has the fix, by pass stops cavatation in block and manifold at t stat works everytime.

 

Thanks for all the help guys. I figured it would be something easy. I'm going to put the bypass on and see what happens.

El Palacko is there a reason you did the de-gas system? Does it work better or is it because your pump doesn't have a tap for the bypass?

 

My radiator is a double pass and doesn't burp out air very well. With yours in the back I could see a similar problem.

 

As I reread your post, the first time it overheated was going down the highway. Then driving into town. Larrys bypass thought is a good one, but when shes hot like going down the road the thermostats open, no bypass water flow anyway. Compression test wont show a minor head gasket leak. I bypass water from the rear corners of the intake to the front corners, but you can run lines to the thermostat housing also. The water passages in the head dead end at the rear of the intake, and thats usually where the air gets trapped.Gotta drill and tap if your intake doesnt have bosses cast in, look at the edelbrock rpm air gap for ideas.I do this on our race motors, no bypass, and they get hot, but dont have air pocket issues at all. Keep trying, good luck.

 

Would have saved a lot of guessing if you mentioned in your first post that you removed the bypass hose....

 

I could be wrong but I think it overheated going down the freeway, because I was caught in traffic and it got up to 250-260. I was just around the corner from where I was going and shut it off shortly after. My thinking is that it getting hot cause a steam pocket or airlock. So when I took off again and got on the freeway it overheated.

I got a chance to talk to my engine builder yesterday. He told me to do the same thing about running the lines to the back of the intake. I figure I'll run the front one first because it'll be easy to try and see if it works. If it doesn't I'll pull the intake and drill and tap it, so I can run the ones to the back.

That's funny because if I would of know the problem was not having a bypass, would I have asked the question? I guess I should of tried messing with it all summer instead of asking questions. The ironic part is you took the time to post on this thread and didn't add any insight.

Plus asking my question I learned what a degas was. What a novel idea asking questions to learn things. Another option if the bypass doesn't solve my problem.

To everybody else thanks for the help and I will reply back to let you know if it fixes my problem.

 

I just wanted to report back. I threw a temporary hose on last night. I drove up to the gas station a few miles away. The temp came up and never spiked. It looks like problem fixed.

 

Good deal then! I always used that hose on my big blocks, kind of odd that it would get left off.

Bob

 

good news...now wait till you get my bill...

 

Another problem solved by the HAMB!

 

Larry, what's email so I can Paypal it to you.

Absolutely on that problem, but I think you were right on the bad head gasket. I put new plugs in it before I took it out last night. It seemed a little lean so I just pulled the plugs out to take a look. The number 2 plug looks like it never fired. Which I believe is the coolant cleaning it off, right?

 

Problem solved, and you only got kicked in the nuts a couple of times !! Good luck and i dig the hot rod truck.

 

Wire or cap is the problem, check ohms on wires if its only one cylinder

 

Could be, I know its a pain, but try my thermostat trick. If there is any slight head gasket leak, you will see tiny bubbles, and from which side.Dont run the motor more than a few minutes with the pump not spinning. I still think thats the root of your problem, I hope im wrong for your sake. Good luck.

 

It would take a lot of coolant to drown out the plug and you'd see it on the plug itself. Have you checked for combustion gases in the coolant?

Bob