I just put in a new Longblock 350. All same external parts, except new water pump, thermostat, and starter. Almost immediatley the engine overheated. I took it bak to the shop who installed. They could not figure it out. They did say they thought it could be anything from radiator to intake manifold. The old 305 didn't overheat why is the 350? I installed a new radiator and thermostat-overheat. I replaced the water pump- overheat. I'm at a lose. Any ideas?? Yes the electric fan is working blowing, sucking in air over the engine. Temp running at 240-250.

 

is it overheating while driving or during idle?

 

More info like said, but random thought. Do they have the timing right?

 

As far as I can tell only whle driving. We haven't just let it sit there and idle long enough to see if it over heats. However I will go out and let it idle for a while and see how long it takes to get to 200 or above. I know driving it it doesn't take long maybe 5-10 minutes to hit 240. Next thing Ill change out the intake manifold and see if gets better coolant flow.

 

Mark, As far as I know they do. Car seems to start and idle just fine and runs just fine. Just overheats.

 

Let us know if it overheats while idling.

What's the condition of the lower radiator hose? Does it collapse under load?

they didn't take the air dam off by chance when installing the motor and not put it back on? this would only cause overheating while driving

 

Mark, I hate to sound like an idiot, but what is an air dam and where does it go? I would hope they didn't leave anything off. Do you think the intake manifold could be the problem? Shop folks thought because it doesn't have rear ports it may not be getting enough water flow.

 

the air dam is under the front of the car. usually bolted to the lower radiator support.

 

best I got for you, pic of my old car.

in front of the front tire, almost looks like a small shadow, but it's a piece of black plastic that hangs down.

it's job is to force up air while driving

 

Mark, Thanks. I ran the car today at idle. At 15 minutes temp gauge was already up to 240. I shut it down at that point. I'm just going to take it to the chevy dealer and see if they can figure it out. Im too much money into this car now. I may replace the intake manifold first and see if that helps. I just read where a clogged catlytic converter could cause this problem too. Although it never happened before with the 305.

 

Quick need advice. Of course with all the stuff I have done above. someone said something about the rotation of my waterpump. I know it is turning in a clockwise fashio, same as the engine. It is connected with a serpentine belt around the Alternator(on passenger side of engine) around the water pump and around the crankshaft pully. Looking from front of engine to back the water pump has a hose on the lower left side which connects to the lower left corner of the radiator. the upper hose is to the Thermostat. Is this the correct rotation? I didn;'t evn bother to look at the fins in the pump. Just assumed when I requested the pump from my parts store they would give me the correct rotation pump. Just a question. thanks to all in advance and to those who have responded. It has helped greatly.

 

Sounds like you might have a faulty thermostat or you have air in your system.

 

Check Hoses see what temp fan is coming on take thermostat out

 

any updates?

 

Mark, Thanks. Only update I have is I have R&R the following: Radiator, Radiator cap, waterpump, thermostat. And after each replacement we drive it and it overheats. Like a dummy this weekend I took my mechanics advice and took off the intake manifold thinking it might be the problem. It wasn't. So now I am going to run it to temp take heat readings of both upper and lower radiator hoses and the heads as well. Then get with the Tech at S&J Engines who built this longblock. I heard I need an air dam but it overheats at idle and driving. Next is to take it to a chevy dealer. Too much money invested to quit now.

 

OK did the heat test. Both upper and lower radiator hoses about 260. couldn't really get to the back of the heads, but the exhaust manifold was at 260. My friend felt that I have a head gasket leak. He did a pressure check on the radiator and it never got to 16PSI and the needle fluxuated (bounced) Also when the car was started it it pumped coolant out the cap with the cap off. Doesn't seem like it should do this. this as done prior to the pressure test. I think the next thing is too R&R the head gaskets. Any thoughts????

 

with both radiator hoses at 260*,that indicates the
radiator is not dissapating the heat-normally the lower
hose will be quite a bit cooler.When the fan runs can you
feel air being blown back toward the engine and is that
air warm or hot?
At least in my experience,head gasket problems are rare
with chevy and each time i have had a blown head
gasket(imports)they did not overheat,but they did
blow steam out the exhaust and miss from the spark
plugs shorting out

 

Well we don't have any steam or smoke coming out of the exhaust. Doesn't sound line it's missing. The fan when running is bliwing towards the engine and i dont think its hot but its warm What would cause the coolant to spurt out or the opening when the engine is running?

 

make sure the fan is blowing the ai towards the motor not towards the front of the car we had someone put on a flex fan backwards and it was pushin air towards the rad. not thru it just athought it should be pulling air theu the front of the rad into the engine com partmant put a rag in front of the rad and see if it gets sucked against the rad

 

Yeah the fan is blowing towards the engine. Ok here is what I have done so far. Changed water pump 3 times. This last one is a brand new one, not Re-man, New radiator, new thermostat, 3 times, new radiator cap. Still overheatong. I'm taking it to the chevy dealer and see if they can tell me the problem.

 

I've read this twice.
The only things I can think of---

correct rotation of the water pump &
how strong does the fan pull thru the radiator??

I don't know when the pumps changed rotating direction,
but I think you would have that covered.

Hope you get to enjoy it soon!!

 

The first thing that comes to mind for me is the timing. I had the same issue with a 78 TA I built. But, if its not that, a bigger engine does generally get hotter. I would suggest either a three or four core aluminum radiator. Or at least adding another fan to your current one.

 

Dude, if its a serpentine belt you need a REVERSE ROTATION WATERPUMP!

Seen it so many times, look of coarse evething is clockwise in a chevy SB, think the back of the belt rides the waterpump, and the cogs run the Alt & crank? follow the logic?

Mustangs (that dirty word) did this way before chevy and alot of rebuilds came to my shop with this problem......my two cents good luck!
Rotten

 

If it's driven by the outside of a serpentine belt, it's turning counterclockwise, opposite to the engine.