a C4 coolong question

[Ty T.]

I know it's a little out of our league here, but hoping someone can help me with a cooling problem on my 93 Corvette. After replacing the water pump it heats up, and in a hurry. I loosened the bleed screws on the thermostat housing and at the bypass hose until coolant ran out. I then thought by some strange coincidence that the thermostat was not opening (another symptom is the lower radiator hose is warm where it comes out of the thermostat housing, but cool where it enters the bottom of the radiator) so I removed the thermostat, and it still overheats. As an afterthought I was afraid I may have reversed the two large hoses on the recovery tank, so I switched them (I currently have the bottom hose running to the heater core), and it still overheats.

I'm stumped, figure it's air locked or could the new water pump be bad?(It's new, not a rebuilt) The water pump is the style with the drive shaft coming in the back of the pump.

Any help greatly appreciated!!
Ty

[Terry M.]

Re: a C4 coolong question

Is the coupling on the back of the water pump installed? Wouldn't be the first time someone forgot to install it.
The reverse flow cooling of the Gen II LT1 helps to make this one of the most difficult to fill cooling systems. It sometimes helps to get the nose of the car as high as possible while bleeding the system at the thermostat housing.
Can't tell you about the heater hoses -- all my experience with this engine is in a B-body, and I suspect the heater hoses are different on the Y-body.

[Ty T.]

Re: a C4 cooling question

By coupling, do you mean the small shaft sticking out the back of the water pump? If so, I remember having to turn it a little to get the water pump to seat flush against the block.
Any way that you know of to tell if the pump is working without removing it?
Thanks for your help Terry.
Ty

[Terry M.]

Re: a C4 cooling question

Ty -- there is a coupling that connects the shaft on the back of the water pump to the shaft that comes out of the timing cover. Yes, one has to rotate the shaft on the back of the pump to align the notches in it with the notches in the coupling. It sounds like you got that right, if the pump easily fit flush against the cylinder case.

I think one can look down behind the pump and see the connection while the engine is operating. Looking at that should tell one if there is power input to the pump. I don't know what happens if one removes the front cover from the pump. I do know those little screws on the front cover will strip out easily. I know someone -- no it wasn't me -- who did that. I've never has the front cover off the pump, but I have a couple of used pumps in the garage. I'll go take one off after work.

I did hear of a case of a water pump purchased from one of the automotive mass merchandisers that was assembled improperly in the rebuilding process. Where did you get your pump?

[Ty T.]

Re: a C4 cooling question

Terry
I'm quite sure I got the shaft in the coupling as I think the coupling stayed with the engine. Had it not, I'm thinking I would have been looking at a shaft similar to what protrudes from the back of the water pump. I recall putting the shaft in to a hole, which I'm thinking was the coupling.

I got my pump at a local parts supplier. They mainly wholesale to repair shops and quality is usually good with them. It is a new pump, not a rebuilt, although I suppose that may not matter.

Another thing I forgot to mention. When I filled the expansion tank, the Haynes manual said to leave the cap off until the engine reaches normal operating tempurature, but it was pushing coolant out the filler neck. Don't know if that has any bearing, but thought I'd let you know.

Thanks again for all your effort.
Ty

[David M.]

Re: a C4 cooling question

You should see coolant moving around with the engine running. I suspect your radiator is clogged with crud. You can try to flush but if its never been done your better off taking it to a radiator shop. They will tell you if its shot or can be rodded out. I'd just replace it if its scaled up.

[Terry M.]

a C4 cooling issues (long)

Sounds like you got the coupling right. As I said it should be possible to see the coupling rotating while the engine is running by looking behind the water pump. I'm not sure how much one can tell, as the outside of the coupling is smooth -- maybe draw a line on the coupling before one starts looking. Might be hard to get in there unless one has a long thin scribe. I'm going at this because it is possible, but not common, for the gears inside the timing cover to fail.

OK -- Pump sounds good quality.

In my experience with B-bodies it is normal for the coolant to expand out of the reservoir when it gets close to operating temperature – especially if there is air trapped in the system. It may now be necessary to run the system through a couple of heat/cooling cycles to remove the entrapped air.

If dead stock originality is not an issue for you, replacing the bleeder screw in the thermostat housing with a 1/8-inch petcock that has an outlet that one can slip a 3/8-inch internal diameter clear plastic hose over might help. Run the plastic hose between the petcock and the coolant reservoir and run the engine with the petcock open until the coolant heats enough to begin to rise in the reservoir. I found that kind of petcock at a NAPA, and the clear plastic tube came from the local ACE. Clearance to the air intake tube is tight, and one will have to run the system through a couple of heat/cooling cycles anyway.


One does not need to buy this bleeder from the above vendor. The brass petcock available from NAPA or any other real parts store will work as well. The pictures in the above link are worth looking at though.

A question about the Y-body engine temperature sender -- the one in the water pump -- how many electrical wires come off it? The reason I ask is the B & D-bodies get the temperature indication from the passenger head, but the computer reads temperature from the water pump sender. I know there are differences between this on F & Y cars, but I don't know which is which. I am raising the possibility that the high temperatures you are seeing may not be the temperature in the entire engine -- especially if there is an air bubble in there. Of course, one doesn't want the air bubble or the high temperatures associated with it.

Be careful of replacement thermostats. Some parts counter people, even some working in Chevrolet dealerships, think all Chevrolet 350 CID engines use the same thermostat. The one for the Gen II LT1 IS different than the one for the standard small block Chevrolet. Overheating will result if the Gen I thermostat is used in the Gen II.


Also it is important to have the proper cap on the coolant reservoir. I'm not sure what the Corvette situation is, but for B-bodies the pre-dexcool cap (15 psi with green dot) is no longer available from Chevrolet. One must use the cap required for Dexcool (18 psi with orange dot). The latter seems to be a good functional replacement. I have not researched availability of aftermarket caps.

[Ty T.]

Re: a C4 cooling issues (long)

Thanks again for the help guys!
Dave the car ran fine, until I changed the pump. Always ran cool. So, I'm not suspecting the radiator at this point. I will however keep it in mind to bring a solution to this problem.

Terry,
Thanks for the info on the bleeder set up. It was hard to tell air from coolant by just loosening the screw. I'll use your advice to bleed it out. As far as the temp sender, I'm at work, but I'll confirm it tonight, but I believe it was 2 wires going to the sender on the front of the water pump. And you are probably correct about the false reading. The reason I say that is that the car reached 240-250 degrees very quickly and when I shut it off, it didn'y act as though it was overheated. If the temp sender is not 2 wire I'll post another message to you. I'll check the coupling to see if it turns too. Thanks so much for your input!
Ty

[Joe L.]

Re: a C4 coolong question

Ty-----

Is the replacement pump a GM or Delco pump? If so, the thermostat and thermostat housing are supplied with the pump. I would not expect a problem with the thermostat if it's new and supplied with the pump.

Just to be sure that you have the correct pump (assuming that it's a Delco), it should be GM #12527739 or Delco #251-554. 1992 pumps are different, although it's unlikely that's what you have since they lack one of the fittings required for 1993-96 and that should have been instantly recognized when you did the installation.

[Ty T.]

Re: a C4 coolong question

Joe,
The pump is under the brand name of "Parts Plus", but I'm not sure who the manufacturer is. The number on it is WP9001. I looked it over before installing it and it looked identical. It did not come with the thermostat or housing, so I used my old one. I also tried it without the thermostat after it overheated to see what it would do and it acted the same. I'll keep playing with it. If you come up with any ideas please let me know. I know you are awfully busy with this forum and I do so appreciate your help.
Signed, "Stumped in Rochester"
Ty