!6lb cap is fine it is not your problem. I also doubt the heater core is the problem it is an auxillary not main flow path.Your problem seems to be the main flow path is not working because the engine is getting hotter than the radiator.

 

Start the car with the cap off (and the car cold!) and look for radiator flow, if the pump is really broken you won't see the jet. It sounds like it has to be the pump; why else would the radiator cool down but the engine overheat?

I'm impressed with your ability to go by the book and not get carried away, I wish I had that composure.

 

Justin, it is difficult to see flow because the car has a surge tank on the firewall (my 61 and other Vettes) have same situation of no cap on radiator. with cap off, you can see level drop and rise if race the engine.

Composure??? I use to beat in frustration on my older Corvettes then they became too valuable. Thanks JOE

 

I can see flow in the surge tank of my 95 when I pull the cap.


Pull your Throttle-body- steam tube connection and see if you have flow there. (IE temporary T-body bypass with out the bypass) red hose on left is steam tube connection to tb and one on right goes to surge tank.

However, I'm going to reiterate that I can see flow in surge tank, the minute I turn the water pump on (I have an EWP) but I use to see flow with the stock pump.

 

So can I it might be a little hard to see with new clean fluid shine a good light on the fluid.

 

OK here's what i would do in the shop---'till the temp is 180 to 185 thermostat should be open----shut the car off-----i would then carefully remove the coolant pressure cap---i would then remove one of the hoses at the heater core---have someone start the car---and take the engine from idle to approx. 2000 rpms---you should see flow --as rpms go up and down spinning the water pump----ie- some increase in flow of coolant as rpms increase either from the hose or the heater core depending on which hose you have off---if no real visible flow and/or increase with rpms---then based on everything that has been done so far---i would suspect as a few others have the water pump the impeller inside may have become loose on the shaft--I've seen this more on aftermarket pumps--I'm not saying this is a common problem---but i have seen it a few times--- anyway sounds like you have no flow and thats where i would start

 

I liked the collapsed radiator hose idea

 

devildog,

Wow, you've really done a good job of trying to pinpoint the problem. My ideas from working with our 94 LT1 are:

1. a vapor lock inside the engine, could prevent coolant flow, even with the water pump working, hence the reason for the vapor tubes on the back side of the heads. To see if the vapor collection tubes from the back of the heads are clear, drain off 1-2 pints of coolant from expansion pressure tank, then start car, and pinch off the rubber hose coming from passenger side top of radiator. You should see coolant running into the expansion pressure tank from one of the two top small hoses connected to it just below the radiator cap (if you follow the rubber hoses, they then connect to small (3/8") metal hose, which 't' to a metal 't', and go towards the rear of the engine for the vapor collection tubes and the throttlebody coolant hose.

2. bad/incorrect thermostat preventing the intermixing of the cooler water from the radiator with the hot water in the engine. The LT1 reverse flow also brings in cooler water from the radiator lower hose and mixes it with the hot water in the engine, as regulated by the thermostat. The thermostat is unique in that it has an extra plunger/plate on the bottom (it is an extension which has a brass cap which just slides over the normal thermostat's plunger), as the thermostat opens, it also closes off the internal recycle passage so that the cooler water is mixed with hot water to minimize shock temp changes to the heads.

So, is the thermostat THE correct one from AC/Delco? Is there a chance that the lower plunger/plate has pulled-off the thermostat main plunger and is overextended, such that it is always closing off the internal passageway. When the thermostat is closed, the lower plunger/plate 'cap' should be completely above/inside the thermostat housing. HOwever, what happened to me on our 94 was weird. After a small overheat (stupidly left cap loose and it spewed antifreeze), the lower plunger/plate was partially forced-off the main plunger (as if back pressure pushed it off, weird). This changed the overall height of the thermostat only slightly, but apparently enough to reduce the flow through the internal passage way. The new thermostat, resulted in the temperature DROPPing 15+ degrees (was running 205-215, now runs 192-196).

3. The 'air bubble' which should be maintained in the pressure tank/coolant system doesn't exist. With all the talk about bleeding the coolant system and not having any air in the system, one thing that is forgotten is that pressure is needed to move internal air pockets around. Water in of itself does not compresses easily, whereas air does. If you study the pressure tank, you will note its design is intended to maintain air in the top of the tank (note how water level should only be to the bottom of the radiator cap lower lip, but there is a higher area in the tank where an air pocket should exist). If by chance there is a slight leak in the tank, or the cap is letting the pressure out into the overflow bottle, or the cap is not being put on immediately, but being left off until the water starts to expand out of the tank. If either of these 3 conditions are occuring, it could delay the pressure build-up and actually cause overheating by allowing the vapor in the block to expand and block the water flow, before the tank pressure builds and counteracts/pushes any air trapped in the block.
I suspect that some of the overheating problems some folks are having is by not putting the cap on immediately to build up pressure and prevent spot boiling in the block/head, along with helping move any 'air' within the system to the logical collection point (pressure tank in our C4, or radiator in other applications).

 

Thanks all, great stuff. Unfortunately I am in Kazakhstan until the 15th and can not checks these latest suggestions. But, I will and post the results (as many too often do not). With all the work from all of you this will be a good thread for future cooling problems.

Joe

 

One last thought, and only because it is dirt cheap and so simple/easy to do. Drill two 1/16" holes (maybe slightly larger) in the flange are of the T/stat. Solves the problem of trapped air not being able to get past the t/stat.

 

Thanks to you all for your suggestions. I just returned from Central Asia and got my 61 ready to run to BG Forum Cruise in next week. Had few minutes to try the steam tube blockage test.

1. I tried the steam tube suggestion and blew air. Did not use compressor just put on hose and blew into it. It seemed clear..air passed into radiator, tank, etc. No resistance.

2. I can see flow in the firewall tank (not great), but if I rev the engine the fluid drops and rises.

Also, felt the two heater hoses after starting, they warmed at same rate.

3. Drove car around neighbor hood with low fan hard wired, temp stayed at 195-205. Drove onto expressway and ran 60mph. Temp started rising a little. exited and returned to my street temp hit 238, High fans came on.

Temp kept rising 258, cooled to 245, then back up to 255 as i pulled in garage. Puked and vented into plastic surge tank. Oil and tranny temps normal, IR temps on block, pump, etc. not above 200

Replace water pump???????????? only thing remaining. Seems like it does not pump a higher rpm.

Thanks all

Joe

 

I think there is a cover/plate on the front of the water pump---behind it is the pump impeller---I'd check that to see if it's loose on the shaft or has partially backed off the shaft --knowing everything you have checked it sure sounds like a flow problem and I'd be leaning towards the water pump.

 

Thanks that makes sense, because it seems to have reduced flow at road rpms. Even though it drops the visual fluid level when the throttle is advanced by hand, it may not continously turn the impeller

joe

 

I don't see the answer to this question.

Are both heater hoses the same temp?

 

yes, i answered it a couple of post above. Yes they both warmed at the same rate when I started it up last night.

thanks

joe

 

I too had overheating problems on my '96. The neck in the expansion tank would become loose when the cooling system was up to operating temperature. When the car was cold the neck was tight. I found it accidently as I checked to make sure the pressure cap was secure and the whole assembly turned! Once I replaced the tank and the system could pressurize the problem was solved. This might be the gremlin hiding in your car causing it to overheat.

Larry

 

Mine was too.It did not spin but had a leak around it.Although I did not get the high heat but it kept pumping out till the low coolant light would come on in a couple of days of driving. It actually had a pinhole leak that showed up after I replaced the cap. If the system is working right and is burped correctly the level in the surge tank will remain the same(full) after a heat cycle. If it has a leak (pinhole?) it will pump into the tank and suck air in after it cools because it is easier to pull the air in than to pull the coolant uphill from the tank.

 

Larry & Dave

Thanks for info about plastic tank leaking. I have to think about that one causing my over heat.

joe

 

I think if your pump was bad, you would NOT get the drop you noted.

Your heater core hoses 'warmed at the same rate'? That doesn't sound convincing. I think you got blockage in the heater core...

How's the heat?

 

It gets heater gets hot quickly

Thanks

joe