cooling question

[72cudaguy]

I seem to have a weird temp. issue. I have a freshly rebuilt 340, auto, 9.5 to 1 compression, somewhat hi performance. After alittle while driving it at high rpm ( highway 3000-3500rpm ) the temp seems to creep up, particularly when I get off the highway and sit at a light or traffic. After looking at the motor during this situation I noticed there was no pressure in the top radiator hose. Being that hot I would think that hose should be pressurized, the temp was around 225. Slowly took the cap off and no pressure.
Running a 160 thermostat, 26in radiator and 7 blade viscous fan. The no pressure thing has me puzzled,  any input is greatly appreciated.  Thanks

Oh ya, noticed this while at the dyno shop so timing and fuel mix is right on.

[quagmire]

You either have a leak somewhere or a faulty radiator cap, those are really the only two reasons that would cause the system to not hold pressure. 

[MEK-Dangerfield]

Yeah, you should always hear air coming out of the radiator when you take off the cap, regardless if the engine is cool or warm. If it was the thermostat, you would boil over quick.

If I were you, I'd go with an 180* thermostat, and a new radiator cap.   

[72cudaguy]

I went to the 160 to try and combat some of the heating issue, would the lack of pressure have anything to do with the lack of flow therefore causing the heat issue ??

[MEK-Dangerfield]

I went to the 160 to try and combat some of the heating issue, would the lack of pressure have anything to do with the lack of flow therefore causing the heat issue ??

Going to a 160 just makes the thermostat open sooner, probably before the engine is warmed up enough.

If you had a lack of flow problem, you would be boiling over at some point. I don't see this as your problem. A 26" radiator with a 7 blade fan is more than enough. I have a 26" radiator on my 440 with a 6 blade fan. Even in hot weather, my temp gauge rarely goes 1/4 of the way up. I still feel the air is getting out somewhere in your case. Look under the hood right after you drive it. Maybe you can hear a hissing from some point.

[quagmire]

Insert Quote
I went to the 160 to try and combat some of the heating issue, would the lack of pressure have anything to do with the lack of flow therefore causing the heat issue ??

No, but if you have a leak and it is low it will obviously heat up.  The system is pressurized to raise the boiling point of the coolant, some cars however do have a tendency to run hotter with a bad cap because of localized boiling near the combustion chambers in the heads that would not occur if it were under pressure.  The water pump is a flow type pump, it does not develop pressure.  It only circulates coolant.  The natural expansion of the hot coolant is what causes pressure to build up, and the cap regulates that pressure.

[Chryco Psycho]

where is the vacuum advance connected to ??

[72cudaguy]

during tuning session on dyno we capped the vacuum advance and thats where we were getting best results

[shadango]

Yeah, you should always hear air coming out of the radiator when you take off the cap, regardless if the engine is cool or warm.

Really?  I dont hear air coming from mine when I open it cool.....

Engine seems to stay at about 170 or so when running.....I do have some rad fluid that comes out of teh overflow now and then or if I park on a hill sideways, driver side downhill......

[moper]

two easy ones. First, leave the cap off and let it idle. You should see coolant flowing when it warms up. If you don't, pull the thermostat and stick in in some water in a pan. If you have a meat thermometer then use that to measure the water and watch for the stat to open. If it doesnt, get a new one. Also, the thermostat is to maintain a temperature when the cooling system has the capacity. The same engine that runs at 160 for hours will run at 195 for hours too. But if the 160 one overheats, it will overheat with any stat, just like adding a 160 where a 180 runs hot. Thermostats have little to do with cooling capacity of a system. Only of the  temp at which the water starts moving. And you will make more power at the crank with a 195 stat properly tuned than a 160 properly tuned.

[72cudaguy]

I put a new radiator cap on and now I have pressure back in the system, that was easy, I also bought a laser temp gun and will monitor where the temp is in certain situations, will also change thermostat back to 180 and see how it goes, thanks for all the input.

[Changin Gears]

There are 2 types of radiator caps.  They are for running with/without an overflow reservoir.  The difference is the little metal thingy that is on the bottom of the cap.  For the non-reservoir systems, the cap required has this metal thing snug on the rubber gasket.  The reservoir version has this piece dangling down.  The reason it dangles down is to let the air purge out of the radiator (no air is supposed to be in the radiator with a reservoir).  The coolant pushes the metal piece up to the rubber gasket when all the air is purged.

This does not work without a reservoir because the radiator always is supposed to have an air pocket for expansion.  The result is no pressure.

[moper]

On a side note, a bad radiator cap is a big deal too... I had one not release when it got hot. It blew the tanks on the radiator on the approach to the Tappan Zee bridge...