88, auto, bolt on mods....

What lbs pressure cap should be on the rad? I replaced 16lbs one with a new one. problem is after some hard driving the cooant is getting pushed back into the overflow.
I've changed all hosed, 180 deg thermo, and have burped this thing about 5 times now but I always seem to get the "low coolant " light and it's a little low. I started watching the overflow and noticed it was pushing back into it.
Thinking I might need a higher pressure cap? What should I do?

Thanks, jeff

 

can you even buy higher pressure than 16#?
i just replaced mine along with a 160 stat, but haven't bled the system yet - ran pretty hot - over 230 today - but nothing is boiling into the overflow tank.

 

Do you have a leak somewhere or is the hose in the overflow tank too short? When the H2O flows into the overflow tank it is supposed to get sucked back up when the water cools back down. It is a closed system and the water should just go back and forth. Anybody disagree???

 

No leaks "found", hose is OEM length to tank. just for some season it appears to be pushing back into the overflow and not back into the rad at cooldown.

Togo,
summit is showing 24 lbs rad cap but I'm thinking thats to much.

 

my 86 had a 17lb Delco cap, but everywhere I checked the feedback was it calls for a 16lb.

 

i would then say that if you don't have a leak now, and use that 24# cap, YOU WILL

i would stick with no more than 17 as mentioned above, i just put a 16# prestone cap from auto zone on my 92 LT1 (it HAD a 15# cap on it before i changed it)

 

A 24 will take out your heat core for sure, get a new cap the same lb as you have - it might have seen better days and there cheap enough.

 

sniperdiver37, The GM Factory Service manual (FSM) for the 88 Vette says the stock radiator cap is a 15psi pressure cap. This is mentioned twice on page 6B-1. Togo is right about using a 24psi cap. When pressure testing the radiator for leaks, the FSM says to use no more than 20psi or the radiator will develop leaks from the test.

After hard driving the coolant should be getting pushed into the overflow tank. As it cools the coolant should be getting pulled back into the radiator. If the correct amount isn't being sucked back into the radiator, I would guess that you have either an incompletely filled radiator, a pressure leak, or a hot spot that is causing the coolant to boil in a limited area in the block creating excessive pressure.

My first thought is that maybe you have not been getting all of the air out of the system when you burp it. Also, have you made SURE that the pressure cap is seating cleanly and completely when it is installed? Absolutely no rust or corrosion at the seat? Check for coolant leaks when the engine is cold. Look for ANY coolant ANYWHERE the hoses and cooling system connections go.

jd_v3tt3, The official GM Dealer Parts & Illustration Guide page 1-03 says that the 85-89 Vettes all used the same radiator pressure cap, GM Part #6410962. That and the notes from the 88 FSM would indicate to me that your 86 should be using a 15psi cap stock. I'm pretty sure that 16psi isn't going to damage the system, so I wouldn't replace it with a 15psi until it needed replacing.

 

Also, pull the small hose between the radiator and the overflow tank and inspect both the hose and the inlet into the tank.

I've found that even the tank inlet can get crudded up over time.

For a while after having my coolant system open the "low coolant" light would come on occasionally and the level in the radiator would be low. I tried repeatedly burping the system and checking for leaks.

Eventually I noticed that after a while the light would go off; after a few months it stopped comming on at all. I really hate it when the car "fixes itself" but happens occasionally. I can only think that somehow I didn't get all the air out of the system, even with several "burpings". Also, maybe the intake gasket design, with those small holes over the rear coolant passages tends to entrap air.

You could have your coolant system and your cap pressure tested for leaks. But it the engine isnt's overheating, see if that "light" goes out after driving for some time or after you let it stand for a while.

 

Vette Specs book says cap is 18

 

As for burping the system, the highest part of the cooling system on my 86 w/ iron heads, is a capillary line that runs atop/alongside of the evap housing. if you cap the metal tube and elevate the rubber hose until you get coolant, then do a quicky swap of the cap and slide hose back on. I have thought about putting a "T" in this line for burping purposes.

 

Mike 88Z51-
Nice reply and I think your right. Leaving it alone to do Vett things seems to be working. It's not overheating, not leaking, all hoses new, new thermo, new cap 16lbs, fresh coolant, and the low coolant light comes on once in a while then goes out.
Can someone talk me through "burping" the system again just to make sure I'm doing the correct way.

This is how I'm doing it.

remove the cap
warm the car to temp, thermo open
rev to about 2000 rpm and fill rad with fluid
replace cap
........
am I missing something?

 

The 88 GM FActory Shop manual says:
"Fill radiator to the base of the radiator fill neck and add sufficient coolant to the recovery tank to raise level to the "FULL" mark. Reinstall recovery tank cap."
"Run engine with the radiator cap removed, until normal operating temp is reached. (Upper radiator hose becomes hot.)"
"With engine idling add coolant until level reaches bottom of filler neck and install radiator cap making certain arrows line up with overflow tube."

The above has always worked for me except one time. The one time it did not work, I resolved the problem by pulling the low coolant sensor and cleaning it and the area where it was seated. No low coolant light problems since.

I'd also follow 65Z01's advice and the overflow hose to make sure it is clear of any gunk or obstruction.


jd_v3tt3, the Vette Specs book could be right and the 88 GM Factory Shop Manual (FSM) could be wrong. I've seen errors in FSMs before. In this case, since the FSM says the same value of 15# in two different places, I'd assume that the FSM is correct. Stant was the OEM source for all of the thermostats used in the early C4s. I believe that they were the source for the radiator caps as well. Their online site recommends part #10230 (16 psi) as a standard replacement, so my guess is that the original cap was either a 15psi or 16psi cap. I would not recommend using an 18psi cap until I got a second confirmation from another reliable source.