mwm1993

I had to swap the otpi on my car (1994 lt1) after it died on me autocrossing a few weeks ago. This past weekend I finally got everything buttoned up and went for a drive. The car runs quite a bit warmer now than it did in the past. It used to hang right around 193-194 all day long (according to the lcd display) and the analog gauge would usually be at or below the middle mark.

I did pull the knock sensors to get most of the old stuff out. The only major change I made was I capped off the throttle body coolant lines figuring it's just one less thing that might leak on my new petris opti. Also I changed plugs, wires, rad cap nothing too interesting.

I did use a pressure tester to help it bleed when cold and I thought I did a pretty decent job getting all the air out with the front up jacked up a bit

The first time I drove it (after a few hot and cold cycles trying to bleed air out and bleeding at the thermostat housing) the car got pretty hot on the highway. It hit over 230 a couple times according to the lcd but the analog gauge stayed to what was pretty normal for me. When I got home from that drive I got a ton of air surging into the tank. I could hear it moving to the tank for over 5 minutes after turning the engine off. This has happened a couple times since and will happen any time the car gets over 220 or so. There is no air at the bleed above the thermostat anymore

I decided to try bleeding by opening up the surge tank, letting the car warm up, and getting revs up to 3k until there were no more bubbles in the stream coming from the water pump, topping off surge tank, putting the cap on, and then shutting it down to cool. The day after that it seemed to be ok (temps around 193) but I drove the car pretty easy. On the way home from work I got on it a little bit, shift at 5k, and a few miles later it started to get hot again. Maybe 220 on lcd but still right at the middle mark on the analog gauge and when I got home and turned the car off there was air surging into the tank again.

I have a feeling there is some boiling happening somewhere it is the only thing in my mind that makes sense to cause this. I thought maybe the new radiator cap may not be holding pressure so I grabbed another one from the parts store and it doesn't seem to have helped.

Tomorrow if I have time I think I'll drain the block again and start over with a new thermostat

I can't help but feel like I'm forgetting something. I can't imagine capping off the lines to the throttle body could make it so much harder to get all the air out. I feel like I'm chasing my tail at this point

Other than the cooling though the car really does run perfect!

antfarmer2

I bet it is the wrong cap both times.Still have the original?

kg4fku

I agree that you should check the cap first.

I have never seen/heard anyone filling a cooling system and then drive it to clear the air out of the system. That is a good way to get stranded.

Usually you let the car idle with the radiator cap off to get the initial air out then replace the cap and let the car get up to full operating temperature. Once the fans kick on, shut the car off and let cool. Once cool, open the cap and add coolant as needed. Fill up over flow and repeat as needed until air is gone. Then drive and check again.

mwm1993

I'll see if I can get my hands on another cap and try that this afternoon. I ran the car three times and let it fully cool down before driving it at all so you can imagine my surprise when there was all this air going into the surge tank.

Unfortunately I ruined the original cap. It had some corrosion on it so I hit it with the wire wheel and the little penny sized piece on the bottom broke off.

antfarmer2

Go to the dealer for this one. and look at the hose going to your tank.

mwm1993

I wish! Ill check that hose, though. I'm waiting for the car to cool down a little bit right now but I'm going to swap to another new radiator cap.

One of the guys I work with had a decent thought. The car has spring clamps on all the hoses that are 22 years old. I'm going to go through and replace any of the ones I removed with regular hose clamps. Maybe they are tight enough not to leak but loose enough to let it suck some air when I get on it

kg4fku

Wouldn't waste my time or money replacing hose clamps. Why?

1. You said it isn't leaking. OK. A cooling system is pressurized when operating. It will leak out before it sucks air in. Additionally, when it cools and does draw a vacuum, it will suck coolant from the over flow bottle if there is coolant in the overflow bottle.

2. Cam style hose clamps have a tendency to pinch hoses and can be over tightened which can crack plastic necks and shred hoses vs. the spring clamps which give constant pressure over the full circumference and cannot be over tightened.

IMO, you are better of buying a radiator cap and starting there.

You can do what you want but you asked for our opinions.

jmgtp

I also eliminated the coolant line to the throttle body and do not have air in the system. Your problem does sound like air but also the temps you report don't seem out of norm for an LT1 - though 220-230 at a stop is normal I wouldn't expect to see those temps at speed with a lot of airflow over the radiator. Is your overflow tank (not the surge) appropriately full? If not, as the system cools it can suck it dry and then pull air in. Same problem if the hoses to the overflow tank are damaged. It can pull air through a crack in those hoses or a missing/damaged clamp as the car cools.

ddahlgren

Put some coolant in the recovery tank if dry other wise air out then air back in.

mwm1993

I ended up replacing the thermostat yesterday but in the process I flushed everything out pretty well. I put probably 30 gallons of water through and got a ton of crap out. The new thermostat does have a little hole for bleeding air and after a few warm up and cool downs she seems back to normal! It is running right at 189-190 according to the lcd gauge and it sticks right about there no matter what I do. Also, no more air surging to the tank after I turn the engine off.

Needless to say I'm pretty happy to have my car back!

Thanks all for the help on this one. I can't say I'm entirely sure what the cause was but I think there was just an air pocket that didn't want to work its way out

antfarmer2

Good job I hope you pulled the knocks out again or you left a bunch of crap in it.

ddahlgren

I have been drilling a 1/8 inch hole in the thermostat to bleed for almost 50 years no matter what car. Yes I am in my 60's LOL. I wonder if it is illegal to drive a Corvette while on medicare LOL?

mwm1993

I did and I was pretty surprised how much came out. I don't think it had been changed since 96