I just finished rebuilding my straight LS motor, I replaced every single hose, new head gasket, resurfaced head, and new head studs.

My problem is that the motor seems to only overheat when I'm driving continuously without breaking, like when I'm on the freeway going 60 for about 5 mins, the temp would go over half way. I would have to slow down then the temp drops. I'm thinking it's the thermostat and just wanted to see what else it could possibly be.

My radiator or cap seems to not be working properly also. After the car overheats half way up the gauge, I stop and I can hear steam leaking from the cap... is this normal? I also noticed that the radiator neck where the cap goes on is damaged. I can turn the cap all the way around without stopping. If the neck is damaged, do I need to replace the whole radiator? Or do you guys think it's just the cap?

BTW: All this happened last night and to fix the leak from the cap, I used some liquid gasket on the cap and I don't hear any more steam from it. Doing this won't damage anything right? Because my girl took the car to work this morning.

 

Actually, I wouldn't put anything on the cap - it is designed to blow off at a certain pressure, and sealing it may allow too much pressure to build up.

You may just need to replace the cap, as the seals on it eventually crack, or the spring may have failed (works like the thermostat does too. If the neck visually looks fine, it is probably just the cap - ~$10 from the typical parts place, $21 from the dealer (may be special order) for the right one. I just replaced mine with the one from the dealer yesterday because a shop told me the seal was bad. That may not solve your overheating problem though, but it is something to check.

If the neck is damaged, you might find a place that will repair it, but I would probably replace it if the damage is significant. You can very easily test the thermostat by taking it out and putting it in a pot of hot water.

When you did the rebuild, did you redo the pistons also? Did you do a compression check on the cylinders? What you describe can be worn rings and it could give symptoms of a blown or leaking head gasket - when you do the head, I believe it is normal to replace the rings at the same time as well, because you're in there already.

 

Yeah you defintly need a new cap. I think the steam is from the coolant boiling off (because it is no longer a pressurized system). The liquid gasket will fix it temporaily but get a new cap ASAP (like now or when your chick comes home from work).

 

Of course the rings were replaced. All cylinders have good compression, no white smoke from the exhaust... I think I need a new thermostat also because the cap was replaced like 7-8 months ago, but the thermostat was never replaced and it has about 212k on it. Ill change out both and see what happens.

BTW: Is it worth it to spend the extra bucks to get a performance thermostat, that opens at a lower temp?

 

If your motor is more or less stock or even with basic bolt ons (I,H,E), the stock thermostat is fine.

 

I'm having a similiar problem . My car will go over 1/2 temp on the freeway after you are driving it a while. It will also do this around town after a while. If you have the AC on the AC will get warm, then the car temp will go up then under 1/2 and the AC will get cool again.

It can be stable with normal temp for a while, then it will go up.

So far I flushed the coolant, got a new cap, checked to see if the fans were coming on(both go on when the AC is on).I'm gonna try bleeding the system. Tried an aftermarket thermostat and its still gets hot.


I don't have any water in the oil/oil in the water or white smoke.

I'm starting to wonder if the AC clutch is slipping, which causes the pressure to build in the condenser and the heat
Could the AC clutch slip which causes the AC to get warm which somehow ads heat to the radiator?

I did the timing belt/bent vavle last year and it ran fine all Summer. I hope it isn't a head gasket or cracked head....

 

Just a follow up, I changed out the thermostat with an oem one and also replaced the cap... So far, the car haven't overheated at all.

I had to flush and bleed out all the air in the cooling system. Everything seems to be running normal now. Thanks for the response guys.

And Guitarssheelies, Have you replaced every single hose in your cooling system? I replaced most of the hose at first because the other hose looked to be okay, then one of the old hose blew up. I'd highly recommend you to replace all of the hose, just to cancel out that possibility.

 

No I didn't replace the hoses except for a few short ones. I bought some aftermarket hoses and I was apalled at how bad they were-you couldn't even use them.By a bad hose, you mean hoses that collapse and block coolant flow?

Unfortunately I think my problem is head gasket related.I noticed some very fine bubbles coming up to the radiator neck with the cap off . This might be from combustion pressure blowing into the water jacket ): . Also one of the plugs looks cleaner than the other 3 which could be a sign of some water in the cyl.There's none of the obvious blown headgasket signs though.I could try pulling a plug lead to see if the bubbles stop.

I'm gonna try retourqeing the head.If that doesn't work its a leak down test and a combustion leak test(chemical that tests coolant for carbon monoxide).

Funny, I was just driving my car at night here and it ran fine on the highway with the AC on, but this afternoon it ran warm in the 93 degree heat...

 

Compression test and leak down test should tell you if you have a bad head gasket. I have this problem right now on my 240...(the search for a new motor is on ) The symptoms you describe are exactly what is going on with it, and they said it is one bad cylinder, not the head gasket.

 

I had a classic overheat/head gasket blow in my 240 and had to drive it back through a snowstorm-but that's far in the past! Later the chain eventually ate into the timing cover and lost coolant and cooked to death end of car

So regarding the leak down test you had-they told you it had a bad cyl and not a bad head gasket? Does that mean you have a cracked head and combustion pressure is leaking into the water jacket? I had cracked heads on 2002's and they ran fine just used a little coolant...

Replacing the head on this car would suck. I'm on the fence with it anyway and it might be better to find a nicer car than spend $'ss for GSR head...