MAJ_Charlie

I recently went to a LS3 based 416 in my 2002 C5. What we noticed was a cooling issue. Yesterday we were finally able to determine it wasn't related to the radiator, water pump, or t-stat. Only runs hot when we turn the AC on. When the compressor clutch kicks on it places a tremendous drag on the engine to the point of almost stalling the car out, but then it gets to a normal idle. Trying to determine if I have a bad compressor, bad condenser, or both. With AC off doing hard pulls it doesn't get above 180 in 90*+ temperatures. Turn the AC on and from the same 180* after a half throttle pull the temp jumped up to 187. At idle the temp continues to rise. AC does blow ice cold. Yesterday while idling before I took it for a drive we watched the temp go from 176 to 203 in a matter of minutes. Turn off the AC the temp drops. When rolling with the AC on as long as I'm not running thru the gears it does cool...just a slow process. The radiator and fans are Dewitt's/Spal. The water pump is a brand new Edelbrock with a 160* SLP t-stat. The fans are definitely coming on and pulling air. The only things that remained from the last LS6 build were the radiator and AC components. Now that we've eliminated the radiator as being the problem we need help in determining why it runs hot with the AC on at idle, why the compressor puts such a drag on the engine when it comes on, why it takes so long for it to cool while rolling. We checked the low and high side pressures and they were 40/300 in 95* ambient on Saturday. I took it to a wash rack yesterday and used a pressure washer on the condenser. Had done that last week with the radiator while it was out. Problem continues to be the temperature rises while at an idle or in stop and go traffic. Once rolling it starts to cool. Only cools at an idle with the AC off. I'm trying to eliminate every possible cause before throwing money out for parts that may not be at fault.

Where things don't make sense are that it runs super cool with the AC off. When the AC is on it blows cold, but the car runs hot at idle or in stop and go traffic. Takes a while for it to cool back down while rolling with the AC on. Anyone else out there dealt with this or something similar?

mmartinez

Sounds normal to me, must cars run hotter when the AC is on, that's because the condenser is taking some of the cooling away from the radiator, and the compressor does add some load to the engine.

73Corvette

Your temps seem pretty normal... if it gets over 210 I would start wanting to see MINE a little cooler, although the consensus is that up to 230 ish is normal... it's hotter than I want to run MY car.

MAJ_Charlie

I would agree if this was a stock LS1 engine; however, after my first built LS6 swap normal idling temp was around 187 on a 95+ degree day with just the Dewitt's radiator, Spal fans, and stock water pump. With the 416 we've tried a Meziere electric 55GPM, back to stock, and now with the Edlebrock mechanical water pump. We've gotten to 230 and beyond just idling or sitting in traffic. It just does not cool when the AC is on and at an idle or sitting in traffic and the temp continues to rise to the point of the radiator becoming heat saturated (been there twice) unless we turn off the AC. The fans are definitely coming on when they are supposed to (programmed for high speed at 180*) and pulling air in, but we just have some issue causing the engine temp to continue to rise. When the AC is off the idle temp is about 176. After driving it around yesterday normal temp was between 178-183...even after a few pulls. Sitting in traffic at a light is a different story. When the AC is on and the temp gets above 210 I turn the AC off and the engine temp cools down while idling. If rolling from a stop it starts to cool on it's own. Very aware the C5s are not very well designed when it comes to cooling. If there was a blockage or restriction in the AC system it wouldn't be blowing ice cold inside the car would it? No leaks in the head gaskets and with three different tests showing no combustion gases in the coolant we're still searching for the root cause since this started back in March with the last engine, but corrected itself.

mmartinez

You may simply need a higher capacity/larger radiator because the LS6 is 5.7l and the LS3 is 6.2l, hence you have more displacement and more heat being generated. My 2 cents.

neutron82

have you tried blowing out the crap between the condenser fins?... if they are clogged then the fans aren't going to pull air through the condenser as effieciently and that would explain the temp rising with the ac on... 203 degrees with the ac on is really not bad at all, if that's as high as it gets then I would leave it alone... now if it starts creeping up to 210 and keeps climbing then I would investigate further

MAJ_Charlie

Can't get any bigger than the Dewitt's.

Yep...used a pressure washer on both. Been investigating this for weeks.

S.C. vette

Are you running the stock LS1/6 fan(s), or _______?

What is your fan set-up?

73Corvette

Ok... it's NOT a FAN problem...
SO, it must be a circulation problem...
Which could be thermostat... water pump... blockage... air pocket...
You wouldn't think a little A/C compressor would add hardly anything as far as pulling to a big block motor...my 5.9 diesel barely knows the A/C is on...
Good luck brother

neutron82

sorry I must have looked over that in your first post... aluminum or iron block?... I would assume aluminum since you say it's ls3 based but just making sure

CactusCat

Couple of things to consider here. The DeWitt's is doing its job as the only time you're having an issue is at idle with AC on. We know the C5 is a bottom feeder. Have you given your car the ability to pull air in while at idle? Sounds like you might need to do a front license plate mod and/or remove the fog light shrouds and use some Z06 front bumper screens. It sounds like there is just not enough air coming in when sitting still to cool both the motor and the AC. You stated that once you start moving, the temps start coming down, so that adds more fuel to that argument. Keep us updated. I have the same setup as you and did the front plate mod and removed the fog light shrouds. See this link. (It's not my car but I have the identical mod)
https://www.corvetteforum.com/forums...cover-mod.html
Thought I'd go ahead and u/l a pic of my mod. White car so I used white molding.

MAJ_Charlie

I thought about that mod, but unfortunately VA requires front and rear license plates. I have a few other ideas to try.

73Corvette

I don't remember who... but, somebody had taken the front brake vents and fabricated some ducts that moved that air over the radiator... but, that's won't help you much since your issues is not moving with a/c on...so i all that I said was really pretty much useless

ajrothm

Its a normal occurrance for the temps to jump up with the AC on... The condenser is right in front of the radiator so it sees a shot of hot air flow pretty quick.

The reason the engine "lugs" so bad or tries to stall is your tune needs some work. There is adjustments to be made for AC ON for idle quality. Sometimes it takes a couple seconds for the idle to catch up when everything is not 100% right in the tune.

AC is often the "straw that broke the camel's back" for the cooling system on these cars... On my car, AC on adds about 15* of additional temp on a 90* day to the coolant temps.

I will say this is pretty out of character for a car that is not supercharged (intercooler blocking the radiator) though. I would continue to look at your air flow components.... Air dam on the bottom, radiator shroud sealing etc etc..

*Its not an AC problem.
*Its not a circulation problem because temps are in check when driving down the highway with the AC on and temps are REALLY good when the AC is off and the car is at speed...Even making WOT pulls isnt heating it up... So everything on the water side is fine.

It really sounds like an air flow problem to me....
All of us supercharged guys fight this stuff constantly every summer... even with the best cooling system components installed. Our problem stems from the intercooler blocking most of the radiator air flow.

Your symptoms mimic ours, only ours are a lot worse and temps a lot higher...

One other personal theory of mine, unfounded mind you, is that the HP spal fans (paddle blade style) are not really strong enough to pull enough air flow through the thick core of the dewitts... I run the same fans on my dewitts, and I have tried two other types of fans besides the dual 12" spal HP fans, the Spals were the best of the 3 types but, I still don't think they are enough.

I just bought dual 12" Spal EXTREME fans....I'll be testing them this weekend. They are supposed to be a lot better pulling air with restriction in front of the radiator.... I'm not optimistic but... its worth a try.