Just changed a 42Kmile LS6 out. Was running just fine. Purpose is for USCA/CAM-S racing. I installed an LS3 525HP Crate Motor and my park/idle temp is <210 on super hot day with >100F ambient. But when I start driving it eventually climbs to 230-234F and struggles to come back down untill I stop again, and then it recovers. This can be country roads or highway, it doesn't matter. I think it is an airflow issue where the 110 extra HP is too much for the non fan-forced level of air flow. I installed a Ron Davis high capacity radiator and I cleaned the AC condensor core during the install. I checked and changed the thermostat when I discovered this issue, both old and new worked in a boiling water pot (with gauge). I also now have trackspec hood vents. I have tried with them blanked off and open. No difference. I was wondering if I had affected the air pressure balance into the radiator. Definitely seems to be not enough air flow without fans. Even a problem on the highway. Max it gets to is around 234F. Ambient is very hot (90-100F) and fairly humid. I was not expecting this issue. My front radiator airdam is in decent shape. I have cleaned the AC condensor with a brush vacuum ad then reverse blast of compressed air, it is clean. Fans work as instructed. I am wondering if a better quality radiator air scoop, like the Big Mouth will help. If that doesn't then I am running out of ideas. Engine was tuned by local tuner. I just got it back on friday, it made 460Hp at the wheels. I was not at the dyno, he reported no temp issues and no retarded timing. I have some engine retard that is going on as well under moderate/hard acceleration. I will be taking it back to sort that out, but I'd like to get this cooling issue resolved. Anyone experienced and solved this. I don't think it is airlock as the LS3 (like the LS6) has an upper small bleed hose piped back to the top of the raditor which also then has a hose to the expansion tank.

 

They are notorious for air lock. It is hard to get the air out. You need to jack up front of car and fill thru top rad hose. Then use a no spill funnel . Over fill funnel and run while attached while squeezing hoses to release air. My swap had these exact symptoms until I got all the air out. Another option is filling under a vacuum . There is a tool for this on Amazon for 80$

 

Posting this as I am sure someone else will bump into this issue.

The problem was with the port size of the two fittings on the passenger side radiator top section on the new Ron Davis Radiator. One is the return from the top of the block and is a small hose (1/4"), the other is the expansion tank feeder hose and is a bit bigger (3/8"). I noticed that the expansion tank had a lot of turbulence (flow) going through it. This bothered me as, at the time not knowing what the radiator passenger side tank looked like inside, it seemed wrong. I opened the old original radiator and these ports go to a closed box and are tiny openings (1/16" and 1/8" maximum I estimate). So I put some vice grip clamps on the hoses to partially close them off, in particular the large one to the expansion tank untill the flow into the expansion tank was pretty low. I went for a drive and the temp was rock solid at around 198-201F under all driving conditions. I am baffled as to why Ron Davis would send me fittings with such large porting. I will call them on Monday and discuss. If I was not carefull and cautious I could have overheated my car. A decent portion of the coolant flow was shortcutting across the top of the radiator through the expansion tank.