It's possible you sealed a low pressure leak related to your coolant change, not an o-ring leak where combustion pressure can reach 1000 psi. And the block weld seals in the direction from inside the cooling passages outward, and the o-ring would leak exh gas into the passages.

Is it possible you fixed the problem when you repaired the leak in the plastic filler neck that you had mentioned?

 

Possible, but doubtful. It was having the puking coolant and temp spikes anytime I would drive it after I added that radiator flush in, until this morning, when I had let the car sit overnight after adding the block seal.

The plastic filler neck still had the tiny pinhole leak yesterday. It had that when I bought the car, which I drove two states away back home, so I don't think that was related. I really do not want to say "yeah, it was a coolant seal" but that is looking like it.

 

So I took the car on another 20~ mile trip. towards the halfway point of the trip, the idle temps started to be around 200 (I was the only car in the drive through line at the pharmacy and sat for maybe three to four minutes.)

Driving back, the warmest they got was 214 while steadily cruising down the highway doing about 60-80 mph. It held that temp on the way back when cruising. When I would slow down and get into residential area speed limits, the temps would drop into the 20X's. I let it idle in the driveway for about a minute or two, and the temp stayed at 200.

This is with the heater core closed, and the AC fan on, the entire time of the trip. This seems a bit high imho...thoughts?

Edit: not that it matters because the stock gauge is essentially useless, but it did not deviate from it's regular position.

 

Those temps are reasonable with AC fans on continuously, and light load on engine. You can't stop flow to heater core, unless you put a shut off valve in the hose to/from it.

 

Well, I took it out for an extended test today, with the AC fan on, After about 25 minutes of driving, I noticed when I got on the highway, the faster I would go, the more the temps would rise. going almost 80 would force the OEM temp gauge to start moving, and push the AEM water temp gauge readings into the 218's. When the car slowed down, the temps settled back to 206.

The car doesn't have an undertray, BUT, I drove it back from Georgia to NC and it was fine before without one. Any ideas?

 

You def need an undertray for the stock cooling system to have a fair chance. Your temps shouldn't be so high on cruise. It's backwards. Your having a weird and fraught experience and I think it would be good to go check in with a reputable specialist to get on an even keel.

 

Finally, the truth comes out.

And you wonder why you are overheating

The reason why temps decrease while your going slow/idling is the motor is not producing a lot of heat at low RPMs and your fan is doing all the heat transfer. The problem is when you increase your RPMs and load on your engine, the fan is not adequate to provide effective heat transfer. At speed, the cooling system relies on forced airflow across the radiator. The only way air will be forced through the radiator is to block any paths of egress, i.e. below the radiator using the undertray

 

With no undertray, the air that flows naturally will more easily go below the radiator and intercooler duct (stock intercooler is ~useless), vs go through them. The AC fans are not sufficient to cool the rad at high speeds/loads. Drag force on the car requires much more hp at high speed, based on speed^3 (cubed), so I'd expect temp gauge to rise as speed gets high. At 80 mph, the engine is running with boost, but it will only be measured before the TB, manifold gauge will be at vacuum. The boost creates a higher "ambient" pressure, for instant supercharging.

The stock gauge location reads true, higher temps. It starts to move at 230F, and is 250F at white line on gauge.

 

What confuses me though, is the way back from the sellers place to where I live, the temps did not go above 214 at the highest, I believe they stayed around 204. This was highway speeds for hours. Why would it suddenly be doing this now, and not then? I'd think it should have had temps increasing at speed when I drove it back.

 

Driving w/o an undertray (for however long it's been without one) may have compromised the cooling system. Have you performed a pressure test? If it can't hold pressure, you may have localized boiling (you may even have it if it does hold pressure). Without pressure, it takes less heat to boil, especially if the ratio of water to coolant is high.

 

A solid pressure test with no leaky fittings and all pinholes, leaks at clamps, etc. traced and fixed is what was needed before the block weld treatment, but I guess it's too late for that now.It's not a one and done sort of test..

 

I did pressure test it at the AST and held it at 14 PSI per rotorsports recommendation. It dropped marginally after a minute and a half. No visible leaks were present externally.

 

Surprised it hasn't been mentioned yet, But i'm going through a similar situation with my FD. I would definitely recommend and try a hydrocarbon test, its pretty cheap to pick up and has a chemical fluid that indicates if there are traces of combustion in the coolant.

 

???
I mentioned it in my 1st reply, #53
"I think you can have the overflow tank sniffed to check for the presence of exhaust gas."

And reply #64
" Many shops have a simple sniffer to detect exhaust in the overflow tank. Simple cheap critical test."

You can lead a .....