Rabbit GTI overheats at track events

83 ITB Gti. After 15 minutes temps climb into the red. About 250 degrees? Don't remember actual numbers as it was last year at Lime Rock and then the car went into storage. I have upgraded to external overflow tank style system. Heater core is bypassed. Radiator is good used. Fans working as they should. Water pump is new on the 1.8L. No oil cooler, but that will change soon. At idle runs okay and fans come on as they should. Could it be running lean? No O2 sensor so no way for me to check air/fuel.

At speed? Lean is possible. Otherwise a restriction in water or air flow, air in the system, possibly too small a radiator, stuck thermostat possibly.

Fans wouldn't normally be a problem while moving, unless they're blocking air flow.

Thermostat

corroded cooling system

Lower radiator hose collapsing.

So, replace the thermostat or just install a restrictor.

Flush the cooling system and replace coolant. Might want to try water only.

Replace both radiator hoses.

How is the water pump ?

Air flow to the radiator ?

Just a few things I can think of.

Lower radiator hose collapsing is very possible, especially with old hoses. Rev the engine toward track rpms while watching the hose. My LeMons 280zx suffered from that before I stuck a coil in the hose. That could also be indicative of reduced flow in the rad.

You check for belt slip? If the pump is new, maybe you forgot to tighten the adjuster?

Stock undertray missing?

Is all the stock rad ducting in place?

Duct the crap out of it so all air must go through the radiator.

Undertray? Hahaha
All hoses are new. I will recheck pump belt. Normal barriers in place around rad. Coolant system was flushed and filled before last year when I upgraded the system to later style with more capacity and overflow bottle. I will drain and refill with mostly water and water wetter.

Are the bypass flaps in the shroud (oh jeez, I'm working from mostly memory) present, and if so, do they open appropriately? If they're stiff/obstructed/etc it seems like it would prevent proper airflow at speed consdierably... The whole arrangement seems more optimized for fans than for at-speed...

Ransom wrote: Are the bypass flaps in the shroud (oh jeez, I'm working from mostly memory) present, and if so, do they open appropriately? If they're stiff/obstructed/etc it seems like it would prevent proper airflow at speed consdierably... The whole arrangement seems more optimized for fans than for at-speed...

Yeah, they're still quite flexible.

Old age ?

vwcorvette wrote: Undertray? Hahaha All hoses are new. I will recheck pump belt. Normal barriers in place around rad. Coolant system was flushed and filled before last year when I upgraded the system to later style with more capacity and overflow bottle. I will drain and refill with mostly water and water wetter.

Don't discount the importance of the undertray if the car was originally equipped with one, turbulence behind the rad can reduce airflow quite a bit. Also make sure any avenues/gaps for air to bypass the rad are blocked. It all helps.

You said USED radiator. Was it flow tested? Guessing this is your issue.

Cheap fix!

Coolest Radiator

No undertray. Ever. It's an 83.
Guess I'll pull the rad and have it checked for flow. And check the waterpump. And the tstat.

+Many on the above comments. All the ITB + ITC guys I know did some cardboard aided engineering to duct the crap out of the radiator. This is a tried a true fix. In addition I run a monster electric fan ala Ebay that flows roughly 4 times what the OEM boat anchor setup flows.