Should you replace both radiators.
 

Hi, my 98 boxster overheats. The supply line to the radiators gets hot but the return line doesn't. After 20 minutes, coolant starts to overflow out of the trunk.

Does anyone know what part of the radiator setup could be responsible for the failure? I think I am going to order both left and right radiators to progress this but thought it would be worth a post to share my experience and ask what others had done regarding diagnosing and repairing a faulty radiator.

So far, the story goes as follows:
- idler pulley next to alternator fails
- car overheats and I get it home on the flatbed
- trunk full of water so I replaced coolant tank
- I guessed that the water pump/thermostat might be responsible for the cooling failure..replaced both and no change.. Car still overheats after 20 mins.

I used the airlift to add the distilled water. I also ran water from the garden hose, through a kitchen plumbing setup to try and flush the radiator. I put the water into the 2 rubber hoses where you would normally drain the coolant. The water flushed through but I guess there is still a rad problem because the return line remained cool.

Any advice appreciated.

Thanks
Dave B

Sounds like you may have an air lock in the rads - have you burped the system?

Under the coolant filler, lift the thin plastic cover and you will find a chrome D shaped lever on top of the pressure release valve. Lift this lever to its vertical position & run the car for at least 2 - 3 complete heat cycles. A heat cycle is a minimum of 15 minute drive or until the thermostat opens.
Be prepared to top up the coolant if it is an air lock as the level will drop as the air is expelled out of the valve.

Thanks Steve. I will try and improve my burping. I must admit that I thought the airlift would have got most of those air locks out but I'll be more than happy to be wrong. Thanks.

The burping is only a suggestion - the airlift is pretty reliable, but......

Just a thought - is your car a tip ?? I think tips have a slightly different way of flushing the cooling system - use the search function as I'm not sure of the specifics, pulling fuses or something.

It's a base 2.5. I'll burp it a couple more times. Thanks.

are both coolant fans working properly? Hi and Low speed? when i checked mine passenger side fan wasnt working at all and the drivers side only on hi speed.

Yes, I did hear the fans come on. I think I have inadequate burping.

The main coolant return pipe doesn't get hot. Are the 2 radiators plumbed in series?..if one radiator gets blocked, does it stop the entire return flow?

Heat
 

If the up stream rad. is blocked it will stop the flow to the second rad. and back to the motor.

I'm not reading this right I think,

'the supply line to the radiator is hot but the water returning to the motor is not. '

Isn't that what the radiators suppose to do?

how are you burping the system?

I used the airlift initially and put about 5 gallons of distilled water in. I rev at 2000 for 15 minutes until the fans come on. Even though the radiator return should be cooler, I think it should at least be warm.

I was going to take it off the jack stands and go up/down some hills to try and burp it that way. Whenever it gets to 200F, it starts to overflow through the radiator cap and I wait 2 hours before trying again.

I would drive with the air bleed open, and check the coolant tank every so often to make sure it is no higher than the max level

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5 gallons of distilled water? Was it mixed with antifreeze?

No, it wasn't. I wanted to do a successful test to avoid wasting antifreeze.

are you still on the original coolant cap or did you upgrade to the 2004?
A really old cap will hold less pressure each time you unscrew it. Should not go below 18 PSIG.
Replace every couple of years to be on the safe side.

Burping will cause you to pull your hair out.
Our resident mechanics have suggested using UV coolant dye to see if you have a weak link somewhere.

Yes, I replaced the cap with gates part#31335 which is rated at 20psi.

The vacuum on the airlift held at 25psi so I was confident that the system wasn't leaking.

I hope I'm just a bad burper....this is the first time I've changed the coolant. Otherwise I will take off the radiators and have them tested...though I did disconnect both large hoses from engine to radiator and hooked up the garden hose. I saw the water come out and assume it went through an unblocked radiator setup.

I would have a pro look at it. If they find nothing then they can't charge too much :)

But if you've got a problem with the coolant system that's not a can you want to keep kicking down the road with this type of engine.

I wonder if you've got a rag stuck in the hose that feeds the water pump? It could also be a hose that's collapsing. Or the new thermostat is stuck closed.