Hello,

I've got a 1988 240DL wagon. Here's my backstory

1. Radiator cracked. Replaced radiator. Ran beautifully. Temp gauge way low, like 1/4. Everything great.

2. Coolant started leaking. Blown water pump seal. Slow leak, still drove okay, but air was leaking into system as water leaking out, intermittent overheating.

3. Replaced water pump seals and gasket, replaced thermostat. No coolant leak, at all. Re-filled, idled with cap off for a few minutes.

4. Test drive! Drove about 10 miles, temp at middle, ok. Suddenly it started overheating!

I suspected air traveling through the system. So I went back to my neighborhood and ran the engine with the coolant reservoir cap off for a good while because I know air in the system can haunt you. I ran the engine with the cap off for way longer than necessary. Temp calmed down. Drove like another 5 miles with the cap off. No way there's air in the system. Coolant is not leaking at all, so I can't imagine there's air sneaking into the system either.

Once I seemed to get the temp calmed down I drove another 10 miles and it was at mid, but as soon as I got back into town and, for example, went up a tall hill or something, temp went STRAIGHT to the top again! I am totally stumped and getting frustrated. Worth noting I have enough oil, car drives fine. No low fluids. Can't imagine there's anything else wrong, problem started with the cooling system, persist with the cooling system.

Everything is hot. Both radiator hoses, radiator itself and return pipe form back of water pump. Does this mean the pump isn't pumping hard enough?

What's going on?