Bad Thermostat?

[Danny]

So when I'm driving my car will heat up to about halfway between the half and 3/4 marks. Then after a 10 maybe 15 seconds it'll drop right back down to the half mark, like the thermostat is taking too long to open. The highest its ever gotten while doing this was the 3/4 mark.

[Adrian_Visser]

You mean hot starting Or when starting cold it just keeps heating up? Or do you mean it heats up in stop and go traffic?

[Danny]

It'll happen under an conditions, all though it happens more frequently at cruising speeds, or so it seems.

[xwill112x]

are you running any engine fan?

[Danny]

Nope, but the e-fan is wired to be on at all times.

[Dj Buttchug]

replace/ drill thermostat. re bleed the cooling system

[nando]

Nope, but the e-fan is wired to be on at all times.

why? you're sucking HP all the time when you don't need to. get a thermoswitch. they're like $20.. :p

[Danny]

Shit I dunno. It was easier than doing all that.

[xwill112x]

i dont have any fan on my m42 car and it kinda does that on long cruises on the highway, but not as hot of temps. it creeps barely up to the middle mark then back down again, and repeats.

[nando]

Shit I dunno. It was easier than doing all that.

what, plugging the fan into the stock aux fan wiring and changing the thermoswitch? lol :)

[Danny]

Its kind of a long story. lol.

[jlevie]

Is the car loosing coolant? If so you may be seeing overheats/overpressure from a small head gasket leak. That will form a bubble in the cooling system, causing the engine to run hot. When the pressure gets high enough the cap lifts, venting coolant and air and the temp comes back down.

[MR E30 325is]

Is the car loosing coolant? If so you may be seeing overheats/overpressure from a small head gasket leak. That will form a bubble in the cooling system, causing the engine to run hot. When the pressure gets high enough the cap lifts, venting coolant and air and the temp comes back down.

I am with this guy.

[Danny]

Yeah its losing coolant, but I attributed it to a slow leak instead of a headgasket.

that's bad news.

[MR E30 325is]

Yeah its losing coolant, but I attributed it to a slow leak instead of a headgasket.

that's bad news.

If its a slow leak, then when the coolant gets too low the temp will go up, thermostat opens, and the temp goes down.

The same exact thing happened in my E30 when i had a 318is rad trying to cool the M50. Exact same symptoms as you.

I swapped in a 325is cooling system, and I was much happier, and the problem disappeared.

Check your coolant level when it starts doing it. If its really low then you have a pretty bad leak.