engine and drivetrain Running hot on freeway, AGAIN! I think its the gauge/sender

[bmw_jeff]

I have replaced everything in the cooling system, incl a used 02 radiator "known to be good". When I drive around town and its not too hot outside, the engine runs up to about 5/8-3/4 with alot of stop and go driving. When I drive on the freeway at 55+mph it gets up near the red on the gauge.

Question: How hot should it feel under the hood if its truely running that hot (near red)? I pulled off the freeway as the needle is just under the red, pull into a gas station and pop the hood. It does not seem hot at all in there. With car running I can literally grab and hold the upper or lower radiator hoses for a couple full seconds before needing to let go due to heat. I can even touch the radiator cap (metal) for a second or two at a time, so this stuff is not burning hot. The housing that the sender screws into is hotter and I can only briefly tap that housing.

I think its the gauge or sender since:

1)it does not seem to be "red hot" under the hood, and

2)my gauge has a poor ground and

3)the temp sender was (I think) designed for 12v cars, mine is 6v and that may throw off the amperage it is sending out to the gauge

The only thing that throws this theory off is that it is much more of a problem on the freeway driving (extended high rpm). It *does* sometimes get hot on local driving, but only with alot of driving time on warm days. Freeway driving only takes about 10-15 miles @ 60mph to heat up to near red, even on a cool day (low 70's). Any opinions, sorry for beating a dead horse on this one but its driving me crazy! Thanks in advance!

Jeff

[BigDog]

Here's two ideas.

1. Check your coolant level after she cools down. If you're low, aka you have a small leak someplace, you'll run hot like that. Happens to me. Fill it up and re-test. Then find the leak, not always soo easy...

2. CHEcK YOUR TIMING. Seriously, if you run too much advance, you will heat up, especially when driving at higher RPM's like on the freeway!

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BigDog

'76 2002

[BrianC]

Here's two ideas.

1. Check your coolant level after she cools down. If you're low, aka you have a small leak someplace, you'll run hot like that. Happens to me. Fill it up and re-test. Then find the leak, not always soo easy...

2. CHEcK YOUR TIMING. Seriously, if you run too much advance, you will heat up, especially when driving at higher RPM's like on the freeway!

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BigDog

'76 2002

But that would mean actually setting the timing with a light so you know its correct

True a small leak will cause overheating, or it could be a sticky T-stat, or a rad cap that isn't sealing well... even new ones can be the culprit.

The best thing to do is install (even temporarily) a gauge with numbers so you know what temp you are at instead of speculating. Pressure test the cooling system to check for possible leaks?

HTH

BC

[cj]

I would definitly be suspect of a "known to be good" radiator. Flow is king in a radiator for both water and air. The other suggestions, cap, leaks, timing and t-stat, I agree need to be checked.

Cris

[rho]

Get a IR temp gun and check the temps at all the usual places... $30-40 should be a good investment that can be used for many other applications as well....

[cj]

Get a IR temp gun and check the temps at all the usual places... $30-40 should be a good investment that can be used for many other applications as well....

Great idea! You can also use it to check the temps on the rad(middle, sides etc.).

Cris

[bmw_jeff]

Thanks all, I guess I'll have to get a IR temp gun to see where its really at. 170-200 deg is normal right? I have no leaks, new cap and new Tstat, but I have not checked the timing. If the timing was off, wouldn't that cuase drivability issues? This is my first "old" car that has to be manually timed and I am not familiar with the process but I have the manual(s) and the light...

[DavinSF]

...gauge solved this problem for me. After changing my t-stat to a cooler one and banging my head over why the car would heat up on the freeway I added the ground wire. Now the temp guage is rock-solid and I'll be changing back to a 'medium' t-stat because she barely gets over the blue at highway speeds. Check the search to find a good writeup on the ground wire addition, it's a must-do in my opinion.

Also, you can use a meat thermometer to check the coolant temp in the top of the radiator. This will give you an inexpensive clue as to how close your temp gauge is reading. Good luck!