Engine Overheat Indication?

[flyer92]

Here's a dumb story...I recently had a BMW mechanic drain and refill the cooling system on my '92 318i vert. When I picked up the car, I was in a rush and didn't check his work, so drove it home (over an hour at highway speeds), with no apparent overheating or leaks. The next day, I checked the radiator only to find that it was bone dry! I added about 1.5 quarts of coolant to bring the level up to the full mark, and am now road-testing to see where the level ends up.


My biggest concern, is that I could have theoretically overheated the engine during my hour-long drive. If the indication of an overheated engine is merely the coolant temp gauge, it is possible that I may have not noticed it. However, if the Check Engine (or other) light illuminates, that's something I definitely would have noticed and would have stopped the car immediately. Unfortunately, the owner's manual and other maintenance manuals don't address this, so just curious if there is an indication along with the red zone in the temp gauge. Please help me put my mind to rest...and thanks!

[e30davie]

All you can do is fill it back up and monitor the temperature.

Blowing the head gasket is not your main concern, those can be replaced. A major overheat will bend the head beyond use.

[cunninglinguist]

Bleeding the M42 can be difficult sometimes. Now that you have it in your care, do a through bleed with the nose of the car up, heat on full, etc. Squeeze upper radiator hose repeatedly to ensure proper passage. There also can be an issue with corrosion or overuse of a sealant at the bleeding port where the thermostat lives.

Monitor the oil cap and the dip stick for "mayo" that could indicate a blown head gasket.

[flyer92]

Didn't have time to bleed the system tonite, but added coolant to a point about 1 inch below the full line and drove for 45 minutes at varying speeds up to approx. 3200 rpm. No apparent leaks, no mayo, and temp needle barely passed the middle index line, even while sitting for several minutes at idle. Seems like the engine may be OK after all, but will continue monitoring. Thoughts?

[e30davie]

if its going to overheat from engine damage its going to do it in the first 15mins or so. She'll be right.

Like my new zealand mate used to say "if in doubt, rev it out".

[flyer92]

Thanks, Davie...totally agree with the 15-minute rule.

As an alternative to bleeding, is it not possible to just drive, let engine cool completely, check fluid level, and add coolant as needed? Granted, this may require a few iterations until it is right, but it's not much more involved than the whole bleeding process. As long as the engine is running cool enough during each drive, I can't see why this wouldn't be OK.

[e30davie]

I dont have any experience with the m42 but I have gotten caught out with various engines over the years with air trapped.

If it were me I would fill it up, do your best to get the air our as per the method outlined in various sources. im sure there is a bleeder screw to crack.

Once you have done your best just let it idle in the driveway and watch it go through its motions. if its blead properly and working like it should the temp gauge should come up to normal operating temp and just hold it pretty much steady. take it for a spin let it cool down and check the level. top up as needed.

I got caught on a WRX engine a few years ago, i couldn't get the damn air bubble out and it was getting hotter than it should just idling (i guess air around the thermostat), gave up and let it sit over night and it magically fixed itself.....happy days:)

[flyer92]

Got it, Davie...many thanks! Given that the engine temp is already running steady in the mid-range, I think I will continue as you advised by running the car, letting it cool, checking the fluid level, and topping up as needed. If the temp and/or fluid levels were more erratic, then I'd continue bleeding the system.

[TobyB]

1.5 qts is ok, as long as it wasn't stupid hot out, and you weren't flogging the engine.

Keep an eye on it, and don't freak if it needs a little more in the next week.

I did a similar thing to my first m42...

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[bmwman91]

There is much consternation in the M42 community about bleeding the air out and how difficult it is. Here's the thing. It is dead simple and takes about 15 seconds from a cold start if the thermostat housing is properly installed.

There is a groove in the housing that is a bypass between the hot & cold sides of the thermostat. Very often, this clogs up with gasket sealer or corrosion, which then means that air cannot escape until the thermostat opens, which means running the engine long enough to (possibly) cause damage if there is a large bubble in the head. I have seen some cases where the groove is totally blocked with corrosion that also looks a lot like the aluminum of the housing itself, so people don't realize that it needs to be chipped out.

In your case, it is potentially not a big deal if the expansion tank was empty. If you only had to add ~1.5 quarts to get it up to the "full" line, then there was probably sufficient coolant in the rest of the system to keep things under control. The temperature needle is the only indicator of overheating, short of serious mechanical failure.

[flyer92]

Thanks. Luckily it only needed about 1.5 qts, but it seemed much more serious because the radiator tank was bone dry when I first discovered this. Good news, is that it wasn't stupid hot out, and I was driving at pretty constant highway speed, just under 3500 rpm for about 1 hour.

[e30davie]

I think cooling systems are alot more resiliant than most people give them credit for. I drove home one day from work and noticed a slight puddle under my car. checked the radiator, it was like half full due to a leaking hose, not just the over flow, but the radiator itself took like 3L of water. Temp was normal and no evidence of overheating. Wasn't exactly a cold day either.

Keep on truckin'

[AndrewBird]

1.5 quarts is nothing. I've ran a car a full gallon low before the temp gauge even moved.

[jaredmac11]

There is much consternation in the M42 community about bleeding the air out and how difficult it is. Here's the thing. It is dead simple and takes about 15 seconds from a cold start if the thermostat housing is properly installed.

There is a groove in the housing that is a bypass between the hot & cold sides of the thermostat. Very often, this clogs up with gasket sealer or corrosion, which then means that air cannot escape until the thermostat opens, which means running the engine long enough to (possibly) cause damage if there is a large bubble in the head. I have seen some cases where the groove is totally blocked with corrosion that also looks a lot like the aluminum of the housing itself, so people don't realize that it needs to be chipped out.

This is a gem of a comment and its a shame its sitting in the middle of a single discussion thread. I dont think RTV silicone gasket should be used at the thermostat housing for this reason. Goop up the timing case and oil pan, sure.

Terrific insight bmwman91.

[bmwman91]

Thanks. I think I have plopped that little nugget into almost every "coolant woes" thread I have come across in the last decade lol. Hopefully it will become more common knowledge!