odd overheating but not really

[vping]

To continue from:

http://www.bimmernut.com/forum/5-ser...tart-up-2.html

I replaced the clutch fan, radiator and overflow tank. All is well. Drove the car for a week two and no overheating issues. Then last Saturday the check engine light came on. I threw the scanner on it and found out I had an o2 sensor fault. I did not want to drive until I got parts.

Friday the parts came in so first thing Saturday am, I got out the ramps, jacked it up and replaced both precat sensors. Easy. Started it up and took it for a drive.

About a mile from home I saw the temp gauge climb to about 1oclock.. I shut it down instantly. Popped the hood, no smells, no visible fluid leaks. I decided to take scree driver and open up the small caps at radiator and water pump neck to purge some air. Started it up, which was no more than a minute from when I shut it down and temp was normal. Drive about a half mile and temp started to climb. Put it in neutral to prepare to pull over and it dropped right back to normal temp, 12 oclock.

I drove the rest of the way home plus a few miles more without issue. Car was filthy so I decide to take it to the car wash. The entire trip was fine, through the car wash, was still running, fine.

the only thing I can remember about both occurances, is that I opened the overflow cap prior to each temperature raise.

Am I letting excess air into the system and causing a bubble to sit where the temp sensor is?

Each time temp goes up and down within seconds.

Vince

[genphreak]

Certainly sounds like an air bubble working its way out. Is the plumbing correct after previous cooling system work? Maybe you have a leaky connection or a pinhole/crack in a cooling system hose.

I don't really know e39s, but if you can monitor the engine coolant temp sensor (not the temp gauge coolant sensor) via the OBC? It will give you a slightly different but more accurate reading and tell you when combustion temps go bad.

Maybe you have a leaky head gasket- in which case stop driving it, pull the head and replace the head gasket before it becomes a big job to do... or the cat absorbs too much coolant and dies (ie the fix requires more than just gasket (VRS kit). You could get/do a special check for pollutants in the coolant to verify the far-fetchedness of any such online diagnosis.

[vping]

Certainly sounds like an air bubble working its way out. Is the plumbing correct after previous cooling system work? Maybe you have a leaky connection or a pinhole/crack in a cooling system hose.

I don't really know e39s, but if you can monitor the engine coolant temp sensor (not the temp gauge coolant sensor) via the OBC? It will give you a slightly different but more accurate reading and tell you when combustion temps go bad.

Maybe you have a leaky head gasket- in which case stop driving it, pull the head and replace the head gasket before it becomes a big job to do... or the cat absorbs too much coolant and dies (ie the fix requires more than just gasket (VRS kit). You could get/do a special check for pollutants in the coolant to verify the far-fetchedness of any such online diagnosis.

How do you check the coolant for contaminants?

Would a leaking head gasket cause the O2 sensor to fail?

[genphreak]

How do you check the coolant for contaminants? Its a chemical test of the coolant, all good shops have the kit to do it in a few minutes.

Would a leaking head gasket cause the O2 sensor to fail?

Yes, the ethylene glycol destroys the sensor very quickly indeed. If you ever do a head gasket, it is a good idea to change the O2 sensor the week afterwards (as a new gasket leaks a little when bedding in)

[vping]

must have been an air bubble. Been driving the car since and not overheating at all. I've yet to throw the scanner on it to check engine temp while driving but will do soon. Still looking into the rough running at start but all things seem to point to the air pump.

[vping]

must have been an air bubble. Been driving the car since and not overheating at all. I've yet to throw the scanner on it to check engine temp while driving but will do soon. Still looking into the rough running at start but all things seem to point to the air pump.

I've been driving the car with no issues and so has my wife. All has been well until yesterday. I was about 5 minutes from home, driving 30mph roads and the temp needle started to move. I shut it down pulled over and looked under the hood. No visible signs of damage. Bled some air from the two small caps on pump and rad. Lower hose was cool and I noticed the overflow tank was a little low. Got back in, started car and gauge was at 12:00. Drove the rest of the way to work with no issues and gauge read normal. Drove all the way home last night in one shot with zero issues.

This morning, I added some fluid to the overflow tank to a bit above the cold line and headed out to work. Same thing happened in the same location same distance from home. I bled a little and then all was normal just like yesterday.

Perhaps when I did the radiator swap I left some air in there?

[Tiger]

What I would do is to upen up the cap and bleeder... and squeeze the upper radiator hose to remove any air trapped.

Make sure you run the heater at full blast hot to get air out of the heater core.

If that doesn't help, then I would hook up a coolant pressure tester to see if the pressure holds.

If it does hold, then start up the car and see if the pressure levels out at certain PSI... if it keeps rising say past 22PSI... then that is another issue.

[Bill R.]

If you are still getting the rough running for the first 15 seconds or so , i would check the coolant for contamination ie blown head gasket. If you know somebody with a
exhaust gas analyzer or a shop that has one you can get them to put the sniffer at the expansion tank with the cap off and running. If they show a hydrocarbon reading much above the outside air you have a blown head gasket.
When you shut the engine off hot or worm the cooling system has pressure in it still which pushes some coolant into the bad cylinder , then when you start it up after its been sitting this small amount of coolant in the cylinder causing it to miss and run rough until its all pushed out.
Alternately some autozones have a loaner tool to check the coolant for the presence of exhaust gas, the tool is a loaner but you have to buy the chemicals for it from them.
As the engine runs with a blown headgasket it will keep putting exhaust into the cooling system which will cause air pockets like you've noticed as well. My first suspicion is a blown head gasket especially if it have been really overheated in the past.

[93 525 Paul]

I used to have a Lincoln LS and it required extensive bleeding of the coolant system or it would act exactly as you describe. Bleed it until you are certain it is air free and you will be good

[genphreak]

All M50 and M52s auto-bleed. If it gets a bubble that causes a temperature variation, it can quickly lead to a catastrophic failure. The root cause is probably head gasket as Bill R says. Plenty of E39s have problems that probably start out with warning signs not unlike this...

Root cause may be something else, but these symptoms could easily end up cause HG failure if they are allowed to continue.