M73 probably head gasket failure?

[charcoal]

Hello,
So for a little while I had coolant disappearing, it would go fast some times and other times it would take weeks to go down a noticeable amount. I done the old test for combustion gasses in the coolant and it was clean so I thought that was the end of it and figured it was a minor leak somewhere. That is until a routine oil change lately and it was light brown, obvious water contamination. Inside the expansion tank there appears to be some of this brown watered down oil as shown below. Would I be correct in expecting this to be a head gasket failure? I just can't quite get my head around how it might sometimes need a coolant top up after a couple of days and sometimes go weeks before it needs any.
723574

[Timm]

Very unlikely - the head gaskets on the M73 are not a weak point unless the engine has been overheated. Are you using Blue BMW coolant?

[charcoal]

Unless the gauge is malfunctioning its never overheated. Doesnt move past centre no matter what. Im using blue but not bmw branded, sadly just the halfords stuff. I read others talking about the oil cooler, maybe that could explain these symptoms?

Edit: In fact now that I think about it I remember when I first noticed the coolant disappearing the temperature gauge did go up but luckily I was close to home and could park it up when the gauge was at about 2/3 towards red. Since then I have periodically checked coolant so it hasnt ran that low and subsequently gone over half way since so gauge definitely works.

[charcoal]

Any other inputs people?

[Timm]

Although never owning a 750i - the only oil/coolant possible mixing point is for the transmission oil and not engine oil (the heat exchanger for the transmission). For peace of mind get yourself a block tester https://www.ebay.co.uk/itm/175891600114?itmmeta=01HS71NT39BY2YBDC4JBD7H7RF&hash=item28f3f4d2f2:g:56QAAOxyxpxQ7csv&itmprp=enc%3AAQAJAAAA8NtVROnJN4CbF4RFAhY%2FSjgZxKW Ea2160J6PqfBvhWvyQlMsfE1TyB8qxxM3QZAOo92YAbDPBocwv 3Tngq2zZm%2FZjPjPOTrZkODt2BO69t5RGzfua5Dfr3agvj55B 3MUxLsWl0M4kcdYn%2FydIVk2e56pXDVo4MLkJgC9%2Fah9eCG GbMlISbpSbozqAeHZinym4QOr8U0yNaNNtrWTqQ01n2Q9f%2BY YRgcdnFLl8mGhVjP1RFQa2wmcCu6E7CqzS6V1GPh0x7JdoqHm6 an1%2FkRX0FYUEOQ3ckmc1IzJvQ5O%2Fncd%2B5Xnw0utwoRM7 YzIADp42w%3D%3D%7Ctkp%3ABk9SR-qh1-HJYw

And check for exhaust gasses at the expansion tank

[charcoal]

So I actually did that test a couple of weeks ago, I'll insert a picture here but the fluid did not even remotely change colour to indicate combustion gasses. This was taken after it got up to temperature and holding the revs at 3000. This is why I'm so bamboozled by how there is so much water in the oil to make it turn brown and oil in header tank. Maybe the only explanation for the former I can think of is that the fresh oil is just taking the colour of any residual oil left after the oil change as I have maybe driven it only 25 miles since. I did have the transmission oil and filter changed because it was making that scrapy rattle sound on startup, do you think if oil was getting in from the transmission it could've caused that noise? Also still doesn't explain how so much water got in the engine, I thought maybe it was condensation but seemed an awful lot for just that

[a777fan]

With those results my money is on a failed transmission oil cooler.

[charcoal]

Thanks yes if this is a possibility I will definitely look further into it and see if I can confirm. I just had a quick search on the transmission oil cooler but I'm getting two wildly different looking things come up, one is a box with two pipes coming out and mounted under the engine and the other at least from the parts diagram is the traditional radiator like thing with pipes leading to the auto box, what is the difference between these two and which is the one you're referring to?

[CPHES]

Can you use a long cotton swab, chopstick with a paper towel ( sorry Chinese person solution ), and get some of the residue in the expansion tank? You have a 98 car, so that auto trans cooler should be a separate loop - not going to the radiator - pls look and confirm. If that is the case, it is not an AT leak issue.

[TheAngryBear]

So two extreme possibilities here but technically possible:
1. Have you had significant rainfall (wait, you are in UK so likely) and felt ANY changes in your brakes? The drains for the fresh air cabin and the windshield empty to the area of the brake booster. If that areas drain gets plugged (leaves, dirt, etc) in theory you could get water into the booster. Then the vacuum could evaporate it into the engine (bad enough and just suck it in but hopefully you would have noticed a non -responsive brake pedal at that point). The combined evap and saturated air COULD provide enough for some condensation internally post engine operation. (Again, possible but low likelihood)
2. Any issues with your secondary air system? I think the M73 has the same setup as the M73TU but a different pump… but if you have issues with the SAS non-return valves being stuck open… in theory the exhaust and/or crank vapors can condense in the pipe. Normally if you have too much it will drain down to and destroy your SAS pump. But if you just get a bit then it can collect in the pipe. That has a small hole to allow anything to drain BUT if it is clogged (dirt on interior/exterior) in theory you could get some condensation collected there and “blown in” by the pump when it activates. (Again, possible but not very likely).

As for the stuff in the cooling system… I like the chopstick test and see if it is oil sludge or just corrosion and other gunk. Depending on the age of the cooling system (waterpump and tstat included) it may just be buildup/film. Yes, normal changes keep it at bay but it still happens. And that same stuff may be a culprit of your sporadic leak as it could provide a temp plug to a radiator pinhole for low loss periods and get washed away and leak from the radiator (with evaporation no leaving significant residue) for the high periods of loss.

Just possible scenarios to explore and see if there are any other symptoms.
On the one hand, each is pretty ridiculous…
On the other, my blown SAS fuse read as having continuity before I replaced it…


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[charcoal]

Thanks for the replies, sorry I been away on a work trip but now I am back to the car, I've attached a picture where I scrapped out a bit of the brown water in the expansion tank, it had the viscosity of water. I also had a look at the radiator and it appears to have a separate radiator for the transmission, I appeached pictures of both. Do you know if the engine oil is cooled inside the main radiator too? As I had a friend who suggested they could be mixing inside the radiator.
723839723840

Thanks Angry bear so it has been very wet in the UK this winter, especially in the past two months, I have noticed that the under bonnet soundproofing is pretty wet actually. I did take it for a 30 minute drive in the hopes it was condensation and after parking it up and leaving for another 30 minutes the windscreen was all covered in condensation which seemed a bit odd (dry warm day). The breaking is perfectly fine though and the brake servo doesn't look like its being affected by water. As for the secondary air system have you got any recommendations for how to check it? With the stuff in the cooling system I'm unsure if it is dirt or oil but its definitely not sludgy. I'm not sure on the age of the cooling system but in the 3 years I have owned the car I haven't done anything to it if that helps. Do you know which fuse is the sas one as I'm not finding it on the fuse list. How could you tell after replacing the fuse that it fixed the issue? I am going to try and get hold of a smoke machine to see if I can get to the bottom of this, will update once some tests have been performed.

[TheAngryBear]

So not a familiar with the M73 as I have the TU but here are some attempts

The SAS (for me) has two relays and a fuse. The relays are in the ebox under the DMEs. The fuse is under the passenger floor (US version, maybe driver side on UK version?) where the power distribution center is. The seat needs to be at least tipped back (rails unbolted) to access it. The fuse is a large 25/50 amp and not the fuses you re thinking about in the traditional fuse areas.

If your windshield/windscreen has condensation on it like that, likely you have a leak letting water into the cabin. Most often it is the sunroof drains or the door card seals.

I would advise changing your coolant. Impossible to tell over pictures but I have a good feeling that is just “crud” like corrosion etc.

I think the M73 didn’t have a water cooled alternator, so if you have one you are the TU version. To check, is you alternator “encased” or “open”? Like Timm said, the TU could mix transmission oil through the cooler but not engine oil. The heat exchanger your picture shows runs to just in front of the radiator.

Again, unlikely as several things come together but if the oil cooler is leaking AND you have a pinhole in the radiator that “self plugs” you could get some traces in the coolant… but if these low odds things are happening to you, then go buy a lotto ticket and don’t worry about money again.


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[charcoal]

I see, I wasnt even aware of a fuse box in that location, will do a bit of research on it before I start taking things apart. Is there any symptoms of this system failed that you can tell from just operating various parts of the car? Thats no worries I wasnt sure if maybe there was an issue with the condensation getting inside the hvac system also, I had a closer look and the strip of metal that runs under the windscreen, the rubber that seals it with the windscreen is rotten and is leaving the windscreen basically unsealed!

I agree with changing the coolant, its basically just been surviving from top ups over the past two years so I hate to think what shite is inside there. Will get that sorted asap. By the looks of it I think it may be water cooled but I wasnt exactly sure what I was looking for, after looking at them online I will check again in the daylight and update here. That does sound very coincidental to have these issues combined. As the radiator doesnt appear to have engine oil cooling capabilities I think that youre probably right that its a combination of multiple issues. Still waiting on a smoke machine but hopefully that should give us some answers at least to where the coolants going.

[TheAngryBear]

The water cooled alternator is an upgrade of the N/TU system. I’ve seen both nomenclatures (M73N and M73TU) and believe it came out sometime during the 98 production. The last image in this page shows the housing https://ia801005.us.archive.org/11/items/BMWTechnicalTrainingDocuments/ST035%201999%20System%20Diagnosis/M62TU%26M73TU%20Water%20Cooled%20Gen.pdf
So if you look at the bottom front left of the engine (left from behind the drivers wheel, bottom near the ground, front near the bumper) if you see that housing and pipe to it you are water cooled.

As for the under floor fuse box, this video is for the E39 but it also applies to the E38, start watching about 3:55-4:00. https://m.youtube.com/watch?v=aJt6uIUII3g
Or pg 12 of this file https://ia801005.us.archive.org/11/items/BMWTechnicalTrainingDocuments/ST052%20Body%20Electronics%20II/02a_Power%20Supply%20and%20Bus%20Systems.pdf

As for the windscreen seal on the base, it isn’t pretty but easily replaced.
http://www.bimmerboard.com/forums/posts/822748
Just keep in mind variances between model years like nut sizes etc.

There is an additional sealing practice but of course the only place I have seen the writeup is on a now “BMW-blocked” page…

Generally though, that is for the engine e-box getting flooded. Cabin intrusions are generally either the sunroof drains or window/door seals. Post 45 https://www.bimmerfest.com/threads/tab-enters-the-world-of-performance-luxury-in-a-750il-battlestar.1268410/page-2?nested_view=1&sortby=oldest shows what I did for my drains. And yes the wrong “channel” but as it is mine and going for the solution hoping I am not in trouble…


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[charcoal]

Many thanks for the picture, yes that is the alternator that I have. Might be a silly question but how do you get the carpet up without damaging/cutting it? I had a quick look also but it wasn't immediately clear to to remove it. After seeing some of the pictures showing how bad they can get its definitely a good idea.
That little guide is perfect, it does indeed look easy, I got one on order so providing the rain stops we can see if that stops the condensation! What do you mean by additional sealing practice? I did do a few tests (spraying with the garden hose lol) to see if the sunroof leaks and I'm fairly certain it doesn't and combined with the fact that this underscreen seal is completely rotten to the point where there is a visible gap that looks full of condensation leading to the outside world I wouldn't be surprised if this is where it's coming in. Other than this mini sidequest I got going I ordered some of that uv leak detection fluid in the end. When having a look I did notice that the bottom of the radiator was rusty and in one place in particular the fins were pretty damaged, it wasn't clear if it's leaking from any of these points but the special fluid should give an answer to that.

[TheAngryBear]

The power distribution center is interesting as is the second battery. https://ia801005.us.archive.org/11/items/BMWTechnicalTrainingDocuments/ST052%20Body%20Electronics%20II/02a_Power%20Supply%20and%20Bus%20Systems.pdf

To access the power distribution center, info the seat bolts to the floor and tilt the seat back. That allows you to slide the carpet forward and access it. There is foam and insulation there as well. For the cleanest access, remove the seat from the vehicle.


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[charcoal]

Perfect thanks, this will also help my mother with her E39 as that had a pool of water underneath the carpets and wouldnt be surprised if her issues were caused by this also.