Welcome to Tech Talk, where our resident BMW tech Nick Owen answers all your pressing BMW technical questions, frustrations, and issues. Have a question for Nick? Email him at techtalk@roundel.org. Let’s get into it.
THAT WAYWARD COOLANT
I have a 2004 325Ci with 139,250 miles. I have owned the car for two years and have addressed the delayed maintenance issues. Last year I replaced the radiator, water pump, thermostat, expansion tank, and associated hoses. I regularly check coolant and oil levels. Yesterday I checked the coolant level and needed to add at least a quart of coolant. Where does the coolant go in a sealed system? There is no evidence of leaks, and I do not smell any odor of coolant. Last week when I checked the coolant level it was fine, and I have probably driven less than 200 miles in that time. Any suggestions?
Don Ford
The mystery of disappearing coolant in the E46 can be a deceiving one. I have serviced and topped off fluids on many M54 engines, but for some reason the coolant reservoir always needed to be topped off to reach “maximum” fluid level. However, there was no low-coolant light on, and there was no evidence of a leak—nor was there a customer complaint.
My theory is that just because the fluid level was not at “max” does not mean there is a leak. The system naturally bleeds off pressure because the coolant/water mix will slowly evaporate and burn off, because the coolant reservoir cap is set for two bar of pressure before releasing—which it will do very minimally. You won’t notice it, but the fluid level slowly drops because the cap is venting. This is completely normal.
Of course, you may have a slow leak somewhere under the car or the radiator; maybe the transmission-oil heat exchanger (if you have an auto) or one of the coolant hoses is only 99% secured. A cooling-system pressure test is useful in this circumstance, because you could pressurize the system and see how fast it bleeds down, and also identify the leak. Finally—and this is a low probability—if there is a failing head gasket that hasn’t reached the point of being noticeable, then it’s possible that the engine is slowly consuming coolant.
(Update from Don: I took advantage of the Advance Auto loan-a-tool program and borrowed a coolant pressure-testing kit. The coolant system held 1.5 bar of pressure for two minutes with little to no pressure drop, so it looks like a new pressure cap takes care of things.)
DIFFERENTIAL DESPERATION
I drive a fairly rare E60 530i with a six-speed manual transmission (B997023). Long story short, my differential is beginning to fail and needs to be replaced. If my car is rare, it seems that the part (33107560592) might qualify for unicorn status; there are no U.S. diffs on eBay, and I have spoken to many salvage yards, most with a network: no joy. I don’t want drive this car any more than necessary, which puts my summer road trip in jeopardy.
I’m at the point where, maybe, I need to jettison this cherished family member. Can you offer any advice?
On a related topic, I’ve been noodling the idea of replacing this car with an F10, the last 5 Series with three-pedal models. I had the idea of finding an M5, maybe the last model year, with a six-speed. My trusted mechanic says it’s an amazing car, but that transmission, not so much. The problem, he explained, is that every time you disengage the clutch, the car loses all boost, and there’s a lag to get back on the cam. What say you?
David R. Good
Phoenix, Arizona
While you may have a unicorn E60, finding NLA parts can be a headache! You could try contacting Diffsonline.com and see what they would charge for a rebuild of your failing differential. It might not be cost-effective, but it’s a solution.
As for the F10, it’s a solid chassis—a great platform no matter the engine. The F10 M5 in a manual is a hard one, though; I have seen one and driven it. Personally, I liked it. I did not notice the drop in boost between shifts because I don’t think it’s that noticeable; the turbos are small enough to spool right back up. I have heard zero complaints about the manual F10 M5. While the DTC transmission is quite good, it’s boring—however, a manual F10 with an N55 engine (535i) might be just the ticket, all things considered. That S63 V8 in the M5 is expensive to maintain.
(Update: David found a diff in Canada.)
SOFT-TOP SENSORS
In March 2019, I purchased, from the original owner, who had a fleet, (it was time to rotate his stock, if you will), a 2009 650i convertible with a mere 36,000 miles on it. After replacing the original tires and putting in fresh fluids all around, I began to enjoy open-air motoring here in New England, especially in the spring and fall. I also replaced the hydraulic roof pump and have that fluid checked at each dealer-performed oil change.
The car now has 51,000 miles, and now, being retired, I’ve vowed to use it more, putting on the last 3,000 miles in just the past two months.
On several occasions I’ve encountered the dreaded “Top Not Locked” dash warning, which, as you know, locks the trunk and rear window. I have performed, with my pudgy little fingers, the TSB instructing the elongation of the mounting screws that hold the sensor mount in place, with minimal success. I have also replaced the sensor with a true BMW unit. Several forums have suggested tapping the left bat wing where the sensor resides on the skeleton of the top; sometimes it works, most times it does not.
An EE friend measured the sensors’ electrical characteristics and determined that the capacitance changes with rotation, but it is very sensitive; heat plays a role in changing the capacitance, hence the TSB suggestion to rotate it farther from the minimum and maximum angle of rotation. I, however, have found it is more likely to present a lockout condition when the car interior is hot, or if the top is down and sitting in the sun for a while. I surmise this because if I freeze the interior with the AC when the top is up and the sensor has sent the lockout message, after about ten minutes of cold air in the cabin, all is well.
Now, obviously, this won’t work if the top is stowed. The EE friend suggests trying to insulate the sensor, or mount a small computer-type fan (1×1 inch square) to the sensor to circulate air, or to carry a can of compressed air used for a computer to chill the sensor, which I can reach from the left side of the cabin, if I want to open the roof. I was thinking of routing a piece of surgical tubing with enough slack to traverse the tops’ movement into the storage position from an AC duct—a bit extreme and time-consuming, but maybe successful in keeping the sensor somewhat cool(er).
Any thoughts or experience?
Nick Ciarlo
Wallingford, Connecticut
Oh, the dreaded E64 convertible-top issue! I have seen this one a few times, but the TSB of adjusting the sensor worked every time. Have you taken the car to a shop to determine the sensor position in relation to the other sensors to ensure that this sensor is the issue? I have had to modify that sensor beyond what BMW instructs in order for it to consistently work properly. While you are able to determine that the issue is influenced by temperature, that indicates to me that the electric windings of the sensor could be compromised—at least enough to let temperature influence its operation, which, in theory, should not matter unless extreme temperatures are reached.
I personally hate convertible-top issues, and in the few dozen times I’ve dealt with them at the dealership, I’m lucky to have the resources of ISTA to interrogate the convertible-top module and look at the status of all the sensors to determine the issue. Call it cheating, but in these circumstances, it’s not a favorable job to take on as a flat-rate dealer tech. But it is a delicate component of the car, which requires patience.
I like the compressed-air idea. Running a tube from an A/C duct sounds like it could work, but with all the moving components in the convertible top, it needs to be 100 percent clear of anything that could somehow interfere with the linkage during transitions. This might be something you or someone else needs to play with to get just right so it works every time. Again, I hate convertible tops.