Most often, the loose nut in any old BMW is behind the wheel. Driving a virtually worthless Bavarian sports sedan is liberating, and can give typically sensible drivers the confidence to do some not-so-sensible things. However, if you drive certain BMWs hard enough, another nut can come loose loose with the potential for disastrous consequences.
So what does this nut do? Does it hold the steering wheel on, keep the differential input yoke attached, or hold the crank pulley to the engine? Nope, but it’s just as critical as anything you could imagine. I’m talking about the oil pump nut.
Under prolonged hard use, the oil pump sprocket retaining nut (number 11 in the image below) can back out on M54, M52, M50, and S52 engines installed in, well, pretty much every inline-six-powered BMW made between 1990 2006. In fact, it’s easier to list exceptions: The E32 735i, the E34 535i, the E46 M3, and the 2000 to 2002 BMW Z3 M Roadster and Coupe.
Mind you, this issue is most commonly reported on models with the M54B30 three-liter inline-six, such as the E46 330i, E39 530i, and pre-facelift E85 Z4 3.0i, so owners of cars with those engines are particularly attentive to this issue, with a litany of forum threads dedicated to the topic.
Before we get into what happens when the oil pump nut backs out, we need to answer why the M54B30 seems to be the most affected. Well, there may be some bias in failure reporting since the three-liter variant is the most powerful M54 and therefore the most preferred engine for performance driving. Performance driving causes this particular failure, so it makes sense that an outsized number of oil pump nut failures happen on the most powerful motor. Perhaps unsurprisingly, if you drive a 2.5-liter car hard enough, the oil pump can still back off.
One forum post might not seem like the greatest of evidence, but it comes from a reliable source. Let’s just click that link and see where it goes.
Yep, it’s Throttle House with Thomas but without James. Small world! So, what actually causes the oil pump nut to back off in the first place? Well, our friends at FCP Euro have succinctly detailed exactly why this failure mode happens.
Now, the reason why the oil pump nut has the tendency to come loose on the engines mentioned above is mostly due to the lack of a chain guide/tensioner for the oil pump chain. It does have some slack in it (which isn’t a problem per se), but the problem ultimately comes down to how the sprocket is mounted to the pump. The nut itself is relatively small and isn’t torqued very tightly (only 25Nm). The nut is reverse threaded which means it tightens as you spin it counterclockwise, and while this does help keep the nut in place, it’s not a surefire guarantee. When any of the listed engines go through constant and rapid RPM changes (such as being used on the track), the slack on the oil pump chain in a way acts as an impact gun on the nut, so over time, it can come loose.
Needless to say, the oil pump nut coming loose is a very bad thing because once it lets go; nothing’s holding the oil pump sprocket in, which means it can slide off of its splines and stop spinning the oil pump. Considering that every moving part inside the engine needs a film of oil to not self-destruct, losing the pressure that delivers that film of oil results in disastrous consequences. With the average 20-year-old BMW having the Kelley Blue Book value of a used Trojan, once that oil nut backs off, it could be the end of the car.
Even crazier? BMW may have known all along. E46 330i models exclusively with the ZHP performance package got an oil pump nut with thread locker applied. All other cars with the M54 inline-six? No thread locker. For those wondering about part numbers, the ZHP oil pump nut is part number 11417897238, while the standard oil pump nut is part number 0711990857. However, thread locker isn’t foolproof, so the aftermarket has come up with its own solutions.
One such solution is to safety wire the oil pump nut to the oil pump sprocket. It’s cheap, it’s relatively easy, it requires no special tools, and it’s a complementary upgrade to replacing a leaky oil pan gasket. Some people prefer to weld the oil pump nut to the shaft of the oil pump, but that’s really up to whatever you feel comfortable with.
If you have more than, I don’t know, $20, there are other, fancier upgrades out there that use extra hardware to prevent the sprocket from coming loose. How about three Allen bolts instead of a nut on a splined shaft? Yeah, those ought to hold really nicely. Granted, these solutions are probably overkill, but if you’re building an engine rather than just replacing an oil pan gasket and doing a complementary upgrade, why not?
If you aren’t actively competing in motorsports or doing a ton of trackdays, you probably don’t have to worry about your BMW’s oil pump nut coming loose. However, if you’re a particularly enthusiastic driver with a current Snell-rated helmet and own an older BMW, you might want to tend to your oil pump nut before you’re replacing an engine in your driveway.
(Photo credits: BMW, Amazon, E46 Fanatics, eBay)
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So the M54 in my E53 X5 should be OK as there’s no way I am tracking this 4500lb trucky brick. It’s a manual (!) and has very short gearing so the revs do get up there moving this tank, but not much over 4,500rpm. But noted for if I ever need to pull the pan.
An all-metal lock nut would do the trick here.
Well, there go my desires of owning a six-cylinder E39 wagon.
This sounds like a pretty easy fix now that we are aware of this issue.
If the engine runs smooth and compression is good, you can buy with confidence that this particular issue won’t be what kills your bmw.
Shoulda used Loctite 620 sleeve retainer on that nut. The threads will pull off before that nut backs off once that stuff sets up, it’s absolutely brutal.
Wouldn’t the allen bolts also be prone to vibrating loose? If the vibrations are backing off a reverse threaded nut I would think they could also do the job on a few bolts.
The difference is that the oem nut is threaded onto the shaft, so forces applied by the chain as the engine revs up and down are applying a force that directly turns the nut.
The Allen bolts are arrayed around the shaft, so the turning of the gear isn’t actually trying to turn the bolts.
Good to know since I have a Z3 with the M54.
similar issues plague plenty of other makes and models.as the owner of a 2002 Nissan Sentra SE-R Spec-V, I was a victim of the infamous ( at least among our small B15 Sentra community) Butterfly Screw.
The screws holding the plate of the variable intake butterfly valve could back out, and get sucked into the engine, scoring the walls and generally destroying the block. If you did a little maintenance with Loctite, you were generally in the clear. of course I had not gotten the memo. thankfully Nissan replaced that engine under warranty because it was within the mileage range of their goodwill program.
I later got a second engine thanks to Nissan’s inability to make an Exhaust manifold Pre-cat that didn’t disintegrate and get sucked into the engine. that time i had to find my own used block.
I loved my 13 years with that car, but one could never claim it was well built or reliable.
surprisingly my 6 years with my 2012 Fiat 500 Abarth have been better, with only moderate issues unrelated to the main useage of the vehicle for transport.
I remember that pre-cat issue, they knew about it early or maybe even pre production, as the issue even affected press vehicles.
With that knowledge, nissan just put it into production as is.
a great article for Know Nut November
COTD.
slow clap.. definitely COTD
This was a very common issue on S52 engines, which started in the US in ’96 (and S50 in ’95), as well. So, it’s not like BMW wasn’t aware of the issue.
You mean there’s a future without meaningless chassis codes? Oh hallelujah!! May the Dead Nut strike them down…
Yet another motivation to swap a LS in.
Wait, the Booty Patrol was almost too much, but having a killer nut is fine? Haha.
Isn’t this No Nut November?!
(David probably
1. needs that explained and
2. has the sense not to recognize such a thing, especially now.)
Well, if the killer nut backs off THAT far, whew, you’ve got a hell of a problem on your hands.
The more I read about modern BMW car engines I wonder how they lost their way. The 247 Airhead motorcycle engine from the 70s is nigh indestructible and achieves Volvo level mileage. The average 21st century BMW needs expensive repair at 50,000 miles because a plastic piece got brittle and failed or a nut backed off
I have a Honda minivan with 300k miles on the original coil packs. The BMW M62 needs replacements every few thousand miles. Maybe they should give up making coils and just buy them from Honda.
Supposedly the reason the 3.0L engines have it the worst is because of crank harmonics. The 3.2L S52 and 3.0L M54 have the same crankshaft specs, and they achieve much of their displacement through a longer stroke. When you spend a lot of time at higher RPM’s with that crankshaft, it can cause the oil pump nut to back off. Safety wiring the oil pump nut is a simple fix, but it just moves the failure to the next weakest link, which is the oil pump shaft. People that race the 3.0L M54 and 3.2L S52 engines will usually run a reinforced oil pump with multiple bolts holding the sprocket on, an aftermarket chain tensioner to the keep the oil pump chain from whipping around, and an ATI Super Damper to help keep the harmonics in check.
Thought this was going to be about the early swirl flap design, but then remembered US never got the glorious early M57 lump
Wow that’s nuts
This would appear to be a perfect application for Nord-lock washers. https://youtu.be/IKwWu2w1gGk?si=UHdgh4S7l4MFSVT7 Every size of bolt / thread pitch has it’s own worst amplitude of vibration for loosening it, and it appears that this application reaches that for this nut under racing rpms.
I doth protest.
This is a(n) (automotive) news site, isn’t it ?
WHERE IS THE NEWS ?
That BMW engines for the last 20-ish years would have factory engineered, inexpensive and amazing in simplicity self-destroyers built in their engines at no charge is common knowledge, and no news at all.
From weak bearings on s85 and s65 engines – ok, at least those can be detected and fixed proactively, through $7 seals and plugs on V8s that start leaking and require labor hours in the double digits to replace, to serpentine belt ingestion for the n54 and n55 engines (where a broken serpentine belt, instead of harmlessly falling, has no other way but to be ingested in the crankcase via the front crankcase seal, because why not).
The beauty of the serpentine belt joy is that it is combined with another engineering feat: a leaking oil filter housing which would make the belt oily and prone to slippage and self-destruction. It’s a thing of beauty. Two usually harmless (if annoying) things – an oil leak and/or a broken serpentine belt – becoming cold blooded engine killers.
It’s a BMW. If you own one, do the work and keep explaining how it’s worth it and how nobody understands and how you don’t buy a BMW to save money and how Lexus is for boring people.
If you drive Japanese – do visit BMW forums from time to time, like a healthy iced bath after a sauna, then run back to safety.
Carry on 🙂
Nuts to that.
Yep, the safety wired nut is one of the few aftermarket/performance parts on the M50 I swapped into my 24 Hours of Lemons racer.
SHHHHH!
I am studiously ignoring the flickering oil light at idle when it’s hot….
The nut holding the toilet handle in our guest bathroom keeps backing itself off too. I might try this fix for that.
Any excuse to break out the welder, right?
Nail polish is a hillbilly thread locker that there’s a chance you might have some of near a toilet. Hopefully that nut isn’t seeing too many poo-s-i of force applied
Worth it. That guest bathroom toilet is probably worth more than most BMWs.
Probably not as shitty either.