Do we have an engine oiling problem with our BMW 435i?

Even when we first waded into this 2015 BMW 435i project, we warned you that we may encounter some genetic challenges, particularly with track use of its N55 engine.

The N55–BMW’s replacement and evolution of the turbocharged N54 3.0-liter inline-six–is regarded as a sporty, torquey engine, but it also has a bit of a reputation–as many recent BMW inline sixes do–for being hard on the original equipment rod bearings.

We expected this, as it’s kind of what you deal with on late-model BMWs, but in the case of the N55, there was an exacerbating condition that we were realizing may or may not exist that could shorten the life of that bottom end even more.

Owners of N55-powered BMWs seemed to be reporting bottom-end failures on track at a troublesome rate. Of course, everything seems scary on the internet, and anecdotes are not data–all engines occasionally fail and of course the owners of those failed mills will be more likely to go online to vent their frustrations–but there was enough noise here for us to want to explore things a bit further.

So we got with our friends at BimmerWorld to devise a strategy to objectively figure out whether we were actually facing a potential issue or just succumbing to internet-induced paranoia.

BimmerWorld asked us to log a few engine parameters while on track so we could review the data. Particularly, we wanted to examine oil pressure–the lifeblood of any engine–in relation to throttle position, boost and rpm.

Our MHD Tuning app makes data logging simple, particularly since several seemingly important engine functions are not available through OE gauges. Oil pressure, for example, is monitored by the car, but the driver can’t track this important function at a glance.

The MHD system not only allows real-time monitoring of oil pressure–and every other function accessed by the OBD system–but also produces time-based logs of this data for easy later review.

Our logs looked, unfortunately, problematic.

While we’d love to rationalize away some of what we’re seeing on these logs, when viewed in the context of the anecdotal reports of a higher-than-desired failure rate, they definitely point to the need for a solution.

First, let’s talk a bit about some baseline numbers when it comes to oil pressure for N55s engines–or, at least our N55 engine.

These engines run fairly high oil pressures. Following BimmerWorld’s recommendation, we run 10W-40 Redline, slightly thicker than the 5W-30 oil that BMW specifies. Why this deviation? We live in Florida, which is close to the sun, while the car sees a lot of track use.

[Treating our BMW 435i to its first taste of fresh fluids]

At hot idle, our oil pressure runs between 40 and 45 psi. Cruise pressure is roughly 55 to 65 psi, and full-throttle pressure can exceed 90 psi.

So, when our data logs revealed several dips down near and even below 30 psi, red flags went up.

“30 psi itself doesn’t particularly scare me,” said BimmerWorld’s James Clay, a guy who probably has a pile of lightly used N55s out behind his shop. “That number tells me the engine is still circulating oil, but what I really want to know is what’s being asked of the engine when the pressure is that low. Is it simple freewheeling off throttle and off boost, or are we putting strain on it when it has insufficient lubrication?”

Okay, well, let’s examine this particular graph and see if we can answer James’ questions.

This particular graph, taken at our official test track, the Florida International Rally & Motorsport Park in Keystone Heights, shows the sequence of events the engine is experiencing on the entry to a corner.

[Surprise: Our BMW 435i broke a belt during its first track day]

At around the 198-second mark (time is graphed along the X-axis), we lift off the throttle and initiate braking. Less than a second later, the oil pressure drops from more than 90 psi to around 40 psi. Boost is reduced gradually, dropping to near zero by 201 seconds.

At the 200-second mark, though, we get back on the throttle–and hard. Boost starts to climb a fraction of a second later, and the oil pressure? It drops again.

Okay, that’s not great.

You can see a very slight upward wiggle of the oil pressure that coincides with throttle opening and rpm rise, but it immediately drops off again to its lowest point in the sequence. Boost has hit 10 psi while the oil pressure hovers briefly at its lowest point and the throttle is fully open.

What does that mean? When the rod bearings are pushing against the crank their hardest due to boost and maximum power request, the oil system is struggling to keep juice on those bearings.

That little rise-dip in the oil pressure graph looks an awful lot like cavitation, and the BimmerWorld crew agrees. The inherent design of the oil pickup–placed well to the rear of the oil pan–and the fact that this is occurring in the braking phase would seem to strengthen our hypothesis.

Basically, under braking, the oil sloshes forward in the pan, away from the pickup, and when a demand for oil pressure is made again, there’s nothing there for the pickup to pick up, further lowering the pressure as the pump basically loses prime until oil can wash back to the rear of the pan.

Oof.

So, are we completely screwed here? Well, we’ll point out that plenty of N55 owners report thousands of trouble-free track miles with zero failures. But the data is what it is, and continuing to track this car in its current condition seems like an unnecessary risk.

But, as you’ve already seen, the car is on a trailer in a hotel parking lot, and if we told you that parking lot is on the way to BimmerWorld’s headquarters in Dublin, Virginia, you’d believe us, right?

See, the N55 as equipped in the 435i may have some measurable shortcomings when it comes to track capability, N55s and their variants were used by BMW in many of their more track-capable cars, and even one of their factory race cars. And the technology they used to keep those engines alive was not exotic racing tech, but off-the-shelf OEM hardware.

So here we are, attempting to furl this red flag and allow the N55 to run as intended. It’s a simple fix, but not exactly an easy fix, but that pattern applies to so many things in the world of BMWs. Stay tuned as we solve this problem. Then everything will be fine. Right? RIGHT?

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