How A Tiny $1 Part Killed A $4000 Engine

New York-based, 54-year-old welding inspector Tom Otto uses his white 2014 BMW i3 every day for his 96-mile round-trip commute. Because his car only gets about 65 miles of EV range (less in the winter), Tom relies on the gasoline “range extender” under his rear cargo floor to get him all the way home.

Unfortunately, in April, while on his way home, the engine just cut out, yielding a “drivetrain error” message on the screen. Trying to restart the engine led to a “horrendous noise coming from the rear of the car, and the engine shut down after about two or three seconds of runtime,” Otto told me. Yikes! So the handy wrencher decided to dig into the problem, ultimately finding that his engine ended up grenaded due to a tiny part probably worth only a buck.

I noticed this post on one of the BMW i3 pages I frequent on Facebook. “I’m sure many of you saw where my Rex engine failed at 124,000 miles (1103 hours of runtime),” Otto begins. “While I admit this is quite a high number of hours, I was not pleased to find the failure point to be a $65 part.”

“Upon disassembling the engine, I found the timing chain to have considerable slack in it,” he continued in the post. “This led me to believe that the chain tensioner failed. I disassembled the chain tensioner today, finding that there is a tension spring on the inside that is meant to apply constant pressure to the screw mechanism that keeps tension on the timing chain guide. The end of the tension spring failed where it engages with the screw. A $0.75 part (estimated) caused the failure of a $4000 unit (estimated).”

To give a bit of background: On an internal combustion engine, the “timing” whatever — whether it’s a timing belt or chain or gears — is what mechanically couples the rotation of the camshaft (which sets the location of the valves at a given point in the combustion cycle) with the rotation of the crankshaft (which sets the location of the pistons at a given point in the combustion cycle). On some engines called “interference engines,” if the timing belt/chain/gears fail and the valves’ positions are no longer properly synched with the pistons’ positions, pistons can actually collide with the valves, causing major damage to the engine.

That’s precisely what happened with Otton’s BMW i3 range extender. The chain that goes over the camshaft/crankshaft sprockets came loose, and thus, the location of the pistons relative to the valves was no longer properly synchronized. The result? Well:

It may not look like much, but you can see in the top photos that the exhaust valves have bent, and if you look at the bottom photo (particularly towards the bottom of the piston), you can see silver marks from where those exhaust valves hit the piston.

What caused this? Well, the actual chain itself didn’t break, it just got too much slack in it, causing the engine to “jump time,” which means it basically skipped a tooth or or two on one of the sprockets.

If you look at the image directly above, you’ll see that the timing chain is tensioned via two plastic chain guides, one of which is pushed by a tensioner, which is marked part number 18 in the schematic below:

That tensioner, which you can buy for $65, looks like this:

But what frustrated Otto so much (and rightly so) is that the reason this tensioner failed, and thus allowed the timing chain to go slack, skipping teeth on sprockets, allowing exhaust valves to collide with a piston, is that a dirt-cheap part within the tensioner failed. This spring:

“I had originally thought that the tensioner might have been a hydraulic unit like I have seen in other engines from other manufacturers,” Otto told me. “But, no, the timing chain tension relies solely upon a mechanical chain tensioner deriving its forces from a very small diameter tension spring.”

“You can just barely see the coils of the spring within that gap [in the photo of the full tensioner],” he continued. “The top end of that coil spring engages in the body close to the plunger end, the other end terminates in a 90° angle and engages in a slot which is accessible under the 8 mm bolt that is on the end of the unit.”

Apparently getting to the tensioner is no easy job. “It is not conveniently located for easy change out either. The chain tensioner is bolted to the top end of the block on the timing chain side which resides on the end of the engine that is in close proximity to the electric motor,” Otto wrote to me over Facebook Messenger. “It’s also on the backside of the motor engine as you’re looking at it from the back of the car. Though it is only held in by 2 T30 screws and a gasket to maintain an oil tight seal of the engine.”

Otto managed to find a new BMW i3 range extender with starter/generator built in for just $1,500 plus shipping. I took him — someone who has “always been active with Auto repair, small engine repair, wood boat restoration, vintage outboard restoration etc.” (Otto wrote to me that “people often wonder how I came to know all of the things I do about repairing cars, it’s because I never had the expendable income nor time to pay someone else to do something that I could do myself.”) — five days to do the job. Total cost of the repair was $3,000 — significantly cheaper than what BMW charges for a complete new engine.

Could Otto have saved those $3000 by pursuing the i3’s humongous 15 year, 150,000 mile warranty in CARB states like New York? Perhaps. But, Otto told me, he’s never taken a car to a dealership, he needed the car to get to work, and he didn’t want to waste the time.

It’s possible that the tensioner has been revised after 2014, and that later i3s won’t have this issue. I’m hoping so. Regardless, Otto concluded by telling me: “My posts are more of a cautionary tale, to hopefully help others not experiencing a catastrophic failure.”

A failure caused by a dirt-cheap spring. How absurd! But it’s yet another example of how, while a well-designed timing chain system is much better than a typical timing belt system, a poorly designed timing chain system is just the worst of all words.