'91 318i M42 Chain Tensioner

[AlexJ]

Good morning all,

First post here, but long time lurker. I've learned more from this place about my car than almost anywhere else, including YouTube.

I know the chain tensioner in the M42/M44 has been covered in great detail, but a search hasn't turned up my question.

Issue: Racket at front of engine. '91 318i w/ 140k mile. Suspect timing system. I've replaced the original tensioner with the Febi equivalent of the M44 tensioner (https://www.turnermotorsport.com/p-3...ain-tensioner/)

My understanding is that oil pressure in the tensioner cylinder is doing the majority of the work while the engine is running, and that the spring, along with what oil remains in the cyclinder, is what keeps the tensioner extended when the car is off. If that's the case, then it seems to me that it can be very difficult to truly determine running chain slop/deflection (measured between the cam sprockets) while the engine is off because there is no oil pressure.

All that said, I'm aware my assumptions could be wrong. I'm guessing that the spring is not stiff enough on its own to properly tension the chain, and that oil pressure is required. Am I wrong?

Now, to get to the point, I removed the tensioner and inserted a nylon tipped rod into the hole, applied pressure, and checked chain tightness between cam sprockets. Lo and behold, barely any deflection to speak of. I figure this helps me narrow the problem down to a faulty tensioner or low oil pressure. The one major unknown here is that I don't know if the amount of pressure I applied exceeds some design limit that is intentionally outside of the capabilities of the tensioner.

Thoughts?

[bmwman91]

The spring in the tensioner is sufficient to get the chain tight enough to set the cam timing under normal circumstances. It should be pretty difficult to push it in (fully extended) and get the big threaded cap back on.

The exception to this would be if the guides are badly worn so that the chain is extra loose and requiring the tensioner piston to be extended a lot further. Have you had a chance to pop the upper timing case cover off and get a look at the top half of the guides?

Does the rattle go away or get worse as RPM increases?

If it gets worse with RPM. then I might start thinking about a bad tensioner or low oil pressure. The other thing low oil pressure can cause is lifter noise if they are not pumping up properly.

[roguetoaster]

You know, I don't think that checking chain tension between the cam sprockets (where there is usually a guide) will produce any slop except when the chain itself is worn out. I say this as the chain there is trapped between the cam sprockets, which shouldn't move with a chain deflection check, and isn't influenced by the tensioner at that position.

Consider that your other guides/chain may be worn, beyond what a new tensioner can solve, or that a bolt may have worked loose behind the timing cover, causing your noise, or as said above that your tensioner may be defective. Other good culprits might be the belt driven accessories, which are easy to rule out.

[AlexJ]

bmwman91 I wouldn't say it was difficult to get the cap back on, but not that easy either. I was able to do it with bare hands and without removing anything to make room.

It's seems to go away with higher RPM.

I'm popping the timing cover off sometime this week. If not Friday, then Sunday. I'll take some pictures and report back.

roguetoaster That makes sense. We'll see what happens when I get behind the timing cover.

Thanks to you both for your input.

[AlexJ]

Alrighty. I was able to knock it out today. Now that I've got it apart, it seems pretty obvious that the chain needs replacing. That said, I'd greatly appreciate your feedback.

Hopefully the video attaches properly.

[roguetoaster]

I don't really recall how much slop a new chain has, but that does seem like a lot.

It's not terribly difficult to disassemble the rest of the case once you remove the compressor and the crank bolt so you can see if there are other issues before ordering parts.

[AlexJ]

Word. I'll open it up on Sunday and report back.

Thanks.

[AlexJ]

Alright.

Got it open. Took awhile. Fan clutch nut seized, so went to remove water pump, which was also seized. Attempted to remove using the threaded holes but snapped an ear off. Chiseled away at the casing with a pry bar and hammer to get it to spin, hoping to break up any corrosion locking it in. In the end, I bought a 36" pry bar, stuck it behind the fan clutch, levered it off the oil pan, and went to town with a dead blow. Call me Archimedes.

As far as I can tell, everything guides, gears, and sprockets are in good shape. The tensioner guide itself isn't very worn at all, and with a new tensioner, doubtful it's anything other than the chain, which has 140k on it.

Anyone know how long a new chain is?

[roguetoaster]

I think the last Iwis chain I got had the length on the box, so maybe search some packaging photos for that. Don't forget anti-seize on the fan clutch threads when reassembling, the same around the water pump around the outside of the gasket might be smart too. As a last idea, put a tool on each fastener in the timing case that you can reach, including guide bolts, and check to see if one is stripped and just barely floating in the hole.

[AlexJ]

I think the last Iwis chain I got had the length on the box, so maybe search some packaging photos for that. Don't forget anti-seize on the fan clutch threads when reassembling, the same around the water pump around the outside of the gasket might be smart too. As a last idea, put a tool on each fastener in the timing case that you can reach, including guide bolts, and check to see if one is stripped and just barely floating in the hole.

All good ideas, especially about the bolts.

I'll let y'all know how things go from here. Now that I've got it this far apart, I think I'm going to strip a bunch of other things off and super clean this engine bay. Maybe start a build thread.

[AlexJ]

Picture time.

Slide rail betwixt le sprockets.



Chain tensioner rail (all other surfaces intact)



Crankshaft timing sprocket rail. Actual galling on this piece, near the end that points towards the deflection wheel and the chain tensioner rail.



Guide rail. Cracks throughout, easily identified on the top edge of the piece. Many of them go deep into the rail, likely to the bottom (bottom meaning into the screen).



And just in case someone out there can come up with the length of a new chain.



All bolts have been checked, nothing loose or stripped.

What do you think?

[AlexJ]

For reference:

[TobyB]

So, for timing wear, I always start with the sprockets. If the crank sprocket hasn't been polished at the bottom of the pocket, I don't go
any farther- the whole thing is going to be fine, barring any misalignment or material failure.
Then the chain rollers- if they're not uniformly shiny, then they're not particularly worn, and the chain's going to be ok.

The part that really needs attention is that lower idler sprocket. That's killed more M42's than anything else.
The late E36 parts replace it- but then you need most of them, as the cover, guides, chain etc are all different.

BMW copied Alfa in this- and boy, if there was ever someone to NOT copy, it was Alfa...

So in your shoes, I'd replace that lower idler sprocket, and button it back up, assuming the other sprockets
aren't showing much wear. The M42 cam drive is way overbuilt- as long as it got oil, it's just not going to wear out
in normal usage.

t
plus, the parts to replace it all cost far more than 4 junkyard M42s out of later E36s, and from those you'll
get 3 sets of viable timing drive parts...

[AlexJ]

Thoughts?

[bmwman91]

Wow, so your timing chain assembly is very likely the original parts. The long guide rail was changed to be all plastic, so the fact that you have the metal+rubber one is an indication that it has never been replaced (also note that when changing to the newer part, you need a 5mm longer top bolt...if you use the original shorter one, it WILL strip out of the head after a thousand miles).

As far as the rust on things, did you leave them out in the rain after removal, or did they look that way when you pulled them out? If they were that bad when removed, then the engine has a rough history. Personally, I would replace everything (at least everything that you can still buy) simply based on the rust.

The sprockets are not too bad looking, and can probably be reused. I would give them a good scrub with some degreaser and re-post pics of the teeth though. It is a little hard to tell how worn the teeth are. Since the sprockets are NLA, you may have to reuse them. I think that the tensioner rail that the tensioner piston presses on is also NLA, but check around. Here and there someone has some new-old-stock parts on eBay. For the chain, just replace it. If you are up for a much larger project, see if you can find a 1994-1995 M42 timing case and swap it in. It eliminates the deflector sprocket in favor of another plastic guide.