M20 - No compression (broke/replaced timing belt)

[13Stripes]

Hello fellow E30'ers,

Let me start by saying I've read nearly every post on this, and other, forums over the past month or so that relate to a broken E30 timing belt. There are so many enthusiasts on these boards that serve as a wealth of knowledge, and for that - I'm appreciative. Also, this post is a bit lengthy, but stick with me - I want to be thorough.

From the beginning:
- former E30 owner, and was eager to rejoin the community
- purchased a 1990 325i vert in Jan 2015 (rough interior, minor body imperfections)
- spent spring/summer collecting various parts (interior, seats, new oil cooler)

Labor day weekend, I took the car out for a drive after replacing a bent oil cooler, changing oil, changing spark plugs, installing new brackets for 87 IS front valence (and mounting valence), along with a thorough cleaning. It was a great day, with the top down, zipping around some back roads.

Following day, we were out for another drive and the car died turning into a parking lot not far from home. After a relatively brief diagnosis, it was clear I had a broken timing belt.

*At this point, hindsight is definitely 20-20 : yes, I realize I should have planned to do the timing belt by this point (8 months owning the car) since the maintenance was not well documented.

Research begins. Most forum threads indicated head damage is common. I decide to swap the belt and go from there.

Cleared my Saturday schedule, and performed the following:
-replaced timing belt (adjusted timing back to marks on cam/head, harmonic balancer/block)
-replaced water pump
-replaced alternator belt
-replaced power steering belt
-replaced crankshaft position sensor (while I was in there)
-replaced distributor and rotor

Buttoned everything back up, and tried it out.
-Crank, no start
-One small backfire, but never started
-Confirmed spark and fuel at the cylinder
-Engine sounds odd, which I determine (after research) is no compression
-Using compression gauge, I get 0 psi across all 6 cylinders
-Postulate that timing may still be slightly off, causing no compression

Few weeks later (12/19), clear another Saturday:
-remove all parts (rad, fan, belts, timing cover) to check timing marks
-timing marks line up

I pulled the valve cover off, and discover #5 intake valve rocker is broken, as well as section of head where rocker shaft passes through. (Pictures attached). I wasn't surprised to see a broken rocker, but was less than thrilled to discover damage to the head itself.

With the valve cover off, I turned the crank by hand and watched the camshaft turn all 6 cylinders. I was able to watch intake and exhaust valves open/close for each cylinder, with the exception of intake on #5 since the rocker is broken off.

I have many questions about replacing the head, which valves/springs to use, and if anyone has feedback on the bimmerheads cam (or others) - but let's come back to that.

**The thing that I am still most confused about:
- why am I not getting compression across ANY cylinder? I've worked around many cars. Having a broken timing belt, and subsequent broken rocker, would certainly impact the car from running WELL. However, I never saw the car even try to start again. With damage to rocker shaft at #1 piston, and broken #5 rocker, either of those could justify lack of compression. But what about the other 4 cylinders? With the cam turning, valves opening/closing on time, spark and fuel (at the assumed correct time), I still think there should be SOME compression or perhaps even an attempt at fire/combustion.

E30 experts - what am I missing??

I will be pulling the head to check for damage to pistons, and from there determine if I'm just buying a head to rebuild or picking up a whole block. However, since this is a hobby/project car for me - I still feel the need to understand the lack of compression!


-Steve

[Seawolf]

Is your camshaft actually turning on cylinders 2- 6 ?

I've seen a camshaft snap after the #1 cylinder bearing when a piston/valve collision has occured on that cylinder.

Either way that head has to come off to assess how much damage has been done - unfortunately it's usually substantial.

[13Stripes]

Is your camshaft actually turning on cylinders 2- 6 ?

I've seen a camshaft snap after the #1 cylinder bearing when a piston/valve collision has occured on that cylinder.

Either way that head has to come off to assess how much damage has been done - unfortunately it's usually substantial.

Yes, I should have been more detailed about that. I will update the original post.

With the valve cover off, I turned the crank by hand and watched the camshaft turn at all 6 cylinders. I was able to watch intake and exhaust valves open/close for each cylinder, with the exception of intake on #5 since the rocker is broken off. I also confirmed spark and fuel at each cylinder (while troubleshooting, prior to discovering the broken rocker/head issues).

I'm expecting a fair amount of work to be done. But with the cam turning, valves opening/closing on time, spark and fuel (at the assumed correct time), I still think there should be SOME compression or perhaps even an attempt at fire/combustion. Again, this is purely driven by my curiosity - not any expectation that I will be solving a problem by finding compression.

[spdracrm3]

Most likely bent all the intake valves(can and does sometime happen depending how belt breaks/skips), you can check clearance at rockers as you rotate motor (excessive clearance=bent valve). Its a boat anchor anyway

Sent from my SM-G360V using Tapatalk

[AndrewBird]

There is no way an M20 can survive a timing belt breaking without damage. It's physically impossible. You need a new head, possibly new pistons/block if more was damaged.

[jeffnhiscars]

There is no way an M20 can survive a timing belt breaking without damage. It's physically impossible. You need a new head, possibly new pistons/block if more was damaged.

This.

100% mandatory to remove the head after the belt breaks to replace numerous bent valves, possibly guides and in many cases such as this one, the entire head.

If the Pistons survived consider yourself lucky.

[TobyB]

valve bendage is certain.

t

[13Stripes]

Most likely bent all the intake valves(can and does sometime happen depending how belt breaks/skips), you can check clearance at rockers as you rotate motor (excessive clearance=bent valve). Its a boat anchor anyway

Sent from my SM-G360V using Tapatalk

Interesting. I hadn't considered that it would be possible to literally bend ALL of the valves, but assumed that at least some of them would be bent. Initially, I thought that if the belt broke then only a subset of the valves would be in a position where they were "out of time" and in danger of a piston collision. After thinking about this some more, even at 1000RPM there would be at least 16 rotations of the crank per second, which makes it seem more likely that inertial motion could continue to spin the cam enough that every valve may open at the wrong time and be damaged.

I may take your advice and check clearance. Any one have experience with a broken timing belt bending all valves?

This.

100% mandatory to remove the head after the belt breaks to replace numerous bent valves, possibly guides and in many cases such as this one, the entire head.

If the Pistons survived consider yourself lucky.

I sincerely appreciate the feedback from all of the replies but I want to reiterate - I understand that there's significant damage most likely. This thread isn't questioning whether I need to remove the head, or even "how much trouble am I in", but more focused on understanding what exactly happened here.

It's a hobby car and when I've got a learning experience at hand such as this, I am just looking to get the most out of it!

[turbo55]

I would think that if at least one valve is bent on each cylinder, then the combustion chamber will always be open, either to the intake or exhaust. Therefore, you would have no compression.

[RobDog]

My dad's timing belt skipped 2 teeth. I thought maybe I could set the timing back 2 teeth and hope the valves were not toast. The engine would turn over slowly and made a whirling sound also. The engine had no stroke/compression sound like a normal engine. When cranking a healthy engine over you can hear the number of cranks/revolutions of the engine. This engine had no such sound. I took the head off and found 6 bent intake valves. The pistons bent every intake valve just slightly enough to have zero compression on all 6 cylinders.

[Cabriolet]

buy a bentley's manual
the head is toast
you will need another good head. have it decked and pressure tested.
you will need new BMW head bolts and head gasket.
then replace the timing belt, water pump, get bmw coolant and an oil change.
it takes about a week for most first timers and will run you about 800 to fix it correctly.

[13Stripes]

My dad's timing belt skipped 2 teeth. I thought maybe I could set the timing back 2 teeth and hope the valves were not toast. The engine would turn over slowly and made a whirling sound also. The engine had no stroke/compression sound like a normal engine. When cranking a healthy engine over you can hear the number of cranks/revolutions of the engine. This engine had no such sound. I took the head off and found 6 bent intake valves. The pistons bent every intake valve just slightly enough to have zero compression on all 6 cylinders.

Got it. That's insightful. I most likely have bent valves in each cylinder then also, and explains no compression. Thanks!

buy a bentley's manual
the head is toast
you will need another good head. have it decked and pressure tested.
you will need new BMW head bolts and head gasket.
then replace the timing belt, water pump, get bmw coolant and an oil change.
it takes about a week for most first timers and will run you about 800 to fix it correctly.

I have a Bentley's manual. It's fantastic.
Timing belt and water pump are brand new.
Already started shopping for a head, however, sounds like there is strong potential for piston damage.

Starting my shopping list now. Not building a race car, but while I'm spending cash, I'd like to make it a little faster. I'll hit the search function today for more info, but if anybody is scrolling through here and has recommendations/experience with swapping in a replacement engine with some upgrades, I'd like to hear what you've done:

Replacement head (or complete engine)
Rebuild head with bimmerheads cam and better-than-stock rocker arms/valves?

[turbo55]

From what I've heard you want a new timing belt too, those are one time use. The belt alone is pretty cheap.

[101]

Agree with everything said above.

The M20 engine in my E28 snapped a timing belt at low engine rpm's. The pistons had some minor valve "kissing", but were otherwise OK for reuse. I did not try to do anything with the head - because eta. Found a used head and went on with life.

Depending on your budget and the amount of engine damage you find once the head is removed - you can replace the head with a used head (after having it checked at a machine shop), swap the complete engine with a used one (if pistons / bores are damaged), or go for a higher performance head such as the one offered by "Bimmerheads".

In any case use quality gaskets and rebuild parts. I use all OEM BMW gaskets and head bolts in my rebuilds.

[modernist]

In any case use quality gaskets and rebuild parts. I use all OEM BMW gaskets and head bolts in my rebuilds.

Yup, especially head bolts. Elring gasket sets are excellent as well. Stay away from Victor Reinz head gaskets.