97 M3 Cylinder#5 low/no compression

[E30Gus]

Hey everyone
I recently bought my M3 with 190k on it.
I was driving it the other day (to VIR no less) and all of a sudden I loose power, runs rough, and it throws the check engine light. I pull over and check to see if there is anything obvious going on then limp home a few miles away.
My first thought was a bad coil pack or plug since I recently changed the plugs. It is neither. Then I do a dry compression check on #5 and it shows 45psi.

After a bunch of reading I find that I could have dropped a valve, have a bad lifter, or the least likely have bad rings. I did a quick wet compression test cranking the engine over by hand on the crank nut and I got 0psi. I don't know if that is the correct way of doing it or not.

I pulled the valve cover and I see nothing out of the ordinary going on. I realize that some things you cant diagnose with just a visual inspection.

One question is how would a collapsed lifter cause low compression if the valve is normally closed?

Also any ideas as to what I should check next before pulling the head and sending it to the machine shop?

I'm really hoping I am overlooking something simple and don't have to do head work.

Thanks
G

[Dakotademon7]

I would pressurize the cylinder with compressed air and see if its blowing past the piston or the valves. If it's the valves, you will know witch one by where the air is leaking from.

[E30Gus]

I would pressurize the cylinder with compressed air and see if its blowing past the piston or the valves. If it's the valves, you will know witch one by where the air is leaking from.

I just did this with about 40psi and found it is one of the intake valves. So would that mean it is the valve retainers that fell off? And if so do I NEED to pull the head to fix this?

FWIW I can see the valve stem with a mirror and one seems to be lower than the other judging by the wear marks on the stem.

[Dakotademon7]

I can't really tell you what to do next. You will have to inspect the head to determine why. It could be a number of things. I would just pull the head because you will want to pull the valve to make sure it's not bent and make sure there's no damage to the top of the piston.

[E30Gus]

OK thanks!
I guess with the cams coming out I might as well take the head out and get it checked over, replace the valve seals, all the retainers, and check the valves/seats. I have the vanos rattle/seal kit ready to go as well. Then hopefully the engine will last until ~300k

Can I re-use the head bolts? LOL just kidding.

[E30Gus]

Got another question...What exactly is the "upgrade" for the valve keepers and retainers so I don't drop any more valves in the future? And I was told it would be a good idea to replace all the exhaust valves due to the age and the fact that they have been heat cycled so many times. Doesn't seem 100% correct to me. I always thought if the valve was straight and was going to get a 3 angle job then run with it.

[matthugie]

The weak valve retainers were only an early s50 issue that they fixed after about 8 months of production. I haven't heard of people having issues with the s52 retainers.

[E30Gus]

The weak valve retainers were only an early s50 issue that they fixed after about 8 months of production. I haven't heard of people having issues with the s52 retainers.

I'm not 100% certain it is a dropped valve but something is causing a leak on the top end. The head will be off tonight then I can look into it more.

[UNHCLL]

I just did this with about 40psi and found it is one of the intake valves.

When you did this did you make sure that cylinder was a TDC?
If it wasn't, this test is inconclusive.

[E30Gus]

When you did this did you make sure that cylinder was a TDC?
If it wasn't, this test is inconclusive.

Yes! hey I'm from Rye. Went to Portsmouth HS

[E30Gus]

I guess i am "lucky" The HG blew between 5 and 6. I've never had a HG go this way so put this new learning experience in the books.

[justin_l_]

Well, either way you were going to pull the head, at least this way there shouldn't be any real damage.

[E30Gus]

There is evidence of a small piece of metal floating around in the cylinder but the damage is very minimal. I took the cams off yesterday and I am a little bit worried about the amount of scoring on the cam's journals/bearing surfaces. I may have to post a pic to get some opinions.

[E30Gus]

So this is where the HG poped.
Car is all back together with fresh head work and rebuilt Vanos running very smooth and feels like it has more power.

I'm going to do an oil change here soon as cheap insurance since I cleaned the pistons pretty well.