[Trick440]

I have an n55 manual and was racing it when I tried shifting from 3rd to 4th, I accidentally shift from 3rd to 2nd. RPM went to the moon, car immediately lost power and coasted to a stop. It would not start anymore. Its called a 'Money Shift' because you blow your motor up.

I took it to a shop that does BWM and mid to high tier sports cars. They did a compression test and it failed. I forget the numbers but basically every cylinder was down with some at zero. We decided to go with a used 39k mile motor. They get this new motor install and now this replacement motor is doing the same thing. It won't start and it failed compression test. wtf?

So now I have the old motor at my house and I'm opening it up out of curiosity and to potentially rebuild it as an extra motor. I get the head off today and the valves and pistons look fine. Nothing is showing that the valves hit the piston or anything that to me would look like a blown motor. I spun the motor to test the crank and everything seemed to work as it should. Now I'm not a mechanic by trade but I do do it has a hobby and this is starting to sound like my original motor isn't bad.

My guess would be it jumped timing possibly and that's why the no compression. But the kicker is why would the replacement motor not be starting either?

So now I'm going to take the head to the mechanic and show him it looks fine along with photos of the top of the pistons and see what he says.

I'm just lost with this and could use any advice you might want to share.

[khj24]

Not a tech by any means, but a little confused that they would run a compression test and say the motor is toast immediately. In my mind before or after the test you would scope down the plug holes to see if the pistons and the valves got into a fight which doesn't look to be your case. If it's freely cranking without obvious piston/valve damage that would lead to timing issues being an obvious thing to look into? I wonder if you spun your crank hub like the s55 guys which threw your timing. I'm kinda with you on this if I'm reading into it correctly. No idea what's going on with the new engine though.

[thejeremyman9]

So is their compression tester junk?

But seriously, you do compression test, then you follow up with leakdown test afterwards if there is an issue to see WHY you failed the compression test - intake valves, exhaust valves, rings? Did they do a leakdown?

Spun crank hub is a possibility, and you could have gotten lucky and spun it only enough to go out of time but not to have piston-valve contact. Again, this is why you do a leakdown test.

How are the cylinder walls in the old motor you have apart? Post some pics.

And tell the shop to do a leakdown on the new motor thats in the car.

[harkes]

How does the cylinder walls look? Normally a piston ring failure would crack the piston but maybe your cylinder walls are scored?

Could definitely be a timing thing...

FYI a cold compression test should read ~190psi (what mine did).

A timing issue on the old engine does not explain the missing compression on the replacement - probably a bad compression tester meaning that your old engine might be ok...

[thejeremyman9]

Originally Posted by harkes

How does the cylinder walls look? Normally a piston ring failure would crack the piston but maybe your cylinder walls are scored?

Could definitely be a timing thing...

FYI a cold compression test should read ~190psi (what mine did).

A timing issue on the old engine does not explain the missing compression on the replacement - probably a bad compression tester meaning that your old engine might be ok...

Sounds like harkes and I are on the same page, basically same thing I said in my post above but he also provided some baseline numbers for comparison.

Post up some pics of cylinder walls.

Also ask the shop why they did not do a leakdown.

[sspade]

If the engine jumped timing it would have bent the valves.

The N55 is an interference motor.

[thejeremyman9]

Originally Posted by sspade

If the engine jumped timing it would have bent the valves.

The N55 is an interference motor.

Correct, but it depends on how much the crank hub might have spun. IIRC it can spin enough to lose time (and compression), without piston-valve contact. Its like jumping a tooth or two on a timing belt for other interference engines.