Any engine experts that can help re some technical questions on an bmw s85 motor?

I'm not familiar with your motor specifically, but I do build engines...

When you say zero compression in all cylinders, do you mean you are doing a compression check (with the throttle body held open) and the readings are around zero PSI??

Also, has anyone done a leak down test in the cylinders, as opposed to just a straight compression check?

Do you happen to know the individual piston to cylinder wall clearances for each individual engine bore?

Also, is this a direct gas injection motor or a port fuel injected motor?

Ray

 

https://en.wikipedia.org/wiki/BMW_S85

Quite the motor you're dealing with over there. I wonder if the ITB's are opening properly?

Ray

 

Ray is correct, leak down will tell you where the issue is. I had similar issue, bent every intake valve was the issue.

Compression test tells you nothing about diagnostics. Exept you have an issue. Leak down is next.

Sent from my SM-G990U using FerrariChat.com mobile app

 

I have been working on restoring a 2007 M5 and have a pretty good understanding of the engine. I was able to get a Troy 5.5L rebuild. It has been great but I should have just replaced everything else when I put the motor in. Since the rebuild I think I have replace most of the ignition and air control systems and had to work on the diagnostic of each.

I am not an engine builder and not sure if something in the engine rebuild could cause the compression to come and go. Normally when I see compression go it doesn't just come back. Are you able to use a boreacsope to make sure there are no damage in the cylinders? Any fuel in the oil? If there is something mechanical wrong from the rebuild causing a loss of compression I would think it would either show up on the cylinder walls or in the oil.

It could have something to do with the ITBs. My understanding is engine doesn't open the throttle bodes until over a a few thousand rpms. Under that it relays on the idle control valves. The throttle actuators do go bad. Each of mine went bad intermittently and it was frustrating to verify before just throwing cash at them.

Do you have an check engine lights?
Is the engine going into limp mode? If so what rpm is it limiting the engine to?
How many miles on the rest of the engine components (ionic control module, throttle actuators, idle control valves, plugs and coils, fuel innjectors)?
Are you able to monitor with INPA?
Have you tested for a small vacume leak that could be sealing itself when the engine gets up to temp?


Sent from my SM-G973U using Tapatalk

 

Was the engine rebuilt because of a rod bearing, fuel hydrolock, or some other reason?

Sent from my SM-G973U using Tapatalk

 

Wow 2 spun bearing on a new build. Doesn't give much confidence in the build.

Generally my misfires have have been caused by the ionic control module or coils going out. I haven't had any luck with non bmw coils. The engine uses the spark plug as a sensor to measure the quality of the combustion and as a knock sensor.

Also a vacuum leak can cause it to misfire.

There is a vanos high pressure oil line that should have been replaced when the engine was rebuilt, and the vanos bleed procedure should have been done a few times via INPA.

Also the engine shouldn't be rotated backwards since it can damage the vanos pump.

Sent from my SM-G973U using Tapatalk

 

Maybe the timing is slightly off and the vanos adjustment is compensating for it after the engine is warm. Seems like that could explain the weird compression. Would have to be extremely lucky to get the timing off that would cause this but not have the valves hit the pistons.

I think you could use INPA to log the different sensors and get a much better idea of what exactly is wrong. There is a lot of data you can get out of the engine. If you are able to do that I could look at the values my engine is giving and see where the differences are.

Sent from my SM-G973U using Tapatalk

 

If you had a spun bearing just after a rebuild, that brings into question a lot. Either there is a serious lubrication issue, or someone in the chain of rebuilding doesn't know how to use a micromoeter or something. My concern would be that tolerances were not accurate when boring/honing the cylinders relative to the piston sizes.

Usually failure to rev (not counting ECU intervention for some reason) relates either to breathing restrictions some place in either the intake system and/or the exhaust system. The other possibility would be having valve timing relative to the crankshaft off a bit. I don't know your motor first hand, but skimming through Wiki, it looks rather advanced. I would also be concerned that something could be damaged in the electronic ignition relative to the ECU understanding crank position. You've got to be very careful about some of those crank trigger components.

As far as "zero compression", where are you getting this figure from? Having "zero" compression in even one cylinder would be alarming, let alone all of them. Even if you have a hole in the top of your piston, a compression check would probably still be able to muster 10 or 20 PSI on the gauge. Even with a spun bearing, you can usually get 40 or 80 PSI.

Spun bearing is very bad on a rebuild... something had to have been way off the mark. Bearing clearance usually is held to around .001" per 1 inch of crank journal diameter, so we are talking clearances here of around .0020 to .0025 typically on a performance engine. That's far too small to allow any properly installed bearing to spin over on itself.

Ray

 

Anyone talented enough to rebuild your V8 correctly ain't working at a BMW dealership. They are most likely off working in Formula 1 or MotoGP.

Ray

 

That sounds almost like a coolant temp sensor or something is sending the ECU(s) incorrect information during the warm up phase. I don't know the specifics of your particular motor, but keep in mind that gas has to atomize to burn correctly. When things are cold, motors usually jump through a lot of hoops to try to get the show on the road and warm everything up so the fuel will vaporize better. I wonder if some circuit relative to helping that is out of wack or not functioning.

If you restart the engine after it's up to temp, does it still run fine?

Ray

 

Well said.

Compression test tells you there is an issue.

Leak down test helps you identify what the issue actually is.

I just did a compression check on my Toyota.

Cylinder #1 - 185
Cylinder #2 - 135
Cylinder #3 - don't remember because I was in shock from seeing the readings on cylinder #2, which is normally ~ 180
Cylinder #4 - 160 - double shock, since it's normally 178 +/- 2 PSI

So off the cylinder head came... discovered lots of bad things. All the compression rings on all the pistons were broken. Piston #2 had holes burned into the upper ring lands. Part of the ring made its escape into the bore/cylinder head chamber and went to town. Here's the current fun over here:



I think on the BMW, you need someone there who knows what they are doing. Get some compression readings and more data. Also, if anyone worth their salt rebuilt your motor, there shouldn't be any problem getting the rebuild information on the motor - for example, what were the individual cylinder wall clearances, etc. I don't think that's your issue, but you never know. I was just watching a YouTube video series where this guy was rebuilding his Toyota truck and he sent the block out to be re-bored. It came back and he checked it: block was bored incorrectly by .002". Turns out the shop he used allowed "the new guy" to bore it and he wasn't paying attention to something. They ended up having to buy the guy a whole new block.

So be careful out there.

Ray

 

I think a leak-down test should be your first diagonist test. The VANOS system doesn't control the valve lift; it changes the valve timing. If there is very low compression when the engine is cold, the valves many not be completely closing. The hydraulic tappets should compensate for this, but IMO your problem is in the cylinder heads with the valve train.

 

As Wmuno mentioned above, valves are always something to consider. If the valves were replaced (new seats, multi angle valve job, valves replaced, reground) but not properly lapped in, they can sometimes not seal well and leak. I'd be interested to know two things regarding the valves:

1. Was anything done and if so, what
2. Were they lapped in or just cut/ground - and what was the target seat width

Stuff like VVTI and VANOS always makes me nervous. Simple motors usually are more reliable motors in my experience.

Ray

 

Tight valve guides? (hanging valves at different temps)

 

Loose valve guides mainly would affect the seating of the valves on the seats (which would allow compression to escape). It would negatively affect performance, but usually not to the level Craze is experiencing. Whatever is going wrong with his motor has to be something fairly big, for example drastically wrong cam timing, some major blockage in the exhaust system or some failure in the ITB's opening, etc. Also, incorrectly set ignition timing could do it.

Valve guides not being tight is like the icing on the cake. In this situation, we are missing the cake I think.

Ray

 

Right, that's why I said tight valve guides, which would hold the valves open.

 

You mentioned heads reconditioned. Did it get new guides and valves? I've seen this exact same thing (on other engines not S85) when the guides were not honed for clearance.

(which would change depending on temp due to different thermal expansion of the valves and heads/guides)