little knocking on cold engine

[abit]

Hi, what could cause little knocking sound when starting cold engine, and when its colder it takes little bit longer when it shut's up :D on summer time when it's about 15 degrees celsius it takes about 15secons little knocking and then shuts up as engine starting to warm up.
all started last year, when I built m20b28 stroker, m52b28 crank, m20b20 rods and m20b25 pistons, after about 15000 km's was driven on that engine, it started to do this thing.
then engine was disassembled for winter rebuild, nothing wrong was noticed, just piston rings were pretty worn out (honed block, but didn't changed rings, not smart, but, done that :D), thought that rings was the reason.
So changed all bearings, piston rings etc, but already at first start at winter sound was still there.
rod small end bushings looked ok. rockers are dialed in 0.25, valvetrain sounds smooth and silent.
only thing was 2 rockers, that had liiitle more play than others, they both had one streak from one side thru to other, but I don't think, that that could cause that.
Form me, It feels like sound comes from bottom end, but from where...

[nomansland92]

Did you blueprint the motor when it was apart? Maybe just piston slap

[abit]

Did you blueprint the motor when it was apart? Maybe just piston slap

block was measured and it was ok, 6th cylinder was bit worse than others, but still in norm.

measured also piston play in cyl bore like pushing piston to one side and measuring gap on other side, then got other pistons, from low mileage engine, and gap was the same

[nomansland92]

Did you use a torque plate when honing the cylinder?

[abit]

Did you use a torque plate when honing the cylinder?

It was done in machine shop, don't know if they used torque plate, I know that honing was done using flex hone technology

[nomansland92]

Try pulling each plug wire individually when it's cold to narrow down which cylinder is making the noice. When you pull the correct plug the knock will most likely stop since there will be no combustion.

[I got it, yes!!]

Mine does this too. Piston slap. It goes away when warm. Has done it for 3 years now

[redlightpete]

My m20 has been doing this since I bought it, 15 years and 180,000 miles ago. Potentially not a big issue.

[abit]

ook, thanks

[TobyB]

15 seconds?

do you get oil pressure as soon as it starts?

I have a 2002 that does this- it's snotted out one of the bypass o- rings,
and oil leaks down when it sits for a while...

t

[abit]

15 seconds?

do you get oil pressure as soon as it starts?

I have a 2002 that does this- it's snotted out one of the bypass o- rings,
and oil leaks down when it sits for a while...

t

ok, in not so warm days it goes away about in minute.

oil pressure light comes off as soon as the engine starts up, sometimes when engine don't want to start on moment, pressure is enough to turn off light even with starter.

edit: ahh, I know what you think, this is mostly heard about m30 engines, when some valve in oil filter housing goes down and every engine start is like when you freshly changed oil :D but this in now that.

but where in m20 these bypass rings/valves is located anyway ?

[jlevie]

The first task is to identify where the sound is coming from. While that can be a DIY by someone with a lot of BMW engine experience using an automotive stethoscope, for most folks this is a job for a pro.

You've done a lot of work on the engine, but I don't know if it was done correctly. In particular the rod and main journals on the crank must be measured to a tenth and the bearing "color" chosen to match the journal dimensions for the correct clearance. The journals must also be checked for out of round wear and new rod bolts used. The rods and their caps must be matched pairs.

The Oil Pressure Relief (OPR) valve screws into the block. Any time you rebuild an engine you should use a new BMW OE pump, OPR, and seals. Also when rebuilding an engine it is critical to check the intermediate shaft bearings for wear. Replacing them is not a trivial task and requires a special order of the bearings from BMW. Worn intermediate shaft bearings have a big affect on oil pressure.

The oil pressure light is not particularly useful. The light goes out at about 7psi, which pretty much tells you nothing. The only way to get good data is to at least temporarily replace the switch with a pressure gauge and see what the pressure is on a cold and hot engine and while driving. You should see at least 60psi at 6k on a hot engine.

[ADEN]

Mine (stock m20b20) is knocking like a diesel engine till temp gets out of the blue area it is like this for years, engine has been rebuilt 2 years ago.I thought this is normal till i saw this thread.