Need advice from experience for an M42 rebuild

[roguetoaster]

Those numbers are fine, ideally there would be less variation between cylinders, and you'd typically see about that amount of change hot versus cold.

[JonsE30]

yeah the first time was a rented one, this one is from amazon that had some good ratings. i took some pics and zoomed in and was off buy a little. i've read people getting over 200

cylinder 1 170
cylinder 2 177
cylinder 3 180
cylinder 4 176


update: took readings again after its cooled down for several hours

cylinder 1 185
cylinder 2 190
cylinder 3 197
cylinder 4 190

are these readings still good?

[roguetoaster]

Aren't those the same numbers from the cold test? I would've expected some change, but it could be a different tester giving different readings.

[JonsE30]

bmwman91 did the compression test while hot. i am getting 175-180 per cylinder.

[bmwman91]

Nice, yeah that is great to hear. Also, try doing the compression test when it is hot (after removing all spark plugs) and post the numbers. I'm curious to see what the values for a freshly built stock M42 are.

[JonsE30]

Yep got it. Compression tested again and it’s all good. Replaced it put everything back and tested the cooling system. Finally no new leaks. Temp held steady around half. Took it around the neighborhood and no overheating. Gonna take another compression test after to cools down afterwards cuz I’m paranoid. Good news is that the oil doesnt look milky.

[bmwman91]

Nope, not the right o-ring. The correct one is a lot beefier and fills the slot it rests in, protruding past the outside radius of the plastic as well. Make sure to get he Genuine BMW water pipe, not the aftermarket one, since the aftermarket one is prone to cracking. The parts you want:
11531714738
11531709157

[JonsE30]

I got leaking from the water pipe connected to the block. Is this the right o-ring? And is it installed correctly? Sent from my iPhone using Tapatalk

[JonsE30]

I’ll get the cooling system in ASAP. Damn I hope i didn't warp or crack the head. This engine is definitely schooling me.

[bmwman91]

Hmm, well 10 minutes with no coolant is a bit of a long time. The temp sender for the cluster isn't necessarily reading accurately since it is immersed in air rather than flowing coolant, hot spots can form since heat i sbasically only conducting in the metal at that point, etc. Anyway, your compression test likely ruled out a warped head, so it may well be bad gas or just normal condensation from combustion. Honestly, the main thing at this point is to get that coolant system back on there and give it a drive.

Look at it this way...worst case you have to pull the head and get it surfaced again to deal with warping, or it cracked and you need to find a good used one to get surfaced. You'd be in for $250-500 in parts and machine shop labor at that point which sucks, but isn't the end of the world.

[roguetoaster]

Likely water in the fuel from ethanol being hydroscopic. Fuel these days seems to be good for about 6 months in a full tank in an older car, although it will often still combust as gasoline is hydrophobic. You seemingly get much less storage time if the tank has more air space.

[JonsE30]

roguetoaster probably the longest i kept the engine running is probably 10 minutes so i can get the oil hot so i can find leaks. once i saw the temp reach almost half i shut it off.

bmwman91 the ambient temp was maybe 60-65, it was earlier this morning like 8AM when i strarted it up this morning.

i was researching online images to see if i get the sensors mixed up, the one closest to the front of the engine is a light blue sensor, which has a grey connector plugged to it. the orange one is in the middle of the head which has a black connector to it. the brown sensor plugged to the oil filter housing that has a black connector to it that looks like it was Y split from the gray connector going to the the blue sensor.

i dont have an oil warning light when the engine is idling/running. i have a quarter tank of gas left in the tank and it's been sitting there for 2 years during this whole rebuild if that means anything. does it run "less-rich" once i started driving it more? or is it something i can adjust right now? i don't have a check engine light.

[bmwman91]

The gray plug (with 2 populated terminals) goes to the coolant temp sensor for the ECU (also with 2 terminals). It is part of the same branch as the oil pressure switch plug (1 populated terminal). Then there is the plug for the coolant temperature sender for the cluster (1 populated terminal) and its plug is separate from the aforementioned two. I did once swap the two 1-pin plugs (which was scary since it caused the oil light to be on and the temp needle to fly into the red), but the one that matters in your case is the 2-pin one since the ECU reads that to define warm-up enrichments.

And yeah, if there is no coolant in the car then at least you know it isn't burning any! What is the longest that you ran the engine for without the cooling system?

If the HG was blown, I would expect at least 2 cylinders to read a bit low. When I had it go out between cylinders 2 & 3, I got ZERO compression on them.There were hairline cracks between 1-2 and 3-4 as well, and I was getting like 60PSI (cold engine). If you are going to fail, I guess you have to go big lol.

What are the ambient temperatures when you are seeing the "smoke"? I'd guess that it is just water condensation from the cold exhaust and rich running, which normally oges away once the engine warms up (which you have not had the chance to do yet).

[roguetoaster]

Could be any of the above, or water vapor from the exhaust system, probably not bad plugs. Plugs under the intake should have aged wiring that make them "belong" to only one sensor, either way, there's one one pin connector and one two pin connector IIRC which go to corresponding sensors.

*added something that was unclear before

[JonsE30]

ive noticed when i started the car this morning. the car was blowing white smoke, but i have good compression and my coolant system isnt in yet so it's not coolant, it's probably oil. if my headgasket blew wouldn't i get lower compression? i've read somewhere and i think it was commented here that the plugs under the intake would be switched and make it's run rich. could this be it. it's not like as bold or strong like when it blew before the rebuild.

or could it be the bad gas sitting in the tank? bad plugs? all the above? haha.