Low vacuum readings

[Mixzzz]

Recently I had to take of the head of my M20B25 off (its in 1988 e34 tho) due to the fact that the head gasket had corroded and one of the rings collapsed causing pressure to escape to the cooling system... I ended up getting another head anyway because the valve seats were heavily worn. Got done everything I could:
head cleaned and a bit shaved but so slightly I didn't need the thicker HG
valves got lapped and few of them changed
somehow machine shop fixed the valve stems or valve guides because theres no movement anymore there
valve stem tops cut off a bit so theyre flat
rocker arms sanded so theyre smooth again
got another cam because mine was worn

The cam I put in was from M20B20 but according to this link the only difference was that it was 6 degress advanced. We even checked ourselves and they really seemed to be almost the same. Since this cam was near perfect we decided to put it in.

The car now has been running great and quiet for almost 200km and it does feel like it has more power in low range which is ideal for city driving but the idle is rough. I also put in M52B25 injectors since I had a refurbised set laying around and my original ones were dirty and one wasn't really closing. Also Im running first gen LPG and that also seems to have rough idle, maybe a little better, but still rough.
Vacuum lines all have been replaced including the vent hose that comes from the head cover. Dipstick oring is the same, but I inspected it and it seems ok, when putting back it felt like it went in there pretty snug. So far I haven't replaced only oil filler cap, dipstick orings, and throttle body gasket and the gasket of the thing that goes on other side of the manifold opposite the throttle body.

Yesterday I measured the vacuum and it read 52kpa. Somewhere I found that it should be 57-72kpa according to bentley. Before rebuild it was somewhere about 60kpa I think. Now after more research it found that cam advancing can decrese the idle quality and decrease the vacuum. Is that true or theres still a vacuum leak somewhere?

If I really need to change the cam I would rather get adjustable cam gear and just retard it 6 degrees. Are any of the ebay options that are usable?

[digger]

the big thing that reduces engine vacuum is cam overlap. when you advance the cam the overlap doesn't change. ive advanced cams 8* before and there was no real difference in idle quality (i never checked vacuum though). you can get an adjustable cam gear for the m20 but i would be looking elsewhere for issues first. do a smoke test, etc

[Mixzzz]

the big thing that reduces engine vacuum is cam overlap. when you advance the cam the overlap doesn't change. ive advanced cams 8* before and there was no real difference in idle quality (i never checked vacuum though). you can get an adjustable cam gear for the m20 but i would be looking elsewhere for issues first. do a smoke test, etc

For some reason I can't find a place in my country where I could do a proper smoke test But about the vacuum vs cam timing I read when advancing cam you obviously change the time when exhaust valve closes causing more pressure to be left in the cylinder and causing less air to be sucked in. Isn't that true or the timing change is so small it shouldnt really changing anything?

[digger]

when you advance the cam you change all events the same amount so exhaust opens earlier and closes earlier. the differences to vacuum should be relatively small, it wont suddenly start to idle roughly in my experience with relatively small change

[Mixzzz]

when you advance the cam you change all events the same amount so exhaust opens earlier and closes earlier. the differences to vacuum should be relatively small, it wont suddenly start to idle roughly in my experience with relatively small change

Hmm, any tips on DIY smoke test? :D

[digger]

youtube will have some

[Mixzzz]

youtube will have some

Im going to do a compression test tomorrow, but my dad doubts theres a problem in compression since my engine is not using oil(after 9Kkm it has used only 0.6l and some of that leaked out due to small crack in HG). Is it possible for compression rings to go bad but the oil control ring could still be good? Also if my compression readings are good, should I still try to do the leakdown test?

[digger]

Using oil has nothing to do with good or bad compression. It's a good test to do. Depending what you find a leakdown might also help

[Mixzzz]

Using oil has nothing to do with good or bad compression. It's a good test to do. Depending what you find a leakdown might also help

If I'm losing compression the only place where it could leak is past compression rings cause I doubt that valves would could be leaking already after the machine shop.

[Mixzzz]

the big thing that reduces engine vacuum is cam overlap. when you advance the cam the overlap doesn't change. ive advanced cams 8* before and there was no real difference in idle quality (i never checked vacuum though). you can get an adjustable cam gear for the m20 but i would be looking elsewhere for issues first. do a smoke test, etc

Well, today we plugged literally every hole that it had and the vacuum didn't change a bit, it was at 52kpa, maybe went up to 55kpa but not for long. Had to use silicone on the thing that screws onto manifold with three screws and has a tube running to fpa. I am really starting to think that it has to do something with the cam timing, some more sources confirm, that when running a retarded cam you get better vacuum and smoother idle.

[digger]

the engine the cam came from was a stock 2L, they seem to idle fine they use basically identical electronics to the m20b25 but different chip. the smaller engine wouldn't cope as well as a bigger engine.

people use a 272 cam upgrade sometimes which actually increases overlap, it idles not quite as smooth but perfectly fine if the rest of the system is in good condition

if you had it as far off as a tooth i could see some issues but a few degrees i cant see it causing a 20% change, note without hearing how it sounds and the exact symptoms it it is hard to diagnose. maybe the stock motronic is much more fussy than the ECU i use but like i said it worked fine in a m20b20 engine

on a 4V per cylinder engine with separate inlet and exhaust cam it would make a noticeable difference

you can get an adjustable cam gear and new timing belt and set it up in the straight up position, its not that difficult. you might find you prefer it retarded for how it drives

what does the compression test say? you would hope the valves are sealing that is for sure...

what are the specs of the M52b25 injectors?

[Mixzzz]

the engine the cam came from was a stock 2L, they seem to idle fine they use basically identical electronics to the m20b25 but different chip. the smaller engine wouldn't cope as well as a bigger engine.

people use a 272 cam upgrade sometimes which actually increases overlap, it idles not quite as smooth but perfectly fine if the rest of the system is in good condition

if you had it as far off as a tooth i could see some issues but a few degrees i cant see it causing a 20% change, note without hearing how it sounds and the exact symptoms it it is hard to diagnose. maybe the stock motronic is much more fussy than the ECU i use but like i said it worked fine in a m20b20 engine

on a 4V per cylinder engine with separate inlet and exhaust cam it would make a noticeable difference

you can get an adjustable cam gear and new timing belt and set it up in the straight up position, its not that difficult. you might find you prefer it retarded for how it drives

what does the compression test say? you would hope the valves are sealing that is for sure...

what are the specs of the M52b25 injectors?

M52b25 injectors are 17.5lb/hr 4 point spray. I know they're larger than stock but ECU adapts easily.

Turns the cause was valve clearances. Today we adjusted them 0.20mm at camshaft side. On some valves the gaps were 0.10mm and smaller - last time we adjusted them when head was on table. Problem solved, the idle is good now on LPG, need to clean injectors for gas to work properly but otherwise it's good. Haven't checked vacuum yet tho

[digger]

Yeah you always need to adjust them after you torque the head onto the block. Tight clearances increase overlap so Vacuum should improve making it correct

[Mixzzz]

Yeah you always need to adjust them after you torque the head onto the block. Tight clearances increase overlap so Vacuum should improve making it correct

http://www.realoem.com/bmw/enUS/showparts?id=HC11-EUR-03-1988-E34-BMW-525i&diagId=11_0861

I was wondering whether part #1 is just a tube that just splits or there should be something in there? Because mine is empty and I quite can't seem to understand the point of it.

[digger]

that link takes me to the realOEM homepage so not sure the part you mean