m30b35 rough as a dogs arse when cold

Fri Oct 12, 2012 11:23 am

My m30 is still bloody rough when cold, sounds like its running a massive cam.

Interestingly if I push the revs past 2.5k when its cold and then let them drop it begins to run smooth.

What is up with this damn engine?

Fri Oct 12, 2012 5:34 pm

My m20 used to do that so everytime it was cold i reved it up then let it idle. never got to the bottom of it though sorry

Fri Oct 12, 2012 6:01 pm

So its obviously an occurring problem, did you ever have any problems with your alternator Mike? I think mine might be an alternator problem, possibly not giving enough charge at idle and needing a slight boost.?

Sat Oct 13, 2012 8:17 am

Also have a look at the vacuum system as if it is blocked this can happen. And also a leak round the inlet can do this.

Sat Oct 13, 2012 10:08 am

Cheers Zaust, I'd have another look at the vacuum system. Strangly when I give it that blip to make it idle smooth, the cluster lights go brighter all of a sudden.

Sat Oct 13, 2012 2:54 pm

Never had an issue with my alternator and all my dash lights stayed the same, no getting brighter or flickering

Sat Oct 13, 2012 3:20 pm

Have you checked the throttle position switch (TPS)? It needs to be working correctly to instruct the ECU to switch to the idle map.

It's fairly normal for the alternator to initially put out a low charge until it's revved a bit - most need a bit of a rev to get them to kick in properly.

Sat Oct 13, 2012 3:29 pm

Yeah TPS is calibrated correctly, I started it up today and it didn't seem that bad. I'll keep an eye on it.!

Loving the M30 chip by the way Maurice, my wife doesn't like my car though. She said it keeps thrusting her head into the headrest... lol

Sat Oct 13, 2012 3:44 pm

Good to hear that, Chris.

My 1988 F-reg 535iSE also had some issues with cold starts. It had an aftermarket adjustable fuel pressure regulator fitted by a previous owner, so I fitted a standard BMW one (from an M30B35) and it made things worse!

It went very well otherwise though, so I didn't bother tinkering with it too much.

Sat Oct 13, 2012 5:07 pm

Remember you saying about your 535 dodgy FPR, mine seems to run better when I take the vac hose off mine and block the vac hose. Seems to smooth out. How do you test a FPR?

Sat Oct 13, 2012 5:13 pm

I suppose the only real way to test it is hook up a pressure gauge to the fuel injector rail to ensure it gives the correct pressure, i.e., maximum 3 bar (43.5 psi).

Oddly, both M20B25 and M30B35 engines use the same 3.0 bar FPR.

I found this snippet on another forum:
"At idle pressure should be 37-38psi, with the vacuum hose disconnected pressure should be 43-44psi."

So, we can assume that with the hose disconnected, or with low inlet manifold vacuum (i.e., when accelerating hard), the pressure will be at the maximum 3 bar.

Tue Nov 06, 2012 2:59 pm

Thought I'd give a quick update on this issue I've been having.

The horrible 5 cylinder like rough idle when cold has been sorted, previous owner had wound the air screw on the AFM a quarter turn from all the way in....

I've leaned it out 4 turns, which gives a smooth idle and good throttle response, I'll go visit a mate of mine with a gas analyzer and set it properly soon.

Glad to have finally figured it out.! Thanks for all your input guys, you're a decent bunch.!