M42 Rough Idle/Loss of power at lower RPM

[bmwman91]

Fuel was dripping from where the hoses are clamped onto the nipples on the filter body? That is not ideal lol. If it was coming from both sides, then there is possibly some obstruction somewhere in the line between the filter and the fuel rail. You have a very weird problem on your hands...the symptoms are pretty hard to make sense of!

[ScottySbk]

Fuel was dripping from where the hoses are clamped onto the nipples on the filter body? That is not ideal lol. If it was coming from both sides, then there is possibly some obstruction somewhere in the line between the filter and the fuel rail. You have a very weird problem on your hands...the symptoms are pretty hard to make sense of!

exactly, but the thing was i check fuel pressure before and after the filter and it was 29-30 psi. If after replacing them doesn’t fix anything you got any other ideas ? I’ve done vacuum lines and intake gaskets, injectors, cts, fuel filter, fuel pump, and spark plugs. What about bad ignition coils or so?

[bmwman91]

exactly, but the thing was i check fuel pressure before and after the filter and it was 29-30 psi. If after replacing them doesn’t fix anything you got any other ideas ? I’ve done vacuum lines and intake gaskets, injectors, cts, fuel filter, fuel pump, and spark plugs. What about bad ignition coils or so?

Any updates on this?

[jsnppp]

Great thread. My M42 is currently taking 4-5 seconds to start in the morning and I think it may be fuel pressure related. All other drivability/idle is good.

Thanks folks.

[ScottySbk]

So sadly I thought it was fuel pressure related and maybe the new pump I replaced went bad as the fuel pressure seemed off to me, I drove it after replacing the pump and it drove fine besides one little hiccup. The issue came bad shortly after and i’m kinda lost at this point :(

[bmwman91]

Per your original post, you have a 318i, so am I correct in assuming that your battery is under the hood and not in the trunk?

How old are your spark plug wires? Those can have insulation failures and arc to the head sometimes. The ignition coils do eventually fail as well which can definitely lead to weird intermittent issues. Sometimes you can see if they failed by looking on the molded epoxy body for bulges and/or bubbling, although this is not always the case.

[ScottySbk]

Per your original post, you have a 318i, so am I correct in assuming that your battery is under the hood and not in the trunk?

How old are your spark plug wires? Those can have insulation failures and arc to the head sometimes. The ignition coils do eventually fail as well which can definitely lead to weird intermittent issues. Sometimes you can see if they failed by looking on the molded epoxy body for bulges and/or bubbling, although this is not always the case.

No so it’s in the trunk, wire wise I check with a multimeter and also moved the wires around to see if there were any changes via the multimeter on all 4. I’m sure that doesn’t confirm they are good 100% but i’m sure it’ll tell whether they haven’t any damaged wiring and such. Super lost tbh :(. They’re a shop super close that doesn’t german cars down the road, depending on the issue i’d really just like them to diagnose it but idk how much that’ll cost. don’t care at this point was just hoping I was capable of figure it out lmao

[bmwman91]

OK, so if the battery is in the trunk then there is one possible issue related to that that has happened to people, and it leads to very similar problems.

You will see 2 wires coming from the positive terminal of the battery...a really big one, and a smaller one. About 50cm from the terminal, along the smaller wire, there is an inline 50A fuse crimped onto it. This wire is a dedicated power line for the ECU and all related engine electronics. It has happened that corrosion can attack the crimped connections to the fuse and cause intermittent power loss to the ECU.

There are a couple of ways to check. The first one, which is a lot easier but less reliable, is to test the voltage up at the terminal block on the firewall. You will see both the large and smaller power wires come out of the firewall on the right side (US passenger side), and you will see that they are connected to separate terminal blocks. The 2 blocks are electrically separate. See what the voltage measures as on the upper block where the smaller wire is, specially if the car won't start or is running badly.

The second way to check which is harder but more certain is to unwrap all of the tape from the power wires in the trunk and fully inspect the 50A fuse and crimp connections. Obviously, do this with them unplugged from the battery. If the fuse has gone bad, you can get a new one (BMW PN 12411706111).

[jsnppp]

Didn't know about that fuse, now I want to check mine regardless! Thanks.