M42 Rough Idle/Loss of power at lower RPM

[jsnppp]

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

[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).

[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]

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]

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 :(

[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.

[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?

[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]

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]

It would not be unheard of for the pump to be dying even if it is relatively new since URO is one of the "bargain" brands. My experience has been that they are fine for plastic and non-mechanical parts, but I am not really confident in their electrical parts.

As far as the pump, there is not much that you can check with a multimeter, but here are a couple of things.
1) Unplug it and measure the resistance across the terminals. It should be approximately zero Ohms, or whatever the resistance of the test leads is (usually <0.5 ohms).
2) Plug it back in, peel back the boot on the connector and measure the voltage across the terminals when it is running. You should be reading very close to the battery voltage (>13V if the car is running). If you are reading significantly less than the battery voltage, then there is a problem with the wiring or the fuel pump relay.

Being that it sounds like you are having a fuel delivery problem and the engine is running lean, I would seriously avoid driving it hard at all as extreme lean running is dangerous for the engine. The M42 is really tolerant of lean burns in that it won't start pinging until you are dangerously lean (if at all), but running the throttle hard is definitely not doing anything good for combustion chamber temperatures.

Alright last thing man, ohms and voltage were good but it had a fuel drip right at the fuel filter. This drip happened at both ends of it. It’s happened before but it’s very inconsistent sometimes happening at both ends, sometimes at one and sometimes never. im gonna replace them but i can’t imagine how this would cause weak acceleration normally when cold or after sitting for some odd hours and then drive fine and NOT leak after a few minutes. Suppose it could be messing with pressure as well right ? My MAIN question is how it causes the issue at a specific time rather then all the time or more inconsistently. Thanks for all the help, gonna replace them tmro and see how it goes

[ScottySbk]

It would not be unheard of for the pump to be dying even if it is relatively new since URO is one of the "bargain" brands. My experience has been that they are fine for plastic and non-mechanical parts, but I am not really confident in their electrical parts.

As far as the pump, there is not much that you can check with a multimeter, but here are a couple of things.
1) Unplug it and measure the resistance across the terminals. It should be approximately zero Ohms, or whatever the resistance of the test leads is (usually <0.5 ohms).
2) Plug it back in, peel back the boot on the connector and measure the voltage across the terminals when it is running. You should be reading very close to the battery voltage (>13V if the car is running). If you are reading significantly less than the battery voltage, then there is a problem with the wiring or the fuel pump relay.

Being that it sounds like you are having a fuel delivery problem and the engine is running lean, I would seriously avoid driving it hard at all as extreme lean running is dangerous for the engine. The M42 is really tolerant of lean burns in that it won't start pinging until you are dangerously lean (if at all), but running the throttle hard is definitely not doing anything good for combustion chamber temperatures.

Ok thanks, i’ll do that tonight when i’m home. Also what about any possible debri in the tank messing with the pump ?I also swapped the relay with the 02 sensor relay and no change. if it ain’t the pump i’ll be absolutely fucking lost. It was the last on my list because i replaced it.

[bmwman91]

It would not be unheard of for the pump to be dying even if it is relatively new since URO is one of the "bargain" brands. My experience has been that they are fine for plastic and non-mechanical parts, but I am not really confident in their electrical parts.

As far as the pump, there is not much that you can check with a multimeter, but here are a couple of things.
1) Unplug it and measure the resistance across the terminals. It should be approximately zero Ohms, or whatever the resistance of the test leads is (usually <0.5 ohms).
2) Plug it back in, peel back the boot on the connector and measure the voltage across the terminals when it is running. You should be reading very close to the battery voltage (>13V if the car is running). If you are reading significantly less than the battery voltage, then there is a problem with the wiring or the fuel pump relay.

Being that it sounds like you are having a fuel delivery problem and the engine is running lean, I would seriously avoid driving it hard at all as extreme lean running is dangerous for the engine. The M42 is really tolerant of lean burns in that it won't start pinging until you are dangerously lean (if at all), but running the throttle hard is definitely not doing anything good for combustion chamber temperatures.

[ScottySbk]

yeah just now i drove it and it’s popping like crazy before 2k rpm then once it’s past that it’ll drive and idle ok but once the car is parked afterward and idles great it’ll throw a lean/rich code and when at WOT it’ll throw it but go away when i let off. Suppose i’ll pick up a multimeter, how could i test the pump ? never used a multimeter before, can’t be too hard tho. i know my filter isn’t clogged because i checked it a few days ago, it’s a URO fuel pump.

[bmwman91]

Ok, yeah there is definitely a fuel supply issue in that case. Clamping the return line should increase the pressure for sure, and the only thing that I can think of which would cause pressure to not increase is a dying fuel pump. Even a plugged fuel filter should register some increase with the return clamped.

Your symptoms make a little more sense now, I think. When cold the O2 sensor is ignored so the car would run pretty lean with low pressure. Once warmed up the O2 sensor can detect the lean condition and the ECU will increase injector pulse widths to compensate.

[ScottySbk]

If you were reading 29PSI at idle, that is normal. The 3BAR / 45PSI rating is the pressure that the fuel is at relative to the intake manifold, and when idling it is pretty close to a vacuum, so you would only expect to see ~30PSI relative to the atmosphere.

The only thing that really comes to mind is that maybe the coolant temp sensor is dying. Try measuring the resistance across its terminals when it is cold to see what the value is. Also pull back the boot on the plug for it and make sure that the wires are not damaged where they are crimped into the terminals.

I replaced the coolant temp sensor already and it acts the same :| dang ok tho, yeah i clamped the return line and got no pressure increase and also pulled the vacuum line to the FPR and got no increase. When idling and revving is there suppose to be an increase ? cause it did not move a bit when revving at idle. Honestly lost at this point.