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Discussion Starter #1
Hi guys,

I think I may have found the culprit to this but wanted to run it past you guys before spending cash.

Car 2004 325ci auto, mileage 153000, well maintained.

noticed the last few weeks the car has been running rough when cold, happened before in winter but never in summer, then yesterday the idle speed raised to 850ish rpm without warning.

I checked The car with INPA when I got home, no codes whatsoever, altered the idle speed manually on INPA, it dropped to the selected speed (600rpm) immediately, indicating the ICV is fine and not stuck. reset throttle adaptations also, still the same. Throttle pedal potentiometer & angle looks ok also.

Got the OBDfusion going, noticed I’m getting negative Long term fuel trims (-3.5 to -4) when historically it’s been around 0.

Did a bit of research on the net and decided to question the fuel pressure, hooked a tyre gauge on there (not ideal I know) read about 1 Bar after been parked for a few hours, primed the pump, checked again, about 3 Bar. Then started the engine, the pressure went off the gauge, which is calibrated to 4 Bar, the needle was bouncing at the end of the dial, so have no idea what the actual pressure is. It should be 3.5 Bar yeah?

I replaced the fuel pump (Siemens) and filter (fram) last year.

I‘m guessing the high idle speed is due to there being too much fuel getting to the injectors and the mixture can’t be weakend sufficiently at those low RPMs, so the DME is raising the idle to compensate. So I’m thinking of replacing the conveniently placed fuel filter again.

Does this ring true to you guys or am I way off? As mentioned the gauge was a tyre gauge so not massively convinced about its accuracy.

cheers,

Chris.
 

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2004 330Ci 85k miles
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Use a proper fuel pressure gauge to get an accurate reading. Typical fuel pressure should be 50 PSI, which is around 3.45 bar.

Fram fuel filter? I would use Kayser, Hengst, or other OE brand. But that should’t affect fuel pressure.

What is the MAF sensor value in grams/second at hot idle? At the normal 700 RPMs you’d expect around 3.25 g/s. An over-reporting MAF will cause negative fuel trims.
 

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Discussion Starter #3 (Edited)
Hi, thanks for the reply.

When warm the MAF was reading 4.4g/s in neutral, bearing in mind the raised 850rpm idle speed, I thought nothing of that?

The last time I checked, it was at 3.29g/s at 700rpm in neutral, when warm, that was February.

Chris.
 

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2003 330cic, 2003 325iT
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Fram fuel filter? I would use Kayser, Hengst, or other OE brand. But that shouldn’t affect fuel pressure.
I may be misunderstanding, but the fuel pressure regulator is in the fuel filter, making that the first place to look if you had pressure that was too high.
 

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Discussion Starter #5
That was my train of thought also.

Anyway, situation update.

Drove to work in the car, 10 miles, mostly motorway with roadworks, so 50mph. Arrived at work and the idle had reduced from 850 to 750rpm in neutral, the LTFTs were similar to mentioned previously and the air mass meter was showing 3.9g/s. I went under the hood and connected my tyre gauge (haha) to the fuel rail, pressure now showing 3.7 Bar but fluctuating at high frequency by about 0.1 Bar, presumably from the fuel pump pulsing?? is this normal or should it be regulated down to smooth pressure reading?

Did my stuff at work, came out and started the car, still idling at about 750 and not dropping when I place in D, as it normally would do. Drove home, slightly different route but similar. By the time I got home the car was behaving perfectly normally - 700rpm in neutral, drops to about 600 in drive.

Didn't get any logs or check the fuel pressure as it's late and dark.

Ordered a new Hengst filter as was suggested, will fit when it arrives and feed back.

Thanks again for the input.
 

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I may be misunderstanding, but the fuel pressure regulator is in the fuel filter, making that the first place to look if you had pressure that was too high.
Ah yes, totally forgot that the FPR is on the fuel filter! Thanks for the correction.
 

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The manufacturer spec for fuel pressure is 3.5 +/-0.2 bar . AFAIK most E46 owners around the world measured 3.7 bar actually including me on my M52TU engine. But the needle on fuel pressure gauge's not supposed to fluctuate. Should stay steady across entire RPM range. Maybe that's just because you're not using proper gauge.

I don't know OBDFusion app but I've never seen generic OBD2 software that could monitor additive adaption value. Additive adaption value is basically your LTFT at idle. So you should watch adaption values and lambda integrators in Inpa to get proper picture of fuel trims at idle. Lambda integrators are basically what STFT in OBD2 software is calculated from.

Fuel filter itself can't make an engine run rich. Integrated fuel regulator could. But those don't seem to go bad often. And some budget fuel gauge costs less than a new filter with regulator. So why not measure the fuel pressure properly first instead of spending money on a new filter.

I have brand new genuine BMW MAF installed on my 2.0l M52TU and I believe I have no vacuum leaks at least not the ones smoke test would reveal. It reads about 3.15 g/s at idle. So your numbers should be higher than that hard to guess how high though. MAF reading depends on atmospheric pressure and air temperature so it's not even possible to compare two exactly same engines. There's a self diagnostic test for MAF in EasyDIS that will measure MAF idle voltage and give you good voltage range for your car. So maybe you should check that out. But bad MAF's tend to under report the airflow and cause lean conditions under load. Unless it's a cheap aftermarket part. Those are known to over report.

I would measure the fuel pressure properly and check the fuel trim values in Inpa. If 3.7 bar is measured that's within manufacturers specs so nothing wrong with it. And I guess default fuel trim values in DME are set up for 3.5 bar. If the pressure was a bit higher it would make sense that learned fuel trims were a bit negative. As long as additive value in Inpa is low and lambda integrators around 1 there's nothing to worry about. The injection duration has shortened a bit to compensate for still in specs fuel pressure so the LTFT did it's job just as it was supposed to and STFT is also doing it's job by delivering enough O2 and HC for the cats to function properly.

Not sure what would cause of your high RPM be. But if DME wasn't able to take away anymore fuel both long and short term fuel trims should be maxed negative. Max negative STFT is somewhere around -25% ( 27% or 28%? IIRC). And that would throw a rich error code and turn check engine light on.

You verified that DME can command idle speed at desired RPM. It seems that it's commanding raised RPM for some reason. I would monitor coolant temperature while RPM is too high. Maybe your thermostat gets stuck partially open.
 

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Discussion Starter #8
Forgot to update this. Changed the filter/regulator For a Hengst one as recommended, a few weeks back, pressure showing smooth 3.8 Bar, instead of shaking like before. Could be the gauge over reading the actual value

Car is running fine, as it had been apart from that day or two the idle speed went up. I checked the coolant temperature on both the hidden menu and INP, all looked perfect. I haven’t checked the fuel trims Or MAF value at idle since, so can’t comment on that.

Thanks everyone for the input.
 

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Discussion Starter #10
I’m not sure what happened with it, it was only doing it for a couple of days. As mentioned, the pressure was off the gauge to start with and had dropped when I checked after the idle dropped slightly. very strange, only a tyre gauge as I said previously, but the new filter/regulator holds the pressure constantly at 3.8 Bar at idle, instead of it fluctuating at high frequency like when I measured it before.

Next time I go out in it I’ll look at the LTFTs and MAF report back.

Also, forgot about this, the car still runs A little rough when cold before the oxygen sensors kick in (I presume). I don’t remember this happening in previous summers, just winter.

Once, a few years back, when I changed the coolant/water pump/thermostat the idle rose slightly, don’t know why it happened then either but it went back to normal after a quick drive that time.
 
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