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My 2001 m3 has a misfire with no codes thrown, I notice increased fuel consumption and a rough start-up idle at times. I am original owner, the car has 200K plus on odometer, no intake leaks (confirmed with smoke test), throttle bodies cleaned as well as the idle control valve, vanos done, recent new plugs, fuel filter, fuel pump and air filter. Car has original MAF and pre/post O2 sensors (I plan to change pre O2 sensors soon).

In looking at the fuel trims I see negative trim values as follows LTB1 -5.4 and LTB2 - 3.9 on average which improve ny a few points with car at WOT and hot, but do not get to zero.

I tested the fuel pressure today and found some results that I am trying to understand as follows.

With fuel pressure gauge hooked up, turn key and get an initial pressure reading of 73 psi which is just within spec. but then in 5 seconds drops down to 62psi (with multiple runs to confirm).

Start car and the PSI jumps back up to 70 PSI then oscillates back and forth from 73-68 PSI with engine surging in attempt to find stable idle.

In about 10 seconds the engine idle starts to run smooth and the pressure stays at 62psi engine running (not in spec).

Shut car down and the pressure goes down to 58 psi immediately.

Over the next 30 minutes the pressure drops down to zero - will not hold pressure.

Hoping for some help in confirming that these symptoms indicate a failed fuel pressure regulator and that it also is likely the cause of the misfire and perhaps the negative trims.

Any words are greatly appreciated.
When was last time the fuel filter and regulator was changed?
 

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Thank you for the replies,

I will post up first thing in the morning OBDfusion logs for cold start idle, straight highway run and warm idle.

The fuel pump replaced 1.5 years ago was OEM. I am unsure why the pressure goes to the correct 73 PSI pressure initially and then drops in 5 seconds prior to starting and also of the oscillation when searching for stable idle?


I was under the impression that the pressure was to hold after the car was shut off?


The fuel filter and air filter were replaced within 6 months as PM. The fuel regulator valve has never been changed.
Years ago I had a MIL light traced to a bad fuel pressure regulator..it’s part of the fuel filter assembly on my 03M3 vert. It’s rather large compared to old-school fuel filters, not to mention the sintered-bronze metal plug inserts you used to see on Rochester quadrajet carbs. Those went away with in-line filters and larger fuel filters and independent-of-fuelpump pressure regulators.
one other problem could be the vacuum line from the fuel pressure regulator to the engine. These cars are like 20 years old and rubber/plastic doesn’t like age or ozone or UV.
 

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Thank you for the reply, I am going to have another look and will change out the inexpensive vacuum line as pm. I am considering purchasing a pressure regulator but hoping for some feedback on the data in my last post prior to ordering a new regulator which is still low cost at ~$55.
Yes, I was somewhat chagrined to pay over $80 I think for the fuel filter regulator assembly from the BMW dealer a few years ago ( too lazy to pull the 2” thick paper file and thumb through it!) but I will say it was trivial if tedious to change it out under the car. Only thing I really did was to clamp both fuel lines or plug them on either end of the assembly to minimize fuel spill. In my case the replacement was pm after a fuel pump replacement but I haven’t had any fuel oroblems since.
 

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update - I changed out the fuel pressure regulator and when I pressure test I get about the same reading as the old regulator of 64 psi. If I remove the vacuum line pressure increases about 10 psi. If I do an active test of the fuel pump with my Foxwell 510 when activated the fuel pressure jumps up to 71 psi which is within spec. Not sure why turning key on or idling car does not yield 71 psi . Today I swapped fuel pumps with a known good one and do not notice a difference other than the pressure holds after the car shuts off per Bentley manual, so I guess the pump check valve was bad but pump is OK. Still a very slight misfire felt and no codes. Car feels powerful as hell and pull evenly through range.

With OBDfusion I see intermittent fuel status 8 mostly on deceleration - additionally there are some erratic drop-outs of data from the following: both pre and post cat sensors absolute throttle position percent and short term fuel trims all at the same time. MAF reading never drop out and seem consistent?? This happens when spirited driving, not when steady Hyw cruising or at cold start and warm idle??

I am a bit lost at this point wondering if it could be pre cat o2's but also wondering why both drop out for 5-10 seconds at the same time? Also wondering if it could be the DME? Fuel trims remain negative ~5 bank 1 and neg 3 bank 2 on average.

I am going to disconnect pre cat sensors tomorrow and see if that changes any variables before buying new 02 sensors. Spirited driving log attached for drop-out references.

Any ideas on where to go next?

With 200K on the OEM pre-cat 02 sensors it's time. They're usually good to over 100K (mine have 116K) miles but 200K is really pushing it.
 

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Update - I changed out the precat o2 sensors and the fuel trims remained negative but fuel consumption was markedly improved. I decided to change out the MAF with a new Bosch unit and the fuel trims have now gone from negative to positive so the MAF was lazy. Fuel consumption has again improved. Smoked tested and there are no intake leaks.

The car still has a very minor vibration at idle so I think the last piece of this likely has to do with clogged or inefficient fuel injector spray patterns. The fuel filter is only a few months old so I doubt that is an issue but this weekend I am going to run a complete electrical test of the fuel pump relay, fuel pump and do another pump pressure and flow test to confirm the injectors hold pressure by clamping the return line. Either way I will be sending the injectors out to be cleaned as at 200+ I suspect it will be money well spent. I will update once I get the trims to close to or at zero. Again, any input is appreciated and thank you to those who have given insight thus far.
I think before I sent them out I would try cleaning them myself. Lots of stuff on youtube. You have to pull the fuel rail/injectors as an assembly I believe before you can remove individual injectors. Worth trying the below cleaning before going further:

New injectors are like 150 apiece, you can buy a rebuilt set of six for about 300 on amazon.
 

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Thanks SloopJohn. I recently ran a few tanks of Liquid Moly Jectron and one of Valve clean , I noticed some improvement but slight vibration is still there, it never was for all these years. If the car sits for a few days it will sputter or take a few extra cranks on cold start with vibration that gets better when warm but is still often there. Vibration is more noticeable with AC on.

Per OBDfusion the idle is now more consistent and will go down from 1100 cold to about 860 and does not search up and down for steady idle as it did before the new MAF. Forum member and NJ native suggested I run the Moly Proline Fuel system cleaner as it has worked well for a few tanks - this guy knows his stuff. It is ordered and I will give it a shot. I have put an ungodly amount of money into this car I have loved over the last year, so I am having no issues running a cleaner instead of sending injectors out or buying $$.

Sapote - the MAF values have indeed gone down at idle as I test with the new MAF. I see ~4.4 g/s at 860 rpm now. My wording was not correct as "lazy" - perhaps - too high at times and too low also so yes, so erratic or hyperactive is a better choice.

I still see fuel system status 8 too often for 5 or so seconds often on engine work or speed transitions often when aggressive in transitions according to the logging ?

The absolute throttle position values seem to drop at times, is this normal to see on an OBDFusion tps log, or a sign of a throttle position sensor is going bad? I changed all three TPS sensors about 40K miles ago along with the Throttle actuator. When I test with the Foxwell unit the TPS values appear to report cleanly through the whole range. Additionally, I changed all 3 of them a few years back..... No codes or EML light

After the new MAF I now have +3 and+4 fuel trims instead of neg - 4 and -5, no intake leaks and VCG is tight and new.

I will get is sorted completely hopefully sooner than later and I will continue to report back as this info may help another 3/4 ass diy mechanic like me with a 200K + mile M3, thanks for the guidance.

# the M must live, it has been such a great car for so long !!
It's fantastic that the engine has lived so long and hard without valve work, cams, new rings, etc.
 
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