1996 523i - low power, poor fuel economy, negative fuel trims - I'm at my wits' end.

[ferguscan]

Hi all,

Here's a conundrum for all the gurus. Symptoms:

- low power (especially at low RPM)
- rubbish fuel economy (< 20 mpg combined average)
- negative fuel trims (additive and multiplicative) on both banks

This is a long-standing issue, and has slowly gotten worse over the years (i.e. fuel consumption has gradually and consistently increased over time). What I have replaced along the way (all OEM parts):

- air filter
- spark plugs
- ignition coils (three at a time)
- fuel injectors
- cam sensor
- crank sensor
- FPR
- fuel filter
- fuel pump
- O2 sensors
- MAF
- various intake hoses and gaskets
- cat failed (probably because of this problem) and was replaced with a generic one - made no difference to fuel consumption, though I did get some power back for a while
- VANOS seals
- cleaned ICV
- fuel tank ventilation valve

What I have observed:

- VANOS works when excited using INPA (i.e. car runs rough when turning the VANOS on)
- no codes
- intake air temp and coolant temp are both reading fine in INPA
- MAF reading seems sensible
- throttle position sensor reading seems sensible, and varies with throttle position
- strapped on a fuel pressure gauge and went for a drive - pressure was in spec the whole time. However, I did notice something strange at idle. The pressure would creep up by a few PSI, and then fall back down suddenly (staying within spec the whole time). Happens every couple of seconds. This corresponded to subtle blips on the tacho. When it's doing this, and I'm stopped at traffic lights, I can ease my foot off the brake so the car is inching forward, and it will lurch forward every couple of seconds. The change in fuel pressure seems like it might be caused by changing manifold vacuum. I replaced the vacuum line to the FPR, but no difference.
- ignition coils are a mix of Bremi and Bosch. I have strapped an oscilloscope to the coils, and observed the ignition waveforms. The waveforms differ between Bosch and Bremi, but both seemed sensible. Does anyone have a known good waveform from an I6 E39 that I can use for comparison?
- sometimes when starting the car, the engine seems to catch too early (after maybe one revolution only), and coughs a bit. Still starts every time, but it feels a bit odd to me.
- no smoke from the tail pipe

Thoughts:

- I thought the engine might be mis-timed (even though the issue pre-dates my changing the VANOS seals). I have opened it up and re-timed it several times, being careful, using all the right tools. No change

I am properly at my wits' end on this. I don't know what else to test or try. Can anyone offer some guidance or suggestions?

Thanks very much,

Stuart

[Marcus Sky]

A fellow 523i -96er...

I really hope that someone can help you with this issue.

Greets
Marcus

[gmak]

Dear ferguscan:

Don't hold your breath. I have had similar issues and not found any help on any of the forums. Running rich is an alien concept to these BMW'ers. :-) I've read your post carefully and I am impressed with the detective work you have done. The following is just me laying out my thoughts to get organized and provide background. You may want to skip the numbered points and go to the last paragraph. I've been down the same road as you - through not as far yet.

1. My fuel trims were always negative. At one point they led to a CEL. I couldn't solve and went to an Indy. They said that the MAF was reporting too much air (voltage was too high). This was a new MAF (VDO Siemens). I put back in the old MAF and the CEL went away. I sent back the new MAF and they sent me a newer one. I put it in the car and so far no CEL - but the fuel trim is negative still - quite so.

2. There is air, fuel, and spark in ignition. For negative trim, the DME is injecting too much fuel for the amount of air and then getting scolded by the O2 sensors. At some point, the fuel trim goes beyond the limits of adjustment and the CEL is triggered.
3. Air is reported by the MAF. If it is reporting too much, then negative trim will be the result. If there is some kind of back pressure on the exhaust side that interferes with the vacuum in the engine, then not all of the air will be pulled in through the piston /valve cycle. I have been told that this could be a leak on the exhaust manifold side, or 'blocked' catalytic converters. As well, it could be some kind of gasket leak on the exhaust side of, say the valve cover. (Not lecturing, just making sure we're on the same page here: When gases are exhausted from the engine, it creates a vacuum on the intake side, that when those valves are opened, pulls air in.
4. The other option is that the injectors are leaky and letting more fuel in than they should be.

Given all the work you have done, I would suggest looking for hairline fractures on the exhaust manifold - possibly around the O2 sensors. When the weather turns to spring up here, I'm going to check my gasket between the tin man (the SAP valve) and the exhaust manifold. I'm also going to run the engine, intermittently block off the exhaust and look for leaks on that side. If all that fails to show a leak, I have a valve cover gasket that I can put in - was going to replace the lifters anyway. After that, it comes down to the Head gasket and damage from the distant past due to some overheating incident (I'm not the first owner) as a possible source of the failing vacuum.

If it's not the MAF (swapping in another one that is from a car without negative trim could confirm absolutely - but voltage reading is just as good), and it's not the injectors (fuel pressure says no), then it's something preventing the intake of all the air, or your DME itself is the crazy part. I even went so far as to change the throttle sensor to make sure that it was working fine. In any case the air is being pulled in - not pushed - so either the MAF is lying, the DME is crazy, or some of the air is not getting pulled into the combustion chambers (backpressure from the exhaust side - cats or leaks/hairline fractures, or leaky containment - gasket, valve cover crack, some issue with the valves, head cover / gasket)

[ferguscan]

Hi gmak,

Thanks for the reply. Some points and questions:

- How does your car run?
- What kind of fuel economy do you get?
- Do you have similar negative trims on both banks (like me)? If so, I doubt it's an exhaust manifold leak (since each manifold would need to have a leak of identical size).
- I've replaced my MAF (w/ VDO part). No change.
- Have you looked into ignition timing or cam/crank timing for your engine?
- Have you re-timed your engine at any point (e.g. done VANOS seals)?
- I replaced my valve cover gasket when I did the VANOS. No change.
- When did your issue start? Was any work done on the car immediately before the issue began?
- Have you done a compression test on your engine? I have, and my numbers were good and consistent across the cylinders (around 190 PSI if I recall). I felt at the time, however, that the pressure took a long time (i.e. lots of cranks) to max out for each cylinder.

Thanks,

Stuart

[rf900rkw]

The fuel pressure is not causing the vacuum, tis the other way around. The fuel pressure is following the vacuum.

Negative fuel trims are rare. Very rare. Thus not many people have from-the-hip responses.


Negative trims, not enough O2 for the fuel quantity. Either the air flow is being over reported, or the fuel flow is more than expected. You have covered every base on these two fronts I can think of.

That leaves a third option that Gmak touched upon. The reported air isn't as full of O2 as expected.. In other words the air doesn't burn. Exhaust reversion into the intake manifold. You '96 uses a HFM2 which isn't direction sensitive like the HFM5. Thus a slug of exhaust blown back into the intake will be measured and fueled twice, and yet not help with the combustion.
This would be a function of cam timing or exhaust restrictions. I normally discount this as exhaust restrictions will show up as lack of large throttle power long before they start causing light throttle reversion. But you seem to have ruled out everything else.

You can make a backpressure gauge by brazing a bit of tubing into an old O2 sensor metal base and connecting it up to an old fuel pressure gauge.

[edjack]

Good call on the backpressure gauge. The gauge to use is the compound vacuum/pressure gauge, common when fuel pumps delivered about 5 psi.

If the back pressure is more than 1-2 psi at 2500 RPM, the cats are clogged.

Clogged cats can also set the code for random multiple misfires, at least in my case.

[rf900rkw]

Since it seems to be affected both banks equally (is it??), I'm thinking it will be downstream after all exhaust has merged to one pipe.

[gmak]

Hi gmak,

Thanks for the reply. Some points and questions:

- How does your car run?
- What kind of fuel economy do you get?
- Do you have similar negative trims on both banks (like me)? If so, I doubt it's an exhaust manifold leak (since each manifold would need to have a leak of identical size).
- I've replaced my MAF (w/ VDO part). No change.
- Have you looked into ignition timing or cam/crank timing for your engine?
- Have you re-timed your engine at any point (e.g. done VANOS seals)?
- I replaced my valve cover gasket when I did the VANOS. No change.
- When did your issue start? Was any work done on the car immediately before the issue began?
- Have you done a compression test on your engine? I have, and my numbers were good and consistent across the cylinders (around 190 PSI if I recall). I felt at the time, however, that the pressure took a long time (i.e. lots of cranks) to max out for each cylinder.

Thanks,

Stuart

1. My car runs fine. But I have no basis for comparison - ie what does a 'perfect' 1998 528i run like. I have put in new spark plugs and plasma coil and I find that I'm getting the full benefit of whatever torque there is. I'm worried because of the increase in the negative trim and that it may lead to more problems down the road. I've done pretty much everything - all sensors (as part of preventative maintenance and the age of the car), including O2 sensors. I still have good acceleration from stop and good torque in the 4000 - 5000 rpm range (running sport mode).
2. About 18 - 20 mpg
3. Negative trims on both banks. used to be low (< -7), now are up around -30 last I looked.
4. Haven't checked ignition timing. I was planning to do tappets / lifters and check valve cover gasket this summer and check timing at that point.
5. Vanos seals, valve cover gasket, and vanos oil hose were done by dealer as part of oil leak chase about 3 years ago. Before I decided car was old enough for me to learn to work on it myself.
6. I've been aware of the issue only since getting INPA and looking at the numbers - so I don't know when it started, only that the negative adaptation has gotten bigger.
7. No compression test.

As rf900rkw has confirmed - running rich is quite rare. It is likely a symptom of something else, something weird and hard to find. It may even be that we have different issues with the same symptom. rf900rkw is very very knowledgeable and pragmatic. He may have something. Also wouldn't hurt to check the cats.

We are cursed with rare and interesting problems.

[Poolman]

Can you drive the car with the MAF unplugged ,,drive around for 30 min or so and then take readings (with the maf still unplugged) and see if readings are still negative ?

[rf900rkw]

Good thought, but the adaptions are what the MAF reports vs what the O2's report. With the MAF unplugged, the adaptions should not update.

[Poolman]

Okay,,how about this--temp sensor that is located in the engine head,,it's reading like the engine is cold and giving a rich mixture all the time,,instead of giving the proper temp after the engine has heated up? Had that happen once. Since this car has a mech stat ,,that fails open,,don't think it could be that,,but with all he's touched on,,didn't see anything about touched on this subject.

[gmak]

This is a long shot - but check the ICV to make sure it is seated in the grommet properly and that the vacuum hoses that come into the manifold between it and the temperature sensor are hooked up ok.

[ferguscan]

Thanks for all the replies everyone...


The fuel pressure is not causing the vacuum, tis the other way around. The fuel pressure is following the vacuum.

Negative fuel trims are rare. Very rare. Thus not many people have from-the-hip responses.


Negative trims, not enough O2 for the fuel quantity. Either the air flow is being over reported, or the fuel flow is more than expected. You have covered every base on these two fronts I can think of.

That leaves a third option that Gmak touched upon. The reported air isn't as full of O2 as expected.. In other words the air doesn't burn. Exhaust reversion into the intake manifold. You '96 uses a HFM2 which isn't direction sensitive like the HFM5. Thus a slug of exhaust blown back into the intake will be measured and fueled twice, and yet not help with the combustion.
This would be a function of cam timing or exhaust restrictions. I normally discount this as exhaust restrictions will show up as lack of large throttle power long before they start causing light throttle reversion. But you seem to have ruled out everything else.

You can make a backpressure gauge by brazing a bit of tubing into an old O2 sensor metal base and connecting it up to an old fuel pressure gauge.

Regarding fuel pressure chasing the vacuum, I thought that was the case (since it stayed in spec the whole time).

What do you mean by "lack of large throttle power long before they start causing light throttle reversion"? Not following you there.

I did check for exhaust back-pressure at one point (I have a gauge), using one of the O2 sensor holes. There was none. I could check closer to the cat (by drilling a hole)?

Regarding cam timing... if it was incorrectly timed (and I really really don't think it is), would any particular direction (i.e. retarded or advanced) cause combustion problems more than the other?


Good call on the backpressure gauge. The gauge to use is the compound vacuum/pressure gauge, common when fuel pumps delivered about 5 psi.

If the back pressure is more than 1-2 psi at 2500 RPM, the cats are clogged.

Clogged cats can also set the code for random multiple misfires, at least in my case.

** When I checked back-pressure at the O2 sensor hole, there was none. But that cat has died since, and was replaced. I got some power back at that point, but the negative fuel trims (and overall gas mileage) didn't budge.


Since it seems to be affected both banks equally (is it??), I'm thinking it will be downstream after all exhaust has merged to one pipe.

** I only have one cat, after the pipes merge.


Good thought, but the adaptions are what the MAF reports vs what the O2's report. With the MAF unplugged, the adaptions should not update.

** Correct, they don't update. Car doesn't drive any better with the MAF unplugged (MAF is new anyway). In fact, it might be a little worse.


Okay,,how about this--temp sensor that is located in the engine head,,it's reading like the engine is cold and giving a rich mixture all the time,,instead of giving the proper temp after the engine has heated up? Had that happen once. Since this car has a mech stat ,,that fails open,,don't think it could be that,,but with all he's touched on,,didn't see anything about touched on this subject.

** INPA reports sensible values for the coolant temp sensor and the intake air temp sensor.


This is a long shot - but check the ICV to make sure it is seated in the grommet properly and that the vacuum hoses that come into the manifold between it and the temperature sensor are hooked up ok.

** ICV is seated fine. I had it out just the other day (replacing vacuum hose to the FPR).

Thanks!

Stuart

[gmak]

Just looking over your OP again, you did get some power back for a while after changing the cat. There might be something to the back pressure idea. < 20 mpg combined isn't that bad at all ( at least from a 528 perspective) - maybe this is a misleading clue...

The lack of power at low revs is something to think about - Too little air for proper combustion until things really 'get going; and, so is the 'lurching' when stopped - Usually a 'hunting' idle is a symptom of vacuum leak, no? Perhaps it's also a symptom the other way - vacuum blockage.

Just trying to think outside the box or inside the DME as it were.

[skborders]

Have you checked the evap system for signs of fuel contamination entering the intake?

[ferguscan]

Have you checked the evap system for signs of fuel contamination entering the intake?

** I tried pinching off the vac line coming from the tank ventilation valve. No change.

[Poolman]

All the stuff you have reported on --it's got to be something wrong with the OBC..see if you can pull one from a wrecked car and plug it in--or maybe a clogged exhaust system--if they don't sniff you exhaust over there--remove the cat's,,you stated the power came back after replacing one.

[gmak]

Just to add some more information to the mix regarding my negative trim issues.

I had taken the fuel pressure a while ago and it seemed normal, so I was focused on air (spark being assumed to be ok, given how my car runs). I bought a new maf, given the age of the car and was rewarded with a CEL and extreme negative trim. I put back in the old maf and everything was fine. Apparently the new maf had voltage issues, which was confirmed when I sent it back and got a second new one. It's in the car and everything is fine.

I check for leaks on the exhaust side - nada. (Remember this is after proactively replacing sensors etc).

Finally, I decided to check fuel pressure again. Maybe I didn't do it right the first time 2 or so years ago, but now the fuel pressure reading is high. I'll be checking the Fuel Pressure Regulator (FPR) later this week. Mine is under a splash shield next to the fuel filter, so it's a bit more of an annoyance than all you lucky people with the FPR next to the fuel rail. My thought is that it isn't working at all, given the high pressure and that when I take off the vacuum hose and plug it, there will be no change in reading.

If this doesn't turn out to be the problem, my last resort is that I'm going to go check that the pre-cat O2 sensors are torqued in properly.

Maybe this update and any results I find will help Ferguscan with his issues

[EURO E39]

Subscribed...mirror symptoms on a 96 Euro 528....poor MPG, sucking vacuum sound near injectors/ FPR, similar parts tried..VDO MAF, pre-cat Bosch O2 sensors, all vacuum boots, ICV+grommet, VC gasket, TB gaskets (orange and black)...T-stat was stuck open and not warming up past 80C...new Bahr has me up to 97-99C now..still not solving the slightly high idle (750rpm) and bag mileage...Still getting a consistent 12mpgs city...barely 15 on a highway drive..at wits end w/o INPA...what else can I do?

Tried spraying everything -water, carb cleaner,etc..around everywhere recommended to get a change in RPM...no effect.

1) IAT sensor under the intake manifold next to ICV and CCV plug ins...I tested with multimeter with heat applied...appears functioning..grommet/gasket might be a leaker?
2) FPR on the fuel rail?...replaced vacuum line to intake manifold..
3) CPS: camshaft position sensor?...reaching? timing?
Any help appreciated..thanks!

[michaltt]

Any updates ? Having same issue on e46 325. Replaced spark plugs, maf, o2, catless headers, cps, dis , vanos seals, cleaned icv, tb, replaced vcg etc.

Still having power loss (not big), vibration in gear and negative fuel trims. Since mine are not equal on both banks i think that i will replace injectors now and fuel filter.

[EURO E39]

How common is a vacuum leak around the fuel injector grommet? I think I hear sucking air at idle ---that changes/goes away with a hand twisted throttle rev..
I tried spraying everything imaginable(except propane) in and around there....no rpm changes.

Whoever solves this one will make a lot of I-6 drivers happy.

[gmak]

Guy: Listen!
Spraying won't help in our case.
Negative fuel trims mean that you are running RICH. There is no vacuum leak. It's saying that there is too much fuel and not enough air (as measured by O2 sensors) for what the computer believes it is getting via the air intake and injecting via the fuel rail. I've been all over the place on this one. I've finally gotten to the point where it looks like the fuel pressure is too high.

This makes sense:

1. Our vehicles have high RPMs at idle.
2. Lower MPG than we would expect.
3. Probably a drop off in torque / power briefly at higher speeds.

Check the fuel pressure at the rail at idle. It should be around 52 psi (3.5 bar) based on everything I've read here. Some say it's supposed to be 52 psi as the difference with the pressure at the intake manifold which is a negative 10 psi meaning it should be 42. I have no clue about that. I know that at 68 psi, I am too high. When the engine is rev'd, it then drops to 62 psi.

I've hesitated to check the Fuel Pressure Regulator, because in my car (the only year those stoned BMW engineers did this) I have the 3/2 valve and the FPR is above a splash shield next to the 3/2 valve and fuel filter under the car below the driver's seat. I've had other issues (parasitic battery drain that may be from multiple sources), so I'm only getting around to it now.

CHECK YOUR FUEL PRESSURE at the fuel rail schrader valve. If you're lucky enough to have an FPR just off of the fuel rail near the firewall, then you can easily pull the vacuum hose off of it, pinch it to prevent other issues at the intake manifold, and see if there is any change in pressure. If your pressure is high, and doing the vacuum hose thing on the FPR makes little or no change, then it's the FPR - probably a torn diaphragm or worn spring inside - and you replace it. Also, when you pull the vacuum hose (it's a little one) off the FPR, check for fuel in it - a sure sign that something is wrong.

Summary: RICH = too much fuel or too little air. Most likely cause is too much fuel via pressure. Start there.

[michaltt]

Ok so, as i stated above, i have same symptoms as OP. Im also chasing my own tail.

Funny thing is that after i put new MAF (VDO) in, my fuel trims dropped to -7 on both banks. As far as i remember, i had problem with fuel trims just on bank1. It was sitting around -1.2 and bank2 was +2. I have probably same problem with new MAF - it have problems with voltage and overreporting air flow because if i put my old MAF back, fuel trims are back in normal range (around zero). Not sure on which info i have to rely on. On new VDO maf or old VDO maf...

Anyway, i replaced :

Rattling cats --replaced--> o2 sensors (http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Dautomotive&field-keywords=o2%2Bsensor) (oebosch) --replaced--> MAF (OE VDO)--replaced--> spark plugs (NGK) --replaced--> ignition coils --replaced--> injectors --replaced--> thermostat --replaced--> fuel filter