Intermittent SES / fuel trim codes
 

Hi guys,

I have an '06 3.0 with 100k. Recently did a major engine service--OFHG, Vanos, coolant pipes and hoses, water pump, thermostat, and installed the GAS CCV replacement kit. I've driven around 3k miles since I completed this project without any issues. Around 200 miles ago, I also replaced the O2 sensors and MAF preventively (no codes). I used a VDO MAF and Bosch O2 sensors.

Two days ago I had an SES pop up with E3/E4 Fuel Trim bank 1 and 2 permissible range exceeded, as well as cylinder misfires on 1, 4, 5, and 6.

I cleared the codes and the light didn't come back on until my drive to work today. Today, only the E3/E4 Fuel Trim bank 1 and 2 permissible range exceeded codes popped up.

What could cause these codes? Could I possibly have a vacuum leak causing these codes? I didn't smoke test the engine after my big project, but again I've driven about 3000 miles since then and haven't had any problems.

I have the Foxwell NT510 scan tool, but I've never used it to read live data/fuel trims before.

I'm having a similar problem and exhaust leak is my suspect. I need to do a smoke test.

You can reset adaptations to push off the error and see if it resolves you will usually get a 2-4 week reprieve from the error. Look in the service>drive menu for adaptation reset

OP you very likely have a vacuum leak. A smoke test is in order, but before you do I would inspect /reconnect the following connections

-intake boot to throttle body
-every one of the CCV connections including bottom to dipstick and air distribution pipes on top of intake.
-purge valve hoses
-caps on side of intake and the one on the CCV

Can anyone point me to instructions for how to view the fuel trims on the Foxwell tool?

You didn't buy a Siemens MAF. What is the airflow at idle g/s.

I smoke tested this morning for over 10 minutes and found no leaks at all.

The airflow at idle on the new VDO MAF was 7.28 g/s. I swapped back to my old Siemens MAF (which I replaced preventively, so luckily I kept it just in case I needed a spare). The airflow at idle on the OEM BMW Siemens MAF is 4.48 g/s. Which one is correct or better?

I was getting NEGATIVE 8.8-10.5 short term fuel trims with the new VDO MAF at idle installed. With the OEM MAF reinstalled I am getting short term trims that fluctuate between. -0.80 and + 0.80. When I increase to 2500 rpm, the short term trims go to between +3 and +5.

Maybe the new VDO MAF is bad? I guess time will tell if the SES light pops back up. I'd like to investigate this issue further using fuel trims and now with my old OEM MAF reinstalled. What should I do next?

I would say the original Siemens MAF is correctly reporting the aiirflow.

a variation from -0.80 to +0.80 will not set off a SES light.

"A rule of thumb on a MAF's air flow rate at 500 rpm is 1 gram per second per liter of engine displacement."

This rule of thumb only works for an engine that has relatively no load on it, e.g. no A/C on and no heavy electrics (like cooling fan). So since you have a 3.0L engine, that means at 1000 rpms, you should be seeing about 6 g/s of air being reported by the MAF. This is the easiest to do with just the throttle. Then you can check it by then rev'ing slowly to 1500 rpms, 2000 rpms and record what readings you are getting. This should follow a linear fashion, so you can also interpolate data if you are running at say 700 rpms or something in between.

Obviously there will be pumping losses and and friction involved, so more likely than not the mass flow rate is going to be slightly higher.

Make sense?

OP, it seems obvious that "new" MAF is the culprit here.
With your old MAF your fuel trims are almost perfect but with new one they're considereably off.
At 10-15% +/- fuel trim, the ECU will trigger CEL.

P.S.

I've had my AutoEnginuity Scan Tool since 2009 and it's excellent for reading short or long term fuel trims. Even 12 years later, I don't see any better scan tools in this price range, not to mention if they wuold last 12 years.

Only to a Rocket Scientist. So why not just answer his question about which MAF is reporting the correct airflow? The old Siemens or the new VDO?

So just from your explanation, the new VDO is not correct since a BMW M54 idles around 700 rpm so 7.8 g/s is Waaaaay too much.