looking for BHW ECU pin-out diagram (MAF pins)

Helping my bro track down a tricky problem with his Passat and would like to confirm the wiring between the MAF and the ECU is OK.

No luck searching here or via Google.

TIA!

What is this "tricky problem"?

Alldata sourced:

Four wires to MAF.

Pin 2 comes from fuse S231

Pin 3 goes to terminal 50 at the ECU

Pin 4 goes to terminal 40

Pin 5 goes to terminal 82

The ECU connector is the 94 pin one, which should be the larger of the two.

Thanks very much Brian... none of my Bentleys cover the Passat and coverage on the interwebs is scarce too.

In terms of the actual issue(s) at hand, the Readers Digest version is something like this:

Car is a Canadian 2005 Passat with BHW engine. It presented initially with three main issues:

1) Irregular idle and lots of clacking from the valve train
2) Overboosting when pushed even a bit, triggering limp mode
3) Very low power off idle to about 2000 RPM, lots of black smoke, extremely noticeable when engine cold, gets better as the engine reaches operating temperature but still makes the car unsafe when merging etc.

Two fault codes: Overboost and MAF Implausible Signal

Cam was examined, extreme wear to lobes and lifters. Cam/lifters/etc replaced, timing set bang on, torsion adjusted to 0.5, problem 1 solved.

In an effort to solve #2 and #3:

- Airway confirmed clear of obstructions, new air filter installed
- Charge hoses tight, no leaks
- EGR removed and cleaned, intake inspected, no blockages
- IC inspected, no obvious leaks or other damage
- MAF appeared very dirty, replaced with BHW-specific Bosch OEM
- All vacuum tubing replaced, N75 valve tested
- Owner decided to replace turbo, new OEM Garrett


Problem 2 is now gone, but problem 3 remains: extreme lack of power/bogging with engine below 2000 RPM, tons of black smoke. Once the engine gets over 2000 RPM power comes on smoothly, no overboosting. Worse when engine cold. MAF Implausible Signal code comes back a few minutes after being cleared. Car runs pretty much the same with the MAF unplugged.

Given that we've checked/repaired/replaced all the obvious suspects we are now looking at the less-than-obvious, MAF wiring being one of them, since the ECU is still complaining about the signal from a perfectly good MAF.

Open to suggestions...running out of rocks to turn over.

Check to make sure the MAP is the correct one by part number, as I have seen people stick ones from BEWs in there before and they are NOT the same.

Check to make sure the EGR valve is completely seated closed. You can do the output test of the EGR valve with it plugged in but hanging loose and watch the pintle go in and out. It should do so smoothly and without restriction.

Make sure the intake flap moves freely, should be fine though as that would set a different DTC.

Start the engine, run it (drive it a bit) until it is at operating temp. Then pull up the EGR Measuring Blocks (I forget which group, but it is in the first 10).

It will show the MAF actual, MAF requested, and EGR duty cycle in percent. Rev the engine a couple times. Let it stabilize. The actual and requested should be matching very closely. Note the EGR duty. Let it sit and idle, may take 3+ minutes or so. Eventually, the EGR will shut off, duty cycle will change, and the MAF actual should bump way up. I'll say 250 when EGR is on at idle, and when it goes off should jump to maybe 450 or so (I forget the exact numbers, but it will be an obvious change). The pitch of the engine will change too. It will get noticeably more diesel sounding. Rev the engine, it should go back to EGR on with actual MAF matching requested.

Let me know what you find.

Will do, thanks. MAF part number was confirmed to be the specific one for this engine when ordered, but dunno if we confirmed we were given the exact part number we ordered.

MAP, not MAF. Although I'd assume you'd know if the MAP was replaced prior to all this, but sometimes when troubleshooting it is best not to assume anything.

"Trust, but verify"
---Ronald Reagan

oilhammer said:

... sometimes when troubleshooting it is best not to assume anything.
"Trust, but verify"
---Ronald Reagan

Click to expand...

A'int that the truth. Been burned too many times when I assumed that something was never disturbed.

Another of my Mantras is when in doubt, check whatever was messed with last time someone touched the vehicle.

Well, guess what: EGR not closing all the way when cycled. Pintle was sitting a bit crooked and letting air by on one side. Just little, and only noticeable by shining a flashlight thru it, but my brother swapped in the EGR from his BEW Jetta and the bogging issue w/ black smoke was gone, like magic.

The only strange thing is that he then put the bad EGR in his Jetta and the car behaves normally. Perhaps the BHW engine is more sensitive to false air?

We're not going to look a gift horse in the mouth though...the big issue is solved.

Thanks for your help OH... how did you know?!

Yep, common. If you take the EGR apart, the top electric portion slides off after the Torx screws are removed, you can see inside. Often soot over time wicks up the shaft and causes the internal pivot to stick.

Sometimes you can clean them up and get them working again, but that will only be temporary.

Well thanks again for your help.

The EGR was removed and cleaned early on in the diagnosis chain, and appeared to be closed, but we never thought to actually watch it move... and that extra step made all the difference.

You are most welcome. The reason that you had a MAF related DTC is because the ECU "assumed" the EGR valve was closed, because its internal position sensor was feeding back a closed position, but the actual valve was not. So the MAF value was always off of what it expected to be, as some of the air going into the engine was sneaking in past the stuck open EGR.

On a gasoline engine, it likely would not have even been able to idle, but diesels are very different. They normally run with the EGR open at idle when demanded, but the ECU needs to be in control of this, and it does so by using the MAF.

It has recently come to my attention that the oxygen sensor is also used to double check EGR operation, however I am still not 100% sure how a failure mode would be interpreted within the system.

Your exact symptom you described with the car, low power off idle, smoky, stumbly, etc. is a pretty common telltale sign of a slightly stuck open EGR valve on the BEW, BHW, and BRM PD TDIs sold here.

Glad you got it sorted out!