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VW MKIII-A3/B4 TDIs This is a discussion about MKIII-A3/MkIII Jetta/Golf (<99.5) and B4 Passats (96,97) TDI's. Non TDI related postings will be moved or removed.

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Old March 12th, 2018, 17:34   #1
Join Date: Sep 2017
Location: Stamford, CT
Default Bad MAF?

I recently swapped a 1Z out of a 97 Passat into my EuroVan. This required me to lengthen a few sensor harnesses, including the MAF sensor.

Everything is well except one code that I'm getting, which is implausible signal from the MAF. Here is what's going on:

Start engine, MAF (specified) reads 250mg/str, actual reads 29.9mg/str. If I give some slight throttle, the actual will drop down to about 10, and then return to 29.9 at idle. If I give it enough throttle to reach around 2500rpm, the reading will drop very low, then stick to 548.6mg/str, which remains unchanged. After this happens, RPM will have no effect on the actual reading until the engine is restarted. Reading never goes above 29.9 until it sticks at 548.6.

I'm 99% sure I have no boost or vacuum leaks, but I assume I would just see a slightly lower number, which would rise in relation to RPM, not fall slightly.

I triple checked my wiring. Everything is fine. From my understanding, there is only one signal wire. I am getting 5v and 12v to the proper pins. Ground pins are also fine.

Is there anything else that could be causing this? Injection pump timing is a fair bit off, but I assume that wouldn't have an effect on my intake air mass (at least this major). Engine starts and runs well both cold and warm.

Are you supposed to be able to see the "hot wire" inside the sensor housing? I can't seem to find one in there. I had a cluster short out on me through the "sender ground" before I installed the B4 cluster. I wonder if the wire got hot and burned up. That's my theory, unless of course I burnt up some segments in the ECU, which I find unlikely due to the fact that everything else is working properly.

I have a new MAF on order already. Just wondering if there is anything else to check out.

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Old March 12th, 2018, 19:35   #2
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Join Date: Aug 2011
Location: NJ 08002
Fuel Economy: Dismal

I think one of the ways to test the MAF is to unplug it and see if it runs better.

'96 B4V: Kerma tune and PP520s
'97 B4: Stock
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Old March 13th, 2018, 06:49   #3
Join Date: Jan 2001
Location: mi 48836
Fuel Economy: 53 max, 42 min, average 46

Those reading indicate an intake clog or a failed MAF sensor or wiring error. Thogh I am not sure an engine would run if it was accurately reading 10mg/str.
548.6 mg/str is the ECU putting the engine in limp mode from the implausible reading. Unplugging the sensor should make it run like the limp mode...because it is.
Restart resets the sequence.

Take the the air filter and air box out of the picture.
Detach the MAF sensor from the air box leaving it attached to the intake hose and try it. If that works, the check the air box screen and filter for clogging. Critters love that area for making winter lodging.

See if the MAP sensor (group 11 actual) reads well below 1000mb at idle. If it is down into 850 or lower then it is probably an intake clog.
1999.5 a4 Jetta, port matched, RC2, sprint 520's, 1.5" lift kit
1996 Passat B4V, coat hanger supported exhaust system,
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Last edited by JETaah; March 13th, 2018 at 06:59.
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Old March 13th, 2018, 17:31   #4
Join Date: Sep 2017
Location: Stamford, CT

The van has a brand new air filter, and there are no restrictions upstream of the MAF.

When the engine is restarted, it first reads 29.9, then the number DECREASES to around 10 when the engine is revved, meaning the reading decreases in relation to RPM (air mass) increase.

I haven't had a chance to check MAP readings, but I recall them being within bounds of the specified value. I have a boost gauge and it shoots right to 10-15psi with light to moderate throttle, which as far as I'm concerned, would indicate no significant restriction.

I'm somewhat interested in what other peoples experience with hot wire MAF failures have been on their TDIs. As far as I know, they tend to be pretty stout units.
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Old March 13th, 2018, 22:17   #5
Steve Addy
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Join Date: Aug 2002
Location: Iowa
TDI(s): 97 Mk3

If the MAF has been exposed to oil, say from a K&N type air cleaner element they can give faulty readings. I would first try some MAF cleaner and see what happens after that.

As Todd said the easiest test is to unplug the MAF and see if it runs better, if it does that's a good sign that the MAF is faulty. There's also the test you can do with VCDS recording performance.

The Mk3 / B4 Pierburg MAF's test to be very reliable. I'm not saying they are bullet proof, but compared with the Bosch units that followed on the Mk4 cars they were stellar performers.

97 Jetta TDI, 90 B3V TDI, 92 MB 300SD

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Old March 14th, 2018, 18:47   #6
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Join Date: Nov 2007
Location: Nobleboro, Maine
Fuel Economy: And the miles just keep ticking away

I have seen 3 of them that failed due to the oil film from K&N air failters.
'97 B4: 453,000 miles Malone 3, PP520's, Stage 2 cam, SBC3, ARP studs, 2 1/4" exhaust, 2.5" Cat, no mufflers or EGR.
'96 B4V: 356,000 miles Malone 2+, PP520's, 6 speed, 02J short shifter, E-Codes, G60 brakes, Evo plate, DMF, stainless exhaust, heated leathers, cold weather package
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Old March 18th, 2018, 09:15   #7
Join Date: Sep 2017
Location: Stamford, CT

'Twas the MAF. Got a cheaper knockoff from RockAuto that seems to be working well. Comparing the two, it seems as if the hot wire inside the original burned up. Weird.

Injection timing is spot on and I have plenty of boost now. Thanks for your help guys.
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