Otto Mechanic

Quite a few post here mention disconnecting the MAF while trying to diagnose misfire problems. I'm working my way back through a recent water bridge re-seal on my '85 S3 (US) and the first thing to try was this test. I removed the air box, disconnected the MAF cable and re-installed.

Prior to this the car would start, but die in seconds. After unplugging the MAF it starts right up and held idle.

After searching, I came up with lots of posts that describe the test, but none that describe what it really means. Is this the conclusive sign of a bad MAF or just a branch in the decision tree?

Thanks,

GregBBRD

The MAF is a constant....good or bad.

If you disconnect it, hook it back up and it works different (again, good or bad), that would mean, to me, that one or more of the connections had resistance.

I'd clean the contacts and spray on some Wurth Contact Cleaner and Oil.

FredR

Disconnecting the MAF tells the brains that a default condition exists and in such mode the MAF sends a signal to the system to generate a fixed load signal equivalent to support operation at a very light load factor. This signal is designed to support operation such that car can roll at a low speed but not much more than that hence the term "limp home mode". The car will run rich at idle and then quickly lean out as the throttle is opened much more than just off idle.
i
You did not state in your post whether the car now runs ok or whether it is just the idle that is stable. If it runs ok chances are you have a fault in the connector plug or wiring. If the car operates but is rich at idle and will not pull the skin off a rice pudding then the implication is the MAF is shot [or the connector is bad]. Thus pulling the MAF plug tells the operator a fair bit about what may be going on but not necessarily everything. Why the condition existed but was not detected by the brains initially I am not so sure - maybe it is getting some kind of corrupted signal that fools the brains? Pulling the plug for sure generates the limp home mode scenario.

Otto Mechanic

Hey Greg, we meet yet again

Thank you for your reply. I did clean the contacts using Stan's favorite; DeOxit. That was the second pass on the problem, I should have done it before putting it together the first time, but at least I remembered to do it the second time.

Alas, it had no effect. What I'm hearing from you and Fred is I either have a MAF problem, which I can rule out with a spare MAF, or a harness/connector problem. Based on physical inspection my best guess would be the harness, which is tightly kinked and has been removed and replaced many times over the past 5 years I've been engaged in this project. The rubber boot is still blocking a full visual inspection, but it's cracked open in three places and appears fragile.

Thank you both for your consideration. My next move will be to test the MAF (easy) then the MAF harness unless advised otherwise by equally skilled diagnosticians.

For a more complete history of this problem, feel free to look at a previous thread here: https://rennlist.com/forums/928-foru...l-failure.html

Best Regards,

Otto Mechanic

Fred -

I haven't yet taken it off the lift to test it's chutzpah. It starts and maintains a high idle (near 2000 rpm) with the MAF disconnected. With the MAF connected it starts, runs to about 2000 rpm, then dies. I can feather the throttle in both modes and have run it past 4000, where it starts to detonate (backfire).

So, not sure yet if it runs OK w/o the MAF, but can say it runs.

Sounds like the harness?

Thanks very much for your help,

FredR

Scott,

Unless I missed it, you did not state whether you have a manual or an automatic transmission -not that it makes much difference. The automatic stall speed is 2000 rpms so my presumption [right or wrong] is that Porsche designed the default set point to coincide with that stall speed knowing that the engine pretty much runs to 2k rpm as soon as one hits the pedal and at 2k rpms the car will happily run in top gear and get one home. The engine will support a lean burn to some extent and then at some point it will simply give up the ghost as it were.

Sounds to me like your LH is not getting a viable modulating signal - just question of whether it is the hard wiring elements or the MAF itself. I revamped my wiring around the connector about 5 years ago - it was a mess under the connector and not much better until it got back into the main loom where it seemed fine. I cut back the wires into the loom, crimped a new section of wire and redid the terminals on the connector that I reused. I also purchased a new connector that remains in my spare parts bin waiting to be fitted but to date just not needed. I used the crimps because they were easy to fit- was a bit concerned about whether they might corrupt the signal but that was seemingly not the case- at least I can see what is going on MAF reading and fuelling wise with ST2 [Sharktuner] .

Otto Mechanic

Hey Fred -

Thanks for relating your experience, it sounds as if I should try my spare MAF set to the factory spec of 382 ohms, see if that does the trick, then tear into the harness, which would of course be a much more difficult procedure, especially since I suffer from an L5/S1 disc herniation that makes bending over the engine bay for long periods very painful .

Can I assume you spliced in lug connectors on the wires or did you discover a way to open up the old male connector? Did you find a replacement boot somewhere or use shrink wrap/electrical tape to close?

I truly appreciate your effort to relate all this and yes, my car is an automatic. It normally idles at 750 though.

Best Regards,
Scott.

FredR

Scott,

Removing the pins from the connector is easy- you just need a safety pin or a paperclip to push against the barb that holds each pin in place. Upon reinstatement I used a self vulcanising tape over the connector as I had nothing else available to me at the time.

Replacement is the best option and connectors come with a new boot or you can find replacement boots as these connectors are standard 7 pin Powertimer connectors [I think that is their name].

Otto Mechanic

Fred -

Many thanks again. I'll try locating a new connector, though mine seems re-usable now I know how to take it apart. If there's damage to the harness I expect it to be at the kink, which is about an inch back from the connector.

I do have some self vulcanizing silicone tape my daughter bought last week to repair a leaking fuel line on a "new to her" 2005 Volvo S40 (I know, I kept her from using it) that should work fine to replace the boot.

I'll let everyone know how this eventually works out.

Regards,
Scott.

SeanR

You can have a MAF that "tests" out ohm wise but still doesn't work correctly. Lots more internal to those than the 2 pins you check for ohmages. One of the reason I keep a JDS rebuilt on hand for testing.

Otto Mechanic

Yeah, I'd planned to send my spare out for rebuild, I think JDS has a US subsidiary somewhere in Oregon. I pulled the MAF and boxed it up but never shipped it. Did the same with a set of 8 injectors that I meant to send to Witchhunter but never have.

I'll try the spare anyway before sending it to JDS.

Otto Mechanic

Mine seems to be a 6 pin and "Powertimer" seems to have gotten me close enough with DuckDuckGo; I think its an AMP/Bosch 6 pin Male (plug) Junior Power Timer (JPT) connector. I can find plenty of female (socket) connectors but so far haven't found any males, which is sort of odd. I may have misunderstood the gender of the connectors, it's difficult to tell from the pictures I've found which one is considered "male" or "female".

I'm proceeding with the plan to use your re-build instructions on my existing hardware.

Thanks again,

FredR

Scott,

Check that plug out carefully. As I recall it is a bit weird in that there are 5 cables utilised with 6 pins visible in the MAF side of the connector.

Indeed they are a 6 pin junior power timer connector with 5 wires connected- just checked my spare MAF. Clearly my memory is a bit jaded- must remember to cross check details like that before posting! Now you have me wondering whether I ordered the correct part number of terminals wise-purchased mine off a UK Ebay supplier when I was last in the UK-spur of the moment thing - must check my parts bin. Give Roger a bell and he will fix you up with the correct part should you need such- probably better to replace it says he who did not!.

The terminals on the MAF are male so the terminals in the connector are female terminals thus I presume the connector would be called a "female connector." Maybe they are hermaphroditic?

Heed Sean's advice about the MAF and checking it out. As I recall Louie is still the JDS agent in the US- you can check that out on John's website- http://jdsporsche.com. I understand Louie has facilities to check the MAF for correct operation [or otherwise]. Having visited John at his place in Cambridge I can highly recommend his work and on the plus side he is a very gent to boot and does a lot to support this list. John tested one of my spare MAF's for me some years ago.

John Speake

Roger stocks the 6 way MAF connector kits. I suggest you pull back the boot on the 6way cable form socket and check for bare wires.

Otto Mechanic

Will do John, thanks. I'll check your site for Louie's address and send out my spare.

Regards,