MAF readings - non linear

AMSRoadRunner · 06:33 PM

1986 911 3.2 - no cat - is running like crap. dies at lights, (intermittently) surges from low rpm acceleration (intermittently) seems like it is running hot (but oil temp is ok)

I became suspicious of the MAF. After testing I am almost more confused. The ohm meter is always getting a reading but the readings seem like they are oscillating.

Temp side checked out perfectly.

Any suggestions?

Thanks!

Andrew

Tippy · 03-24-2015, 06:13 PM

I didn't know '73 911's had MAF's.

Elombard · 03-24-2015, 06:31 PM

I assume you are talking about the 86? Have you seen the deal on the wear groove on the circuit board? You have to bend the sweep arm to a fresh spot? Might be worth popping the cover off to see if it looks like its worn through. There is some information posted about the procedure and details if you search.

Quicksilver · 03-24-2015, 08:55 PM

Technical whiny bit: The 3.2 doesn't have a Mass Airflow Sensor. It has a "barn door" Air Flow Sensor...
The air flow sensor isn't doing anything at idle. When the idle switch is contacted it is all on the idle control valve to meter air. You can test the switch easy enough to be sure the DME knows to go to the idle circuit. You can also see if the throttle is releasing enough that the switch is engaged. There are a number of threads on testing the idle control valve you can search on.

But ...
Lots of things to test but I'm suspicious of a vacuum leak in the intake gaskets. Check that the allen bolts are torqued to 18 ft/lbs. Even if it is torqued correctly it still could be cracked gaskets.

Easy test is to get an IR thermometer, put the car up on jack stands, start the engine while cold and after 15 seconds or so get the temps of the various exhaust ports. They should be pretty equal.
If there are cold ports you know that something isn't burning there. A misfiring plugs and bad injectors could also show up as a cold port.

ischmitz · ischmitz's Garage

Go here Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com Very good procedure involving a 9V battery to test the AFM. The Ohmmeter is not using enough current to get reliable readings. Once the test with the 9V battery shows flat spots follow instructions on how to try and fix that by:

- Adding a wire to improve the contact between shaft and wiper
- offset wiper position on thick film resistor to address wear on resistive track

Ingo

NY65912 · 03-25-2015, 02:32 AM

I sent my AFM to Bavarian Restoration, they provide a data sheet of the refurbished unit. THe engine is very happy and linear in acceleration and idle beautifully.

BTW, I also sent my DME to Ingo (Ischmitz) to be evaluated and the necessary repairs performed as needed. Although the car ran very well prior to this work, Ingo found, under microscopic imaging, potential fissures developing in the solder joints which could lead to intermittent conection problems.

As our cars are approaching 30-35 years of age, I thought I would be proactive in heading off any potential problems down the road.

Smoove1010 · Smoove1010's Garage

Bumpy idle, acceleration flat-spots, hesitation, idle hunting - these are all popular topics on the forum, and there are many posts. They seem to fall into two categories - vacuum leak troubleshooting and AFM troubleshooting, though the posts go off on many related (and sometimes pertinent) tangents. Here are some threads that I found to be valuable.

Vacuum Leaks:
vacuum leak you probably didn't ever think of

Vacuum leak - check your brake booster line

Another Vacuum leak question

Stop SMOKIN! The Cure 3.2

AFM, AFR & Idle Adjustments
idle bounce

1986 911 idle surge

Euro 3.2 Air/Fuel Mixture ?

It might be worthwhile to search for threads on the idle control valve and idle switch - these are two of the basics that are very easy to test.

Here's how I solved similar problems on my bucking Carrera: AFM Mixture - Am I That Lean?

Good luck!
GK

Quicksilver · 03-25-2015, 06:49 AM

^ +1 ^
Great post Smoove1010! "Test, Test, and when done TEST!" (I hate seeing people throwing parts blindly at problems.)

When I had my "Rubber Explosion®" incident I discovered more then one hose with large cracks in it behind the throttle body. There are a lot of hoses and swaged fittings back there hardening over time!

WYDBODY1986 · WYDBODY1986's Garage

Where can you hook up a Vacuum Gauge to check for a leak and what readings would you expect to see if you had one? 86 911 like the OP

Tippy · 03-25-2015, 08:47 AM

Quote:

I didn't know '73 911's had MAF's.

86 I see. Wasn't there when I was being facetious. 😜

Elombard · 03-25-2015, 08:57 AM

^I figured he must of changed his sig :-) I was thinking "What does Tippy know?" Is there a story here....hmmmm

AMSRoadRunner · 03-27-2015, 02:52 PM

Thanks for all the helpful leads guys!!

Update:
I tested the MAF as per:
Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk* ©2001 www.the944.com

then I had a beer and tested it again with the same results. Voltage bounces all over the place and drops off when wide open.

Not looking good for my MAF.

Going to give Bavarian Restoration a call in the AM. I feel like that is my best bet. (I know, I know perhaps not the best place to talk about my feelings.)

Andrew

Hil911 · Hil911's Garage

I apologise if this is rubbish, but I remember testing the barndoor flow meter in either my Toyota MR2 or my GTiR and it does indeed have an oscillating signal and that is correct.

In those cars the signal is not supposed to be linear.