964 running rich again..last time maf sensor..

[bigchillcar]

car is doing what it did back in late spring, running rich intermittently, blowing black smoke and dying without the gas pedal. last time it was intermittent, too. one drive would be fine, the next cutting out. my wrench played around with cleaning the connection at my maf sensor. i'd add that i have a k&n filter (he doesn't like them, partly because they fit loosely..maybe contributing?) i understand that a bracket was made to hold them firmly in place, but that even these have been known to break?

any leads to follow? should i find and disconnect the maf sensor and see if there's corrosion on it - again??

[ischmitz]

I had a couple of intermittent issues with my 964 3.6 engine recently. Turned out I had a loose connector at the AFM and a bad connection or intermittent failure of the O2 sensor. With the O2 sensor signal missing the car was undriveable. It would not idle at all and cut out completely for seconds at a time during cruise. It felt like a no-spark event. Hooking up a hammer (Bosch KTS301) quickly identified the culprits.

So what happened was that the O2 signal would drop to zero for seconds and completely throw the mixture off. As a result the car lost power (felt like no spark) during cruise. Most times the signal loss was not long enough to throw up the CEL. The loose connector on the AFM evently came unplugged and did throw up the CEL. With the DME in limp mode it was next to impossible to get it to idle.

Another item to think of is that hot-wire MAF sensors do not like oil in the intake. It contaminates the filament. Unfortunately, K&N filter require oil to function. I would loose the K&N and go back to a stock filter if possible.

Ingo

[bazar01]

That's going to be tough to diagnose in the absence of the Bosch tool.

I would check for proper fuel pressure first at the rail to make sure the FPR is good. Do a visual on the FPR vac line for presence of fuel.
Check the AFM connector and make sure it's firmly seated with the clip. Do you really have a MAF instead of AFM?
Do a resistance test on the new CHT both cold and warm and compare with specs.
Backprobe the O2 sensor while engine running at idle and note DC millivolt reading .

Also test the ignition system. If you have a timing light, you can use that to check that both circuits are putting out consistent sparks.
Maybe you have inconsistent spark causing too much unburned gas in the exhaust.

Good luck.

[bigchillcar]

thanks, guys. my gut tells me that it's related to my k&n air filter..wrench definitely doesn't care for them..it was sort of the problem last time and the symptoms are the same. bazar - could be i'm calling the sensor by the wrong name. is it the afm sensor and not maf sensor at the intake?

[Don Plumley]

I believe in a 964 its really an AFM - big barn door. In 993's they have a more typical hot wire MAF.

[ischmitz]

If your air metering is stock then you have a barn-door style AFM (flapper box) as Don said. MAF (mass air flow) sensors are stock on 993 and later. There are some aftermarket MAF kits for the 964 though for $$$$.

So with the stock AFM the K&N filter does not cause a problem with oil. The reason that your mechanic and others do not like them is that their efficiency is worse compared to the stock filter. On top of that I believe some have found the stock air box draws colder air due to its nozzle location. The reason some like them is the intake sound....

Back to your issue: Do all the tests others have suggested. I think that either the AFM (or its connection) or the O2 sensor (or its connection) are the prime suspects. In my case the issue would show up at random after going days to week without anything. That pointed to the connectors. It made it all the more difficult to pin down without the Bosch tester. Every time you unplug something to measure and put it back together the failure "hides" for a while because the contacts clean themselves to some extend in the proces. You might want to get some electronics contact spray and a small brush and clean the connectors thoroughly.

BTW: A bad or missing CHT would cause cold-start issues and lean running when cold.

Hope that helps,
Ingo

[bigchillcar]

Originally Posted by ischmitz

If your air metering is stock then you have a barn-door style AFM (flapper box) as Don said. MAF (mass air flow) sensors are stock on 993 and later. There are some aftermarket MAF kits for the 964 though for $$$$.

So with the stock AFM the K&N filter does not cause a problem with oil. The reason that your mechanic and others do not like them is that their efficiency is worse compared to the stock filter. On top of that I believe some have found the stock air box draws colder air due to its nozzle location. The reason some like them is the intake sound....

Back to your issue: Do all the tests others have suggested. I think that either the AFM (or its connection) or the O2 sensor (or its connection) are the prime suspects. In my case the issue would show up at random after going days to week without anything. That pointed to the connectors. It made it all the more difficult to pin down without the Bosch tester. Every time you unplug something to measure and put it back together the failure "hides" for a while because the contacts clean themselves to some extend in the proces. You might want to get some electronics contact spray and a small brush and clean the connectors thoroughly.

BTW: A bad or missing CHT would cause cold-start issues and lean running when cold.

Hope that helps,
Ingo

ah ha..this sounds like my problem, and i agree with all the points about the k&n - like it for the sound. last time it was running rich, dumping raw fuel through the exhaust, it was awful..black smoke and even a small fire i had to extinguish when i tried to make it to the wrench's shop. wrench actually came to my house (not a typo)..and what he did was to clean the afm connections, i'm almost certain. he had to remove the filter to get to the connector and he cleaned the contacts with a wire brush i have. right now, the problem has mysteriously vanished..so i need to know what the afm connector looks like, where to find it exactly and i'll clean that sucker..

[ischmitz]

Ryan,

I think you are right. The connector is a 5-pin female connector held in place with a spring clip. It connects to the air metering device (where the K&N is mounted to) facing toward the firewall. I would remove the both the K&N air filter and the air flow meter as well. To get to the connector you have to feel your way around the AFM. The spade-style pins on the AFM could use a cleaning, tool.

Let us know what you find and how things go.
Ingo

[bigchillcar]

Originally Posted by ischmitz

Ryan,

I think you are right. The connector is a 5-pin female connector held in place with a spring clip. It connects to the air metering device (where the K&N is mounted to) facing toward the firewall. I would remove the both the K&N air filter and the air flow meter as well. To get to the connector you have to feel your way around the AFM. The spade-style pins on the AFM could use a cleaning, tool.

Let us know what you find and how things go.
Ingo

ingo..excellent! thanks! i probably can't get to it until the weekend, but will report asap..and carry my fire extinguisher up front until then just in case.

[bigchillcar]

it wouldn't be oil contamination from the k&n causing it would it..?

[ischmitz]

Nope, oil won't have any negative effect on the barndoor-style AFM. It might actually lubricate it making it work better (just kidding). Let us know what you find.

[bigchillcar]

late getting to this job, as i really haven't had any symptoms lately. but i did remove the k&n filter to get access to the backside of the air metering device and disconnected the connector (it wasn't loose),looked at it..it looked back at me...it didn't look dirty, so i reconnected. so far no change..everything running normally. however, if i do hit the throttle hard, it will show some black smoke and i seem to be getting only about 17-18 mpg, even running mostly on the highway around 70mph. sounds like it's running rich to me..shouldn't i be seeing closer to 20-22 mpg?

[ischmitz]

22 - 24 mpg sounds about right for highway driving. Any chance your O2 sensor is not working properly? To diagnose you need to measure its output voltage as bazar explained. Also, if you have access to a wideband O2 meter you might want to check your mixture. I believe the C2 has a port upstream from the cat.