3.2 Carrera Bucking @2200 RPM

[Scott Wolthuis]

The car, 1984 Carrera 200K plus miles.
With a light sustained load on the engine as it passes by 2100-2200 RPM it begins to stumble/buck. If held at this RPM it will continue, if I accelerate just slightly the bucking stops and it pulls away as expected.
I also noticed that on deceleration as it passes by the same RPM range it seems to surge/buck some as well.

Some things I’ve checked –
AFM modified and tested per http://frwilk.com/944dme/
The air temp sensor is out of spec. range indicating a warmer engine. (Has anyone got a line on the individual temp sensor vs. a rebuilt AFM?)
Plugs
Plug Wires
Cap & Rotor
Speed & Reference sensors (Both) are tight and check per ohm spec.

From what I’ve found here and if I’ve read it correctly some have indicated that the speed sensors sometimes have a break-up in the signal at a certain RPM but that doesn’t seem to explain the symptom going away when the engine load is increased???

[avendlerdp]

Many threads on this. Try your Cylinder Head Temp. Sensor.

-Alex

[Scott Wolthuis]

Cyl. head temp sensor was replaced in 2000 with the two wire version...

[450knotOffice]

Your post mentioned that you tested the AFM but did you check the adjustment of the idle mixture screw on it? I know it sounds like apples and oranges but apparently if the idle mixture is off a bit along with the idle speed setting the car can develop a sort of lurching stumble at very low throttle settings. Mine sure did. Once the car was warmed up to normal operating temperature it would buck any time I used very light throttle and had the revs around 2000 to 2500 or so.

I checked all sorts of things out and finally gave up and took my car to my Porsche mechanic. He found that the idle mixture and idle speed had been mis-adjusted and returned them to their proper settings. The car has run perfectly ever since.

[Hugh R]

On a Motronic car, how do you adjust idle mixture. Speed is jumping those two pins and turning a screw in front, but mixture?

[Dutchie]

Mine did the the same from 1500 to about 2500 rpm. I had the injectors cleaned and the problem was gone. Hope this helps.

[indigowhale]

Look at my Thread "Carrera Balkiness Season", you may find the same fix that I did.

Best of luck, this is a very common problem with the DME cars. Unfortunately, most of us go replacing parts first.

[450knotOffice]

This picture came out of Wayne's 101 Projects book. The green arrow points to the base idle adjustment location on the AFM. I believe it takes an allen wrench and is buried up inside a little hole on the underside of the AFM.

[indigowhale]

That allen screw on the AFM (turns about 15 revs total) when turned in will enrich your mixture. Tie a string around the allen wrench because it is very easy to drop it in the engine when trying to turn this screw. It does not turn very easily. If you look at the thread I referenced, you will get full details on how to set your mix with only the use of a voltemeter. My car was running lean and producing the same results. Even passed emissions with flying colors. I changed all the usual pieces: AFM, HTS, Plugs, Wires, DME chip, etc.

[911carrera1984]

have a leakdown performed just to be sure. i had stumbling at the low end and after lots of relaced parts and wasted time it turned out to be a bad valve on number 4.

[Hugh R]

Thanks guys. Nice to know, occasionally I have bucking problems, but not very often.

[Scott Wolthuis]

Some rennlist threads also indicated this was a result of running too rich. Since my O2 sensor has well surpassed its 60k service interval I’ve got one on the way. So far I adjusted the AFM 3 turns toward lean with a drive between each turn. Un plugged the O2 for a 4th drive and it still have some bucking although it seems a bit milder. After I get the O2 sensor installed I’ll go to the inspection station and adjust the mixture per spec. and go from there.

A few years ago this car developed the drastic hunt at idle not knowing the O2 maintance history I replaced it. The car still had the approx. 1500 rpm variation at idle. So, I drank a couple beers and decided to tap into and monitor the O2 voltage while in closed loop. Then I unhooked the O2 and adjusted the AFM mixture until it replicated the closed loop voltage and the problem was solved. This may not be worth mentioning but it worked…

[Scott Wolthuis]

Apparently being rich was the problem, good call Scott (450knotOffice). I had made the previous adjustments in quick succession in one outing. From the next day on, the bucking was gone. I replaced the O2 sensor yesterday and haven't made it to adjust the mixture yet but too rich appears to have been the problem...

Thanks
Scott