Hi all, first post. New baby: '87 Carrera 3.2.

Experiencing a stumble/missing when accelerating - description below. Possible O2 sensor issue (air/fuel mix issue)?

Here is what I am experiencing: when i lug thorough 1st, then accelerate hard through 2nd gear, it stumbles / misses repeatedly through 2nd, every 500-1000rpm, right from 2000 up to 5500. If I lug through 2nd, then accelerate hard through 3rd, same experience through 3rd (although not as noticable / not as many misses). In any case, after upshifting I don't seem to experience it in the next gear up. Hence my theory that it may be related to fuel mix after having lugged through the prior gear (as opposed to ignition), not sure if that makes sense.

As a next step, I am thinking of checking voltage variation of the O2 sensor at different throttle/rpms to see if it is not responding properly as new and is starting to fail due to age - seems to be original based on invoices. 75k miles. (I know it is at least partially functional because when I unplug it, the idle definitely changes) Thoughts? am I way off? Anyone with similar experience?


- Pulled a plug and it looked good (tan color).
- Distributor cap, rotor look good. Original chip.
- 91, 94 octane same experience.

Brian
'87 Carrera

 

Hello Brian. I chased a similar problem on the 84' I used to have. I replaced the O2 sensor, the cylinder head temp. sensor and cleaned the AFM (I also adjusted the wiper arm in the AFM). I started with the cylinder head temp. sensor and that didn't fix it so I moved on to the O2 sensor and the AFM. That did the trick (I also replaced the cap, rotor, plugs, plug wires adn fuel filter along the way). I hope that helps.

Chris

 

I had a similar problem on my 84 last year at wide open throttle when I got over 3500 RPM. Turned out to be the fuel pump. At full throttle it was only pushing 10psi. Should be 35-40 psi if I remember correctly. Figured it out after connecting a pressure gauge to the fuel rail, taping it to the left quarter window and going for a drive. Replaced the fuel pump which the mechanic thinks may have been original and problem solved.

 

Hi folks, thanks for the inputs. The issue became more frequent over several weeks (car was new to me, and I believe it was not driven often in recent past prior to my ownership).

I started to suspect ignition / spark plug wires as well.

The new wires solved it ! Yay. In love again.

Interestingly, I did not see any sparks initially after several checks in the garage in the dark, until finally spraying somewhat more generously some water mist along the wires - then finally I did see very faint sparks in the area where the wires run around the top of the fan shroud. :-)

Just for fun, here are some other steps I took prior to detecting and ordering the new wires:

- checked the AFM voltage output consistency using the 9v battery test per the944.com info thread; the process turned out to be quite simple and not very time consuming at all. Was all ok. I didn't open the afm, or clean or move the arm. I simply observed very consistent voltage increase with no drops
- checked the O2 sensor voltage output, and it was also ok, ranged approx .1v to .8v per the related threads. The only tricky step was contacting the voltage meter electrodes to the o2 plug. Was glad it seemed fine.
- Founds a vacuum line hanging behind the airbox area, which had disconnected from the y connector behind throttle body. Reconnected that, but did not notice any running change really.


Anyhow, problem solved in the end. Thanks for the ideas, and happy motoring !

Brian
'87 Carrera

 

bring it out to starbucks saturday on the queensway .. we are all out back

 

I'm very impressed that you mentally checked down the list, and did the u-turn needed to find the problem.

Great work! BTW, the vacuum line going to the fuel pressure regulator has a huge impact on the way the car runs. Without it, you're running very rich.

 

A new set of wires, cap, and rotor did the trick for me early on when performance seemed to be lacking. I also had the dreaded no start after a car wash. Distributor cap was original equipment and filling up with water.

Speaking of that hose..... I'm going to be checking that this weekend. My first thoughts was that PIXXI's problem was a vacuum leak between the manifold and heads. That was one of my big issues. Changed out a lot of parts chasing that one down.

My engine doesn't always fire when I first tap the key. I checked for spark with a timing light and have ruled that out. My next guess is fuel pressure.

....also have new parking brake shoes to install this weekend.

 

Fuel pressure is fairly easy to check on 3.2 911s. The fuel gauge simply attaches to the fuel rail, unlike the CIS cars. Hard starting can be a weak coil too, so an old spark plug properly grounded, with a remote starter switch can visually check for spark. I have a nice spark tester that I use. Also, improper reference sensor gap (too much gap) can cause hard starting. The gap is not that sensitive, but if it's way off, then starting can be touch and go.

 

I did think a lot about vacuum and tried to 'listen' for vacuum around the intake manifolds with a piece of tubing (i was a little afraid to use the ether / quickstart / brake cleaner spray trick to listen for elevated idle, given my suspicion of bad spark wires) and instead of hearing vacuum, I heard pressure!. Turned out it I a small crack in the air fan cowling. So I temporarily i gave up on the vacuum leak check. Just enjoying smooth running ignition for now. ( may still replace plugs soon - did clean and regap all of them, but maybe there is a benefit there.

I forgot to mention something maybe relevant: 4 of the 6 plugs had the brass screw-on tip for the spark plug electrode tip, onto which the ignition wire cap is pushed, were noticeably loose. I gently tightened them before putting in the new wires that may have helped, but not sure.

Cheers, Brian
'87 Carrera