3,500 rpm

sojahseh · 11-06-2013, 05:17 PM

Is there something that happens to the timing or fuel at 3,500 RPM?

I have an 85 Carrera and Is struggles pretty severely at low speed. It feels like it is missing pretty bad, and bucks violently sometimes.

If you get it to cross 3500 it is like a switch goes off in the car and it runs beautifully. It's like magic, I swear.

This situation is totally repeatable, and the car runs great in the top of the rev range, but it's treacherous to have to pull out into traffic with the car. It's that bad.

Thoughts?

steely · 06:08 PM

Please see if someone else has a suggestion or do some research on this answer before acting, but....

it seems to me that the AFM could be a suspect - it has a carbon trace on a phenolic board that acts as a potentiometer directly tied to the "barn" door. Over time, the wiper cuts (wears) a grove in the portion of the trace that is most used, although I forget offhand what the usually RPMs would be that are associated with this section - but I think yours is a candidate- some can gingerly bend the wiper in order to place it on a nice portion of the trace.

There is a voltage measurement you can perform in order to test/refute this with a nine volt battery.

on edit: link fixed
http://www.the944.com/afm.htm

sojahseh · 06:50 PM

Dan,

That link does't seem to bring me anywhere but the top level of the Pelican boards.

Maybe buying a used AFM would be a way to prove the theory, certainly cheaper than buying a new one.

Someone with some Porsche knowledge suggested it might be the injectors, but it seems to me if it were the injectors, it wouldn't stop at 3,500.

Right?

Tippy · 11-06-2013, 06:55 PM

I had the same exact issue as you and spent 6 months swapping parts to no avail. Something was cutting the injectors off until I got over 3000RPM. Never did figure out and went aftermarket EMS.

Good luck.

Smoove1010 · Smoove1010's Garage

There are many, many threads on bucking, hesitation, hurky-jerkies, etc. Do a search and you'll have hours of reading and many suggestions. My issues are 99% solved, and it was a combination of different issues, including O2 sensor, AFM needing the track-adjustment, vacuum leaks, and base idle not being set properly. I also re-did my top-end which resulted in a big improvement. These cars are old, and there are many aging parts that could cause this.

I like using Google for searching the forum: in the Google search box, enter your search term followed by "site:forums.pelicanparts.com/porsche-911-technical-forum/"

Example: "AFM site:forums.pelicanparts.com/porsche-911-technical-forum/"

BTW - here's a thread on the AFM adjustment:
FR Wilke's AFM mod a success (and easy)