I'm trying to cover all bases with my car. What things control and adjust fuel mixture on the 944? O2, DME Temp sensor, AFM.

 

Thtrottle position, rpm.

 

incoming air is measured by the AFM, and a signal is sent to the DME. DME then takes note of the RPM and has a table telling it how much fuel to send out for a given "load" (airflow vs RPM vs change in RPM rate)
this fuel amount is adjusted a certain % depending on the temperature of the engine coolant, and the O2 sensor signal coming back from the exhaust.

throttle position only comes into play on a non-turbo car at idle or wide-open throttle, when the DME defaults to fixed fuel/spark tables and essentially ignores the AFM.

 

Just to build on what Spencer said and speaking of the early cars, the percent of fuel versus incoming air, i.e. the mixture, is obtained by the duration that the fuel injectors are open. They are "batch fired", which means all 4 are open and closed the same amount of time. How long they stay open is determined by an analog based algorithm ensconced in the DME architecture. Note that the vast majority of one's driving probably occurs in light throttle cruise, where the engine is just having to overcome the air and rolling resistance of the car. In this case, the O2 sensor constantly observes the exhuast gases and tweaks the mixture to obtain stochiometric mixture, typically 14.7 parts air to 1 part fuel (with gasoline). This gives best fuel economy. When you start up a hill or stomp the pedal to the floor, the DME will override the O2 sensor and use it's "look up table" to select an appropriate mixture for these more demanding conditions. Yes, it is all based on air flow, air temp, coolant temp, rpm, and at a secondary level, fuel pressure.

There is really no adjustment of the mixture possible, if that is what you are wondering about. Although folks have tried things like setting the fuel pressure higher, which provides a richer mixture across the board. In the early 80's, some Porsches had mechanical fuel injection, which did have a mixture screw and of course carburators had mixture screws.

 

Did you re-position the arms on the AFM yet or still resisting this advice?

Another possibility (not mentioned) is that you have a vacuum leak. Vacuum controls the fuel pressure regulator/dampener.

Both of these are mentioned on Clarks as indicated in both of your previous threads.

 

With the way you word it yes I'm resisting the advice, I find it hard to believe my AFM is sending mixed signals due to the tests I've preformed according to Clarks Garage. I'd like to note I've discovered the O2 sensor is disconnected, which I understand it makes the car run on pre-determined fuel maps. However if my DME thinks its getting a signal from the O2 sensor it could try to account for something that isn't even plugged in. I'm going to try and connect my O2 sensor today, where it plugs in is broken so I will need to do something about that to plug it in.

 

I honestly feel like this is one of the more likely causes of your problem.

Ive experienced it myself and the symptoms seem very similar to what you are experiencing (stuttering and hesitation at certain throttle positions or rpms, with no real pattern). Its not difficult to do, and the contact strips getting worn is a very very common thing on these old AFM's that have had those pickup points running back and forth over the course of 30 something years.

Is it 100% for sure the cause of your problems? I cant say.. but its definitely a common issue and may not fully show itself in a voltage check (moving the arm too quickly wont show a voltage drop/break in the contact strip during the test for instance because the multimeter refreshes too slowly). To me its worth doing as part of a process of elimination and frankly, just common preventative maintenance on an old part. There is no other component on these cars more responsible for telling the DME the proper air fuel ratio than the AFM. Its absolutely critical the AFM is calculating the location of the barn door correctly because this is what meters air volume, and air volume determines the amount of fuel the DME injects (along with RPM and so forth). This is exactly why Clarks mentions it as the cause for stuttering and hesitation, and why it recommends repositioning the arms.

Ultimately its your car and your choice, Im just trying to help with the most prudent and relevant advice (based on a trusted resource and real world experience) and keep you from going down rabbit holes or replacing unneeded parts.

 

Follow up with my car, issue is solved, I was just a fool lol.

Link: https://rennlist.com/forums/924-931-...-no-start.html

 

What ended up being the fix? The last post was about you looking for a DME temp sensor?

 

This thread will no longer be updated, best hope to anyone in the future referencing this.