944 Fuel Injection - How does it work?

[George_Mayson]

Hello! Recently my 944 started stinking of a rich mixture, common problem I know so I ordered up my oxygen sensor thinking that was the most likely culprit and thought no more of it until I got the car up on the lift and found my '83 model has no oxygen sensor.

Supposedly being a final year engineering student I was suitably embarrassed that I hadn't even checked this, but now I'm going about it the right way and trying to test all the sensors that affect the computer's mixture control before attempting a repair or ordering the replacement part.

So my question is this - which sensors on my car affect the computer's mixture control? Obviously the MAF air flow meter and the DME temperature sensor will affect it. But what about the speed and reference sensors? Are there any other sensors that might be failing thus forcing the computer into a rich base mode?

Would like to get this sorted soon as the car consumes fuel at an alarming rate - used a quarter of a tank to do about 40 miles...

Symptoms:-
Extremely poor fuel economy
Exhaust stinks of rich mixture hot OR cold
Car idles poorly at all times, but worse when cold. It's doing ~500rpm idle when cold and ~700rpm hot.
As soon as you touch the throttle, car bursts up and runs fine, if a little lumpy around 1000rpm.
Exhaust quite often pops/backfires on the over-run.

Would appreciate any thoughts on this! Thank you

George,
Dundee, Scotland.

 

[Eldavo]

The temp sensor on the top of the block at the front can fail and make the car think the ambient temperature is too cold - thus richening the mixture. It's a relatively cheap fix and pretty easy to get to (especially on a NA car) so that would be my first port of call.

I'd also be checking the fuel pressure regulator as that seems to be a relatively common fault these days - get a gauge on the end of the fuel rail when running to check. Take off the big nut at the front but be careful not to lose the ball bearing in the end or forget to put it back in when done.

 

[George_Mayson]

Thanks for the advice, I've now checked the temp sensor as per clark's garage and the sensor is out of tolerance, but not by a huge amount. I've also checked the air flow meter which seems to be working perfectly.

I'm going to replace the temp sensor and see what effect that has, although the fuel pressure regulator would be a very plausable candidate for my way-too-rich mixture. (Higher than specified fuel pressure = rich mixture?)

The problem seemed to start happening quite suddenly so that makes me think an electrical sensor?

Thanks again for your help,
George

 

[GlennS]

Do not overlook your throttle position sensor.