cfc928gt

This is my sons '89 vw fox which has CIS fuel injection. The CO is twice the allowed, HC just barely passed, NOX is fine.
HC - GPM 2.8642 actual 3.0 allowed
CO - GPM 41.377 actual 20.0 allowed
CO2 - GPM 257.5889 actual
NOx - GPM 1.9507 actual 6.0 allowed

So it appears it's running too rich. My question is this. Can I lower the CO by adjusting the air flow sensor plate? In the fox manual it implies it only lowers it for idle. Will lowering CO also reduce HC a bit? Any thoughts or ideas welcomed. Thanks!

Rich9928p

It sounds like it is grossly over fueled. Adjusting the a/f mixuture could solve the problem. Other potential problems could be

- improper control pressure which is provided by the warm up fuel pressure regulator (if it is similar to the 928 Bosch K-Jetronic system) - you need gauges to verify that the system pressures are correct

- bad spray patterns from the fuel injectors [fuel isn't atomized well, so more fuel must be sprayed in to keep the engine running, excess fuel puddles up resulting in too rich of a mixture.

- clogged up fuel distributor, too little flow to some injector circuits requires more flow to go to others, resulting in over fueling and too much CO.

cfc928gt

Rich, thanks for your input. I do know it has a warm up reg so that may be suspect. All the plugs looked the same, a little black from the mixture.

Rich9928p

Look around on the web for primers on Bosch K-Jetronic fuel injection. The system is pretty straight forward, however the engineers had to figure out how to get warm up enrichment and idle speed increase when cold. So they played tricks do that, hence the WUR, aux. air valve, etc. In other cars (not Porsche 928) they added external lambda control schemes for smog control.

Key measurements for system health are system pressure and control pressure [which changes with engine temp - this is where the cold start enrichment is done].

In 928s, often the fuel injectors get clogged and don't spray well (there are screens inside the fuel injectors, and the proper cleaning requires the cleaning fluid to be flowed through in the opposite direction, most shops don't do this). I also found small screens between the top of the fuel distributor and the fuel lines - one more thing to get clogged so I removed them [I'm not aware of a source for replacements]. CIS fuel injectors are less expensive then the electronic ones, so buy'em while they're still available.

macreel

cfc928gt... have you set the CO adjustment screw on
the airflow sensor lever arm (takes a long
3 mm allen wrench)?

Checked the fuel pressures ??


G'luck.

Dennis Wilson

How old is the O2 sensor? May help to replace it. Also, if the O2 sensor or frequency valve wire is broken the system will go to a default rich mixture.

Dennis

jpitman2

Get Ben Watson 'Bosch Fuel Injection', paper back, Amazon ~$15, covers data on all CIS installs, with system and control pressures, diagnosing problems etc. For Audi Fox, cold control 14-24, hot 49-55, system 65-75psi.
Good value.
jp 83 Euro S AT 50k