# CIS mixture adjustment?



## danielson (Feb 13, 2011)

I am wondering if there is any way of adjusting the mixture or in my case (leaning) a cis system to deliver less fuel overall? possibly by adjusting the air flapper or maby something simple on the fuel distribution head?


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## cuppie (May 4, 2005)

All CIS systems have a mixture adjustment screw on the airflow meter - so, yes, it can be done.

BUT! 
1- what's the goal?
2- what's the problem?
3- what specific system is this (year/model/engine)?

Mixture adjustments should be a very, very rare thing to do - and, only done after ensuring that there are no othe problems that will affect mixture (incorrect fuel pressure(s), intake leaks.)
And, if it's an electronic system (K-Jetronic/Lambda, KE-Jetronic, KE-Motronic - anything with an O2 sensor), it'll simply try to correct out your (mis)tuning of it.


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## ziddey (Apr 16, 2006)

also, which kind of cis system are we talking about. the original cis basic was the only version that did not have a feedback circuit. with all other cis variants, an oxygen sensor is used to keep the mixture around stoich. Obviously, if you adjust the mixture beyond the scope that the system can compensate, you'll still be lean/rich. And of course you can unplug the o2 sensor as well.

What is the goal here? Long distance driving, trying to conserve fuel?


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## danielson (Feb 13, 2011)

the system is a 1987 jetta 1.8L 8v, and I am looking to possibly tweek the system to burn less fuel since I am not looking for performance or power of any sort, but just something that will ensure this thing isnt pushing more fuel then it absolutely needs to...


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## cuppie (May 4, 2005)

And, that one has O2 feedback control. Assuming that the O2 sensor itself is still functional, it'll try to undo your "tweaking" (read: mis-tuning) of the system (up to its adaptation limits.)
And, you'll probably make it run worse. If your state has I/M emissions testing, you'll also run the (rather high) risk of failing your next sniff test, too.

If everything is in proper working order (fuel pressures are correct, injectors aren't all gakked up, no intake leaks, clean air filter, engine grounds clean, coolant-temp sensor working, and a healthy (working, recently replaced - they do have a service life!) O2 sensor, that car really should get quite good gas mileage.
What's yours getting (average tank, and an all-highway tank)?

{edit}
Wait a minute! You're the same guy with dying fuel pumps!
Fix that (with a new filter, too!), per the recommendations in your other thread.
If you haven't yet done so (and, don't know how old it is), replace the O2 sensor - their rated service life is 60,000 miles; after that, accuracy can't be guaranteed (and, when they start failing, they tend to drive the mixture 'rich.')
- Check the thing over for intake leaks - remove and inspect the intake boot for cracks; replace if needed.
- Inspect (and, if necessary, replace) any cracked vacuum hoses.
- Replace your oil dipstick, dipstick funnel, oil cap, and engine breather hose if they're cracked/broken (these are intake leaks!)
- Check the rest of the intake tract for leaks - spraying carb cleaner at the sealing points will show where it's leaking. Common failure: injector seals.

Fix what's broke - aside from your failing fuel pumps, everything mentioned above is _cheap_ (and easy) to fix.
Once that's done, re-evaluate your fuel consumption.
{/edit}


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