Go-carter

Hi!

I've had a problem with massive fuel consumption on my 928, on long relaxed trips it still made 7-8mpg. Seeing that, together with six black plugs, and some other things I figured that maybe the injectors were stuck open, or didn't close properly.

Fitted rebuilt 047 from eBay yesterday, and when I fired it up it smoked like CRAZY! Didn't dare to run it more than a minute, but was a huge cloud of black/gray smoke out of the exhaust. But the engine itself ran great, revving perfectly, no stumbling.

This leaves me to two conclusions:

1: The intake tubes/heads/cylinders are full of old fuel, oil, carbon build-up etc. from the rich running it has had before, and now that it's actually running right it clears everything up

2: The new injectors are worthless, and spraying fuel all the time.

What do you think? I'm going to do some more testing this afternoon.

WallyP

3. The control pressures are screwed up.

Fabio421

+1 Have you checked system pressure and control pressure both hot and cold?

SeanR

Give this link a try.

http://www.pelicanparts.com/911/tech..._Bosch_CIS.htm

Dennis Wilson

Have you tried leaning out your A/F mixture?

Dennis

dr bob

You can test the individual CIS injectors off the car with a simple test rig: A fuel pump, some plumbing, and a large glass jar. Put a hole in the lid of the jar and fit a grommet the noxxle will just pass through. A couple small vent holes are needed too. Use paint thinner through the pump, attached to the injector, and observe the spray pattern. Paint thinner will do some cleaning on the way through. You should see a nice cone-shaped fog of droplets from the nozzle. you can also verify the balance of the nozzles by timing the spray and measuring the amount of fluid that passes through in that time.

CIS does not depend on the "injector" nozzles for anything more than dispersing the fuel in a fog just above the intake valve. They aren't used for metering flow at all, so a rich condition won't be caused by the nozzles. Flow is determined by the metering unit, commonly referred to as the 'fuel distributor', and that flow is also determined by control pressure that's managed by the warm-up regulator. The basic system pressure is managed by a spring-loaded pressure control valve in the metering unit.

The ONLY way to determine proper function of the pressure control valve and the warm-up control presure regulator is with a set of CIS pressure gauges.

Only after proper control and system pressures are set, actual mixture settings require a CO meter of some sort in the exhaust stream. There are wide-band exhaust-gas oxygen sensors and meters available that make the measurements relatively simple. But that system requires a port in the hot exhaust piping right behind the engine, where the sensor can see hot exhaust before any cats or mufflers.


Conclusion: Buy/beg/borrow/rent a set of CIS gauges. IIRC, Roger has them available in his 928rus store. You may also find a local owner that has a set. They are common to virtually all CIS cars, including MB, Saab, Volvo, etc from the mid-70's to early '80's, meaning that a good local repair shop is likely to have a set. Once that's done, you can adjust the airflow sensor plate to get the final CO numbers dialed in perfectly.

In my limited experience, there is no substitute or shortcut method to setting up the system with gauges and the CO meter. Some folks can back in to a group of compromise settings that might allow the car to run, but the only way to get things right across all conditions is to start with a clean system, set up with correct pressures, then tuned for correct exhuast mixture. Anything else is like throwing darts at a small target in an blacked-out room.

cpayne

I had poor MPG and performance it turned out to be bad coil wire. The car wasn't firing on all cylinders The injectors were dumping fuel that wasn't burning. Car felt rough at low RPMs at high you could tell something was off but ran out well just poor mpg. Changed the wires . MPG fixed as well as rich fuel smell. I'm a noob, so please listen to the others first. Just posted this in case it's one of those Sling Blade moments " It ain't got no gas in it."

http://www.youtube.com/watch?v=G-tCIRJH9p0

Go-carter

Yup, assumed this was the road to go... It's just about impossible to get hold of any CIS gauges around here.

But I promise, next thing on list is cheking the control pressure.. thanks for the document!

EDIT: Plugs, wires, distributor and rotor is brand spanking new So the CIS is the only sinner.

Dennis Wilson

You can order the CIS test gauges from Ideolas site or JC Whitney. Prior to doing that, it's possible the thermotime switch is shorting to ground and causing the cold start valve to run continuously. Try unplugging it after startup to see if the smoking stops. Also, it is possible a PO has over richened the A/F mixture. Try leaning the mixture in small increments. Since it starts and accelerates OK, the system and control pressures are close, if not within, acceptable range.

Dennis

Go-carter

The thing is, prior to the injector swap I disconnected the cold start injector to test things out, and the car STILL started on cold mornings, with -10*C. This made me think that even control pressure couldn't be that off, and it has to be the injectors.

But anyhow, I'm going to buy myself a set of gauges

jpitman2

There was a time when JCWhitney would not ship outside USA when I wanted a set of gauges. I had an order for parts in with 928Intl, so I had Whitneys send the gauges to them, and they included them with my other parts.
jp 83 Euro S AT 53k, good CIS.

karl ruiter

Roger has the gauge sets, but I don't know if he could get them to you. You could build your own with a regular fuel pressure gauge, but you would need some way to adapt it to the plumbing. If you had a salvage yard where you could pull some of the lines off a old volvo, you could fabricate something pretty easily. I'm suprised you cannot find a gauge set locally, though, given that the same injection system was used by volve, benz, vw, etc.

Go-carter

They're selling this at my local car-parts shop:



Could it work?

I also have a 2,8 V6 with CIS, so I could probably take some things off that.

gbgastowers

That looks like it would work but it also looks very expensive. $250 or more is my guess.

Go-carter

Yes, it actually costs $330.

But now I found a CIS tes gauge kit on ebay for $100, so I think I'll try that one