CIS Injection during start up / cranking

strictly · 10-12-2017, 11:06 AM

Can anyone tell me categorically (not theoretically), that if you disable the CDi, and hot wire the fuel pump on a CIS (K jet) system, that the injectors will inject while cranking/starting?

74 911 2.7S

I am getting no flow from the injectors during cranking, I have plugged the injector holes, and observed the air flow sensor plate movment, the plate lifts but only a few mm. My system and control pressures are fine. The cold start valve works (through the TTS). I presume i have a decent air leak. I have noticed the air flow plate flutters during cranking (pulses). My rest pressures are good, but presumably the fuel accumulator also serves as a damper? I have the 76 one in one out

I presume the injectors inject (by movement of the air flow plate), and the cold start valve sprays extra fuel

OR

do the injectors NOT inject during cranking, as there is not enough movement of the air plate; In which case the cold start valve provides enough fuel to start the car, and then once running it has enough vacuum to lift the air flow meter, which only then fires the injectors. If not, then that would not explain how a car would ever start hot

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I have checked a motronic 3.2, and the injectors definitely spray during cranking, with ignition disabled

ossiblue · 10-12-2017, 11:36 AM

The injectors will flow when the sensor plate rises far enough off of its rest stop. It is a mechanical connection between the sensor arm and the pin in the fuel distributor which allows the injectors to flow. There is enough vacuum in an in spec system to fire the injectors during cranking. In your case, it's possible an air leak is preventing a sufficient lift or something in the fuel distributor is gummed up.

Have you tried running the pump and lifting the sensor plate by hand? If you do that and the injectors still don't flow, there's likely a problem at the FD. If they do flow, your problem points more to a huge air leak.

strictly · 10-12-2017, 01:21 PM

Quote:

Originally Posted by ossiblue

Have you tried running the pump and lifting the sensor plate by hand? If you do that and the injectors still don't flow, there's likely a problem at the FD. If they do flow, your problem points more to a huge air leak.

Yes get flow and usual injector squeel when lift plate by hand (FP running). Okay so....i should definitely have flow when cranking....okay that is important, as i definatly do not....my FD was rebuilt (by a pro) a year ago, new fuel tank, etc. The resistance feels normal, and i have checked the plate height at rest and that it is not binding. So either i am not getting enough suck due to an air leak, or the FD or air flow mechanism is not working some how hmmmmmmmm

keep posting idea and i will keep chasing an answer. Im preety good with CIS generally.

T77911S · 10-13-2017, 04:06 AM

first off, do you a problem starting?

its all about the mixture setting, air flow and control pressure.

if the mixture is too lean that is an issue right there.
the second part of this is hard to explain in writing.
starting, the sensor plate is not going to move much, obviously. not taking into account any changes in control pressure, the sensor plate is going to open a certain amount, lets say 1mm for X amount of air with the mixture set correctly. that will move the plunger lets say 1mm.
now lean it out. the plate moves 1mm but the plunger only moved .7mm
with such small plunger movement when starting it can make a big difference since it right on the edge of letting fuel go to the injectors.
an easier way of saying it is if you lean it out, the more air needed to start fuel flow.
NOW, don't go richen the mixture. you need to check other things.
air leaks and fuel pressures, timing all need to be checked first.

raise the control pressure and the sensor plate movement is even smaller. you need to check cold CP.

one other check you can do. with the engine off and the pump on, check for any play in the sensor plate. as soon as you push on it you should feel resistance

eastbay · 10-13-2017, 08:20 AM

sounds like you may be too lean on the mixture, if you don't have a gas tester this may help.

Dave Kost · Dave Kost's Garage

A word of caution

1975 and earlier 911 CIS systems don't have a fuel pump relay's which means you can easily hydrolock the engine. If the plunger in the fuel distributor freezes in the up position and the key is in the on position, ( i.e. not cranking), the injectors will fill the cylinders with gas.

Also, check the points on the ignition distributor are not closed.