dr bob

My instant diagnosis method goes to Randy's experience. A leaking fuel metering/"distributor" unit is allowing fuel to weep into the cylinders, allowing vapor lock on what's normally the liquid side, as well as putting fuel into cylinders. You can check plugs after 20 min soak and sniff for fuel there and in the cylinder(s). Otherwise you'll want to borrow or buy the pressure gauges and the Probst book anyway if you want to keep the car running and happy both short- and long-term. Coupled with a decent WB02 setup, you'll master CIS quickly. Or go to sleep crying ...

CIS is wonderful when it's used and driven regularly so it stays clean. Because it depends on careful balance of fuel and control pressures, and is sensitive to blind changes and adjustments, dirt and fiddly folks are usually the biggest enemies of good CIS.

am

Thank's a lot Dr Bob and Randy. I'm waiting for the check valve to put on fuel line I ordered to continue working on my car. To check the plugs after the crucial non starting period is an excellent idea.
Should I put injectors cleaner to help lower the chances of having problems since I winterize my 928 for several months?
Another theorical question and forgive my ignorance: Since the injectors are not electrically powered on CIS cars, what makes them inject at the right time?

Randy V

The CIS system uses a fuel distributor that acts much like an ignition distributor, sending pressurized fuel to each injector in sequence.

The fuel distributor can also be a source of trouble as it is a delicate piece of equipment that can be impacted by dirty fuel clogging the small screens located in each of the 8 distributor ports.

I recommend you do a google search for CIS fuel system overview. You need to understand how the overall system functions in order to troubleshoot and repair it.

LT Texan

The C in CIS is for continuous.

Once the pressure in the line to each injector rises about 45psi (I think), they start spraying.

The fuel distributor is one big variable pressure regulator. The more fuel you need, the higher it raises the pressure in the lines to the injectors.

am

Thank's for excellent explanations. I know the next question is not related to my problem but by curiosity... What synchronise the CIS to pressurise the right injector at the right time?

Rich9928p

As said before, the "C" in CIS = Continuous, the injectors are ALWAYS spraying. Think of a carburetor, it is adding fuel to the air stream at all times ... CIS does the same. The intake air plate moves a piston that varies the rate of fuel flow that is sprayed by the injectors, it is proportional to the flow of air into the engine. The warm-up regulator fine tunes the control pressure that enables the subtle enrichment tuning needed for cold running condition.

The 928 electronic fuel injection (L-Jetronic and LH-Jetronic) is a batch fire system, all eight injectors fire at the same time. More modern injection systems provide sequential injection where injectors opening is synchronized to the intake stroke of each cylinder .... but that was done not on the Porsche 928.

am

So if a leaking injector could be my problem the only injector that could leak is the cold start injector no?

checkmate1996

yes, any injector could leak when the motor is off. However, when the motor is running, CIS literally means continuous injection system - so in essence they are always 'leaking'. Generally, CIS give bad spray patterns when old or clogged which is more around poor performance than anything.

checkmate1996

lol - i just realized RICH be me to the punch!

am

If I understood well, a CIS in good condition holds the gas pressure when the engine is OFF ?

GregBBRD

Trying to fix CIS injection without a pressure gauge is like trying to figure out what color something is, without your eyes open.

"Your" problem is a very typical CIS problem.....and one of the first questions I ask, when someone is trying to tell me what their CIS vehicle is doing wrong.


"So, if you get your car warm and go into the grocery store for 15 minutes....does the car re-start?"

Of course, the answer is usually "No".

99.9% of the time, this means there is a pressure leak.....and that pressure leak can happen anywhere. Fuel pump check valve, fuel accumulator, injectors, fuel distributor, warm-up regulator, cold start valve, leaky fuel line.

The "system" must retain a certain fuel pressure for a certain amount of time. This exact specification is in the literature, but is about 1.5 bar for 20 minutes. Yours probably drops to 0 bar, in 20 seconds.

What is happening is the fuel that is in the system is being forced, from the engine heat, back to the tank. The heat in the fuel lines then turns the remaining fuel into vapor. The result is like running out of fuel and having vapor lock at the same instance....the car must be cranked a very long time....or cool down to re-start.

Without a pressure gauge, you can only replace one component at a time, until you fix the car. With a pressure gauge, you can isolate which component is bad, using a couple of fuel line clamps.

For instance, to test the fuel pump check valve, you install the gauge and monitor fuel pressure. Have someone shut off the car, the same instant that you "clamp" the fuel feed line shut. If the pressure holds....you have a bad check valve....if the pressure still drops, the problem is somewhere else.

You can do the same test for the accumulator....simply crimp off the fuel line between the engine and the accumulator. If the pressure still drops....you have a problem somewhere else. If the pressure holds, you have a bad accumulator (or a bad fuel pump check valve, which hopefully you tested first.)

Warm-up regulator or fuel distributor leaking? Crimp off the return line.

Injectors or cold start leaking? Crimp off both the return line and the feed line at the same instant.

With a little bit of logic and a pressure gauge, you can figure this out pretty easily. Without either...you are going to "throw money" at the vehicle until you get lucky.

am

At this point I'm trying to understand perfectly how the system works. This forum is better than many books because of the great experience and knowledge of the persons who want to help by writing back.
I'll get the gauges and books it takes for the trouble shooting in time.

LT Texan

this!

checkmate1996

am- in our most polite way ... You must have the book and gauges. These tell you exactly where the problem lies. Again, think of calling a surgeon to diagnose heart problems over the phone. Without a blood pressure test, etc he has no baseline. Same thing!!!! We need baseline numbers !!! How many more of these!!!!!!!!!!! Do i need to put down...

jpitman2

The Watson book is excellent, but hard to find cheap now. get it or similar, and the gauge set (Roger T does a good one), and make systematic tests. I had similar hot start issues once. Fixed for some time with a new accumulator, then the half hot start temp sensor that fires a solenoid failed - I don't think your model had this part. Pressure testing will tell you a Lot!
jp 83 Euro S AT 55k