CIS experts - help please, I'm drowning (in gas)

cowtown · 05-09-2003, 07:30 AM

Hi guys,
I'm still trying to get my '77 930 engine to idle well, but I have fuel problems, and just don't have the experience to know how to solve them:

--I guess I don't really understand how the fuel distributor is supposed to behave when there is fuel presure but no air going through the metering plate.

I hooked up my new pressure tester last night, and ran the pump without starting the car. The cylinders filled up with gas within 30 seconds, and I'm not sure why. Are the injector check valves supposed to stop this, or is the fuel distributor supposed to prevent ANY fuel flow when the air meter plate is closed?

I've played with the mixture screw, but maybe need to try again. Is backing off the screw (counterclockwise?) supposed to decrease fuel delivery, and prevent the injectors from spraying when there is no air going through the plate? Does control pressure play a role in this situation, or do I concentrate on the distributor adjustment?

--I followed the directions with the fuel pressure gauge and got a cold control pressure of 11psi (0.8bar) at 20C, a bit low. This measurement is valid without starting the engine, correct? I've got so much fuel in the oil that I need to change it before starting up again, but want to solve the fuel leakage problem first.

--Finally, can someone clarify the function of the valve on the fuel tester. When I close it, pressure goes to 55psi. Is this the way to measure normal fuel pressure (not control pressure)?

What to do? I'm clueless, but trying to learn. Thank you for any advice.
Colin

RoninLB · 05-09-2003, 07:42 AM

Quote:

Originally posted by cowtown

I hooked up my new pressure tester last night, and ran the pump without starting the car. The cylinders filled up with gas within 30 seconds, and I'm not sure why.

the CIS will always weep fuel, even when that specific cyl. is not firing.. there is no elec. sol. on/off control.. so you flooded the poor beast.. The "C" in CIS means continuous............Ron

MotoSook · 05-09-2003, 07:48 AM

Check the fuel pressure again, the right way...the injectors open on pressure. So if you are spraying, not weeping, gas, there is too much pressure tot he injectors! Make sure you have all the lines connected correctly. I'll try to scan the instructions for the tester for you. But it is essentially a 3-way valve that allows you to isolate the WUR return pressure (I think memory is correct on that).
Got to run...e-mail you later.

cowtown · 05-09-2003, 07:49 AM

Ron, but if the airflow plate is all the way closed, how much gas is actually delivered to the injectors? And shouldn't the injector check valves prevent flooding?

This is kind of what I'm trying to figure out...do I have an injector problem, or a fuel distributor problem.
Thanks,
Colin

dougcl · 05-09-2003, 08:03 AM

Colin, there could be a problem with the sensor plate rest position.

Doug
75 911S

RoninLB · 05-09-2003, 08:09 AM

"Ron, but if the airflow plate is all the way closed, how much gas is actually delivered to the injectors? And shouldn't the injector check valves prevent flooding?"
--- if the plate is closed the fuel pump should be off and the fuel pressure to the injector will not be enough to overcome the injector open/close.. if it leaks with pump off then dirty injector. you can also check the spray pattern by removing all the injectors and putting them into containers and eyeballing the pattern when you lift the airflow plate

"This is kind of what I'm trying to figure out...do I have an injector problem, or a fuel distributor problem."
--- if a problem, it's most likely an injector problem.. if the injector spray doesn't look good then maybe a couple of tanks of Techron will clean everything up.. I do the Techron act before every oil change, and Redline fuel treatment with every tank.. a $6 bottle lasts about 2k miles.. CIS is varnish/water/dirt sensitive

Colin / Ron

dougcl · 05-09-2003, 08:23 AM

Colin, it is possible that your fuel pump runs even when the engine isn't running. If it is true that all of your cylinders filled up with the engine off and the fuel pump running, then it's probably not injectors, since this would require that all of your injectors are simultaneously bad. The problem is more likely to be in something common to all the injectors.

For testing purposes, you can pull all of your injectors and let them run into containers (baby bottles) to monitor the spray, volume while the pump runs. Careful with the gas if you go this route, for obvious reasons.
Doug
75 911S

sammyg2 · 05-09-2003, 08:37 AM

When the air sensor plate is sitting on it's rest stop you shouldn't really see any fuel out of the injectors. As soon as it moves at all it will go to idle circuit which will deliver only enough fuel to support an idle.

The injectors fire any time there is sufficient pressure to them to overcome the spring in the injector, IIRC it's around 3.5 bar.

The mixture adjustment screw in the distributor adjusts a needle screw and changes the relationship of air plate position to fuel pressure delivered (I know that's a little simplified, but work with me here).

A simple way to remember which way the screw needs to be turned is "lean out".
Obviously to make it more rich would mean to screw it in.
For me it's easier to remember lean out as to make it more lean I need to screw it out.

A very small amount of adjustment makes a large difference. I read somewhere that total adjustment must be limited to 1 full turn max.

If I were you I would first make sure that the air sensor plate is adjusted so that it sits in the correct posistion at rest before I went any further. There's a spring thingy that it's suppose to rest on when no air is flowing. This is critical.
Then I would start checking fuel pressures and see if the injectors were actually firing when they are not supposed to be or if the source of fuel is from the WUR. If they are firing when they shouldn't be there are at least three different possible causes I can think of:
1) weak injectors
2) misadjusted mixture screw
3) worn, clogged, or sticking fuel distributor.

There are some good CIS books out there, I have one called "gasoline fuel-injection system K-jetronic technical instruction" from bosch.
It is not 911 specific but does a pretty good job of explaining functions. It doesn't have all the details but does explain the theory.
I'd be glad to look up specifics if you have questions on the function.

cowtown · 05-09-2003, 08:43 AM

Originally posted by dougcl
Colin, it is possible that your fuel pump runs even when the engine isn't running.
Yes, on the '75 chassis there was no provision for the fuel pump switch...I'm going to hook up the one on the '77 engine but for now it runs when the key is turned no matter what.

If it is true that all of your cylinders filled up with the engine off and the fuel pump running, then it's probably not injectors, since this would require that all of your injectors are simultaneously bad. The problem is more likely to be in something common to all the injectors.
This is also what I was thinking, but the engine sat 20 years, so who knows...I have the injectors sitting in Techron right now and will do the spray test tonight.

Do you think turning the mixture screw should correct the injectors spraying even with the air plate at rest? I checked the rest position and it's in spec (<0.5mm from venturi top).

Souk, I would appreciate help with the tester. I think I did follow all the hookup directions in the shop manual and that came with the tester...but I'm confused about the purpose of the valve on the tester.

Thanks

jason2guy · 05-09-2003, 08:51 AM

check this out

http://www.phat-gti.com/downloads/boschtech-12d.pdf

Superman · 05-09-2003, 08:57 AM

Yes, injectors commonly weep but no, there should not be gas spraying when the air flow sensor plate is closed. There should be none. In fact, when installing a new system, the procedure involves first turning the adjustmetn screw all the way one way, and then, with the FP running and the sensor plate at rest, turn the screw just until fuel starts bubbling out of one or more injector ports (with the injector hoses disconnected). Back off the adjusting screw until fuel stops coming out. Finishing installing injector lines.

So no, there should not be fuel pressure going to the injector lines with the air flow sensor plate at rest. Back off that adjusting screw.

The other condition that may be present is a fuel distributor plunger that is not moving freely in its bore. Be VERY VERY careful with this plunger if you remove it. Its fit in the fuel distributor bore is so critical that fuel, at 60 psi, does not leak between the plunger and the bore! There is no o-ring or other seal!

targa80 · targa80's Garage

Cowtown I sent you a PM

cowtown · 05-09-2003, 09:14 AM

Gerald, Jason, Super,
Ah ha! Thanks for all that information. Now I've got a plan.

I'll follow the procedure Super outlined about setting the mixture screw. Then if the injectors stop spraying I can do the spray test tonight, and then a real pressure test.

I have already R&Red the fuel distributor plunger and it now moves freely. It was really gunked up.

I'm not sure if the WUR is good and have no intention of taking it apart. Hopefully adjusting the fuel distributor and possibly replacing the injectors is all it will take.

I'll let you know how it goes. Thanks everyone!

beepbeep · 05-09-2003, 09:31 AM

Thomas Owen · 05-09-2003, 09:43 AM

Colin - I have a couple of real good books on the CIS system that I studied and used for troubleshooting back in my pre-Motronic days. I will gladly send them to you, perhaps they will help. I know in my case, with the '78, '80, and '81 I had they proved valuable. In all 3 cases I was able to solve problems that had plagued the PO and end up with a decent running car.
The CIS can be difficult to troubleshoot and these books give you a good foundation. If you want them, send me a PM and I will loan them to you.
Regards,

cowtown · 05-09-2003, 10:01 AM

Thomas, thanks very much for your generous offer...could you tell me the titles of the books? I would feel a lot more comfortable buying copies for myself because I'll probably want to keep them forever (and I tend to get car-related books dirty because I have a bookshelf in the garage).

Beep, your diagram illustrates why I'm confused about the mixture screw. My Yellow Bosch book shows the same diagram, but my Turbo's air plate moves down, not up, as airflow increases. So I'm having trouble visualizing how the mixture screw interacts with the arm.

bigchillcar · bigchillcar's Garage

Hi Colin,

With CIS, airflow across the sensor is what directly meters fuel as shown in the above diagram. At start this is accomplished by the starter - it turns the crank, etc. and the fan starts turning, drawing air in and into the airbox and so forth. After ignition, the starter isn't needed and internal combustion takes over responsibility for 'turning the engine'. This initial airflow, precipitated by the starter motor is what should cause the air sensor plate to rise and meter fuel for start with assistance from the 7th injector...the cold start valve. The cold start valve is governed by the thermotimer which decides in a 'millisecond' whether the temperature at the chain covers is < 90F or so and fire this 'en-riching' cold start valve. Once started, the WUR immediately begins the job of leaning the mixture, and through its bi-metal, temperature sensing strip, adjusts system pressure according to variances in temp. as the block it resides upon experiences temp. change.

My point to all of this? Don't overlook the possibility of a stuck-open cold start valve.

Ryan

cowtown · 05-09-2003, 10:42 AM

Thanks Ryan,
A week ago, when I first got the fuel lines hooked up, I discovered the coldstart injector was indeed dumping fuel, even though it's unplugged. The fuel line to that injector is now capped at the fuel distributor. I'm trying to eliminate as many components as possible before I replace anything...I've got a lot more time than money!

Colin

Thomas Owen · 05-09-2003, 12:45 PM

The 2 books are:
"Solving Bosch CIS Problems" by James Weber
"Bosch Fuel Injection and Engine Management" by Charles Probst.
The first one being the most helpful I have found.
Good luck,

studeb · 05-09-2003, 01:13 PM

Quote:

Originally posted by cowtown
[/b]
This is also what I was thinking, but the engine sat 20 years, so who knows...I have the injectors sitting in Techron right now and will do the spray test tonight.

Souk, I would appreciate help with the tester. I think I did follow all the hookup directions in the shop manual and that came with the tester...but I'm confused about the purpose of the valve on the tester.

i really know little about these things, but if you engine sat for 20, and the fuel distributor was gunked up, then your injectors are probably gunked up too. Clean and back flush them. Before you turn the adjuster screw anymore find out if the mixture control is different for the turbo, i know the plunger is upside down, but don't know about the screw.

As for the tester, what kind? The valve may be a bleed off screw to release pressure in the assembly, so that when you disconnect you do not spray gas everywhere.