Fuel System Troubleshooting & Tuning
The BMW 320i and 323i are the only BMW’s to be equipped with CIS, aka Bosch K-jetronic. CIS stands for “Continuous Injection System”, so named because the injectors are mechanical valves that spray constantly rather than being pulsed the way efi injectors are. CIS was also used on many other European cars from the 70’s, ‘80’s, and even the ‘90’s, including Porsches, Vw’s, Audi’s, Ferraris, Lambo’s, Volvo, Mercedes and the Delorean to name a few. In its day the system was considered state of the art, though it’s strength is emissions, not performance. This system is almost entirely mechanical, in good tune is very robust, and is amazingly adaptive to performance modifications.
First off, if you’re trying to trouble shoot a problem or are just asking a question, knowing what each component is and what it does will be a big help to you and to us as we try to answer your questions. Read and study the links below to learn what things are and what they do. There are a few components that often go by different names. The Control Pressure regulator (CPR) is the same thing as the Warm-up Regulator (WUR). The Air Flow Meter (AFM) is also called the Lift Plate or Sensor Plate. The auxiliary air valve is also called the Idle Air Valve. I generally refer to the fuel distributor as the FD, the air flow meter as the AFM, and warm up regulator as the WUR.
Bosch K-Jetronic Fuel Injection Manual - link to bosch factory manual detailing components and function of CIS components. It doesn't get any better than this.
Bosch K-jet in detail (volvo specific) .pdf format
K-jet.org
Auto solve writeup on K-jet
CIS trouble shooting:
If you’re having CIS problems, the most valuable book you can buy is “How to tune and Modify Bosch Fuel Injection” by Ben Watson. If you buy this book, you’ll also need to get a CIS pressure test kit. For only $60, JCWhitney sells them for less than half the price that Bavauto and other BMW suppliers do.
CIS pressure test kit
Vacuum Leaks
If you’re having drivability issues such as hard starts, stalling, or no start, the first thing to do is check for vacuum leaks, then check again, then once more for good measure. The K-jet system is extremely sensitive to vacuum leaks, if there is even a small one the sensor plate won’t lift high enough and your mixture will be lean, sometimes to the point that the motor won’t even run
Common places to look are the oil cap seal, cracked/loose vacuum lines, dipstick o-rings, crank case vent lines, intake boot, etc.
To check for leaks, spray starting fluid around all suspect areas with the motor running, or if it doesn’t run do it while a helper cranks the motor over. If there is a leak the motor will suck the ether in through the leak and the idle will rise or the motor will sputter briefly. If in doubt, replace old rubber hoses with new to be sure.
Hard Cold start/Rough running when cold
The most common 2 problems I see being asked are hard cold starts and hard warm starts.
A hard cold start accompanied by a rough idle when cold is often caused by an incorrect (lean) mixture when the motor is cold, which is often due to a dirty or misadjusted WUR. A dirty WUR has a greater effect on a cold control pressure, and thus cold fuel mixture, because the WUR needs to flow more fuel to allow for the lower control pressure that a cold motor needs. As the WUR warms up, the internal valve closes anyway, so any restriction caused by dirt won’t effect fuel flow as much.
Often just cleaning the WUR can help. If this doesn’t help, then your options are to adjust your mixture to be richer, install another WUR, or do a full pressure test and make your WUR adjustable to bring it back in spec.
WUR Servicing
Hard warm start
A hard warm start is often due to an internal system leak causing a drop is rest pressure. This results in vapor lock on a hot engine. The causes for this leak down are a leaking fuel pump check valve, leaking system pressure regulator o-rings, leaking accumulator, or leaking cold start injector. The accumulator can be tested easily by disconnecting the vent line and jumpering the fuel pump. It's easy to tell if there's a problem with the pressure regulator through simple inspection or the o-rings. If the cold start injector is leaking it will need replaced.
1 - Leaking fuel pump check valve - The most common cause, costs about $15. A leaking check valve will usually only cause hard warm starts.
2 – Leaking Pressure Regulator - How to replace pressure regulator o-rings
3 - Leaking Accumulator - A small leak in the accumulator can cause hard warm starts but if it's large enough it can leak off enough pressure to keep the car from starting at all. To check for an accumulator leak, disconnect the small rubber line from the back of the accumulator and plug it with a golf tee or bolt and clamp it. Shove some fuel hose onto the nipple on the back of the accumulator and run it into a gas can. Jumper the fuel pump. Some fuel will spray out of the accumulator, this is normal as the rear chamber will have some fuel in it from the in-tank pump (the fuel line we disconnected). If the fuel flow does not stop then the diaphragm is leaking and it needs replaced.
4 – Leaking injectors or cold start injector - To check for an injector or cold start injector leak, remove the injector and place it in a jar and jumper the pump. If it drips it leaks. If your main injectors are all dripping slightly, you may first want to try backing your mixture setting off a tad to make sure it's not just set too rich.
Jumpering the Fuel Pump
Most fuel system component testing you’ll need to do require you jumper the fuel pump.
To jumper the pump you have to turn the ignition on for the pump to run. Jumper terminal 15 which is hot when the start-run bus is on (ignition switch on) and 87, which is the hot lead to the fuel pumps, WUR, and aux air valve. Note, i've recently found out from other members the orientation of the relay plug can be different than what was on my car, so make sure you connect correct terminals by checking the orientation of the plug:
Re-adjust Base Mixture Setting
If you’ve done major fuel system work such as swapping fuel distributor or AFM or you have a non-running car it’s a good idea to re-set your base mixture setting. To do this, jumper the fuel pump. Turn the mixture screw clockwise (rich) until you here the injectors start to spray. Back off till they stop, then back off another 1/8 turn. This should get the mixture setting close enough for the car to at least run.
Fine Tune Mixture Setting
If you clean your WUR, swap fuel distributors or AFM, replace the fuel pump or even filter, or install hotrod parts like header or cam, expect to have to do some fine tuning of your mixture for it to be perfect or even run afterward.
I adjusted my mixture by ear and feel for years before I had a wideband O2 gage. It's not difficult to get a smooth running car with no gages or measuring equipment. With the motor running and oxygen sensor UNPLUGGED, adjust rich (CW) until idle starts to slow or stumble, then adjust lean (CCW) till it does the same, then adjust to half way between those 2 points.
If you want to be more accurate you can adjust the mixture by reading the voltage from the oxygen sensor. Unplug the O2 sensor first so the lambda system doesn't interfere with your adjustments. Make sure the motor is hot, the O2 sensor has to be above 600 degrees for an accurate measurement. Bringing the motor up to operating temp by idling won't get it hot enough, you'll have to drive. Adjust the mixture until the O2 sensor voltage is at .45v.
You can also back probe the frequency valve with the O2 sensor plugged in. With a dwell meter you can read the duty cycle of the valve, you target value is 45 degrees.
On a stock 320i you could also install a narrow band air/fuel ratio gage which only costs $30 and uses the stock oxygen sensor. It is accurate right around stoichiometric ratio, which is really all you need to worry about on a stock motor, and is adequate to set your base mixture with. At WOT the fuel system will go richer than what it can read, but it's not a safety issue and the fuel system was designed to cope with an NA motor.
CIS Pressure Specs
For those that have a CIS pressure gage and just need to know the pressure specs, here they are for the 320i:
Fuel pump delivery (as measured through the fuel return line): 750 cc's/30 seconds
Cold control pressure:
77-79 320i:
50 deg = 10 psi
75 deg = 18 psi
100 deg = 26 psi
80-83 320i:
50 deg = 17 psi
75 deg = 26 psi
100 deg = 35 psi
System Pressure:
65-75 psi
Warm control pressure:
39-45 psi
Rest pressure (leak test): above 22 psi for 20 minutes
injector opening pressure: 45 psi
Complete CIS Test Procedure
The pressure gage takes the place of the fuel line that runs from the top of the fuel distributor to the inlet of the WUR (the larger of the 2 ports). Make sure your shut off valve is on the WUR side of the pressure gage.
1) Measure cold control pressure first. Make sure valve on your gage valve is open and the motor is stone cold. Disconnect the plug on the WUR so it doesn't heat up to warm control pressure while you measure. Jumper the fuel pump. You'll need to estimate what the cold control pressure is based on outside temp.
2) Measure system pressure next. With the pump still jumpered, close the valve. You're now blocking flow to the WUR and the pressure above the control plunger will rise to equal the pressure in the lower chamber of the FD (system pressure).
3) Next measure warm control pressure. Open the valve, start the car and let it come up to operating temp. If you can't start it, plug the WUR back in watch control pressure rise until it stops moving.
4) Last, measure rest pressure (leak test). unjumper the fuel pump and watch the gage. It should stay above 22 psi for 20 minutes. If your rest pressure drops then fuel is leaking either at the check valve, system pressure regulator, accumulator, cold start injector, or WUR. See the warm start section above for procedures on how to test each component
5) Injector flow test: Pull the injectors and put them each in a jar or graduated cylinder and spray for 10-20 seconds. Check for good spray pattern out of each injector, total volume delivered should be equal
Tuning the WUR:
You should never have to buy a new WUR unless the internal valve is rustys or the heating element is broken. You can make the WUR adjustable, this link is a good start but I consider it incomplete so I'll write my own DIY when I get a chance:
How to tune your WUR
Here's a diagram I drew up on how to make both cold and warm control pressure adjustable. This is how my WUR is set up.
Custom CIS Tuning Theory Discussion
Plumbing a Boost Sensing WUR
Volvo 240 turbo WUR Connections
Fuel Distributor Rebuild
Porsche 928 Distributor Rebuild
Fuel Leak Around Tanks
Fuel Tank O-ring and vent line replacement
Carb Swap
Goin' Carbed
Weber Downdraft installation guide
Tuning DCOE's
Weber Tuning Tips
Last edited by jrcook320; 01-30-2010 at 12:12 AM..
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12-29-2008, 03:52 AM
'81 320i turbo | t25, 931 CIS, 250hp, 13.92@100mph | 
