Fuel Injection, Technical Tips,

1986-90 5000/200TQ-CIS, 1991 200TQ 20V Motronic


One of the first steps in trouble-shooting the CIS fuel injection system is to ensure that the CIS fuel injection system pressures are correct (system pressure and warm up regulator control pressure). You need some special fittings to connect a fuel injection pressure gauge to the CIS system. JC Whitney apparently sells a low cost pressure gauge setup with the correct fittings for this.

CAUTION: The fuel system has fuel under pressure approaching 90 pounds per square inch (psi) !! EXTREME FIRE HAZARD!

Use extreme caution when loosening fuel fittings as residual pressure may cause fuel to squirt out into your face or body. Goggles should be worn any time you are working around gasoline. Be extremely careful as gasoline fumes can ignite from nearby flame or spark sources. Incandescent drop light bulbs often will break when dropped and may ignite gasoline fumes nearby.

The gauge is normally connected between the pressure supply line that connects to the warm up regulator and the warm up regulator itself. The gauge needs a shut off valve to isolate the warm up regulator from the gauge to check the system pressure. The System Pressure can be checked after fitting the proper gauge, with the engine not running but with the fuel pump turned on.

If you insert a fuse into the top of the fuel pump relay and then turn on the ignition, this will start the ECU Output tests and energize the fuel pump without the engine running. If you are not familiar with this method to energize the fuel pump by inserting a fuse into the fuel pump relay, then go to the Engine Control Unit (ECU) Fault Code, Output Test section for more details on the Output tests that begin by energizing the fuel pump.

The Control Pressure should be checked when the engine is cold, by connecting the gauges and then starting the engine and noting the control pressure change as the engine warms up. NOTE: The shut off valve is left open during the control pressure check. If the control pressure does not rise as the engine warms up, check the resistance of the heating element inside the warm up regulator as it may be defective. Also check the warm up regulator electrical connector for +12V supply with the engine running.

Check the Bentley manual for the specific procedure and recommended pressures, typical system pressures are between 6.0 and 6.5 bar, warm engine control pressure usually is around 3.6-3.8 bar. Cold engine control pressure depends on the ambient temperature, usually the cold control pressure is between 1.5-1.8 bar at 20C (68F) shop temperature.

If your fuel pressures are incorrect, you may want to first replace the fuel filter and then recheck the pressures.

The Residual Pressure should be checked. This is done by connecting the fuel pressure gauge as discussed above for measuring the control pressure, but then shut off the engine. Note the gauge pressure readout at the 10, and 20 minute intervals after the engine has been turned off. The residual pressure should be at least 3.0 bar (43.5 psi) after 10 mins, 2.8 bar (40.6psi) after 20 mins.

Hard starting from a loss of residual fuel system pressure, can be caused by a leaky fuel pump check valve, a leak in the frequency valve, a leaky cold start valve or from leaky injectors.

The pressure regulator valve inside the fuel distributor can also be leaking on the 1986-88 5000TQ (early 1989 200TQ) and this will cause a loss of residual pressure. There is a repair kit which has the O-rings and seals along with the pressure regulator inside the fuel distributor. Contact me for details.

Here is a photo of the pressure regulator plunger, spring and valve assembly, that is inside the fuel distributor. Shims are used to adjust the system pressure. Also shown at the top of the photo, is the later 1989 200TQ "Plug" that replaces this regulator assembly. The later 1989-90 200TQ (1989-91 200T FWD) uses the external canister type fuel pressure regulator which is mounted on the outside of the fuel distributor/air cleaner housing, next to the fender.

MC-2 Engine, external Fuel Pressure Regulator details

The later 1989-90 200TQ with dual knock sensor MC engine (MC-2 engine) uses the external canister type fuel pressure regulator, and this regulator can also leak after shut down and cause the loss of residual pressure on these vehicles. The warm up regulator and fuel distributor return line that sends fuel back to the tank is routed through this externally mounted regulator, and the regulator is designed to hold pressure in this return line after shut down.

On these later 1989-90 200TQ (1989-91 200T FWD) vehicles with the externally mounted pressure regulator, I have noticed on some vehicles during high ambient temperatures that the residual pressure will start out at about 3.5 bar after shut down, but actually will rise up over 4.2 bar about 10 minutes later from heat soak. I theorized, that this rise in residual pressure could cause fuel to be sprayed out of the injectors and cause delayed hot starting problems. This was a problem I was having with my car, it would take a couple of seconds to start when the engine was warm/hot. The starting problem was not very severe, it was just a little annoying and the exhaust would smell a little rich after each start as well.

I have experimented with adjusting the internal spring pressure acting on the check valve inside this canister type external regulator. Increasing the spring pressure on the check valve, helps reduce the initial amount of residual pressure and this eliminated the occurrence of the rising residual pressure. This pressure regulator adjustment has eliminated the delayed hot starting problems I had, which caused the car to take a couple of seconds of cranking before it would start.

Note: This assumes you have already installed new injectors, as leaky Viton ® tipped injectors on high mileage vehicles, normally causes the hot and cold starting problem.

Here is a photo of the parts inside this external fuel pressure regulator, the regulator was destroyed in order to see what was inside.

If you find that your residual fuel pressure is going up too high, and you are getting some hot start problems with mild engine flooding (rich mixture), and you have already replaced all of your fuel injectors, then you may want to try adjusting the residual pressure. Replacing the fuel pressure regulator may also fix the problem.

The external fuel pressure regulator residual pressure can be adjusted, by removing the right front plastic fender liner to gain access to this regulator. Here is a photo taken underneath the fender, showing the location of this regulator on the 1989-90 200TQ, (1989-91 200T FWD). The hose from the decel valve to the CIS intake boot was removed for easier access.

If you break loose and remove the fuel line fitting shown, you can insert a long allen wrench inside the fuel fitting on the regulator, and turn the allen screw "IN" or clockwise, 1-2 turns, to lower your residual pressure. Note how much you have turned the screw in case you need to turn it back to the original position.

The allen wrench size is a bit strange, either a 3.5mm or 4.5mm allen wrench is needed if memory serve me.

You will need to re-install the fuel line to check your residual pressure after making this adjustment.

Here is a diagram showing the internal parts in this fuel pressure regulator.

Diagram courtesy of Robert Bosch Corporation

The spring shown in the bottom of the diagram above the return line to the tank, is the spring acting against the return check valve. It is this spring pressure that is being adjusted with the allen screw, and more tension will lower the residual pressure remaining after engine shut down.

I have also noticed that most vehicles have some leakage occurring from the fuel distributor plunger and this will allow a small amount of fuel to drip into the air filter housing area. NOTE: This could become a fire hazard if a back fire occurs and there is a large amount of fuel leaking. The later fuel distributors use a O-Ring seal at the bottom of this plunger, to help keep fuel from leaking out when the engine is shut off, and the plunger drops down on top of the seal. I have not found a replacement O-Ring seal available. Bosch usually wants you to buy a rebuilt fuel distributor.

Fuel Pump Delivery Rate



The Fuel pump delivery rate, (cubic centimeters of fuel pumped out in 30 seconds) should be checked if you are having problems with the engine cutting out at high RPM or when under heavy load in 3rd/4th/5th gear.

NOTE: A clogged fuel filter can also cause the same cutout under medium to high load.

The Bentley manual should be consulted, but basically, if you have 12.0 Volts supplied to the fuel pump, it should pump 760cc's in 30 seconds. With 10 volts supplied to the fuel pump, 450cc's of fuel should be delivered in 30 seconds. The Bentley manual has a chart which shows the fuel delivered versus the fuel pump voltage supplied. (1000cc's or 1 liter, is approximately 1 quart)

The return fuel line that returns fuel back to the fuel tank is used to measure the fuel delivery. This return fuel line is next to the fuel filter supply line, it should be disconnected and the line coming from the fuel distributor should be inserted into a calibrated beaker or other suitable container to measure the fuel delivery. Remove the fuel pump relay from the fuse box. A sealed remote switch should be connected across the fuel pump relay connector large terminals to switch on the fuel pump with a thumb switch. Turn on the fuel pump for 30 seconds and measure the fuel delivered.

Engine misses or cuts out in 3rd/4th/5th gear?

If your engine seems to run fine in the lower gears, but cuts out or feels like it is missing in 3rd gear or higher, you may have a partially plugged fuel filter. Normally the fuel filters are spec'd to go 30,000 miles between changes, but some areas with lousy fuel or if you use fuel with methanol content, you may need to change the filter once a year or every 15,000 miles. If your vehicle has the problem with the fuel tank coating coming loose (see section above) and the fuel pump screen has been removed, you may need to change the fuel filter more often as well.

If you remove the filter and drain out some of the fuel into a glass beaker, you can see if the filter is full of crud, rust, dirt, water etc. Normally you should be able to blow through the filter if you shove a rubber hose onto the inlet side fitting and use your mouth to blow through the filter.


Quality of the Fuel? Water/Dirt/Rust Contamination?

If you are having problems with the running of your car, and everything checks out in the fuel injection and ignition system, and there are no mechanical problems, you may want to take a look at the fuel quality. Using the above procedure for checking fuel pump delivery rate, pump some fuel into a clear glass beaker to see if any water droplets, dirt, crud or rust are present in the fuel.

The CIS and Electronic Fuel Injection systems will go haywire if rust/dirt gets into the injectors or fuel distributor.

Having lousy contaminated fuel will drive you nuts as it can be the last thing you think about when having a driveability problem.

Often times crud can get stirred up in the gas station tanks during a tanker truck dump of fuel. You may want to avoid refueling your car if a tanker truck is present as the station. The winter fuels can also draw any water/moisture in the bottom of your fuel tank into the fuel and then clog up the fuel filter when this moisture hits the filtering element. Some fuel filters use paper elements that are designed to swell up in the presence of water.


As some have mentioned the CIS frequency valve was done away with around the mid 80's when the Naturally Aspirated (NA) Audi engines used the differential pressure regulator on the side of the fuel distributor to adjust the mixture. The Turbo Audi I5 engines sold in the US continued to use the CIS frequency valve in the MC designated engine found in the 1986-88 model 5000T/Q and the 1989-90 200TQ and from 1989-1991 in the FWD 200T. The 1991 200TQ with the 20V Turbo 3B engine threw out the whole CIS system and went to the Motronic with electronically controlled injectors.


When I took apart my warm up regulator to fine tune the control pressure when cold I noticed that there is indeed a reference chamber inside that is used to tweak the control pressure at different altitudes.

The "Bosch Fuel injection and Engine Management" book by Charles Probst has a cutaway view showing this type of warm up regulator. In this book he states:

"Altitude compensation is provided by an aneroid chamber at the bottom of the regulator. This chamber expands as the air gets thinner at higher altitudes; this in turn acts on the control pressure valve to increase control pressure and lean the mixture."

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