Cranking the starter triggers the cold start valve to spray into the intake plenum on early models. Due to flooding problems a thermo-time sensor was added to prevent the cold start valve from spraying fuel for more than 8 seconds as well as when the engine temp was greater than F? The control pin in the fuel distributor moves according to how much air deflects the air sensor plate and is also affected by control pressure. Control pressure acts on the top of the control pin, cold control pressure is low 14 psi depending on model which lessens the resistance of the air sensor plate to rise. As engine temp comes up the control pressure rises and the resistance increases leaning out the mixture hot control pressure is 50 psi.
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Cranking the starter triggers the cold start valve to spray into the intake plenum on early models. Due to flooding problems a thermo-time sensor was added to prevent the cold start valve from spraying fuel for more than 8 seconds as well as when the engine temp was greater than F?
The control pin in the fuel distributor moves according to how much air deflects the air sensor plate and is also affected by control pressure. Control pressure acts on the top of the control pin, cold control pressure is low 14 psi depending on model which lessens the resistance of the air sensor plate to rise. As engine temp comes up the control pressure rises and the resistance increases leaning out the mixture hot control pressure is 50 psi. The guts of the fuel distributor determines how much fuel is output to the injectors.
There are 2 chambers separated by a stainless steel diaphragm, the lower chamber is system pressure set by a small spring and is adjustable but almost never necessary.
When the control pin rises this allows system pressure into the upper chamber, once the pressures in both chambers are equal the spring pressure deflects the diaphragm and lets fuel out to the injectors. Note the diagram below. Later models have a switch that will cutoff the pump if the engine dies. It works off the air sensor plate as long as the plate is off the stop the pump will run. You can bypass the relay if you suspect it is the culprit.
Pumps do go bad but normally give you a warning such as excessive noise. Starts hard cold- This is almost always the warm up regulator. High cold control pressure will make the mixture very lean. The cold start valve will still spray and you will likely fire up for a second or two and then die. In order to see what your control pressure is you will need a set of special gauges which are available at most auto parts houses.
Keep in mind that you have fuel pressure at all times, even with the engine off, so be careful when attaching fuel pressure gauges. Starts hard hot- A couple things to look at are system pressure at rest, and CO setting. Get your engine to normal operating temperature. If you have an O2 sensor unplug it. Use a vacuum gauge on manifold vacuum, not on the distributor line, and using your 3 mm allen key lean out counter-clockwise the mixture until the engine just starts to stumble.
The gauge should read about 15 at this point. You may want to rev the engine a bit between adjustments. If you have a car with an O2 sensor you can use a voltmeter to get a bit closer. Unplug the O2 sensor and tap into the wire that sends the mixture signal, on single wire O2 sensors no problem, on sensors with three wires two are for the heater and the other is the signal.
As you lean out the mixture you will see that the voltage will decrease and vice versa. You can use an analog or digital meter. Be careful when adjusting CO as a small turn of the allen screw translates to a large CO change.
Once you have the CO set plug your O2 sensor back in and road test the car. No fast idle- To change the idle speed on a car with fuel injection you bypass air around the throttle plate, in other words make a controlled vacuum leak. Check out all the hoses that have to do with vacuum for cracks. Erratic Idle- Again check out all vacuum lines for leaks. Some models have a throttle controller sometimes called a decel valve. The decel valve lets the idle down slowly to decrease oxides of nitrogen NOX.
Disable the valve to eliminate it as a problem by plugging the vacuum line at the top of the valve. Newer models, starting in 82 I think, have an idle speed controller. In the past I have tried cleaning and even disassembly to try and avoid the cost of replacement, success is only short lived. Use a small hammer to tap on the idle controller to see if speed changes. What happens to the controller is the bushings get worn and bound not allowing for small changes.
More tips will be added as I remember or other members add their experiences. All Rights Reserved.
BOSCH K-JETRONIC & CIS Fuel Injection Pressure Tester
The CPR is a device used for the enrichment of mixture during the engine warm-up phase. The upper connection hose is attached to the fuel distributor pipe duct. The lower connection hose is attached to the pipe that is connected to the K-Jetronic fuel distributor control input. There is also a 12V power socket on the top of the CPR.
Adjusting the control pressure regulator on the K-Jetronic injection system2
These fuel injection systems were considered to be really reliable and trustworthy. Basically, such conclusion can be made based on a large number of cars that had such injection systems installed by the manufacturers during that time. However, the time has passed and all the automobiles with mechanical fuel injection systems are out of production. Nevertheless, they are still being used by the car owners, thus the older fuel injection systems still need to be diagnosed, serviced and repaired. Experience has proven that those cars that are equipped with mechanical fuel injection systems are hardly welcomed by the car-care centers. Dealing with them is a pretty labor-consuming task that requires a lot of time and spare parts.
Bosch K-Jetronic Fuel Injection