Fuel Injection Operation

Many blame fuel injection for ignition or engine mechanical problems, feeling complicated fuel injection must be the culprit. Fuel injection is actually simplistic, but there is no substitute for a little working knowledge. I will explain fuel injection operation basics:

1. As the distributor turns, points open and close turning on and off injectors that inject fuel into the intake.

2. A mechanical device (air flow meter (1.8) air pressure sensor (1.7, 2.0) senses engine load and speeds up the duration of injection therefore enriching the mixture.

3. Sensors sense engine temperature and incoming air temperature and therefore also help adjust the mixture.

4. The fuel is pumped in a loop at high pressure with non used gas pumped back to the tank. The pump is automatically shut off if the car stalls.

5. The control unit supervises 1-4.



If the fuel injection fails it will be one of these items. However, before we assume a fuel injection problem, we must verify the following:

1. The ignition system is perfect

2. The engine has good compression (at least 100 lbs)

3. The vacuum system is tight

A. High mile engines have low vacuum and cause lean running for 1. 8s and rich running for 1.7, 2.0s.

B. Old vacuum lines and intake runner boots cause vacuum leaks and lean running



4. The wiring is in good shape



With the above checked and double checked we can now assume a fuel injection problem. The most common failure is the fuel pump loop, that will be discussed first.

If the fuel loop is inoperative the car will not run. The 1.7 and 2.0 914's have momentary fuel pump operation when the key is first turned on. If you do not hear the fuel pump run, your 914 will not start. 914 1.8 fuel pumps work as the car is "cranked" thorough the dual relay. Failure on either of these systems is always the circuit not the control unit. We have seen few fuel pumps fail. 1.7 and 2.0 models have a fuse and relay on the relay board in the left front of the engine compartment. The fuse also operates the heater blower motor. Substitution of this fuse or the relay usually corrects fuel pump operation. Other things to consider are the following:

1. Low battery voltage!

2. Broken circuit in the relay board (especially when hot).

3. Corroded fuel pump plug in.

4. Loose engine block ground leads.



The 1.8 fuel pump circuit is less complicated. It powers through the dual relay mounted on the side of the battery stand. (A bad place I may add) Because of this, the relay usually becomes corroded and the fuel pump, looses power. Simple cleaning and reinstalling the dual harness plugs restores the 1.8 fuel circuit to proper operation.

As an added note it is a good idea before you go any further to make sure you have sufficient fuel pressure. Run the fuel pump continuously and check the pressure with a 50lb. gauge connected between the two left (driver side) injectors. 1.7 and 2.0 models should be 28-32 and 1.8 models 35.

The needle needs to read steadily. If it flips or vibrates the fuel line is pinched or the fuel pump or circuit is failing (if the return line is pinched you will have 80lb pressure and very rich running!)

The next area to generally fail is 2, the engine demand sensors. The 1.7, 2.0 has a pressure sensor which senses manifold vacuum and the 1.8 an air flow meter which senses air flow. Each enriches the mixture as the engine demand increases. The 1.7, 2.0 pressure sensor usually fails from age. Its sensing diaphragm breaks and the engine runs rich causing a black sooty tailpipe, poor warm idle, terrible gas mileage, and gas flooded oil! Replacement of this device is the only solution. The 1.8 air flow meter usually fails, because of lean backfire. Poor engine vacuum caused this lean mixture. Backfire warps the sensing flap and it usually sticks open resulting again with the engine running rich and a black sooty tailpipe, poor warm idle, and terrible gas mileage!

The third most common failure is 3, the sensors -- specifically, the head temperature sensor. When this fails, strange things happen. The engine may suddenly "flood out", or may idle, but not accelerate. It may be extremely difficult to restart when hot or not start at all! The simple test is to disconnect the sensor (it enters the engine sheet metal under the right rear intake tube) from the fuel injection wiring harness and plug a lead into the harness and ground it to the negative terminal of the battery. Since when the engine is hot the head temperature sensor is a direct ground we are duplicating this condition. If your 914 has the symptom of suddenly flooding, make sure you disconnect the fuel pump circuit then run the engine out of fuel to "clean it out". Before making the test or head temperature sensor substitution will have no effect.

The final and least likely to fail is the injection triggering mechanism. The 1.7 and 2.0 engines used dual points in the bottom of the distributor and the 1.8, the ignition points! If the trigger points are disconnected (especially after a tune up) the car will not start. If the trigger points are warn or dirty, diagonal cylinders will misfire, idle may be rough, or the will buck upon heavy acceleration. Replacing the points and cleaning the distributor solves all.

Finally, there are many fuel injection components that never fail even though people insist on buying them:

1. Cold start valve. Rarely fail unless physically broken. (But sometimes they leak.)

2. Thermo time switch -- this only works below 41 degrees for 1.7 or below 59 degrees for 1.8.

3. Injectors -- only fail if long dry or leaking between the colored plug area and the body.

4. Control unit -- never usually fails, unless the potentionmeter does not click, or has been under water, or has had a voltage overload.

5. Fuel pressure regulator -- the 1.8 litre diaphragm sometimes breaks.

6. Fuel pump -- only if long dry very, very old or leaking.

7. Air temperature sensor.



And there are some strange injection related problems:

1. High idle: auxiliary air valve not closing, vacuum leaks

2. 1.8 does not idle: oil cap seals need replacing, vacuum lines disconnected

3. Idle hunts (goes up & down): vacuum leaks, auxiliary air valve not closing

4. Will not start when hot: (turns over but will hardly catch) vapor lock-(relocate the fuel pump to the front), engine running too hot. (This hot starting could also be a flooded condition due to head temperature sensor, pressure sensor or leaking cold start valve)

5. Bucking at a steady speed (73 1.7 and all 2.0): Throttle switch worn and momentarily cutting fuel supply-replace

6. 1.8 fuel pump running constantly with key on: Air flow meter flap sticking-replace, repair vacuum leaks.

7. 1.8 with one cylinder dead: resister block connection - resolder broken lead.

8. Two cylinders on the same side dead: ground lead disconnected on the top of the block-reconnect

9. Poor running when cold: Vacuum leaks, head temperature sensor resistance less than 2800 ohms-repair, replace.



The above highlights fuel injection trouble shooting. Much more could be written, but I think I have covered the basics and invite your comments. If you have a problem with your 914 call our tech line at (770) 427-2844 x16

As a final word, again, do not remove your fuel injection - it's really a great system!



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