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Bob UK
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Posted on Wednesday, 31 December, 2014 - 12:32 pm:   Edit PostDelete PostView Post/Check IP

Fuel injection first appeared on semi diesels which were invented before diesels were invented by Herr Diesel. The semi diesel was iinvented by Ackroyd-Stuart in the 1870s. This became Gardener engines.
Bolinder is a semi diesel or hot bulb engine. The modern diesel injector was invented by Robert Bosch in 1926 for Mann.

By the 1950 a few upmarket petrol cars had mechanical injection. At the time if the carb had been invented the day after the carb would have been hailed as a technical marvel.

Nowadays it's the other way round.

How modern injection works.

For the engine to get the right mixture the ecu needs to know how much, air mass, load and rpm there is. A subsystem is engine temp.

Engine temp subsystem is often either on or off. Once a certain value is reached say 40c. The ecu turns off warm up sub routine.

Rpm is usually a signal from the ignition or a signal shared by the ignition like a rev counter. Crankshaft position sensor tells the ecu the rpm and no1 tdc. The camshaft sensor tcells the ecu what cycle the no1 cylinder is in. These are hall effect sensors same as dizzys. My jeep has a cam position sensor in the dizzy. This shows that the ignition is independent of the fuel. The jeep is sequential injection.

Air mass. This gets complicated because atmospheric pressure changes.
so if the position of the throttle is known and how much air is passing over a hot wire or moving a flap and how much vacuum is in the inlet manifold downstream of the throttle plate plus the temperature of that air then at that moment in time the ecu opens the injector just long enough for precisely the right amount fuel.
As you can see the air going in bit needs careful measuring.
However, some systems don't use a mass flow meter. They use a manifold absolute pressure sensor. A fancy way of saying vacuum sensor. The map sensor coupled with the throttle position sensor does the air bit. Chrysler MPi.

The ecu once it has referenced the inputs in nano seconds. It uses a fixed fueling map to correctly pulse the injectors which is an ecu output.

The oxygen sensor. When the car is accelerating slowing down etc and not on a constant throttle setting the ecu uses the fueling map. When on cruise the ecu uses the oxygen sensor. The authority of the map is more than the oxygen sensor.

Injectors are constant pressure and there is a regulator that dumps excess to tank and often a small accumulator, usually a spring type. The function of the accumulator is the same as the brakes. The pressure varies form make to make but 60 psi is I guess average.

How the sensors work.
A regulated voltage normally 5v is supplied from the ecu.
The volts are connected to a resistance that changes. The other end is connected to earth. The third connection is the one that moves and this feeds voltage back to the ecu the ecu then knows where the moving contact is.
Say the moving contact is attached to the throttle plate and the moving contact is nearly touching the 5v end. Then ecu will get say 4.5 volts. The 4.5v means that the throttle is 7/8 open. The opposite is that the moving contact is nearly touching the earth or zero volts side and the ecu gets a 1.5v signal. If the ecu gets 0v then it flags up a fault and engine check light.

The variable resistance or potentiometer could be attacted to a diaphragm in a map sensor or a flap in the air flow meter. Or an oil pressure sensor.

Hot wire. If a wire is heated then the resistance changes it goes up as temp goes up. If it is cooled by air then the resistance change will follow the airflow. Job sorted.

Idle. This is a sub routine. This is operated by a simple switch on the throttle plate. Separate hopefully from the potentiometer. The ecu then goes to idle map which is say 700 rpm. A servo motor drives a bypass screw. The bypass is like the ones on SU carbs. The ecu monitors rpm and adjusts as required. It automatically compensates for aircon and other transitional loads.

The starter motor engaged is also a sub routine.

The above is about as simple as it gets for electronic injection.

But we all know how clever us engineers like to get, we can such show offs.

If the engine has a computer that is working in nanoseconds and engines don't run at.10000000rpm. Then there's spare capacity. So we have room for the ignition and alternator charging and we can squash the cooling fans in. And if we add a extra bit we can fit a fancy do dah in the gearbox.

What goes wrong.
Anything and every thing.

If a faultcode is available then follow that.

If not then first check the ecu from the 5v or whatever reference voltage it is.

To do this go to nearest easy to get at device. Say map sensor. Three wires ground signal and 5v. Check all three for 5v if 5v is present then that bit is working. No volts go to ecu and check output. No volts. Check power supply to ecu including memory supply. Check ecu earth. Power to ecu but no 5v ecu broken. Have seen this once or twice.

If there is volts at the sensors then measure the signal voltage no volts broken sensor. And so on. Volts at one sensor but missing from other ones broken wire in loom and so on, looms have spliced wires a weak spot. Note some looms have individual supply for the 5v for each sensor and have more terminals at the ecu. Other types daisy chain them and then go back to the 5v terminal on the ecu. Like a loop. In the UK we use mains like that, a ring main. Cheap stuff does not go back, so it has a tail end Charlie sensor.

The wiring loom. These fail and it happens quite a bit. It catches out repairers.

So it is important that even with fault codes the loom can be the problem. This first place to look is the plugs especially the ecu one. Look carefully where wires go in the plugs. My no5 injector one fell out.

Injectors. Never apply battery volts to an injector. The injectors must only be worked by the ecu.

Batch injection. An eight cylinder engine may fire the injectors in 4 batches of 2.

A sequencial system fires each injector when the valve is open.

For reasons out of the scope this exposure of injection. It makes absolutely no difference.
and on batch systems I have seen pairs confused and it runs fine.
However some swear that sequential is slightly smoother on idle. There's the Clue as to why there's no difference.

The ecu sends volts to the injector to open. The other connection is to earth.

So using a ohmeter check that the other sides are earthed.

Quite often theses earths are spliced together inside the loom and the earth emerges somewhere convenient. The jeep one is on the block at the back. If an earth is missing then unpick the loom and find the fault solder heat shrink etc. Heat shrink is available big enough to fit the whole loom.

Plugs are well made and reliable and respond to WD40 and petroleum jelly. Poke only carefully with cleaning tools. These plugs are expensive.

With time patience and a circuit diagram it is possible to fix with a ten quid meter.

Must be high impedance. Maplins. Do not ever use a test lamp this has very low impedance and it can pop the ecu.

Note that injection cars still have things like air filters exhaust systems piston and valves. So before assuming and checking for hours check the normal stuff. Don't want to be spending all day for a stupid small vacuum leak to the charcoal canister for instance. Been there. The diagnoses for that one started as a bad throttle switch. Then I thought why is that little rubber pipe not attached to anything. Much cheaper than a new switch. Which wasn't broke I was probing the wrong wire.

Ah forgot. Often it's a real bitch to get a connection and some probing has to done connected. The signal can't be checked if it's disconnected.
Use a sharp needle through the wires insulation into the conductor. A bit of heat gun will repair tiny wee holes a hint of petroleum jelly will protect.

Haynes work shop manulas do a general purpose injection book that fully explains the popular injection systems including RR. Try Amazon for a bargain. 3 average price. Garages have copies concealed under the counter for reference. We wouldn't want to tell the customer we haven't a clue why your car won't go.

(Message approved by david_gore)

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