In the workshop today:
We had a Vauxhall Corsa come in to us that had a very bad misfire and had recently been to another garage and hadn’t been cured.
Firstly we connected to the car with Autel diagnostics threw the o.b.d. (on board diagnostics) connector and scanned the E.C.M. (engine control module) for faults. (Fig.1)
From the following codes we can see that the engine control module has detected that there are problems with the following:-
- P0300-1 Random or multiple cylinder misfire detected. This fault is present (meaning it is currently happening)
- P0303-1 That there has been a misfire on cylinder 3 ( but not currently present, could be an old code)
We can also see the code P0304-1 for the current misfire on cylinder 4. Which is present
- P0325-2 Knock sensor 1 circuit low input but not present (this could be an old code)
- P0650-4 Engine light circuit malfunction,
On switching the ignition on and off and also trying to activate the engine light with diagnostics we have became aware that the engine light does not work
Could this be and additional fault or has someone taken the bulb out to try and cover up the miss fire fault?!
Secondly as we investigate the missfire fault further, on lifting the bonnet we notice that the vehicle has a brand new coil pack fitted and brand new egr valve.
Could these have been previously faulty? Or has someone just been fitting parts hoping for the best or trying to repair the vehicle by trial and error?
We at Branston Filling Station have seen this all to many times before, where garages are un able to properly diagnose a fault or have the diagnostic equipment but don’t know how to understand the fault codes they have been presented. And simply waste customers money by guessing and fitting parts trying to cure the problem without a proper diagnosis.
Having a diagnostic computer (or several) is only half the battle, any one can plug a diagnostic computer into a car and read the faults stored in the modules. But the skill and training of a good technician is all about understanding the faults you have been presented and carrying out further testing to get a 100% accurate diagnosis. Thus saving the customer waisted money on a whole load of un-needed parts fitted to there car, and saving them waisted man hours of labour.
We then cleared the faults on the diagnostic computer to see what returned.
The vehicle will store faults from years ago, if they have never been cleared after previous repairs and problems so it is necessary to clear and see what returns.
The faults that returned were as follows:-
- P0304-1 cylinder 4 misfire detected (present)
- P0650-4 malfunction indicator circuit malfunction
So we start with the misfire code p0304-1
This is saying that cylinder 4 (last cylinder on gearbox end on this vehicle) is misfiring.
First we test the ignition system, we remove the coil pack and spark plugs from the engine (both have already been changed very recently), we then lay the coil pack and plugs connected to the coil pack on the engine casing so that they earth as they would fitted in the engine. We then activate the coil pack with the diagnostic computer. All the spark plugs are sparking with a bright blue spark, so we know this is fine.
Next we test the cylinders for compression. (Fig.2) this tests the state wear of the engine and for any failures in the cylinders.
We screw a compression tester into each cylinder in turn, then i get an assistant to turn the engine over (try to start the vehicle)
The gauge should now go up measuring the compression of the engine (pressure each cylinder makes while cranking). A good engine should read around 150psi
We note the following readings
- Cylinder 1 155 psi
- Cylinder 2 160 psi
- Cylinder 3 152 psi
- Cylinder 4 2 psi
As we can see from the readings there is a big problem with cylinder 4, so next we need to confirm this and see why this cylinder is producing no or very low compression. We do this with whats called “A cylinder leakage test”
A cylinder leakage test is where we put a measured amount of compressed air into the cylinder with an leakage tester, airline and compressor and measure how much leaks out of the cylinder. A good engine will have no leakage however a bad engine will have very high leakage. This is done with the valves closed and the piston at the top (top dead centre) so the cylinder should be air tight and not leaking any air. as seen in the image here.
So next we turn the engine round by hand on the crankshaft and time the engine up so cylinder 4 is at top dead centre (piston at the top valves shut) next we connect the airline to the tool we set the tool and screw it into the spark plug hole and measure the leakage.
As we can see in fig.4 the cylinder is reading virtually 100% leakage this means there is a very big problem with this cylinder.
As fast as the air is being pumped in, it is leaking out. There are only 4 places really that the air can be leaking out:-
- An inlet valve. To test this we remove the intake pipe and listen for hissing.
- An exhaust valve. Again we listen at the exhaust for hissing.
- The piston rings (piston blow by) air leaking past and down the side of the piston. To test this we remove the dip stick and listen for hissing air.
- Head gasket rupture. (Air leaking into the cooling system) We test this by removing the cooling bottle cap looking for hissing or bubbling in the water and checking for loss of water.
All of the above can be seen demonstrated in Fig.3, as the blue escaping gasses
In our case it is problem No 3. Piston and/or ring failure. When removing the dipstick (with the leak off tester still connected pumping air into the cylinder) we can hear a massive amount of hissing.
To confirm this further we can remove the tool and pour a small amount of engine oil into the spark plug hole, which will temporarily seal the piston rings for a few seconds. If we then do another compression test, it should then make the compression shoot up thus proving the rings are faulty.
So we add the oil to cylinder 4, screw in the compression tester, then I get my assistant to turn the vehicle over again. Now for a few seconds the compression shoots up to 90 psi, proving the piston rings are either broken or badly worn. which would require a full engine rebuild.
On contacting the customer they did not want to spend the money on the vehicle and decided to collect it with just a small diagnostic fee of £24 to pay
This is why it is so important to get a correct diagnosis, we may have been able to have saved the customer wasting money on a coil pack and e.g.r. valve (which were probably not faulty in the first place) on a vehicle beyond economical repair.
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