Very bad misfire, old 1.8i Scirocco

[paulhaigh]

VW Scirocco Scala 1989 1.8 injection DX, 130,000 miles (Bosch K-jetronic injection).

I’m seeking guidance in identifying the cause of a problem with my old Scirocco. As you will see this is quite a lengthy posting, but will hopefully give a clear picture of what I’ve checked so far. I’d be grateful if someone who is good with car electrical systems would read it and hopefully the answer might be clear to them.

The symptoms are as follows:

From cold the car will start (without the use of the throttle), it increases in revs to the typical cold start revs, then it dies after a few second. You can repeat this a number of times before it will fail to start at all. If I use jumper leads to another running car it will start and continue to run. I can then disconnect the jump leads and it will continue to run. If I blip the throttle the revs will start to increase but it immediately misfires badly. If I press the throttle gently the engine will rev higher without any major misfire, but at higher revs there is general misfiring. If I attempt to drive the car it will misfire very badly, with a great lack of power, and bit of popping and banging.

The investigations I’ve carried out have confirmed the following:

The battery is in reasonable condition.

The alternator circuit is charging OK.

The cold start valve, thermotime switch, warm up valve, & cold accelerator enrichment all test OK and appear to work.

The hall sensor in the distributor is working OK.

The cam sprocket timing mark is lining up at TDC; i.e. the cam belt hasn’t slipped.

The spark plug and distributor cap are new & OK.

The spark plug leads & rotor arm appear to be in good working order.

The coil resistances measure OK.

The transistorised ignition switch is receiving the correct voltage.

The engine timing is OK and the mechanical and vacuum advance systems appear to work.

The only checks that didn’t give the values in the Haynes manual were as follows:

With the multiplug disconnected from the hall sender in the distributor you then have to measure the voltage across the low tension terminals of the coil when the ignition is switched on. The book states you should get 2 volts. I measured at least double that. It did drop to zero after 1 or 2 seconds at the book states. Then with the centre terminal of the multiplug temporarily earthed you measure the voltage across the outer terminals when you switch on the ignition. The book states that it should be 5 volts, I measured about battery voltage i.e. 12 volts.

I also checked the engine by using a colourtune spark plug. This replaces one of the spark plugs with a glass spark plug that allows you to observe the ignition of the fuel in the cylinder while the engine in running (fascinating) . What this clearly reveals is that when the engine misfires there is a spark at the spark plug, and after the misfire you can observe a bright yellow ignition as the excess fuel burns that didn’t burn during the misfire. When it isn’t misfiring the ignition in the cylinder is a blue colour. The timing lamp also confirms that during the misfire there is a spark – i.e. the timing light flashes as you would expect and is not erratic during the misfire.

All this lead me toward the theory that could the sparks be generally weak. During the cold start it will start, so I assume the coil must be working. What I don’t know is what the transistorised ignition switch does. I can see that this feeds the low tension side of the coil. The popping and banging when I attempt to drive, and the colourtune plug observation have lead me to the conclusion that there is fuel available, but I have not checked the supply pressure.

Does anyone know the answer to this problem?

[kerbside]

The fact your car is still sparking when it misfires could suggest lack of compression (Burnt valve seats, piston rings etc.) or an air leak (no split vacuum hoses? Inlet manifold gasket look ok?)

However. The fact you are getting high readings at the Hall sensor would indicate a short to live. Usually sensors and switches that connect to the ECU operate from 0olts to 5 volts, so if it reads 12volts battery voltag is getting into the circuit. Try tracing the hall sensor wiring back checking for damaged insulation (and subsequently damaged wiring).

Finally, should all above not fix it, try connecting a jump lead between the battery negative and the cylinder head. If it is ok when you do this it means a bad or broken eath wire/terminal.

There! That should keep you busy for a bit! Good luck.

[Tobes49]

Is it ok when warm?? No misfire?

[paulhaigh]

I've run the engine at tick over until it's warm and get the same symptoms.

I've also checked over as much of the vacuum system as is obvious to me. I've pulled off a few vacuum pipes to see that there is good vacuum which appears to be OK. Pulling off certain vacuum pipes causes the engine to stall as you might expect.

The engine is definitely tired as there is a fair amount of positive pressure in the crank case; if I take off the oil filler cap you can see fumes coming out. I'm not sure if this could be the route of the problem. The problem started fairly quickly and appear to affect all cylinders. I would have expected a failure of lack of compression to only affect one cylinder rather than all of them. The cooling water is OK and not getting into the oil.

Does this give any more clues?

[Tobes49]

You got to decide if it fuel or ignition that your loosing. Because your testing everything and it all checking out.

First thing buy some spark testers they look like spark plug caps but fit between the sparkplug and lead, Fit them to all 4 cylinders. Start the engine and see if any disapear when you have the misfire.

These testers are cheap and will help you decide what part your loosing. Item no 6 click here

[kerbside]

Try the breathers. The problem used to appear on older Golfs and Polos with smaller engines. They would cut out when revved. The cause used to be a breather box thingy down the back of the engine (behind cylinder 1 just above the sump) A good soak in thinners/brake cleaner overnight usually helped. Check the breather pipes are clear too. Its not unusual for the pipes to perish inside and block themselves.

To check the vacuum accuratley you really need a gauge but if you try disconnecting the end that goes onto the injection/inlet and sucking. Make sure whatever the other end is connected to moves. Obviously this will show nothing when you try it with sensors etc. but as you say, you have tried playing about with the vacuum pipes etc.

The thing is, the vacuum comes from inside the inlet, so as long as the car is running there is going to be vacuum anyway.