I have an intermittent starting problem on my mk 4 golf which has done 53000 miles, which my local vw garage have been unable to rectify. They now tell me that one of the cylinders is down on compression and this will be confusing the engine management system and causing the problem. They are proposing stripping down the engine which will be expensive and in my opinion probably not the cause. I have had an independent garage do a compression test and the cylinder is only 4% down, they believe it is a waste of money to srtip the engine. I am confused anyone got any ideas?
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As you have probably picked up, intermittent faults are very difficult to pin down. I would be very surprised if being 4% down on compression on one cylinder makes any difference at all to starting.
Are there any patterns which may help in finding the cause? e.g. does it only happen hot/cold wet/dry etc?
Do you always hear the fuel pump run as you switch on?
Aside from these considerations, I would;
a) check the cleanliness and security of all the engine/ecu earths
b) check the condition and security of all engine wiring, including the conditions of all multiplug connections
c) check the security and connections of the relevant fuses and relays (fuel pump relay especially)
number_cruncher
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Not enough info to help you out what do you mean by won't start. Give us lots of details and maybe help is at hand. Your local vw agents are clealry useless 4% is nothing. Now one good test is when it next fails to start get someone to push you and bump start it or not as the case may be. When it does not start is that it for the day or what does start it ?? Are you sure the imobiliser is not stopping you from starting the car. Come back with more info and we'll help. Regards Peter
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I've just been through this for 2 weeks on a 99 GTI, replaced the blue temperature sender unit in the top hose for a green one,checked out the fuel pump relay for dry joints and correct operation on the key turn, it puts the pump on for a few seconds and then drops out, had the cover off the fuel pump and felt it run on key turn,then the starter was always noisy and slow to disengage so replaced that as well in case it was pulling so much power it wouldn't spark. then it ran perfectly for a day before once again failing. This time it failed to start cold wheras it had been failing after a short run previously.
Checked for a spark and there was nothing. Changed the dry coil pack and no difference. In desperation had a proper techy put a diagnostic gizmo which produced no error codes whatsoever except for no rev counter. This I had failed to spot and pointed me to suspect the crank position sensor. I was not enthusiastic about this because my understanding was they fail or they work, but don't go intermittent.
Crank sensor was pulled and it was rather oily and had a chunk of metal stuck on the magnet which was cleaned off and put back in for a try with no success, next I cut into the wiring to test the continuity which was perfect so no cracked wires before finally biting the bullet and replacing it with a pattern replacement at £28. Spark returned and it starts first time, every time.
I almost don't believe all this starting when it felt like it for over 2 weeks was a failing crank sensor, which up to the point of failure worked better stone cold and the engine ran perfectly if it started. But it was..
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