Mk3 GTI 2 litre (AGG) cuts out but will start and rev

[mk3-handicap]

I tried reading all the many threads about “mk3 cuts out”, but this seems a bit different.

The car is Mk3 2-litre Gti with AGG engine.

The car runs fine, good performance, easy starting, smooth to drive, quiet, even tick-over. Doesn't pink or hesitate.

It cuts out occasionally at junctions, r-abouts, in slow-moving traffic, maybe on a leisurely gear-change - when you'd expect it to idle. We've seen this before.

But, if you try to start it immediately, it'll fire and rev, but then dies. You can't catch it by hitting the accelerator again. Once it's revved the rev-counter drops and it's going to stop. It'll repeat that as many times as you like. (It's almost as if it'll work with the key in the 'start' position, but not in the 'run' position. But it's not quite as clear-cut as that – you wouldn't just expect to fit a new ignition switch and fix it.)  Leave it to cool and it'll drive for some way, directly related to how long it was left to cool. From cold it drives about 10 miles. After the fault, if I leave it 10 minutes I might only get a mile before it recurs.

The local auto-electrical outfit swapped the distributor ('some improvement') the coil ('again an improvement') and finally the crank position sensor ('that fixed it'). Total bill 375 pounds. Hmm.

Now I'm no professional, but wouldn't a faulty crank sensor be quick to diagnose from fault codes?

Any suggestion what component failure might allow the car to start on the key, rev to, say, 2000 then stop, repeatedly?

[gtigolfthree]

Hi, and welcome, not sure if you mean that the auto electrician has cured your problem or if it still exists. Cutting out on mk3s is a common problem and if you use the search facility you will get a lot of info on the subject. the most common issue seems to be a dirty throttle body which you can try cleaning with some carb cleaner. Meantime you may wish to look here.
 http://www.golfgtiforum.co.uk/index.php?topic=101400.0

[mk3-handicap]

Thanks for the pointer, gtigolfthree. I did see that one, and the bit in there about the crank sensor. But it leaves me wondering how a crank sensor can fail in this way. I'm not saying it can't, but while I can understand it causing the engine to not run, I can't see how it would allow it to rev then stop, rev then stop. The auto-elec says it was fixed by replacing the crank sensor, but I haven't been able to try it yet. We'll see. I just get a bit uneasy when auto professionals assume that a bit of jargon will cover for their guesswork. This is engineering not snake oil. 

[thai-wronghorse]

Faulty crank sensors are very common on these engines, having dealt with the massively irritating problem first hand i can assure you that it isn't always easy to pick up on either. The main reason for their failure is down to the cable breaking down from heat damage due to their routing against the engine block and beneath the oil filter. The cable literally heats up and cools down so many times in it's life that it becomes very brittle and begins to crumble letting moisture underneath it's plastic sheathing allowing the wires inside to corrode and become intermittent.

[mk3-handicap]

The experience so far in this case is that the crank sensor replacement (the device behind the front engine mount and its connecting cable) seems to have fixed it. The auto-elec agrees that the distributor and coil were replaced needlessly (altho his test equipment pointed to faults that weren't bourne-out by driving the car.) So, more evidence that the crank sensor should be the prime suspect in many strange cutting-out cases.

Perhaps we should maintain a crank sensor change schedule like the one we are used to applying with timing belts  Failure isn't as dangerous to the engine, but it's no fun being stuck halfway across a fast junction with a dead engine. "Just give me about 10 minutes, mate, while it cools and I'll be able to move it."