S2 cutting out ... why?

[zcacogp]

Chaps,

Following hard on the heels of my (hopefully now resolved) Mystery Part Conundrum, my S2 threw something of a wobbly on me this evening. Perhaps less of a wobbly, more of a light sulk. Maybe to punish me for not having put her back together properly ...

She died on me. Whilst idling, in (light) traffic. It was a soft cut (not a hard cut), and was preceded by a bit of slow idling. I immediately tried to re-start and, while she turned over fine, she wouldn't catch on attempt #1 or #2. She did however catch third time 'round, pretty much as normal, and drove home fine.

Context? She had been running for about 5-10 minutes when she did this, and was started from cold. It isn't the first time this has happened; she failed to start (from stone cold) about a month ago, on either the first or second attempt, but did go on the third attempt. I dismissed this at the time, but thinking back it could have been another manifestation of the same problem.

All suggestions welcome. My instinctive reaction (from the low idle and soft cut) is that it is fuel starvation. Are these symptoms of DME relay failure? Or should I be looking at Fuel Pressure Regulators?

FWIW, when trying to re-start it this evening the tacho was bouncing, so the crank position sensor is OK.

Thanks, in advance, for any help.


Oli.

 

[sawood12]

Speed sensor? You can test the resistance or just check that the mounting bracket has not loosened off.

By crank sensor, do you mean reference sensor? If not then check that too in the same way as the speed sensor - though I think with these things you can't start the car and once it is started you are OK.

How is the car at idle normally? If it is erratic on idle then it could be your AFM - this was the problem with mine when I was suffering idling issues.

ISV? maybe give it a squirt of carb cleaner?

I'm fresh out of ideas now! Sorry.

 

[zcacogp]

Scott,

Thanks ... only answer thus far.

Yes, I mean reference sensor - the one that needs to be registering in order for the car to start/run. What is the speed sensor?

It's generally OK on idle. Not totally smooth when it has been used in town for a long time, but noting to remark on. The emissions are OK too, so I am assuming that the AFM is OK.

ISV would explain it dying on idle (but would throw up some more symptoms - like terribly unstable idle, which I don't have) but wouldn't explain why it doesn't re-start OK. Good idea tho'.

Thanks again. Sounds like checking those two sensors is a good way forward, and changing the DME relay (I have a spare, and will swap 'em over.)


Oli.

 

[TTM]

I would be interested to hear what you find as my S2 has been doing the same for several years and I never took the time to investigate.
I can already say it's not related to the AFM as it has been running a SciVision MAF for 6 or 7 years now and this hasn't made any difference on the problem.

I was thinking the coolant temp sensor to the DME (the blue one at the front of the block) could be the culprit, as the engine otherwise idles fine when it's cold. It really is when it's used for prolonged periods in slow traffic that this problem occurs.

It's worth noticing it is not possible to adjust manually the idle on the S2/968 as the throttle body doesn't feature the idle bypass screw - it's all controlled by the DME and the ISV.

On cold start-ups, at what sort of RPM does your engine idle?

 

[zcacogp]

TTM,

Ooooh, temp sensor. Hadn't thought about that. Would that be enough to cause it to cut out, or just idle really badly?

Idle speed control - interesting. I hadn't considered this. Is there not a way of turning off the control to set the desired point? (On injected Golfs of the same age, which had a pretty much identical ISV, you could disconnect a wire going to the control mechanism, adjust the idle using a set screw, and re-connect the wire. The control mechanism then maintained the idle at whatever you had just manually set it to. Is there not something similar on an S2?)

On cold start-ups it idles at around 1000rpm, possibly a smidge higher. It does idle a little roughly at first when cold (+/- 50rpm, perhaps) but settles down after a few minutes.

I think the answer (in my case) is to change the DME relay and see what difference that makes.

Thanks for your help with this, and with the oil leak issue. 'Tis appreciated.


Oli.

 

[TTM]

I looked up and down through the workshop manual which mentions it is possible to bypass the ISV on the 8V engines, but I couldn't find a mention on idle adjustment for the 16V engines.
Well, maybe it's possible to electronically adjust idle with the Bosch Hammer diagnostic tool that connects to the socket located near the ABS controle module, but I'm not really sure as I haven't read about that in detail.

In any case I would first replace the DME relay, then clean the ISV and the connectors to the temp sensor and AFM.

 

[alwigley]

Failing temp sensor on mine started off as dodgy idling/starting then got progressively worse......... very hard to diagnose and was the last thing I tried !

 

[zcacogp]

TTM,

Thanks. Cleaning the contacts is sage advice.

Al - hmmm. Another pointer to the temperature sensor ... and it's probably very cheap to change too. I wonder if it's the same as from a VW of the same age, as I have several of those knocking around.


Oli.

 

[zcacogp]

OK, slight update on this one.

It cut out again yesterday. (Coming out of the Limehouse Link Tunnel - for anyone who knows the area. And I cannot think of a worse place for a car to pack up than there!) Driving along fine one minute, cut out with no power the next minute. And wouldn't re-start (I tried twice).

I hadn't changed the DME at that point so did so then and there - I had the spare in the car. And it re-started and drove fine after that. Whether this fixed the problem or whether the 5 minute pause while I changed it was responsible I don't know, but it has worked OK since then. The idle is still sometimes a bit rough and it does seem to be running a little warm (temperature gauge creeps up when idling, and then the fans cut in and cool it down quite quickly), but I am assuming that this is unrelated.

I'm still not 100% happy that things are as they should be and will watch it with some care. It's due to do some long journeys in the coming month or so, so I am sincerely hoping it won't disgrace itself ...


Oli.

 

[sawood12]

Not sure if the s2 has a speed sensor, but if it has it is mounted right next to the reference sensor, in fact it is an identical sensor.

Regarding your rough idle, have you checked your idle mixture? With my 944 issues the idle far was out and the idle mix screw on the afm had stripped. My symptoms were generally crappy running at idle, irratic idling and sometimes wouldn't idle, it would cut out at traffic lights unless you kept your foot on the throttle.

 

[simch1]

I'd say DME relay was the culprit.

Re the above, I dont think the S2 has a mixture screw on the AFM. If there is a screw is it not idle speed? If the car is late enought to have an 02 sensor I think that sets the mix via DME, with air temp sensor and TPS also playing a part. If no )2 sensor, i think it is just the air temp and TPS that tell the DME when to actiuvate the ICV and mixture etc.

 

[sawood12]

On the turbo the idle speed control is on the throttle body. The idle mix comes from the afm and the screw adjusts the position of the barn door at idle to change the mixture.

 

[robwright]

The S2 AFAIK only has one sensor on the flywheel. There is another on the camshaft I think.

 

[Neil Haughey]

S2 does have a mixture screw on the underside of the AFM behind a plastic blanking plug, won't do anything though if the car has a working O2 sensor connected in.

BTW my race/track car was cutting out but only on track after a period of hard running, fine on the road. We replaced the crank sensor which also cured the cars tendency to not like hot starting. The really odd thing was that the little section of tube that the sensor sits in was crimped, like it had been bashed in at some point. Must have been a very long time ago as the bellhousing hadn't been off in several years.

 

[zcacogp]

Chaps,

Thanks for your answers.

Simch1 - I am hoping that it was the DME relay. If it is/was, then it is solved. We'll see where we go from here.

I don't think the S2 does have an idle adjust screw, but it does have a mixture adjust screw on the AFM - as Dr Haughey said. In which case, crummy idle must be either poor ignition, or poor idle control. I don't think it's the former, but it could be the latter. I am wondering whether the TPS sensor is working, as that may explain a few things ... I am guessing this is just a switch, and therefore easy to check with a DVM. Maybe this would be a sensible way forward.

I think there is only the one sensor on the flywheel, and am not aware of a sensor on the camshafts. Interesting that replacing your sensor changed the hot starting characteristics Neil. I am wondering whether a replacement temp sensor and a replacement flywheel sensor would be a good idea. Is fitting the flywheel sensor any more of a job than unbolting the old one and bolting in a new one? Isn't there some confabulation with distance from the flywheel teeth or somesuch? (And it's a pain to get to, tucked away 'round the back of the engine, half under the scuttle panel.)

I'll keep the thread updated. Thanks for the inputs.


Oli.

 

[robwright]

The speed sensor should sit 0.8mm from the teeth on the starter gear on the top of the clutch housing. This is adjusted by the bracket that the sensor sits in. As long as you don't disturb the bracket then there should be no issue. They are easily accessible with a 10mm socket and 300mm extension.

 

[zcacogp]

Rob,

Thanks.

Oli.

(How much are those sensors? A tenner, or heaps and heaps more?)

 

[simch1]

which way to adjust the mix screw on the AFM to weaken it up then? Anticlockwise??

 

[Neil Haughey]

I honestly can't remember which way does what all I can remember is that its a smallish allen key and you have to cut out the blanking plug the first time.

 

[zcacogp]

I *think* that anticlockwise makes it richer. BUT this is when viewed from above, and the screw is underneath, so it's all back to front ...

If you are thinking of adjusting it then you need to do it with an exhaust probe in anyway, so twiddle it one way and see what happens.

FWIW, I drove the car a lot in traffic today, and a little on the open road, and it performed utterly faultlessly. Rock-steady idle, no cutting out, pulled like a three-year-old in Hamleys. Maybe it was the DME relay after all, and changing it has solved the problem. But say this all quietly - I don't want to jinx it! []

(Now to try and sort out the pesky oil leak. Which is still continuing - more's the pity.)

Thanks again for your input chaps.


Oli.