Peculiarity when starting

[zcacogp]

Chaps,

Something odd happens when I start my S2. It's infrequent (only happens every 25-30 starts, at a guess), and only when starting from hot (never from cold), but it does happen fairly regularly.

The starter turns the engine, which 'catches' reluctantly and runs, but runs only very slowly and very very roughly. I'd guess it idles at around 450-500rpm, and bounces all over the place. It will carry on idling like this for a while (2-10 seconds, but will usually die after around this time) but won't pick up to a normal engine speed, and if you touch the throttle it dies instantly. Annoyingly, if it does it once and you turn it off and try again immediately it will probably do it again; the only solution is either to try repeatedly, or to wait about 30 seconds and try again.

It has never failed to start and doesn't worry me - much! - but it's annoying. (And it also means than anyone in the near vicinity looks at the car and thinks "gosh, that's a nasty old Porsche - he clearly couldn't afford a proper 911!" [:'(])

The car is extremely well maintained and otherwise runs faultlessly. Once started, it goes precisely as it should. The FPR was changed about 2 months ago (for emissions reasons) but this hasn't changed it. The cap, leads, plugs and whatnot are all less than 3 years old. The problem has appeared within the last couple of years but doesn't seem to be getting any worse (or frequent.)

Has anyone seen this happen on a 944 before, and does anyone have any suggestions as to what could be wrong?

Thanks,


Oli.

 

[Suffolk944]

Oli, that sounds very similar to the problem I had with my current car when I first got it - an intermittent hot start problem accompanied by lumpy running and then the thing dying altogether. If I recall correctly it was the Idle Control Valve (am sure that's what it is called ?) that was diagnosed as the problem. Had the part changed and the problem vanished, never to re-occur.

 

[Hairyarse]

Hi Oli

I had that very same issue with my S2 for the few months I owned it! However, it only happened 2 or 3 times so never got to the bottom of it.

A colleague at work has had a very similar problem with his MK4 Golf. Turned out to be a dirty throttle body. Mechanic friend sussed it as soon as he was told the symptoms and removing it and cleaning sorted it. Not quite same symptoms as yours as the revs tended to go higher not lower from memory but all else sounds the same.

As Jon says ISV sounds plausible, I thought TPS too although would expect the problem to be a lot more frequent. You can test the latter easily enough, there's a how to on Clarks

Cheers
Andy

 

[pauljmcnulty]

Had the same with the Legacy and it needed the throttle body cleaning. Apparently, as I'd have no clue if that was what they did or not, but it worked. Less than an hour at a main dealer, so not unreasonable.

 

[bmnelsc]

I've had similar problems with my S2 - but sometimes cold as well as hot. I found that it was related to when I released the ignition from the start position. I know - sounds pretty fishy but it seems consistent. If I released the key from the start position just as the engine was catching (a nano second seemingly - like the engine was 3/4 of the way to TDC and momentum kept it turning to the point of ignition) I would end up with the barely running/unable to rev condition. If I held the start position until the engine caught (not so long as to keep the starter engaged while the engine was actually running) - then no problems.

There is a transition in the circuitry between start and run that affects the fuel pump - may be the situation here where the car starts but the relays have not completed the switchover? Maybe something with fuel where the engine is running on residual line pressure rather than full pump pressure? Just a thought - no empirical evidence.

 

[zcacogp]

Chaps,

Thanks for the replies. The fact that others have had similar problems is reassuring as it means I am not going mad ... !

Jon, Andy, while the idle stabilisation valve (ISV) and throttle position switch (TPS) are interesting ideas, I'm a bit dubious. The ISV is responsible for idle all the time (not just on starting), and it idles without a problem, and when the throttle is opened the TPS turns the ISV off. In my case it idles (badly), and opening the throttle causes the engine to die totally. I don't doubt your experiences but if it is either of those things in my case then I'll be surprised. However I will test the TPS (very easy to do) and see what I find out. Taking the ICV off and cleaning it is a prize pain due to where it is located on the S2 (under the inlet manifold) but I may have a go at some point.

Two votes for cleaning the throttle body. That's easy to do - I'll have a go today. Thanks.

Bruce, interesting. I'll try holding onto the start position for a while longer. When it did it yesterday there could have been a bit of chatter from the relay panel (I wasn't sure) which would tie in with your theory. Wonder how to check/troubleshoot that? New relays all 'round probably. A relay making a bad connection somewhere would probably fit with the symptoms. Is the fuel pump relay a special one, or a regular automotive relay? I'll do some investigating ... thanks.


Oli.

ETA: Ah, there is only one fuel pump relay and that is the DME relay. And that is new (of a year or so ago.) And I think I still have the old one ... perhaps I'll take the old one apart and see if it can be repaired, and swap them over. I understand the usual mode of failure is a dry solder joint, and that's pretty easy to fix. Thanks for the post Bruce, you're started me thinking ....

 

[TTM]

Vacuum leak? O2 sensor?

 

[sulzeruk]

ISV and throttle body. The ISV may be sticking open allowing air to bleed in when hot. It may do the same when cold but a much richer mixture at cold idle would mask the problem. Worth tqaking off and cleaning. The throttle body can be gummed up around the edges which can affect idle on some cars as well.
Alasdair

 

[TTM]

Coolant might be sipping through the seal of the throttle body?

Oli, you could remove the intake manifold and clean all that. Shouldn't cost a penny.

I too had that random issue with mine, but this plus random difficult hot starts all vanished when I rebuilt the top end and thoroughly cleaned the whole intake tract.