Intermittent uneven running problem?

[Poppers19]

Has anyone any experience of the following problem with an S2;

Every once in a while after a journey, distance is not relevant as happens after short or long journeys, but when I go to restart the car like after nipping into a shop or getting petrol, the engine won't rev and is really lumpy. But if I turn her off and leave her for a couple of minutes then start her again she fires into life and her usual perfect self.

As I said this is a real intermittent problem but can be embarrassing especially if it happens at the petrol station so has anyone got any ideas?

Cheers

Peter

1990 S2 Cab

 

[TTM]

Mine used to do the same but I could not pinpoint exactly what it was between a filthy idle control valve, a faulty coolant temperature sensor, and a dodgy AFM.

 

[frankydoom]

Wow - I got the same problem and mine is a 1989 Cab - I was thinking there is a part under the manifold which if cleaned stop's that problem - I haven't yet had the time to do it - as I believe the injectors need to come out -and the rest etc.

Interesting to see what response you get.

Frank


Ahhhh TTM just mentioned that part I meant - - just to hot now to do that valve thing

 

[edh]

TTM has covered the right bases there I think

check the plugs & see if it's running rich (carbon deposits on plugs)

 

[garyadams]

Hi PPeter,

Really interesting this problem as I also have a S2 Cab and last week I limped home from Falkirk to Lincoln, one minute the car would run really well then the next its as lumping as anything. I did have a simialr problem a year or so ago and that was an HT lead sparking to the chassis which gave a similar lumpiness to the running. Guess I'll sqirt some carb cleaner around a few parts before I mess with the AFM and Coolant temp sensor (where ever that is)

Thanks

Gary

 

[J4CKO]

I have this as well, still not sorted, it starts fine from cold and will run all day, will start immediately after being stopped but leave it a few minutes then it doesn't want to rev or idle until it composes itself, again, mine is a cab !

Have tried,

A new AFM, well new to me, didnt work

A new FPR (Brand new, £100 !) didnt work

Put my spare DME Relay in (new), didnt work

Cleaned the ISV, utter pain in the scrotum to get on and off, didnt work.

Checked Temp sensor, seemed to be working, pan of water and a multi meter

I think now its an injector leaking so need to do a leakdown test and see if its losing rail pressure, though wouldnt this manifest itself when cold as well, as surely the rail pressure would have a bearing then, or does it do something different i.e. choke when it starts from cold, surely that would be affected as well, confused !

 

[944 man]

Maintained pressure stops the fuel from vapourising. Without the pressure youll struggle to start, but vapour locks arent an issue on a cold engine.

 

[J4CKO]

Cheers Simon, makes sense,

So, mine, and some of the others with the same issue, does sound like fuel pressure issues.

Not changed an injector before, what is it like to do and will I get my normal bath in unleaded ?

Could do with a spare one/set, anyone selling any known good ones, are they interchangeable with anything else, i.e. Mercs of that era ?

The other theory was the check valve int he fuel pump but was thinking that would be every time I start it.

 

[bmnelsc]

All the replies sound reasonable to be sure and I have experienced similar problems (three times). What I found was that while cranking the engine if I was releasing the key a split second before the engine fired, the engine would catch (on momentum rather than starter driven) and be lumpy and would not rev (e.g. I couldn't "clear her throat" because she wouldn't respond to the throttle). Not very scientific but seems to be true for my car... so maybe early days of ignition switch failure or by releasing the key (and De-energizing the starting circuit) the engine starts before the switchover to normal operating circuits (DME or associated relay didn't "realize" the engine was running rather than cranking)?. I could restart almost immediately with no problems as long as I let the engine fire while cranking rather than having it fire on momentum.

The trouble with this explanation of course is that it is nearly impossible to reproduce (as are most intermittent faults) so when it happens you won't remember anything unusual about the starting sequence - you only pay close attention to it when you are trying a restart after an episode of lumpy running.

In any case - I'd bet on an electrical problem over fuel as it seems to me that a fuel related problem would clear itself after a minute or two even with a bad idle. I found that I had to switch off and start again to clear the problem and on restart, everything was immediately normal.

 

[garyadams]

Hi All

OK going by what TTM said I thought i'd check a few things. First I gave the throttle assemble a good spray and also the pipe that leads to the idle control valve. I then removed the AFM and checked that, as somebody has mentioned before the connector to the AFM is at the bottom but you can't really see what the pins look like without removing it. What I found was that the pins looked tarnished so I cleaned them and gave a liberal spray of WD40. Also sprayed the DME temp connector (I think this is the blue one at the front of the engine) also sprayed the connector on the Throttle.
Took the car for a run this morning and all was perfect no lumpiness or rough idle etc.

So my theory is that due to such dry weather conditions some of the plug connectors start to give intermittant readings or voltage or what ever their purpose is hence removing them and applying a good spray of the magic lubricant seems to do the trick. I did also hold the starter key for a second longer just in case its down to what Bruce mentioned about the engine catching on momentum rather than starter, but there could also be enough vibration when starting the car to correct any dried out plug by just making it have a slightly better contact.

If it plays up again I'll re-think my theory, but in the mean time those with a similar problem if you could just remove the two or three plugs mentioned give them a spray and then see how the car runs

Cheers

Gary

 

[zcacogp]

Interesting thread. Thanks for posting Peter.

I have what sounds like a very similar problem with my S2. I did a thread about it here:

http://www.porscheclubgbforum.com/tm.asp?m=727042&mpage=1&key=&#727042

... which came up with some similar suggestions to cure it, narrowing down on ISV and throttle body. As it is, the problem went away at the end of last year and I didn't do anything about it, but it's back again now. It gave me a load of grief trying to start it a week or so ago, and again yesterday. Always on starting, always when starting from hot, always the same symptoms (slow, lumpy idle, dies when you push the throttle.) It has never failed to clear itself (i.e. it starts OK on the 3rd / 4th / 8th attempt and thereafter runs fine) but it's worrying while it happens.

It sounds like there are several people afflicted by this, and I am heartened by Gary's suggestions of cleaning the connectors. I'll try that, as well as cleaning the ISV and throttle body. For what it's worth, my S2 had a new dizzy cap and rotor arm about 3 years ago and new leads at the same time, and a new FPR and coolant temperature sensor for the MOT last year.


Oli.

 

[J4CKO]

Oli, getting the ISV off is fun, fiddly bastard of a job, I took the throttle housing off rather than the manifold so you have to operate under the manifold to get at the bolts, didnt make any difference on mine and I dont think it generally is the problem, I cleaned mine out with loads of carb cleaner, a bit of black gunk came out but nothign over the top, my thinkign is that once its sorted itself out after it having its dicky fit it idles just fine, if there was a problem with the ISV the idle would be iffy all the time.

I reckon this is injector related at the moment, however, need to do the bodywork before I can get to it.

 

[TTM]

The least painful way I could find to remove the ISV was to remove the fuel damper and its bracket off the inlet so that I could slide my hand below.