Menu toggle

S2 Misfire

P

PJS917

New member
My recently bought S2 has a misfire that I hope someone may be able to help with, the engine runs fine when cold, only misfiring when the engine is warm. Once warm it is very "˜lumpy' on tick over and has a bad miss fire up to about 3.5k revs, it is still present after 4k but not as bad. It does seem to be getting better, and I am running injector cleaner through it as well. I have checked all the electrics and cleaned the temp sender unit but none have made any difference.

Has anyone any ideas what it maybe?

Thanks

Peter
 
Misfire when hot will almost certainly be electrical, as electrics work less well when warm.

How old are the plugs? (Cheap bit .... )

Failing that, HT leads, dizzy cap and rotor arm are the next culprits (much more expensive.)


Oli.
 
Hi Peter,

As Ollie suggested after changing the spark plugs (try Bert at Berlyn Services), next culprits will be rotor arm, distributor cap, ignition leads and coil.

Regards

Dave K.
 
... a very cheap diagnostic for the leads would be to try running it with the bonnet open when it is dark. DON'T TOUCH THE LEADS WITH YOUR FINGERS, but push them around with a stick or something, and see if you can see them sparking to the block (or other earth.) Try spraying the engine with a fine water spray (plant spray bottle or similar) to make things worse.

If you can see sparks, the HT leads are definately toast. (However, if you can't, it doesn't mean they aren't past it!)

Alternately, find a friend locally with a similar car and swap parts across. The good thing about HT problems is that it is very clear when they are fixed, and the parts are easy to change.


Oli.
 
The leads are fairly new (I know this does not mean they are good) but I doubt it is them, I have cleaned all the contacts and the plugs but am begining to suspect that if it is electrical it may be a hairline crack in the dizzy cap. I have a spare that I will try tonight.

Any thing else I need to check?
 
Go with the most likely option first ... so swap that dizzy cap.

I notice from your sig that you also have a 968S ... which may well yield good candidates for diagnostic swaps. (I am guessing that the dizzy cap and rotor arm are the same on the two engines, but could be wrong.)


Oli.
 
Hi Peter,

When the car is cold, the ECU receives a signal from a temp sensor to richen the mixture. If you have weak fuel delivery, this can then be masked when cold and become more evident when hot.

I would suggest you check the fuel pressure and investigate the fuel system carefully (visual inspection). Consider also the fuel consumption. Spark plug colour may also reveal if a cylinder is running lean.

There are a series of diagnostics tests to run on the S2. My only advice is not to start changing things in the hope it will fix it - you need to identify the source of the problem first. Problems like this need to be addressed logically and some careful thought can usually pinpoint where the problem is.

Based on your description, I think it is more likely fuel delivery than electrical.

Regards,
Andrew
 
Andrew, all

It is number 4 injector that does not appear to be earthed properly, I have just swaped no 1 and 4 injector
and the car is still missing on number 4 so it is not the injector, where do the injectors earth too? Could the problem be in the ECU?


(Andrew I need to have a chat about a chip and tune up)
 
I have a quick question how often is the car used, everyday, once a week, once a month, as my project car has a simular car problem but i have a missfire around 5k revs but mine has been laid up for 4 years, (i havent owned the car for this long by the way) i have changed the dissy cap, the rotor arm (not cheap) and plugs no different, and have the redex injector cleaner in for the second time round no different, but i have been running the car every day (not driven) until the fan cuts in and it`s slowly getting better only slightly but better.............
 
The car has not been used much, 1k a year for the last 7 years, in the last 4 days I have done 1/3rd of its previous annual mileage, however it is an injector power problem, I have tonight diagnosed this after changing the dizzy cap, leads etc all to no avail, then started disconecting the injectors and and when no 4 was disconnected the engine note did not change. A quick electrical test showed no earth.

What I need to find out is are the injectors earthed seperatley or on a common central earth. I do not want
to start cutting and messing with electrics and petrol as I seem to recall from school they don't mix well[8|][8|]
 
Whoah! I stand very corrected!

Apologies for misleading you Peter, and thanks AndrewS for the education.


Oli.


 
Hi Oli how are you i havent been around much on the forum recently but it`s funny as i have been around cars engines g-boxes all my life and people have a tendancy to go straight for the major problem before checking the more common stuff that fails more often that is easier to change or swap, your logical way of thinking is an exact approach, tracking down a problem is a tricky proceedure, ps we have more stars than them two maybe the postority mugs may have a purpose afterall[;)].....
 
Rob,

Going for the major stuff first is the obvious think to do - Occams Razor and all that. However, the more I play with the 944, the more I realise that although on paper it is a simple car (4cyl, DOHC, simple injection and ignition system, 5-sp gearbox etc etc etc), it still has the ability to produce some interesting problems. A little knowledge is a dangerous thing, and the symptoms described (misfire when hot) would definately have been a HT issue on my last car (Mk2 Golf), it is obviously NOT the case here! More fool me for jumping in too quick!

Glad that Peter is on the way to fixing the problem tho'.

Good point - what are those stars for? I've never noticed them before, but I seem to have 4? Must be some kind of rating of good looks, eh?


Oli.
 
Further investigation has revealed that the injectors have a permanent live and the ECU switches the earth as and when they are required, as the car runs 'fairly' sweetly from 4k revs onwards my view is that it may be an ECU problem rather than a wiring issue. Does anyone have any other ideas before I try and track down an ECU?
 
if its had so little use in the past 4 years I would be inclined o check continuity of all the connections first which is easily done with a multimeter.
Even removing and replacing all the relays and fuses will clean their contacts a bit if you don't have electrical contact cleaner. Same goes with the ECU I have read elsewhere that oxidised contacts on the ECU connector can cause problems.Ditto the oxygen sensor connector

If only one injector is playing up it does point to a poor electrical or fuel supply to that one pot.

reference to my electrical diag shows that although all 4 injectors share a common electrical return supply and connection (red/blue) cyl 1 & 2 share one supply (grey) and cyl 3 & 4 share another supply (grn/blu) from the ECU
Therefore anything effecting one cylinder would also equally effect one other .

If the problem is only with a single cylinder then that would suggest a faulty plug or blocked or faulty injector.
You could veify this by finding the non working cylinder and swopping out the plug and injector with another cylinder .The problem should then migrate to the other cylinder .

The fact that the engine runs well at higher revs might simply be due to the engine being less sensitive to an unequal fuel supply from the faulty injector at high revs wheras at low revs the slightest discrepancy in fuel supply would have a marked effect on even running
. Phew hope my ramblings are helpful lol



 
Peanut,

Thank you, I have changed injectors around and it made no difference, so I am confident it is not an injector problem, It now can't be the ECU as that controls all 4 injectors equally, so it is either the spark plug, or a broken wire, both of which I will investigate further tonight.

 
Good point - what are those stars for? I've never noticed them before, but I seem to have 4? Must be some kind of rating of good looks, eh?
Click to expand...

Hi Oli,

I believe that 4 stars mean you are now qualified to use the milkshake machine and staff the drive-thru window......[&:]
 
You must log in or register to reply here.

Posts made and opinions expressed are those of the individual forum members

Use of the Forum is subject to the Terms and Conditions

Disclaimer

The opinions expressed on this site are not necessarily those of the Club, who shall have no liability in respect of them or the accuracy of the content. The Club assumes no responsibility for any effects arising from errors or omissions.

Porsche Club Great Britain gives no warranties, guarantees or assurances and makes no representations or recommendations regarding any goods or services advertised on this site. It is the responsibility of visitors to satisfy themselves that goods and/or services supplied by any advertiser are bona fide and in no instance can the Porsche Club Great Britain be held responsible.

When responding to advertisements please ensure that you satisfy yourself of any applicable call charges on numbers not prefixed by usual "landline" STD Codes. Information can be obtained from the operator or the white pages. Before giving out ANY information regarding cars, or any other items for sale, please satisfy yourself that any potential purchaser is bona fide.

Directors of the Board of Porsche Club GB, Club Office Staff, Register Secretaries and Regional Organisers are often requested by Club members to provide information on matters connected with their cars and other matters referred to in the Club Rules. Such information, advice and assistance provided by such persons is given in good faith and is based on the personal experience and knowledge of the individual concerned.

Neither Porsche Club GB, nor any of the aforementioned, shall be under any liability in respect of any such information, advice or assistance given to members. Members are advised to consult qualified specialists for information, advice and assistance on matters connected with their cars at all times.

Porsche
Lockton
Pirelli
Officially approved
Oracle Finance
WEX Esso
Collecting Cars
Archant Dialogue
Back
Top