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944 S2 Intermittent Misfire
https://forums.tipec.net/viewtopic.php?f=25&t=22769		 Page 1 of 2
Author:  Dan! [ Thu Jul 06, 2017 9:28 pm ]
Post subject:  944 S2 Intermittent Misfire
I have an intermittent misfire which also features rev counter bounce when the misfire occurs.

It's worst when going up hills, but sometimes occurs when cornering or if you hit a pothole.
It never occurs when stationary (on flat ground) or when driving at steady speed on the flat in a straight line.
This makes me think it's a loose / bad connection or possibly a bad earth.

The rev counter bounce led me to think it was the speed / reference sensor as that sends the signal to the rev counter.
However I've replaced the speed / reference sensor (there's only one on 16v engines) and the problem hasn't gone away.

It could be a problem with the ECU, although the intermittent nature seems more wiring related.

A friend has suggested the ignition coil, as some engines use the coil for the rev counter signal, but I'm not sure. Perhaps it's not earthing properly.

Any advice welcomed.

Edit: I should clarify, when I say misfire it stutters massively so that the car kangaroo hops. If you're going slow enough it'll stall the engine.
Author:  blackadder30 [ Thu Jul 06, 2017 10:59 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
Sounds similar but not the same as an issue I'm investigating on the 968 where she just cut out altogether on the motorway at 70mph and then a second or so later fired up again.

I've been pointed towards the DME relay and yours seems to be an intermittent connection like a DME failing... Could of course be any connection to fuel pump, coil, ignition switch...

Hope this helps.
Author:  GlennS [ Fri Jul 07, 2017 4:07 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
It's not the DME relay. That has already been swapped :wink:
Author:  blackadder30 [ Fri Jul 07, 2017 8:33 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
It's one if those trial and error ones then... I'll be doing the same this weekend on mine :wink:
Author:  Dan! [ Fri Jul 07, 2017 10:01 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
GlennS wrote:
It's not the DME relay. That has already been swapped :wink:

You say that, but when I started the drive home from Blyton Park after the track day, the car was misfiring terribly, such that I couldn't do more than about 30mph. So I pulled over after about a mile and swapped back to the old DME relay just to see if it made any difference. Luckily it was quite a bit better with the old relay so I drove home like that.

Having changed the speed / reference sensor to no avail, the next thing I need to do is swap back to the new DME relay to see what that does in conjunction with the new sensor.

If that doesn't work I've thought about possibly taking the ecu out of the frame it sits in and wiggling various wires with the engine running to see if any of them cause the misfire.
Author:  GlennS [ Sat Jul 08, 2017 5:56 am ]
Post subject:  Re: 944 S2 Intermittent Misfire
Dan! wrote:
GlennS wrote:
It's not the DME relay. That has already been swapped :wink:

You say that, but when I started the drive home from Blyton Park after the track day, the car was misfiring terribly, such that I couldn't do more than about 30mph. So I pulled over after about a mile and swapped back to the old DME relay just to see if it made any difference. Luckily it was quite a bit better with the old relay so I drove home like that.


Mmm.... that makes me wonder if there's an issue with the wiring in the fuse box. A bad connection, perhaps.
Author:  Endoman [ Sat Jul 08, 2017 3:57 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
After similar problems I changed various components the eventual culprits were fuel pump non return valve and the speed sensor. Symptoms were poor hot starts and occasional misfire/cut out. A new pump with valve cured the starting problem and new ref sensor the misfire, but I did get the injectors cleaned and replaced the temp sensor and FPR as well as checking/ cleaning all the wiring plug contacts.
Author:  Dan! [ Mon Jul 10, 2017 5:45 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
GlennS wrote:
Mmm.... that makes me wonder if there's an issue with the wiring in the fuse box. A bad connection, perhaps.

I think you're onto something here Glenn.

I lifted the relay mounting plate to check the wires underneath and couldn't see anything obviously loose. There was one small bit of exposed cable which I taped up but otherwise looked ok. However there's so many wires going into there and you can't lift it that much without pulling on the wires that it's tricky to tell properly.

Anyway I removed the old DME relay, put some WD40 on the contacts and put the new DME relay in. Started the engine and it ran fine, but when I put the lid back on the fuse box and banged the clips closed it made the engine stutter as it did when the problem first appeared at the track day. One time I actually managed to stall it by banging the fusebox lid clip closed hard! :shock:

Took it for a short drive and it's slightly better with the new DME relay in, but the problem is still there (mainly on left hand bends now).

So my current conclusion is that it's something to do with the wiring for the fusebox and relays, as there's no way you should be able to stall the car just from a small shock load to the fusebox lid.
Author:  flt505 [ Tue Jul 11, 2017 7:44 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
Is it possible that your DB is possibly cracked, making intermittent contact only when not under stress?
Author:  Dan! [ Wed Jul 12, 2017 6:32 am ]
Post subject:  Re: 944 S2 Intermittent Misfire
I'm probably being a dumbass but what is the DB?
Author:  Waylander [ Wed Jul 12, 2017 10:27 am ]
Post subject:  Re: 944 S2 Intermittent Misfire
dogs B???????s ?
Author:  Dan! [ Wed Jul 12, 2017 12:16 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
flt505 wrote:
Is it possible that your DB is possibly cracked, making intermittent contact only when not under stress?

Only thing I can think of for DB is dashboard, but that shouldn't have any effect on the fusebox, relays or ECU because none of those go through the dashboard on the S2.

:roll: at Waylander, I walked right into that one!
Author:  Dan! [ Wed Jul 12, 2017 12:51 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
Continuing on the fusebox and relays theme, I've got a decidedly dodgy looking relay wired into location G18 which can't be helping things:
Image

Image

I'm not sure what it's powering as according to the layout guide there shouldn't be anything in G17, G18 or G19 (it's a non AC model):

Image

I'm going to try unplugging it and see what stops working (if anything).
The car did have a '90s aftermarket Clifford alarm when I bought it which I've had removed (because it used to go off in the middle of the night for no reason), but maybe the extra relay is for that?

Does anyone with a UK S2 have anything in location G18 or G19?
Author:  flt505 [ Wed Jul 12, 2017 5:37 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
Sorry - DB: distribution board - aka fuse box panel. My error of not explaining properly...!
Author:  GlennS [ Wed Jul 12, 2017 8:28 pm ]
Post subject:  Re: 944 S2 Intermittent Misfire
Dan! wrote:
The car did have a '90s aftermarket Clifford alarm when I bought it which I've had removed (because it used to go off in the middle of the night for no reason), but maybe the extra relay is for that?


My bump of suspicion is triggered.

When you fit an alarm / immobiliser it is normal to have it interfere with 2 or 3 circuits essential to starting the car. Ignition circuit and fuel pump are the obvious ones, but what if the installer was a bit of a smart aleck and knew something about Bosch fuel injection? What if he knew that unless the ECU sees 200rpm, it won't enable the fuel pump? One way of ensuring a 944 doesn't start/won't run would be to make the immobiliser break the signal from the speed/reference sensor to the ECU...

If I remember correctly, at Blyton you said the rev counter gave "impossible readings" when the misfire occurs, like there was a bad connection? I wonder whether your shonky, taped-in, relay is a legacy of an immobiliser, is failing and is switching something vital like the speed/reference sensor.
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