Safrane misfire mystery....

[25v6turbo]

O.k so the Safrane has been going well and I have complete lots of little jobs that have needed doing including getting the heater working today at vaste expense with a new heater resistor.

But the issue I have that's bugging me is the slight misfire it has at idle,it only happens at idle,it drives really well and pulls great,no pinking,knocking etc,all fine,BUT it coughs at a standstill at idle.

So I have replaced the fuel,air filters,spark plugs,coolant temp sensor and MAP sensor and I,am running an injector cleaner through it,still has the misfire.

What I,am thinking is because it goes when I,am driving or when the revs are increased its not the cap/rotor/leads as the misfire would surely be that at some stage whilst driving,also I think its not an injector fault again I feel this would show up driving.

When started up stone cold it runs fine for a few minutes,then this misfire appears,no warning lights are displayed.

If I disconnect the MAP sensor when running it stalls straight away,restart the car and it runs perfect for a minute or so,then the misfire is back,its although something gets reset when the MAP sensor is disconnect.It also seems to be maybe temperature related,the bizarre thing is that there was a brand new cat convertor in the boot when I bought it,but the cat seems fine,no rattles etc previous MOT C/O reading look fine aswell,any ideas?

[vaughant]

Coolant temp sensor? That can certainly affect fuelling.

[Crispian_J_Hotson]

Borked lead. Get some flashy lead testers, have a look when it happens. Discovery done this for a bit, only when cold though and that ran as normal after 3 minutes. It has eight of them and eats them for breakfast.

[25v6turbo]

I fitted a coolant temp sensor,brand new,the MAP sensor was S/H but still did the exact thing,what is a Borked lead?

[Crispian_J_Hotson]

An ht lead that has the carbon electrode snapped inside the insulation. 

[25v6turbo]

It seems to do it all the time after say a couple of minutes starting in the morning,not straight away or after say 10 mins.

Its had a coil not long ago aswell I have the invoice,i still changed this for a spare incase the new one was faulty.

If it was a lead would it break down at roughly the same time but still be good whilst on the move?

[Dirk Diggler]

To me something that isn't doing it cold, but starting as it warms (if I read the above correctly) sounds like something electrical (like a lead) breaking down

[Crispian_J_Hotson]

Like I said, the land rover ran fine with a broken lead, most mornings it would start fine and it was fine all day. Then it started happening every day on first start when it was relatively cold but would shortly clear. I can only put this down to either temp related or the vibration difference between running cold v's warm idling. 

I couldn't get a lead for it for a few days so made one because landrover. It didn't like it much on the motorway!

You can run a continuity test with a meter for free but it may mean messing with the leads to get the break to happen which could, well, break the leads. Those HT testers are cheap enough.

[Asimo]

As ever with Renault electrics, check the voltage at the battery both at idle and at a few thousand revs as a first check.

 

Ignition lead breaking down usually is worse with wider throttle opening: more throttle equals more pressure in the cylinder at firing time equals higher voltage before the plug sparks so an engine that idles ok splutters under load. Sometimes you can see this in the dark. If all is healthy you will see nothing but if plug leads etc are shot you may see quite a display of sparks.

 

I would try and isolate the problem to a cylinder, assuming that it is just one of them. Unplug one injector at a time, one plug at a time etc. Then swap parts around, see if the fault moves.

[Crispian_J_Hotson]

Intake air leak?

 

Ben

 

It would idle high.

[Crispian_J_Hotson]

Bare in mind touching or yanking leads off when the engine is running more than likely will end in a bit of a shock... Literally!

[25v6turbo]

I have found a genuine set of leads which I have ordered,and also checked the cap and rotor which are both in a terrible state,both original so have served well,but 2 of the contacts inside the cap have deep grooves in the middle where the rotor has eaten its way through the contacts over the years,its even slightly touched the inside of the actually cap aswell,funny thing is it ran really well today but I guess it should be better with this lot fitted???

[25v6turbo]

I have fitted the new cap/rotor but still no improvement,leads are on there way,IF is disconnect the oxygen sensor is this going to send it into meltdown?

Would have to run for about 5 mins only,this would eliminate a possible issue with the oxy sensor?

Having sat there and studied it closer the pattern is always the same,runs fine at start up the goes into misfire about 3-5 mins then seems to smooth out the longer it runs,full temp and its only just there,does seem heat related when something gets to a certain temp it starts playing up.

[rainagain]

If you unplug the O2 sensor it should just bring the EML light on and go into limp home. Can you plug an OBD code reader in and get what temperature the engine thinks it's running at? HT faults can be funny, I had an Astra once that would idle fine, pull fine at high revs but at mid revs would misfire, even though it's harder to make a spark at high not mid revs. I fitted a new coil pack and this solved the problem.

 

Does it have an air intake temperature sensor? These can fail but in such a way as to give a plausible value, e.g. -10C so no EML light, but the engine thinks it's colder than it actually is so chucks in extra fuel, or it can go the other way and not put in enough. The coolant temperature sensor can of course fail in the same way.

 

I would do what's been recommended above and remove each HT lead in turn until the problem is no worse, than you know which cylinder has the problem. Then you can swap HT leads/injectors and see if the problem moves.

[The Moog]

Is it OBD2 ?

 

If so you can get Torque and check the switching at o2 sensor to see if it is working or not without having to disconnect it.