Activa not starting

[jwalshe]

Hi all,

Car doesn't want to start from cold. Catches and splutters after a while and usually starts.

Changed plugs and leads but it hasn't made any difference.

Any idea where I should be looking next?

Thanks loads!

James

[xantia_v6]

Does it run OK after starting?

[jwalshe]

It goes alright - but there's definitely a little stutter when cruising at around 2500rpm.

[xantia_v6]

I would guess that it is either ECU temperature sensor or Map sensor. Some Lexia live data would make diagnosis easier.

[DHallworth]

My guess was going to be a temperature sensor as well.

David.

[jwalshe]

Thanks chaps! Do you mean the temp sensor has failed or sticking? Do I need to buy a new one? Apologies for the daft question - I'm used to the fairly basic tech of a Citroen Ami!

[Mandrake]

I would guess that it is either ECU temperature sensor or Map sensor. Some Lexia live data would make diagnosis easier.

I'd have to agree - coolant temperature sensor or MAP sensor are the most likely candidates for those symptoms.

Without a working coolant temperature sensor you won't get any cold start enrichment (think, choke from carb days) so the engine will struggle to start but may run OK once warmed up.

The MAP sensor can also cause difficult cold starting although if its the MAP sensor the car would usually start OK if you use a little throttle after turning on the key.

Ideally the car should go onto a Lexia to get the fault codes read so that you don't end up finding the faulty part by trial and error...

[jwalshe]

Damn! Was hoping it was something I could fix myself. Think I'll have to lay the car up for a while - at least until I can get a slot at BL Autos. They're fully booked at the moment it seems. Back to the Ami it is then! My colleagues will be laughing their heads off. Grr!

[xantia_v6]

The relevent temperature sensor is the green one, I think on the thermostat housing on that engine, and not too expensive, so you could tasks a chance and change that.

[Mandrake]

If you have a multi-meter and a little bit of electrical knowledge you can test both sensors without too much difficulty and without a Lexia.

Coolant temperature sensor you would unplug and measure the resistance of it and compare it to a table showing the resistance versus temperature slope - if the resistance value is within the ballpark for the current coolant temperature (compare with the coolant temperature gauge) then it is probably OK. If it measures open or shorted it's definitely faulty.

The map sensor can be tested by a voltmeter, but must be done while it is still connected and the engine is running, which means you need some kind of piercing probe to push along the wire into the back of the plug (called back-probing) so that you can measure while its still connected. Something like a long sewing needle or pin should work.

I actually made up a test cable for doing this kind of back-probing some time ago, using two standard sewing pins soldered onto the end of some microphone audio cable and made tidy with heat shrink, it works a treat:



It can be used on any of the standard automotive type connectors. You carefully push the pin down parallel with the wire right up beside the wire (but not piercing into the wire) - further down inside the rubber insulation of the back of the plug the wire is exposed, so you can contact it without piercing the wire's insulation or leaving a hole as you're pushing the pin between the wire and the rubber.

A map sensor will have three pins - ground, 5 volt reference (supplied from the ECU) and the signal output which varies with manifold pressure. On a turbo engine it will be a "2 bar" sensor which means 5 volts out will be about 2 bars with 0 volts out being no pressure. 2.5 volts would be 1 bar etc...

What you want to see is 5 volts there all the time on the reference pin, and on the signal pin you would expect to see:

* Near 2.5 volts with the key on and engine not running.
* Less than half of this with the engine idling
* 2.5 volts or more when you open the throttle to rev the engine.

If you don't get this the map sensor is probably faulty.

(Note: for others reading this, non turbo cars almost universally use "1 bar" sensors where full 5 volts equals 1 bar = eg atmospheric pressure, but the same principles apply - the voltage is proportional to absolute pressure)

[jwalshe]

Thanks Simon - and everyone else for your advice. Interesting stuff.

James

[DHallworth]

It was a temperature sensor that was causing the CX to start, run very badly for a few seconds and then cut out.

If the coolant sensor is telling the ECU that the temperature is wrong it won't pull in the cold start injector. I worked it out roughly by starting it and letting it cut out several times, after it had done that a few times it would run a little longer each time. Once I'd got the idea in my head I looked at the workshop manual and got the resistance for the sensor and when I measured it it was miles out. I may have a sensor I could send you to try, I'll need to have a look at my spare engine.

Edit - The V6 sensor is different according to service box. The correct green sensor 1920 Q2 is NFP so unless you can find a dealer with one on the shelf you can't get one from Citroen. There is one on eBay for 14 quid though.

Regards

David.

[CitroJim]

On the Activa engine the sensors are absolute sods to get to James - they're on the thermostat housing but right around the back of the engine.

When I ELIT'd it I saw no obvious issue with the temperature sensor but then in all honesty it was only a quick look... I recall it was reporting 77 degrees when we started it up not long after you arrived...

It needs to be watched from cold...

[DHallworth]

Makes me wonder if it's jammed at 77 degrees, Jim.

I remember our old Xantia V6 was stuck on a hot reading and it made it a dog to start. Fitted a new sensor and a few months later that one stuck on cold and made it run horribly when hot!

A genuine sensor seemed to cure it though.

David.

[CitroJim]

That David, is very possible.. I saw 77 degrees and it looked about right for the prevailing conditions... It was only a short look so never watched it to see if it moved..

I did notice James Oxygen Sensor was static though... Recommended he replace it...