All Things V6...

[Mandrake]

A 1988 car probably doesn't have an oxygen sensor, as 98 octane fuel in the UK still contained lead at that time.

Ah, that blows that theory out of the water then, it won't have an oxygen sensor if unleaded petrol wasn't available.

[Mandrake]

A 1988 car probably doesn't have an oxygen sensor, as 98 octane fuel in the UK still contained lead at that time.

Ah, that blows that theory out of the water then, it won't have an oxygen sensor if unleaded petrol wasn't available.

Definitely change the fuel filter...

David said he's already changed that...

[DHallworth]

Sometimes when starting from cold it won't even idle for 5 seconds before it cuts out.

Thanks for the suggestions guys, something else to think about. Prices for CX parts make C6's look cheap to run!!

David.

[Mandrake]

Get a fuel pressure gauge on it David, if it doesn't have an oxygen sensor checking fuel pressure is the first thing I would probably check especially if opening the throttle just makes it stall.

Do you know if it has an in tank pump and a pressurised main fuel line ?

Also that fuel gauge jumping around is rather suspicious - maybe a bad earth to the tank sender which could also affect the power at the pump if it is an in tank pump ?

If you can get at it, measuring voltage directly at the pump would be worth checking too.

[CitroJim]

The tachymetric relay was a common problem on Pug 205 GTis and used to cause a similar issue...

Even easier to check than using a pressure gauge on the fuel line, just put a voltmeter across the fuel pump supply and see if it's maintained...

Not that it will be cut by the tachymetric relay once the revs from the engine fall below about 100rpm...

[DHallworth]

Someone on the CCC forum in the CX section has said he reckon this is either a split pipe or a problem with the AEI.

Another possibility to look into. I'm confident it's not a split pipe as I spent ages checking them last weekend when it wouldn't go.

Cheryl's on a late shift tonight so I'll scratch my head whilst staring at it and see what I can come up with.

David.

[Mandrake]

Even easier to check than using a pressure gauge on the fuel line, just put a voltmeter across the fuel pump supply and see if it's maintained...

Only easier if the top of the fuel pump is accessible without dropping the fuel tank. I have no idea whether that's the case on a CX...

Someone on the CCC forum in the CX section has said he reckon this is either a split pipe or a problem with the AEI.

AEI ? What's that ?

[Mandrake]

Looking at the service manual to get familiar with the engine management I've noticed the following things:

This is a MAF (mass air flow) system not a MAP system. The MAF sensor is in a box down near the left hand side connected to the air intake. (Item 302 on the service manual) It looks like it is an old fashioned "vane" style sensor not a modern heated wire type. It's basically just a rotating flap connected to a potentiometer so easy to test.

There is no throttle position potentiometer, only a "throttle spindle switch", which is mounted right at the front of the engine near the radiator. (192 on the service manual) Early fuel injection systems especially those with MAF sensors often did not have a variable throttle position sensor - all the switch does is reports whether the throttle is at idle or off idle. The idea is that as soon as you press the throttle from idle the switch will trigger and the ECU will give a big squirt of fuel to prevent the engine stumbling, but once you're away from idle the switch does nothing and it's all up to the MAF sensor...

For the engine to idle properly the spindle switch will have to correctly report that the throttle is at idle, as the ECU relies on the switch to know when the throttle is at idle for controlling idle speed. I would definitely measure this throttle position switch with a multimeter to make sure it is OK, it could easily give the symptoms you see. According to the service data it should be closed when at idle and then change to open circuit when the throttle opens.

There is a separate cold start injector. (575)

Fuel supply is an in tank pump with return system, fuel pressure should be 2.5 bars with the vacuum hose disconnected and about 2 bars at idle with the vacuum hose connected. As suspected earlier there is no oxygen sensor.

The idle control valve is a design I haven't seen before - it looks to me almost like a heated bi-metallic strip which is used to push a flap into the bypass air flow to obstruct it. (There is no description I can find, only those language neutral pictograms that Citroen loved to use in the 80's)

Ignition is a transistor assisted two coil waste spark system with no distributor and there is a single crank position sensor for both spark/injection timing on the flywheel teeth, with one lowered tooth, and there is no cam position sensor. (Not needed for a waste spark system)

There is both a knock sensor and a turbo boost pressure sensor for the ECU.

Could be an interesting challenge to pin the problem down but at least the service manual I have seen is very detailed and has full circuit diagrams and information.

[CitroJim]

No need to get right to the pump, just check the supply at the fuse... Or even listen for the pump running

Is the injection system Bosch LU2 Jetronic? Sounds like it might be if it has a 'flapper' AFM...

Will the engine keep running if the throttle is held open...

A lot of problems, if it is LU2 Jetronic, were caused by air leaks anywhere south of the AFM...

[Mandrake]

No need to get right to the pump, just check the supply at the fuse... Or even listen for the pump running

Sure, but that doesn't test for wiring problems from the fuse to the pump causing the pump to run slow or intermittently...(such as a bad earth return near the pump - look at that fuel gauge jumping up and down! ) which is why I suggested a fuel pressure test. If the fuel pressure is fine that immediately eliminates the pump and all the electrical wiring to the pump in one fell swoop. If you test the voltage on the fuse and its OK you haven't really eliminated much, you still don't know if there's a wiring fault or a faulty pump.

Trying to narrow down the problem to fuel or spark first helps divide the number of possibilities in half really quickly. And as this is a conventional external coil system its easy to test the spark from the coils as well with an earthed screw driver to estimate the spark strength from how far the spark will jump!

Is the injection system Bosch LU2 Jetronic? Sounds like it might be if it has a 'flapper' AFM...

Will the engine keep running if the throttle is held open...

A lot of problems, if it is LU2 Jetronic, were caused by air leaks anywhere south of the AFM...

The service manual says its a Bosch "LE Jetronic".

Good point - a split in the corrugated air intake hose downstream of the MAF sensor would cause massive problems and is a common issue on MAF systems - check that rubber air intake hose carefully for splits David - they can sometimes open and close with engine movement!

Found this, which goes into general principles for the L Jetronic: (This is a slightly later version that includes an oxygen sensor but otherwise looks very similar)

http://www.cardiagnostics.be/-now/Educa ... Manual.pdf

The idle controller is called an "electrically heated auxillary-air device" and does indeed seem to work the way I thought it does. You learn something new every day.

[CitroJim]

Ahh, if it's Jetronic are you sure it has a fifth injector for cold running David? I ask as normally they have a SAD (Supplementary Air Device) tio admit more air into the idle circuit when cold/warming up to up the idle speed. These devices are both bi-metallic and water heated. I wonder if that's the device you refer to as the bi-metallic element poking into the idle circuit...

Base Idle speed is regulated normally by a brass screw in the Throttle Body. This sets the idle speed when the engine is hot and then the SAD looks after high idle speed when cold. The ECU knows when it's cold via water temperature and ups the mixture richness by increasing the pulse width to the injectors..

So, also worth checking the temperature sensor.

Jetronic is very sensitive to air leaks as said and also to battery voltage. The ECU is built of discreet components mainly and has no on-board voltage regulator. A low batter voltage will upset it badly...

Also, any 'mayonnaise' in the engine breathers can upset it too...

Generally all Jetronic controlled engines do not slow idle at all well and all in my experience have a tendency to stall and falter when cold unless the SAD is working well and all air leaks are plugged...

Simon, good point about the supply to the pump... Another method, if you can't get to the pump easily, might be to measure it's current draw by remove the fuse and replacing it with an ammeter...

[Mandrake]

Hi Jim,

The CX series 2 manual I'm referring to can be downloaded from here:

http://www.ebaman.com/index.php/remosit ... itroen/CX/

It needs a free login creating and is about 325MB however!

Yes, it does show a cold start injector as well as the "supplementary air device".

According to the description of the L-Jetronic manual I just linked to, its both heat activated from engine heat and electrically heated. So on a cold start the supplementary air path will be completely open to help starting, then as the electric element heats up the bimetalic strip it will bend and close the air path. On the other hand on a hot engine the engine heat will cause it to be already bent closed when the engine is started, so no extra airflow is provided during a hot engine start.

It sounds like hot idle speed may be set by an idle screw on the throttle instead of being ECU controlled like more modern cars ?

[CitroJim]

It sounds like hot idle speed may be set by an idle screw on the throttle instead of being ECU controlled like more modern cars ?

Yes, precisely as I said...

[Mandrake]

It sounds like hot idle speed may be set by an idle screw on the throttle instead of being ECU controlled like more modern cars ?

Yes, precisely as I said...

Sorry, I must have overlooked your comment on the brass screw.

I should add though that while it's good to understand how the idle control and cold start system works, I don't think the supplementary air unit will be the problem as David has said that once it starts spluttering he can't even keep it going by opening the throttle - it still dies, and if anything opening the throttle makes it die even quicker.

My gut feeling on this is it could be a fuel delivery problem, despite the fuel filter being replaced already. Perhaps something like a sick in tank fuel pump (or bad power supply to it) which might be able to build up enough pressure to start the engine, but can't keep up so the engine starts to splutter and die as the fuel pressure sags... Opening the throttle in a fuel restriction situation would just cause it to stall because it would go massively lean when the injectors open for longer and the pump can't keep up.

The manner in which it ran OK for a few seconds then progressively started spluttering and dying despite applying throttle looks suspiciously like what you'll see if you remove the fuel pump fuse on a running engine. (Yes I've done that to my Xantia many times to depressurise the fuel rail, and it behaves very much like David's video as it struggles to keep going and eventually dies after about 6-7 seconds. )

Hence I would start with a fuel pressure reading and go from there. That will definitively finger or rule out a fuel supply issue.

[xantia_v6]

It would also be worth monitoring the output voltage of the AFM (preferably with an analogue meter or oscilloscope) to check that it is not too noisy.