petrol ECU sensors help needed

bxman · Post by **bxman** » 01 Sep 2008, 09:44

I urgently need to discover a cure !!
16 valve Xantia XU7 JP4
Starts perfectly, after 150 yards or so power decays, by then getting engine revs up to approx 3000rpm and/or slipping clutch progress can be made.
by dropping down to second gear, I can keep going rather than progressing at 10mph while the inevitable tail back develops.
After about a mile everything returns to normal power levels and no more problems are experienced .
I suspect it must be a temp sensor but have no idea where it might be located or what to look for .
provided the engine is warm the problem does not occur on restart (2-3 hrs )
It is dual fuel but it does it when it is switched to petrol only.
so I am sure it is not a change over problem.
All suggestions would be most welcome
THANKS.

Post by **CitroJim** » 01 Sep 2008, 12:34

This sounds very much like a dodgy Coolant Temperature Sensor giving the ECU wrong information and thus causing the mixture to be wrong on a semi-cold engine.

It is worth checking the integrity of the sensor connections before swapping it.

bxman · Post by **bxman** » 02 Sep 2008, 10:29

Good of you Jim
Have checked this morning It's the green sensor ?
lowest of the 3 fitted on thermostat housing on mine.
All appeared OK visually. Cleaned it up and plugged it back in several times,
checked the ECU plug for nasties as disclosed by red_dwarfers recent posting
Mine appeared reasonable but problem is still there.
Is it a thermistor ?
should it have a value that will register on a DVM ?
I will look for a replacement and report back.
Thank you very much Patrick

DickieG · Post by **DickieG** » 02 Sep 2008, 10:38

Another cause of the problem could be the MAP sensor being blocked by gunge that may become temporarily dislodged after a period of driving.

Post by **CitroJim** » 02 Sep 2008, 17:55

DickieG wrote:Another cause of the problem could be the MAP sensor being blocked by gunge that may become temporarily dislodged after a period of driving.

Good thinking Richard!

For the record the Coolant Temperature Sensor (the green one) should show a resistance of about 2.6-2.7K at ambient (18 degrees) temperature. This will fall as it gets hotter and rise as it gets colder. It is indeed a thermistor.

I've just taken the reading above from a green Turbo CT sensor I have on the bench although it should be much the same for any green sensor.

deian · Post by **deian** » 02 Sep 2008, 22:49

Now for my turn to throw a sensor in... Not the oxygen sensor is it?

As in working faulty opposite to the normal faults, i.e not providing enrichement when it needs it, rather than providing enrichment at all times (as is the typical O2 sensor faults). But I think the coolant sensor would have a say over this too.

Let's not assume it's one sensor either.

bxman · Post by **bxman** » 02 Sep 2008, 23:41

update 2.

I have swapped the green sensor without the cure I am afraid .
It read 2.8K which looks to have been OK.
Forgive my ignorance, do I simply extract the MAP wash it off in petrol and refit it and does that have a value that I can read ? I imagine any gunge blocking it will not alter the resistance reading?

bxman · Post by **bxman** » 08 Sep 2008, 22:58

Now that it has at last stopped raining I have ventured out and done something
I am optimistic that I have cured the problem. I should know for certain in the morning .
Although I am afraid I cheated a little in that I have changed the whole butterfly block.
This has 4 electrical plugs :- a
white 3 pin attached to the spindle ( a positional potentiometer I think)

a grey 2 pin plug connected to a small device in line with with spindle on air filter side presumably the MAP.

a yellow 2 pin plug which I assume is a temp sensor in that is it is brass and embedded in the body of the casting.

and a black 4 pin plug which looks like a solenoid. It is made in the USA by Airpax for Magneti Marelli.

What do these do and which of them can I test .
As the block itself will in time be needed back on the engine that it came from.

I appreciate the help everyone has given and am sorry to be asking for more.!

Post by **CitroJim** » 09 Sep 2008, 07:08

That's the way to do it

That changes the whole lot of suspects all at once!!!

Off the top of my head, the white one is the throttle pot (may have had a bad track?), The grey (I think) may be the butterfly heater to keep things warm and ice-free in cold weather, the yellow is the inlet air temperature sensor (I think) and the black will be the idle control valve.

When I return home later, I'll dig the relevant diagrams out and confirm/correct my assumptions!

bxman · Post by **bxman** » 09 Sep 2008, 12:04

Thank you yet again Jim .
I am afraid it is still not cured, it played up again this morning , I am now thinking it could possibly be petrol starvation.
Is it possible to fit a gauge to read the fuel pressure and confirm that the electric pump is working correctly?

The Bol mentions " a filter mounted underneath the rear of the vehicle."A4.3 has anyone ever seen one ?

The other thought is are there any unexpected problems in swapping out the ECU unit itself, I imagine it would pay to have the same immobilizer code on both units before the swap.

I do want to avoid getting into any re-programing problem if at all possible.

DickieG · Post by **DickieG** » 09 Sep 2008, 18:01

Whilst my knowledge of BX's is sketchy personally I'd want to eliminate the MAP sensor from the investigation as it can be the cause all manner of erratic running if it isn't working correctly due either the MAP sensor itself being faulty or due to being blocked by gunge.

I recently come across a Xantia that had been into several respected garages for erratic running with numerous expensive parts being replaced when all that was required was a blow through on the connecting vacuum hose and then a clean up.

Unfortunately I can't help you with the location of the sensor on your BX.

I'd have thought that a blocked filter would severely restrict the performance of the engine to the extent that you couldn't rev it to 3,000 rpm as you describe on your first post.

wheeler · Post by **wheeler** » 09 Sep 2008, 18:37

citrojim wrote: Off the top of my head, the white one is the throttle pot (may have had a bad track?), The grey (I think) may be the butterfly heater to keep things warm and ice-free in cold weather, the yellow is the inlet air temperature sensor (I think) and the black will be the idle control valve.

When I return home later, I'll dig the relevant diagrams out and confirm/correct my assumptions!

I think the heater is yellow plug & grey it air inlet temp sensor.

Post by **CitroJim** » 09 Sep 2008, 18:56

Here is the XU7JP4 injection Circuit Diagram

And here is the Location Diagram This shows the physical location of the various sensors and other bits and pieces. They're big diagrams so please be patient whilst tey download.

The MAP sensor is item 1312 on a 3-way Grey plug near the throttle body. In changing the throttle body you won't have changed the MAP sensor. This is further beyond the butterfly. This remains a prime suspect. Give it a clean and then swap it. A MAP sensor, unless in a really bad way, will not raise the K light.

The Throttle Pot is item 1316, The Idle Valve is item 1225, the oxygen sensor is item 1350, the Throttle Body Heater is item 1270, the Inlet Air sensor is item 1240. The Haynes BoL has a complete list of circuit diagram item numbers for anything I have not listed. They also list the connector colour codes. They're the bold two letter designations next to the plugs in the circuit diagram. For instance item 1220 is the Coolant Temperature Sensor and has 2V VE by the plug. This shows it's a 2 way Green (Vert) plug. If you know the French words for colours you can work them out easily. yellow is JN and White BA and so on...

I'd agree with Richard on the fuel filter but for reference, it's located near the tank on the rear subframe on the drivers side. Look underneath and you'll find an aluminium cylinder with pipes each end.

The symptoms seem to suggest the engine is running weak when power is called for. As said, the MAP sensor is prime and then the oxygen sensor (Lambda Sensor). Also, do not discount the possibility of air leaks into the inlet manifold beyond the throttle body. They play havoc with MAP-based fuel injection systems if the MAP readings are duff and fuel injection is calculated using the duff information. Anything connected to the manifold as well as the manifold itself, especially if it's a plastic one, can cause leaks. Holes can sometimes be blown in them from backfires!

A weak mixture caused by MAP or air leak problems will be much worse when the engine is cold.

let us know how it goes please.

Post by **CitroJim** » 09 Sep 2008, 18:59

wheeler wrote: I think the heater is yellow plug & grey it air inlet temp sensor.

Indeed I got it wrong, sorry and thanks wheeler!

but do bear in mind the time of day I wrote that and it was from pure memory

Two out of three ain't bad as Meatloaf might say...

bxman · Post by **bxman** » 11 Sep 2008, 00:07

Another dry day !!
Again I am hopeful of progress many thanks to ms00 for his earlier post in March '07
" I have removed and cleaned the MAP sensor (which I managed without removing the manifold as Mr. Haynes suggests, woohoo!). " This must have saved me hours of time.
I now know what a MAP looks like in fact 2 different Bosch ones
each with a different 3pin plug so not easily interchangeable
P reg on the one T on other.

I found the clip missing and the dreaded green salts on the pins , cleaned and polished and refitted started OK but it took quite a while to sort itself out.which had me worried for a while.
It does seem to tie in now . The problem first occurred after the bonnet had inadvertently been left open in a downpour. So I am crossing my fingers.

The scans were brilliant thanks Jim . I found the filter, it looks like a replacement job I suspect if it comes to it.

thanks to everyone particularly DickieG who looks to have got it right in one.
____________________

SX xantia 16v X2 (LPG)
" 1.9D NA
+ the odd BX