220 tyres and cutting out

[Bilkob]

So, the answer is yes, you can run 220 tyres on the front of a CX, or at least they are fine on my GTI Turbo. It now sports 220s on the front and 200s on the rear, doesnt look too daft and drives fine
It cut out again the other day, in the really hot humid weather we've had, this time while driving at slow speed, after a lot of sitting and idling.
Same symptoms as before, seems to get hot and bothered, the just cuts out. I left the car and walked home- not good for a cyclist!-when i returned a few hours later, on the first crank it wouldnt start, but the second time it fired and drove home fine.
I've noticed another thing. The car STINKS of stale, unburnt fuel when you dri e around at slow speed with windows open. The tank was half full of good fresh fuel, and it isn't noticeably heavy on fuel, on my Ardennes trip it was doing a seat of the pants 25+mpg
So, any (more!) ideas?
I'm now convinced it's fuel related and not sparks, and before anyone asks, yes, it still cuts out if left idling on my drive with fuel cap off............
Help!

[xantia_v6]

From your description, it seems to cut out when the under-bonnet temperature is high. That might point to a faulty component in the ECU.
Does the level of fuel in the tank seems to have any influence on the behaviour?

[Bilkob]

I would tend to agree, except I've tried a different ecu, my current one is now living just behind the headlight, and the car will cut out with the bonnet open......
Unless anyone wants to lend me a series 1 (non intercooler) ecu. I'm very honest and will return/buy said item!

[white exec]

Wonder about vapour-locking as the under-bonnet temp climbs. Is/are the electric fans cutting in at low speed when they should? If the fans are triple-relay controlled, it would be easy to add an extra (adjustable) thermal switch under-bonnet to bring the fans on if things got very hot under there, independent of coolant temperature.

Rover V8 (in the well-stuffed P6B and MGB engine bays) could suffer from vapour-locking something dreadful at crawl/slow road speeds, egged on by a pair of Aga-sized cast-iron exhaust manifolds. Various cures (fitting an electric pump near the tank, fitting electric fans...) including throwing cold water over the o.e. SU mechanical pump, nicely located right next to one of those manifolds!

[Bilkob]

That's sort of what I'm thinking. It is VERY hot when I open the bonnet, but the fans work fine, and cut in and out as they should. Mind you, don't the fans blow the hot radiator air INTO the engine bay??
I did touch the fuel rail and it was REALLY hot, is this correct/normal?
Keep the answers coming, I will sort this, it's too nice a car not to......

[myglaren]

[white exec]

When you say that the fans work fine, do they operate at both low speed and high speed - or just both come on at high speed?

[xantia_v6]

The efficiency of the fuel pump drops when the fuel in the tank is hot (remember that hot fuel from the fuel rail is recirculated to the tank). So a weak pump will appear to fail when the fuel is hot.

If there is any restriction between the tank and the pump (e.g. a blocked gauze screen), you can get vaporisation and vapour lock before the pump, especially when the fuel is warm.

The high pressure in the fuel delivery lines and fuel rail means that there is practically no chance of vapour lock after the pump while the engine is running.

[Bilkob]

2 Very useful replies! I'm not sure if fans are working low speed, I'll run it tomorrow and see. The hot fuel pump idea sounds potentially promising!

[marc61]

I replaced the map sensor on mine recently and that certainly helped, it doesn’t cut out any more. Only cost £11 on eBay from China and I see they’ve just dropped the price further to £9.90! It’s a standard 2 bar map sensor by GM - see EBay item number 263575728987, takes about 3 weeks to arrive.

[wurlycorner]

Turbo CX engine bays always run very hot (joy!)
However...
The only ECU in the engine bay is the ignition ECU. The Injection ECU is in the cabin (passenger footwell), so if you're sure it's fuel related, then the injection ECU will be entirely unaffected by engine bay temp.

The fuel pump is right under the back of the car, immediately in front of the tank, under a metal cover. The fuel filter is also there.
Why would the fuel back there be getting hot?

The cooling fans are twin relay controlled for engine cooling - the third relay is only there if the car is equipped with a/c and only operates if the a/c is switched on (otherwise this relay remains idle and in any event is not part of the engine cooling operation of the fans).

Marc, your response has confused me... CX's don't use a MAP sensor, they use an AFM. It's the big aluminium lump mounted onto the top of the air filter box.
Are you thinking of the turbo boost sensor, mounted up near the header tank? AFAICR This only feeds the boost gauge on the dash, doesn't do anything for the injection/ignition system.

220 TRX's were quite common fitment back in the day - there was a time when Avon only made them in 200 or 220, not 210 (and lots of people bought the Avon's because they were half the price of the Michelins - although they were terrible in comparison!)

[white exec]

The cooling fans are twin relay controlled for engine cooling - the third relay is only there if the car is equipped with a/c and only operates if the a/c is switched on (otherwise this relay remains idle and in any event is not part of the engine cooling operation of the fans).

Is this really correct?
For twin fans to operate at two speeds (series and parallel), three c/o relays are required; it can't be done with just two. The triple-relay installation was used across most of PSA's models of the era, with and without AC. (Single-fan cars had one relay and a series resistor to produce the two fan speeds.)
Switching on AC normally brings the fans on at slow speed, and this is done by an additional relay, operating the slow-speed feed for the 3 relays.

I may be wrong, of course. The wiring diagram will confirm.

[marc61]

Marc, your response has confused me... CX's don't use a MAP sensor, they use an AFM. It's the big aluminium lump mounted onto the top of the air filter box.
Are you thinking of the turbo boost sensor, mounted up near the header tank? AFAICR This only feeds the boost gauge on the dash, doesn't do anything for the injection/ignition system.

Yes the CX uses a MAP sensor (manifold air pressure) to work out engine load and the fuel/air ratio that’s needed - it’s located near the header tank and bulkhead. It uses an AFM (airflow meter) to measure the air flow rate.

[wurlycorner]

The cooling fans are twin relay controlled for engine cooling - the third relay is only there if the car is equipped with a/c and only operates if the a/c is switched on (otherwise this relay remains idle and in any event is not part of the engine cooling operation of the fans).

Is this really correct?
For twin fans to operate at two speeds (series and parallel), three c/o relays are required; it can't be done with just two. The triple-relay installation was used across most of PSA's models of the era, with and without AC. (Single-fan cars had one relay and a series resistor to produce the two fan speeds.)
Switching on AC normally brings the fans on at slow speed, and this is done by an additional relay, operating the slow-speed feed for the 3 relays.

I may be wrong, of course. The wiring diagram will confirm.

Yep, it's correct - one of the relays is a double switching relay (one set of simple open/close contacts, but the other is a two position contact set) according to the diagram.

[wurlycorner]

Yes the CX uses a MAP sensor (manifold air pressure) to work out engine load and the fuel/air ratio that’s needed - it’s located near the header tank and bulkhead. It uses an AFM (airflow meter) to measure the air flow rate.

LE Jetronic doesn't use a MAP sensor.

The "vacuum sensor" feeds into the AEI unit (which drives the turbo boost gauge). It can't affect fuel mixture though, because it doesn't feed that sort of data into the injection ECU.
The Citroen service manual describes the Vacuum sensor as operating as part of the ignition system and describes this affecting ignition advance (+22 degrees @-500mbar from atmospheric, -11 degrees @+500mbar from atmospheric)


The Vacuum sensor is not mentioned as part of the operation of the injection system, only the following components;
142 - ECU
192 - Throttle Spindle Switch
280 - Supplementary air control
302 - Flow meter
345 - Electrovalve
575 - Cold start injector
576 - Injector
683 - Electrical fuel pump
731 - Relay
841 - Coolant Temp Sensor
870 - Slow Thermal Switch
F - Fuel filter
R - Pressure regulator

280 is controlled by the Injection ECU, not the ignition ECU.
It then says (talking about the AFM) "The main characteristic of the device is to meter directly the air drawn in by the engine, which is the only reference for the fuel quantity to be injected".


MAF sensors didn't feature until LH Jetronic and MAP was never part of the L-Jetronic system (nb: I make no claim to know these last facts for a certainty, only just checked them online!!!)