Never trust a French fuel gauge / Why my GTi won't start?

[Melchior]

Hello boys and girls,

As you can probably figure out from the thread title, I have had a problem which I think and hope is of a petroleum distillatilly challenged nature. Allow me to paint a little picture...

Not long after I got the car, I drove from Amsterdam to Brussels. I was a little nervous about the journey anyway as I didn't really know the car and it's old and French. It went fine until around an hour in when the engine started to cut out. On part throttle it would stutter quite violently and then cut totally. On WOT it would accelerate normally. This fun was accompanied by the orange K shape engine management light. SWMBO was due at a business meeting so I was papping it a bit, but the old Frenchie got us there safely.

There is some distinctly odd wiring going on around the coil area. From the loom to the coil has without doubt been replaced at some point. Looks like the loom was cut and a new coil multiplug spliced in. My first thoughts were that the splicing was dodgy but on closer inspection it is shady but firmly connected. Seconds thoughts include suspect injector multiplugs on right side front wing area. Again, on closer inspection they seem ok.

How does this blathering relate to fuel gauge shenanigans, I hear you ask. Good question and one I asked myself last night as the engine tried several times to cut out at low speed. Unlike the Brussels incident, this was in local roads at 50km/h in 4th or 5th gear but under power. Dip the clutch and it would rev normally but try to apply that power to the wheels and forget it. Two or three times we stopped, engine either cut out (and restarted successfully) or carried on running at idle but not under power.

Fuel gauge at this point was reading anywhere between ¼ and R but never lower.

So, before I start faffing around TOO much, have you guys and girls got any tips about funky petrol gauges? Igonring for a minute the motorway cutting-out, do you reckon I really simply ran outta gas this time?!

Seems a lot of words for such a simple question but hey, everyone likes a good yarn on a Friday evening, right?!

Thanks for any thoughts!
M

[Way2go]

Throttle position sensor fault?

[Melchior]

Hmm, not sure about that. Do you know of a way to test it with a multimeter?
Maybe it's worth mentioning that the idle is mostly fine, but sometimes blatantly too low on a cold start and, once warm/hot, drops low after revving a bit. Not so low that it almost stalls, but obviously below idle speed. It quickly picks back up to idle speed though.

[mat_fenwick]

I think, but I don't know for certain, that the TPS is just an on/off switch to detect a closed throttle on the overrun to cut fuel. If the ECU thinks the throttle is closed when it is not, it may cut fuel and cause cutting out. If I'm right then it should be simple to check with a meter.

I doubt you are running out of fuel, but I have had a sender that read falsely higher than it should because the small hole in the bottom of the sender unit (to restrict the flow of fuel to the float and damp the needle movement) had become almost completely blocked. Hence the float was sitting higher than it should, although would drop down to a correct level after a few days.

[Melchior]

Thanks Matt (again)

As I said, I know next to sod-all about these engines. But, if the TPS was an on/off switch, would it even idle? I mean, if the switch was buggered, it would read "off" or "on" all the time, right? Then I'd expect it not to run at all, or to give bit of pop/bang on overrun. Neither of these cases is true which makes me strongly suspect a dodgy fuel level sender/gauge.

The hole in sender info is great, I'd never considered how the gauges are steadied and it makes total sense. No reason it isn't blocked I suppose but (and I should have mentioned it earlier) but the gauge was hovering between 1/4 and R for the last few KM before it cut out at home. The K shape engine management light coming on makes me wonder as well. Would that light under a low/no fuel state? (low/no fuel pressure causing ECU error maybe?)

[mat_fenwick]

It might be better to wait until someone who knows for sure comes along, but IF it's just a simple on/off switch then there would only be some circumstances in which the ECU would cut the fuel. I.e. at low revs, throttle closed you'd still keep the injection pulses going to avoid snatchiness in traffic. TPS stickiness (rather than complete failure in one position) would be more likely to show itself under transient conditions rather than steady state running.

But this is general info rather than BX specific - do you know whether yours is Jetronic or Motronic? The K light would make me suspect fuel rather than ignition if it is Jetronic, as that just controls the fuelling. Don't know if low fuel pressure would trigger the light but if it really was low on fuel, why would it continue to run OK afterwards?

[Melchior]

No idea if Jet or Mo tronic. If someone can tell me how to figure that out, I can tell you. Now you mention it I should add extra info - the K light was coming on during the Brussels incident and that certainly wasn't low/no fuel. I am 95% sure it was the suspect wiring on the LV side of the coil as it stopped after faffing with that.

It then drove a good 5-6km after said faffing without any incident. It was only after a little more town driving did the cutting out appear - this makes me think it is fuel - I know it's [very] low.

if it really was low on fuel, why would it continue to run OK afterwards?

Good question but, in the end, it didn't run OK. Thank goodness it stopped running OK almost right outside our flat! Refused to restart after coasting past many full parking spaces, we had to push it into a free gap. After pushing, it started again so I could straighten it up, but then it only ran for 10-15 secs before I turned the ign off.

My money's on jungle juice. Perhaps coupled with LV coil related wiring dodgyness. It's 1am here so I should get some kip - I want to go get some petrol for it in the morning and try again.

[RobC]

I had two very similar sounding incidents caused by two very different faults: the first time it was a clogged fuel filter (found by the rear wheel next to the fuel pump) that made fuel flow, particularly on a low tank, very sporadic. The second time almost exactly the same symptoms were caused by a failing ignition module. Might be worth checking these if you haven't already. The fuel filter is meant to be changed every 50,000 miles or so, but often gets forgotten since its so hidden away.

Good luck!

[Melchior]

Thanks for all your replies and ideas. I am happy to say that she lives again - begrudgingly started up this morning and made it to the petrol station which thankfully is very close to home. Stuck 20 litres in and off I went. All fine! Although I am getting a very mild miss through the rev range (above 3500 or so, hard to tell as I think the rev counter is inaccurate, of course) and a very weak top end. It 'feels' like fuelling to me and so suspect the fuel filter like RobC mentioned. Although of course it could also be a weak spark... hmmm

[Way2go]

IF it's just a simple on/off switch then there would only be some circumstances in which the ECU would cut the fuel. I.e. at low revs, throttle closed you'd still keep the injection pulses going to avoid snatchiness in traffic. TPS stickiness (rather than complete failure in one position) would be more likely to show itself under transient conditions rather than steady state running.

The injectors operate with a variety of spray patterns and duty cycles dependent upon engine speed/demand under the control of the ECU.
The ECU uses sensors to assess the engine needs, which principally must come from the TPS and the MAP sensors in the case of the Motronic 3.1. Jetronic uses a flow-meter/potentiometer set-up in the airbox along with the TPS.

The injectors can get semi gummed up too as mentioned before and require servicing. Because of spray pattern optimisation, duty cycle and peak flow considerations, the fact that the injectors are working does not mean they are working efficiently. The pintle caps also can develop a carbon build up which can choke the flow.

[Melchior]

failing ignition module.

Funny you should say that! I mentioned in my last post about a mild cutting out around 3500rpm. It's got worse and is now like a very lumpy and rubbish rev limiter at around those revs.

Even in neutral it won't rev higher than that, just coughs and splutters. Idle and up to those revs is pretty much fine.

Not sure if related but cold starting is sometimes a bit iffy too. It starts but idles badly with the orange "K" ECU lamp on. After a short time, maybe 30 secs, it idles normally and the lamp goes out.

Any ideas?

[Melchior]

After some internet bashing, I now wonder if it could be any of these:

Coil pack
Ignition amplifier (does it have one? I found conflicting nonsense threads all over the place)
Fuel pressure (regulator, pump, filter)

Also can someone tell me how to know whether it's a Jetronic or Motronic system? What to look for etc?

edit: it must be Motronic, at least according to this page. http://www.citroencarclub.org.uk/drupal/node/10" onclick="window.open(this.href);return false;

[RobC]

Isn't 'ignition amplifier' just another name for 'ignition module'?

In my experience the cheap aftermarket ignition modules are crap and have had a tendency to cut out at the rev range you describe. I'd recommend Bosch all the way (if you can find one...)

[Melchior]

Err, I don't know! From what I can see, my BX has what I would describe as and think is totally to be expected:

Low voltage wires from loom go to the coil.
1 HT lead to distributor
4 HT leads to each spark plug.

I see no other gubbins, so where's the ignition amplifier/module?

I'm assuming the distributor is mechanical? Could I be simply looking at a new rotor arm and dizzy cap?

[Way2go]

The ignition amplifier lives under the inlet manifold.

Motronic 3.1 does not have a distributor because it operates on wasted spark ignition.

Jetronic uses a distributor with a conventional rotor arm.