CX 25 ignition cut-out

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pete_wood_uk
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CX 25 ignition cut-out

Post by pete_wood_uk »

I'm struggling to track down what I think is a problem with the ignition in my 87 CX 25GTi, and wondered if anyone had any similar experiences? Having lurked on this board for a while (lots of useful stuff for my Xantia, thanks!) I've finally signed up...
The symptoms seem to be that the car runs fine from cold, and will run about 3-4 miles down the road, until it's approaching operating temperature (according to the dashboard water temp gauge). It then cuts out, two or three times, for maybe a second each time, then picks up and carries on. Runs fine for the rest of the journey. This is disturbing, to say the least! This is quite repeatable - well, 4 days out of 4 so far this week, I can predict to the nearest mile where the engine is going to cut.
When it cuts, obviously the overrun keeps the engine spinning (at that stage in the jorney I'm on open roads), and so when it comes back to life it picks up and carries on. Now, as it cuts, I've noticed that the rev counter falls abruptly to zero, springing back up (to 2500 or whatever) as it starts again.
As far as I can see from the wiring diagram, the rev counter is driven from the AEI unit, which is driven from the flywheel sensor (just one in my case). The flywheel sensors are notorious. But....
I understand that the usual failure mode of the flywheel sensor is to go open circuit above some temperature (that's what my last one did). However, in this case, this is just a momentary problem and then it carries on. To be sure, I took the sensor into my office and measured its resistance over temperature from 5C to 70C, and there's no glitches. That's not conclusive, of course, I can't vibrate it in the same way that the engine can, but it seems unlikely that the sensor is at fault.
I can't see how the AEI unit would fail in this way - "normally" electronics fail hard as they warm - a glitch at a certain temperature doesn't sound too plausible.
I've cleaned the contacts that I can see in the ignition loom, and it doesn't seem to have made any difference.
I'd say it smelt like an "injection" problem, maybe some glitch in mixture control as the engine reached full temperature, except for this symptom of the rev counter going to zero. OK, it's an auto-box, so the revs will fall a bit as the engine dies, but not to zero, I think...
I'm stumped. Does anyone have any useful experience that they could share?
Many thanks
Pete
rg
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Post by rg »

Pete,
Have you tried posting on the Yahoo! groups CX forum?
rg
pete_wood_uk
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Post by pete_wood_uk »

No - didn't know there was one! I'll have a go there as well, if I can find it, thanks!
Pete
DoubleChevron
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Post by DoubleChevron »

Due to the random nature of this, it sounds like a dirty connection somewhere.
With the car idling will the motor stop if you gently grab the wiring harnesses and move them around ??
My first port of call would be the flywheels sensors, on an earlier 2sensor car, I think the pulse for the tacho is taken directly from one of these sensors. Check and clean there connections. These can be tested, they should be somewhere between 70 and 100ohms. Pull them out and boil them in some water, if they suddenly go open circuit as they heat up they need replacing (otherwise the car will cuttout whenever it gets very hot under the bonnet).
First thing though pull the spare wheel out, unbolt the cover it sits on. You will now find 2big multi plugs under this, and another two over next to the inlet funnel for the demister. Pull these plugs apart and recrimp all the female ends and using needle files clean up all the connector surfaces.
good luck (you could say I've been here and done this [:o)] ).
seeya,
Shane L.
pete_wood_uk
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Post by pete_wood_uk »

It's not entirely random, that's the trouble. After it does its little trick 3-4 miles after a cold start, it's then fine - I can do 30 miles or more, including baking it in town and getting it thoroughly hot, with no further hassle. So I'm thinking it's got to be something temperature related - which is why I baked the flywheel sensor across the likely range of temperatures. Measured a solid 65-80 ohms. If you get a coldish morning (ie 5C, this April morning), it'll go about a mile further than it did yesterday (15C) before it coughs and splutters and cuts. Once it's done it, it's fine.
I've wiggled every wire and connector in the ignition loom, to no effect. I've cleaned and remade most of the contacts. The AEI system plugs are now remarkably shiny [:)] I thought for a long time that it must be a loose wire, but as it becomes apparent over a few days' use that it's non-random, I'm changing my mind.
The annoying thing is that it's not a "hard" fault - it only does it transiently, so I can't get the multimeter out with the car in the "broken" state.
I suppose one thing that would give these symptoms is a complete power loss to the AEI unit? Does anyone know if there's a feedback path from the injection unit to the AEI or the AEI power? I'm just wondering if the injection is dithering about something as the car reaches temperature (as I mentioned in my first mail), and as a result of that dithering is panicking and turning things off. Just speculation at this point, I haven't touched the injection loom, believing the fault to be ignition-related...
Cheers
Pete
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AndersDK
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Post by AndersDK »

Pete -
Check your battery !
It's known taht if the battery voltage drops down near the 11.5V region, the ECU's tends to do oddies & weirdies.
When engine has run a while, the battery voltage would be topped up from the starter cranking load ...
rg
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Post by rg »

bxbodger
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Post by bxbodger »

It could possibly be an intermittent HT tracking problem; these can often be temperature related as your problem appears to be.Have you made a close visual check of the HT system,leads, etc?,have a very close look in bright light for any carbon tracks and its also worth looking in total darkness as well for any stray HT sparks.
DoubleChevron
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Post by DoubleChevron »

Hi Pete,
you'll need to be methodical and check every likely suspect.
-Start with the earth point above the battery, pull it off, clean it up with a wire brush, file all the terminals that screw onto it until there shiny.
-Pull back the boots on all connectors that may cause problems, eg: it'll run rich as hell and threaten to stall if the air flow sensor connector isn't good. Clean the connectors with needle files. The reason you pull back the rubber boots on the back of the connectors is to see if the pins have pushed back as opposed to pressed onto there connectors. I found my CX actually didn't have the full load switch on the accelerator spindle connected (it was pushed back into the boot).
-Pull back the boots on the injection computer above the battery and pull both connectors of the computer and clean them.
-Does the fuel pump still run when the car stops ??? Clean big blue, the relay under the right front headlight. This relay is critical to the running of the car and is usually the cause of many problems. It also supplys/receives signals from the injection and ignition computers to keep the car running. Next to big blue you will find a standard relay, this is the power supply relay for the injectors themselves. Clean the contacts on this relay.
Have you considered that this may not be electrical ?? Check all the hoses from the air-flow meter to the induction manifold. Is the car turbocharged ?? If so check the hoses down to the blower and across the the intercooler. If there is big problems with this circuit you will find the car will idle perfectly, but stall the instant you touch the accelerator.
--Fuel pressure ?? Maybe your fuel pressure regualator is on it's last legs (have you changed the fuel filter... has the fuel filter EVER been changed in the cars life ??). If you think it's possibly the fuel pressure (I have had problems with the fuel regulator on my car) install a fuel pressure guage in the fuel rail, then next time the car stops jump out and see what pressure it reads.
You have a lot more things to check yet [:o)]
seeya,
Shane L.
pete_wood_uk
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Post by pete_wood_uk »

Just thought I'd add an update (sadly not a postscript) to this. Been on holiday for 2 weeks, but while I was away a new ignition sensor arrived, so I plugged that in last night and the car started from cold and warmed up til fully hot without missing a beat. Excellent.
However, this morning, start from cold, set off down the road, about 4 miles in, exactly as before, it cut out for a couple of seconds, then continued flawlessly. Damn.
So it wasn't the sensor. This morning's cut was long enough for me to see that the rev counter falls to zero, then flutters feebly between zero and a few hundred RPM, before the car comes back to life. All this, of course, with the car in motion and the engine being turned over just fine by the momentum. I'm wondering if it's a power cut to the system? Next step might be to run a lead from the AEI power feed back to the cockpit with an LED on it, or something (I'm sure a few mA won't screw anything up) so that when it cuts at the same place tomorrow morning, I can see if power has been cut to the AEI unit?
Cheers
Pete
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