1987 Citroen CX 2.5 gti non turbo - hesitancy over 35mph

[citroencxsteve64]

Hi all, first post. My CX is difficult to start in the morning but when I come back to it later it starts fine. The main issue though is that the car is fine up to 35 mph but after that it starts lurching and hesitating very noticeably. However when I kick down on the accelerator it hurtles along without any problems but as soon as I drop to around 35 mph it starts hesitating again if I go any faster under normal acceleration. I've changed the crankshaft sensor and found a leak in the air flow hoses at the top of the engine although there could be others but so far made no difference. Paid £130 to a garage who claimed it was sorted but made no difference. Any suggestions would be greatly appreciated!!!

Addition. Does anyone know where I can get a square air filter for this model. The current one is almost dust. Tried the usual suspects I e Ebay, Citroen classics, Franzose, ??

[xantia_v6]

which version of carburettor or fuel injection does it have?

[citroencxsteve64]

I think it's TRI if that makes any sense,

[PaulC5]

Does it do it in all gears at 35 mph so speed related and not rpm related ?

[wurlycorner]

The 25GTi has Bosch L-Jetronic (without cold-start injector) and separate AEI.

None of those symptoms sound crank sensor related, to me.

For the starting issue, as a first-off I would suggest smoke leak checking all the inlet system, operation of the supplementary air control, checking the fuel pressure and the engine temp sensors (the CX25 uses engine temp sensors to enrich the mixture via the normal injectors).
Haynes is fairly helpful for supplementary air control (Chapter 3, item 14) but not for the water temp sensor - better to try and resistance check that through the range as the engine goes from cold-warm (I *think* the oem Citroen service manual gives either static values or a graph?).
If looking at Haynes, don't forget that Chapter 13 also applies for your model, not just what's in Chapter 3.

[citroencxsteve64]

It changes up without any problem, should have mentioned it's an automatic.

[citroencxsteve64]

Thanks Wurleycorner, I'll have a flip through the chapters you mentioned .

[wurlycorner]

No worries.
The 25GTi is the classic Citroen I'm most familiar with tbh (though all the manual Turbo versions, in my case )

In many ways it's a pain to trouble shoot, because it's electronic control (and two separate systems are involved) but there is absolutely no self-diagnostic capability. In other ways, it's quite a simple system by modern standards, so there is less to trouble-shoot and everything is simple to check by resistance etc.

[citroencxsteve64]

Thanks, I've booked it in for a smoke test on Thursday.

[citroencxsteve64]

Afternoon all, the smoke test showed two small leaks which were tightened up and a new hose put in situ. There is a large leak around the oil filler cap. I've found an air filter from Tres Beau in Holland and ordered some other bits. Despite all of this the damn thing still has the same problem, fine up to about 35 mph then super jumpy but kick down if flies off. Think I'll test the injectors for resistance, does anyone know what the reading should be for this model? Other big question is apart from 4 hrs drive down south to find a citroen cx specialist, does anyone know where I can find one in the northwest of england?

[mickthemaverick]

You may find somewhere from this link: https://www.citroenet.org.uk/resources/ ... pairs.html

[citroencxsteve64]

Fantastic, thanks Mick

[wurlycorner]

The leak around the oil filler cap won’t be a problem.
Have you checked the other basics I mentioned? (Fuel pressure, water temp sensors)? Fuel pressure is page 62, section 15 in Haynes.

It would also be worth checking the AFM for smooth operation throughout its range, by manually operating it and measuring the resistance.

Also worth checking that the throttle position switch is correctly working when closed and partially open.
You should have one set of contacts open on idle and the other closed (supplying 12v to pin 2 on the ECU). Then in the middle you should have both contacts open and on fully open throttle, you have the other set of contacts closed (supplying 12v to on 3 on the ECU) and the other open. This is described in chapter 3 page 62.

From looking at the schematic, I *think* what happens on full throttle (“kick-down” as you describe ?) is the ECU might ‘ignore’ the AFM input and revert to standard map, as a result of pin 3 (input to the ECU) going high? If that’s correct , that might be a possible reason why you have this problem under ‘reasonable’ acceleration, but not on full throttle - a fault with the AFM?

There is also apparently an ignition vacuum advance capsule (item 49 on page 290 / item 3 on page 239) which may also be relevant in this scenario, but I have no experience with that.

If there is anything you’d like to me to test on one of my GTi Turbos, I can try and get over to my barn and confirm tomorrow, if you give me a list - bearing in mind I won’t be able to start it, but can test things statically for you.

As for anywhere up north to ask for advice… isn’t Stan Platts up that way?