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Citroën BX – Small air leak causes idling woes!

3 min read

At the end of last year, I set about resolving some of the wiring niggles caused by the engine bay sensor harness. This meant taking the inlet manifold off the car amongst other things. Since then the engine idling and low-speed drive away on the Citroën BX have been terrible. Has the idle control valve failed or do I have an air leak?

Undriveable car

Since pulling the car apart to repair the faulty wiring, driving the BX has been a nightmare. Most notably the car would stall when braking slowly up to lights. Usually, there would be enough time to restart the engine before the light changed, but not always. Not exactly ideal.

With the issue having occurred since carrying out the electrical work, I can assume I have disturbed something in the inlet. Or maybe I have something wrong with the wiring. More than likely there is an air leak causing my idling problems.

Idle Speed Control Valve

I found after some tinkering that disconnecting the Idle Speed Control Valve (ISCV) would hold the idle revs higher and stop the stalling. This is as expected, but I took it off the car, flushed it, and checked its operation on the bench.

Idle speed control valve in open position.
Idle speed control valve in open position.

The valve opened and closed quickly and the rotation was free. The valve is normally supplied by the Motronic ECU with an oscillating pulse to control the opening amount with reasonable precision. Applying 12v is a good test that the body will rotate, but not always a guarantee of operation.

Confirming the problem was an air leak

I checked with the oracles (Doc and Mat) who both pointed out that the description of my problem meant there was an air leak. Probably between the airflow meter and the throttle body. And perhaps the throttle body wasn’t fully closing.

At the time I took a cursory glance, wiggled some pipes, checked hose clamps for tightness by hand and found . . . nothing. I’ve been putting up with it, the performance was fine away from idle and I’ve become too used to slow acceleration pick up in the Freelander.

Cure the air leak, cure the idling

The band is not quite right, does it allow an air leak?
The band is not quite right, does it allow an air leak?

While investigating the cause of another leak [READ MORE] I noticed something peculiar. The band clamp on the throttle body end of the air feed pipe from the airflow meter simply didn’t look right!

The crudely drawn red line in the picture above shows where the edge of the clamp should be. It still felt tight, but it really is in the wrong place. On the opposite side of the hose from where we can see in the picture, the clamp was completely off the hose! Was this the simple route cause of the problem?

Back to idling normally

Well, I’m happy to report that after taking the hose out and repositioning both the hose, a silicon hose from BakerBM, and putting the clamp in the right place, to full tightness, the idle performance is pretty much spot on.

I’ve no idea how I missed something so clearly wrong for so long. Looking at some older pictures of the engine bay, I think the clamp was in the right place originally and simply not tight enough. I’ll certainly be paying more attention in future.

I do love a quick fix, and can’t wait to take the car out for another drive.

M

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