Xantia – Engine hesitation

[lancia58]

Recently I have a phenomena that when I driving at very low speeds ( 12 MPH ) the engine seems to hesitate when the gas pedal is pressed. It feels like the engine is not responding. Keeping pressing the gas pedal suddenly causes the engine to respond and fast acceleration. This phenomena occurs from time to time and not very often. Any one has an idea what can be the reason/s for this phenomena.

The car is 2.0 16v petrol with automatic gear ( AL4 )


Thanks
Zohar

[Mandrake]

MAP sensor perhaps ? While working on my V6 the other day I had disconnected the MAP sensor and forgotten to reconnect it, when I went for a test drive the symptoms were similar to what you describe, accelerating would initially have a huge flat spot then it would suddenly accelerate away. It also wouldn't idle smoothly.

Quite interesting that the car drove at all, I was under the impression that the MAP sensor data was essential for the engine to run in any sort of usable fashion, yet it was still relatively drivable, just suffering from delay/hesitation when applying throttle and not idling smoothly. Power during constant acceleration (steady throttle) was relatively normal, so it seems the MAP sensor data is more important when the throttle is varied quickly.

If that's the case with a MAP sensor completely disconnected a partially faulty one may cause the symptoms you describe. Can anyone else confirm a MAP sensor as the most likely cause of the symptoms ?

[CitroJim]

I'd agree Simon, the MAP sensor is a good initial bet, followed by the oxygen sensor...

Zohar, didn't you ask about the oxygen sensor signal in a recent post?

[lancia58]

I switched the Oxygen sensor 4 month ago, and the described phenomena started 2 weeks ago so I am assuming it is not the Oxygen sensor. Given the phenomena occurs from time to time and not very often, is the MAP sensor still a good candidate ?


Regards
Zohar

[Mandrake]

I would say so. A minor intermittent problem like this is unlikely to log any fault codes, so the only way to tell for sure is probably to swap the MAP sensor unfortunately. I suspect my MAP sensor is also not up to snuff, now that I've seen the symptoms of it missing entirely I do think the occasional hesitation I experience similar to yours at low revs could be my MAP sensor, however I am going to replace my known faulty spark plug leads first and only replace the MAP sensor if the hesitation is still there...

[CitroJim]

Don't believe the oxygen sensor is good Zohar. Chris570 and I witnessed a brand-new one die very quickly in his V6 the other day. I'm not sure if his was a 'good' make or a cheap ebay job.

I have a really cheap ebay one in my XM which seems OK but I'm not expecting a long life from it.

My daughter's old 206 used to get through them too...

But if you find the oxygen sensor good then the MAP sensor remains a good candidate...

[Chris570]

Don't believe the oxygen sensor is good Zohar. Chris570 and I witnessed a brand-new one die very quickly in his V6 the other day. I'm not sure if his was a 'good' make or a cheap ebay job.

I have a really cheap ebay one in my XM which seems OK but I'm not expecting a long life from it.

My daughter's old 206 used to get through them too...

But if you find the oxygen sensor good then the MAP sensor remains a good candidate...

Actually mine was a genuine (boxed) bosch one. not from ebay either

[CitroJim]

Actually mine was a genuine (boxed) bosch one. not from ebay either

Bloody hell and there was me thinking it came from Pikey's Motor Spares

[Mandrake]

One way you can rule out the oxygen sensor Zohar, is to temporarily disconnect it!

I've had mine disconnected for a few weeks now as I suspect the sensor has become sluggish and inaccurate (the common failure mode apparently) and the engine runs a lot better without it including a lot less hesitation when accelerating.

The ECU has a fallback mode that it switches into when the oxygen sensor is deemed faulty (due to being disconnected) such that the engine always runs in open loop mode. Control of the mixture when idling and with low throttle is not quite as accurate as having a good oxygen sensor connected, and may not pass the lambda test at an MOT, but it will be better than having a faulty sensor connected.

So try disconnecting your oxygen sensor for a week or two and see what difference you notice with the hesitation. (Remember to tie the unplugged cable to something so it doesn't fall onto the exhaust and melt...)

[Chris570]

Actually mine was a genuine (boxed) bosch one. not from ebay either

Bloody hell and there was me thinking it came from Pikey's Motor Spares

hence my language

[Mandrake]

Don't believe the oxygen sensor is good Zohar. Chris570 and I witnessed a brand-new one die very quickly in his V6 the other day. I'm not sure if his was a 'good' make or a cheap ebay job.

What were the symptoms that led you to believe it had died again ? And for that matter what were the symptoms that led to replacing the original one...?

[CitroJim]

What were the symptoms that lead you to believe it had died ?

ELIT diagnostics showed a refusal to enter closed-loop mode, static (450mV) sensor output (which is what's seen if the sensor is disconnected) and a permanent oxygen sensor fault.

Not really hard to believe it's thrown in the towel when presented with that little lot...

[handyman]

Faulty ADI and loose, worn or incorrectly gaped sparkplugs give very similar symptoms. I have also found iffy coil packs or worn connectors between coil pack and plug can also give the same symptoms.

H

[Mandrake]

ADI ?

Will be interesting to see if the intermittent hesitation is gone when I change my rear plug leads then...

[CitroJim]

ADI ?

Approved Driving Instructor...

No, it's a system on some 16v engines that changes the inlet manifold characteristics via vacuum operated butterflies in a dynamic way.

The V6 doesn't have this Simon. Not as such anyway. It has a similar system but it's entirely passive.