lumpy idle of Xm

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MAKO
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lumpy idle of Xm

Post by MAKO »

Hi, all.

I noticed the Xm had a lumpy idle a few months ago. cleaned the ICV and throttle butterfly.
the idle got better, but not completely. still a faint fluctuation around 700rpm and sometimes hesitates on acceleration from start.
I hooked it up to a lexia while idling. it's 125k miles on it.

Image
it recognizes the idle and butterfly angle is 0.0. but the angle reading shows 0.3 now and then, the volts also fluctuate in line with the angle.

I replaced the TPS with genuine BOSCH one. fortunately all symptoms have gone.
just for my information, checked with Lexia again.
angle reading's fluctuation were still there, less frequently seemingly.

is this butterfly angle readings' behavior usual insignificant one?

thank you in advance.

Shin
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Re: lumpy idle of Xm

Post by CitroJim »

Shin, this has got to be one for Mandrake :-D

I've never experienced an idle problem with the ES9 engine but a bit of slight surging at idle when hot is entirely normal; it should however pull cleanly away when the throttle is pressed.

Manifold pressure looks normal. Is the oxygen sensor cycling as expected? (i.e. going from around 0050mV to 850mV over the course of a couple of seconds)..

Did you calibrate the TPS after replacement? All you need to do is switch on the ignition with your feet clear of the pedals and wait ten seconds before start. I think you did as the Lexia shows idle but it's worth doing it again...

Mandrake will be along with lots more soon I expect :wink:
Jim

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Re: lumpy idle of Xm

Post by MAKO »

Thank you for your concern, Jim.

yes, I think O2 sensor is working as it should, cycling between 100 and 850.

and initilialized the idling at first engine start.
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Re: lumpy idle of Xm

Post by Mandrake »

Let me get this straight - your symptoms have all gone when replacing the TPS, it's just the Lexia readings your concerned about ?

I wouldn't worry about the throttle butterfly intermittently reading 0.3 degrees at idle instead of 0 degrees - the voltage from the TPS is digitised and the A/D resolution is such that 0.3 degrees corresponds to the least significant bit, so an intermittent toggle from 0 to 0.3 degrees is just +/- 1 significant digit uncertainty during A/D sampling.

When I replaced my TPS recently it does the same thing - it just means that the voltage at idle is straddling at the threshold between two digital values. Try lifting the accelerator pedal with your foot and I bet the reading drops to 0.0.

I do see two minor things of possible concern on your Lexia readout however - the first is your air intake temperature reading. I looked up Kobe, Japan and you're currently getting highs of around 15 degrees ambient so 57 degrees is awfully high. Are you sure you reconnected the air temperature sensor on the air filter box ?

A hot engine bay and air ducting will warm up the air considerably as it passes through the ducting to reach the throttle butterfly - up to 20-25 degrees increase is typical, but an increase from 15 to 57 degrees seems a lot. I'm not familiar with the XM to know whether it has the same layout and configuration of air intake ducting and air filter box as the Xantia (CitroJim would know as he has both) but it would be worth checking the air intake sensor is working properly.

One way you could check it is to connect the Lexia first thing on a cold morning without starting the engine - by then it should have cooled down to ambient temperature so your coolant temperature sensor and air temperature sensor should both read ambient within a few degrees.

The other thing that doesn't look right is an idle speed of 720 rpm - this should be 640 rpm indicated (actually 650 rpm, but the Lexia reading cannot read between 640 and 680 rpm) in the following conditions:

* Engine warmed up
* No accessory load (headlights, blower fan, radiator fans, A/C all off)
* Steering wheel not being turned. (idle speed increases when the power steering pressure increases)

What you want to check is "Idle OCR %" which is on one of the other parameters measurement screens. (Injection I think)

You should see a reading of about 30-35% under the following conditions:

* Engine warmed up
* No accessory load (headlights, blower fan, radiator fans, A/C all off)
* Steering wheel not being turned.
* Gearbox in Neutral

If the reading is below about 25% this means the ICV is more or less fully closed - if the idle speed is still 720 rpm with the ICV well below 30% you either have a vacuum leak or the ICV isn't working properly.

Because the ICV is fully closed the idle speed will fluctuate slightly as the ECU is unable to regulate the air flow to compensate for small idle speed changes. To check whether its a faulty ICV disconnect the hose from the ICV to the air filter box and block it - if the rpm drops well below 640 rpm the ICV is not closing properly when commanded, if its still too high at 720 rpm you have a vacuum leak elsewhere.

Cleaning a dirty throttle plate can drop the Idle OCR % reading by as much as 5% as the (deliberate) air leakage past the throttle plate increases. A dirty throttle plate can disguise the symptoms of a vacuum leak.

I think I have a small vacuum leak somewhere on mine - ever since I cleaned the throttle plate my Idle OCR reading is about 26% in the above stated conditions and although idle is correct at 640 rpm it is only just able to achieve it - blocking the ICV hose only drops rpm to 560 rpm, when it should be almost stalling.

Apart from intake manifold gasket leaks a couple of possible sources of vacuum leak are a leak in the PCV system (the left hand lower hose on the front of the throttle butterfly) or the charcoal canister purge line. (bottom right hand hose on the front of the throttle butterfly) I haven't had a chance to investigate further on mine to see where the leakage is coming from.
Simon

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Re: lumpy idle of Xm

Post by MAKO »

Simon,

I really appreciate your thorough consideration and sorry for my incomplete presentation.

yes I am wondering whether this throttle value's behavior (between 0.0←→0.3) is significant or not.

relieved to hear you also have the same experience about throttle value on Lexia, and disappointed to know Lexia didn't help me, though. I really struggled to set it up on my PC. sounds like it needs a lot of experience and skills to use.
Anyway, I see "0.3" is negligible.

about air intake temperature, this value is with a new aftermarket sensor which I installed just before the TPS. I even replaced MAP sensor both of which didn't cure the symptom.

the idle rpm is basically 640 but switched to 720 when I felt "lumpy". The picture was taken before TPS was replaced.

I will check the temperature in air intake again in the morning.

Thanks a lot.

Shin
'00 citroen xm 3.0l late 24v (rp08599) black
MAKO
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Re: lumpy idle of Xm

Post by MAKO »

Hi,

I have rechecked the temperature by lexia.
the ambient temperature was around 14 degrees,

 before starting engine, air intake temperature was 20 degrees, coolant was 15. might be a error? looks like it tends to read a little higher than it should, indeed.

while driving the town, the intake air was about 30 degrees, the coolant was on their 80's. off course, I only monitored during parking.

I can't explain about the big discrepancy between the intake air sensor and the ambient sensor last time. The sock in front of the air box cleaner was fine.

idle OCR was 50% before the engine starts, then decreased to about 35% gradually.
'00 citroen xm 3.0l late 24v (rp08599) black
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