Weird idling issue!

This is the Forum for all your Peugeot Technical Questions, Problems or Advice.

Moderator: RichardW

Post Reply
User avatar
LFY
Posts: 92
Joined: 20 Sep 2015, 11:14
Location: Iran..
My Cars: Peugeot 406 1.8 16V
x 3

Weird idling issue!

Post by LFY »

Hi everyone,

My car has a weird idling problem which is difficult to diagnose.

The engine itself has no problem (XU7JP4 LFY 1.8 16V Petrol). It has nice power, good consumption, smooth acceleration, etc.

The only problem is when you "LET OFF THE THROTTLE". When I release the throttle, RPM goes down gradually and smoothly but when it reaches below 1000 (approximately between 900 - 800) the engine vibrates and sometimes revs up to 1100 and back to 900 again. After that RPM drops to 800 and everything is normal. The idle itself is a little erratic and not very smooth.

This problem doesn't happen all the time but most of the time the car behaves this way when lifting off the throttle.

I should also add that sometimes (not always), there is a jerkiness when I release the throttle while moving.

I spent lots of time to cure the problem but couldn't solve it.

I suspected vacuum leak. I checked intake and exhaust manifold gaskets, throttle body gasket, brake booster hose, oil vapor hose gasoline vapor hose and they all seemed to have no leaks. I disconnected the whole throttle body from the car and tested it for leaks. I could find a little vacuum leak in throttle shaft seals. I fixed that leak but the problem still exists.

I suspected stepper motor and even replaced it with a brand new one but no improvement on the situation.

I suspected Throttle Position Sensor but according to voltage test it doesn't seem to be faulty.

This issue is not related to temperature because I discovered that it occurs whether the engine is cold (during start-up) or warm (operating temperature).

To help you understand the issue better I recorded a video when the car behaved that way.

In this video I first rev the engine to 2750 RPM and after that I release the throttle completely. If you watch it carefully you will notice self-revving of the engine between 00.07 and 00.09 in the video.

Here is the video:
https://www.mediafire.com/?sjnjjh9djcf1qq0


I also noticed something strange when I released the throttle. I disconnected the intake hose and started up the engine. Now I could see the throat of the throttle body and the tip of the stepper motor and the throttle butterfly. With the engine running, I opened the throttle a bit and released it to see how stepper motor performs while the engine returns back to idle. I saw that when RPM dropped to about 900, the tip of the stepper motor fluctuates back and forth and then after about 2 or 3 seconds it almost stabilizes and the engine revs back to normal idling RPM (which is 800).

I even recorded a video of stepper motor fluctuation! Maybe it helps to diagnose the problem.

Here is the video:
https://www.mediafire.com/?pfmphc3ib8hl1rc

You can see the stepper motor fluctuation in the above video (after I release the throttle) between 00.13 and 00.16 and also between 00.22 and 00.25.


This is my throttle body and the tip of stepper motor:


Image


I also noticed that the engine vibration happens exactly when the fluctuation of stepper motor happens! I know that stepper motor isn't faulty because I replaced it but it behaved like the previous one. What can cause the stepper motor to act like that? Vacuum leak? TPS issue?

Now I feel tired and helpless and I decided to replacing the whole throttle body to solve the problem. :(

But I thought that it is better to consult you before replacing any part.

Thanks in advance for your useful recommendations.
User avatar
LFY
Posts: 92
Joined: 20 Sep 2015, 11:14
Location: Iran..
My Cars: Peugeot 406 1.8 16V
x 3

Re: Weird idling issue!

Post by LFY »

It is disappointing that no one replied to this thread. :(

Update:
I changed throttle body and the problem almost solved because throttle shaft seals were leaking.


I now have only one problem. When I start the car at morning, idle fluctuates a bit (between 900 - 1000) but it happens only when the engine is cold. After about 10 to 15 minutes the idle becomes stable.


I made a video of the aforementioned idle fluctuation.


Video Clip:

https://www.mediafire.com/?7kzhw8u793b9g4a




If anyone could help me to solve it, I will be much appreciated.

It is very strange that it only happens for about 15 mins!

I suspected coolant temp. sensor but it passed my ohm test.

It could be stepper motor but i suspect because if it was, it should happen all the time regardless of temperature.

It is a temperature based problem.

I now suspect TPS, but I am not sure if it proves faulty only at certain temperatures.
Hell Razor5543
Donor 2023
Posts: 13727
Joined: 01 Apr 2012, 09:47
Location: Reading
My Cars: C5 Mk2 VTX+ estate.
x 2993

Re: Weird idling issue!

Post by Hell Razor5543 »

Sorry that nobody responded to the problem, but maybe you are only one of a few to experience it (lots have read the post though). I hope that somebody will help you completely resolve the issue.
James
ex BX 1.9
ex Xantia 2.0HDi SX
ex Xantia 2.0HDi LX
Ex C5 2.0HDi VTR
Ex C5 2.0HDi VTR

C5 2.2HDi VTX+
Yes, I am paranoid, but am I paranoid ENOUGH?
Out amongst the stars, looking for a world of my own!
Post Reply