Fast idle fault

[MikeT]

I've read a lot of threads here concerning this but some advice conflicts so here's how I understand it, please feel free to correct me.

The fast idle system is controlled by the ECU via a solenoid utilising vacuum from the camshaft drive. It's default is operative (fast idle) and achieves a fast idle by advancing the timing (5 degrees) to provide higher revs until the engine reaches a certain temperature. It is also activated if the air con compressor is active to ensure the extra load does not cause a stalled engine.

Regarding my xantia, it has always idled at 1100 rpm and I've not been overly concerned for two reasons.

1. I (wrongly) assumed the fast idle operation was similar to a petrol-type choke/accelerator advance and 2. It made for a smoother transition between on and off accelerator pedal [which is at best rather unpleasant (probably due to worn engine/gearbox mounts?)].

But now I know it's actually a timing advance, I am concerned it's permanent. Some say it's cancelled when the accelerator is pressed but I can't see evidence of this. How does this affect the engine in normal running?

[CitroJim]

Hi Mike,

No, no and thrice NO, the fast idle is not done by advancing the timing. Let us quash that myth right now. That applies to both the Bosch and Lucas pumps.

It is done by slightly "opening the throttle". Recall when we looked at the dismantled pump the fast-idle lever on the back of the pump operates, indirectly, the same lever as the main throttle.

The only timing adjustment is via the timing servo on the underside of the pump under ECU control.

Idle speed adjustments is accomplised by adjustment of the throttle lever stop screw. There are two, the one facing you is for maximum speed (i.e. Maximum throttle) and a screw on the rear of the pump adjusts the idle.

I expect your fast idle is actually caused by a stuck electrovalve/vacuum canister. Don't try adjusting the throttle stop screw until you've checked the vacuum/electrical side of the fast idle system.

[CitroJim]

I should have added that the timing is a little advanced when the engine is cold but this is set by the ECU under guidance from the state of the coolant temperature sensor. The ECU consults its "Cold Map" when the coolant is below 60C.

Cold adavance is beneficial as it takes longer to get the diesel (or veg) burning when everything is cold so starting everything a little earlier gets it burning just at the right time. In fact I think a little more advance being applied when burning veg may well be beneficial. The shame is, it cannot be adjusted on a semi-electronic pump.

[MikeT]

Hi Mike,

No, no and thrice NO, the fast idle is not done by advancing the timing. Let us quash that myth right now. That applies to both the Bosch and Lucas pumps.

It is done by slightly "opening the throttle".

Hi Jim (back online now thanks to kind sis and bil)....So I was right in my first assumption, the several threads I read last night must all be wrong.

Recall when we looked at the dismantled pump the fast-idle lever on the back of the pump operates, indirectly, the same lever as the main throttle.

The only timing adjustment is via the timing servo on the underside of the pump under ECU control.

Yeah but.... recall I told you anything to do with "timing" confuses the hell out of me And now we've got fuel pump timing added into the fold my mind goes blank

Idle speed adjustments is accomplised by adjustment of the throttle lever stop screw. There are two, the one facing you is for maximum speed (i.e. Maximum throttle) and a screw on the rear of the pump adjusts the idle.

Yes, I understood that from BOL but was reluctant to make any such adjustments as I also know there is this other system (via the solenoid/vacuum).

I expect your fast idle is actually caused by a stuck electrovalve/vacuum canister. Don't try adjusting the throttle stop screw until you've checked the vacuum/electrical side of the fast idle system.

Is this valve/canister what I call the solenoid? The one next to the EGR solenoid? or are you referring to another part of the system and how do you check for sticking?

[CitroJim]

Timing, a quick lesson Mike

To get best engine performance it is necessary, be it petrol or diesel, to get the flammable charge burning at the right time. Ideally, you need the charge to be getting to the peak of its burn as the piston just passes top-dead-centre (TDC). Start the fire too early and the piston has to push against a rapidly expanding and burning charge, causing knocking and loss of power or start it burning too late and the piston is too far past TDC for the charge to do much useful work and most goes out the exhaust valve still burning. The latter is known as retarded timing. Hence why a badly retarded engine can run a red-hot exhaust manifold.

Now, fuel takes a finite time to start burning. In a diesel the period between injection and the fire lighting is known as the injection (or ignition) delay and is measured in milliseconds. So, to ensure that the fuel is burning well at the right time it is injected a little before the piston reaches TDC. The point at which it is injected is measured in crankshaft degrees before TDC. This is known as the ignition advance. Ignition timing might be quoted as say, 10 degrees BTDC at 1500rpm.

The injection delay is a constant but engine speed is not so as the engine speeds up, the injection has to take place earlier to ensure the fuel is still burning optimally at the right time. The injection is advanced with increasing engine speed. Cold diesel takes longer to get burning, hence cold-advance.

In your pump, timing is under control of the ECU totally. It looks at how fast the engine is running , how far the throttle is open and how hot the coolant is. Using this information, it looks up in its table of values (its map) the optimal ignition advance for that particular set of conditions and sets the pump timing accordingly on the servo. It uses the injector lift sensor signal to confirm the timing it has set is correct and adjusts if necessary. This part works in a feedback loop.

To test the fast idle: Fast idle is achieved by pulling on the cable and thus moving the lever toward the rear of the pump. The cable is attached to a vacuum diaphragm (on the end of the cable in a circular housing attached to the inlet manifold). When vacuum is present on one side of the diaphragm, the cable will pull and increase the idle. Vacuum to the diaphragm is controlled by the electrovalve next to the EGR electrovalve up on the bulkhead. So, to test, firstly disconnect the vacuum pipe to the diaphragm and feel with your finger for the presence of a vacuum. The idle may fall if the vacuum was present. Next, disconncet the electrical feed to the electrovalve and observe/feel for vacuum. You may have a duff diaphragm or a duff electrovalve allowing vacuum through all the time. You can feel vacuum from the pump on yor fingertip quite easily and then trace it through the system. Vacuum should be present at the fast-idle diaphragm when the engine is cold only.

Hope that helps Mike.

[CitroJim]

Mike,

another cause of high idle: I've just been looking at another pump and noticed the spring that keeps the fast idle lever in the slow position is broken and the natural tendency of the fast idle lever is to drift to the fast setting if the spring is not doing its job. The spring is a coil wrapped around the shaft and will be hard to spot with the pump in-situ but you should be able to feel if it is there or not by operating the lever by hand.

[MikeT]

I should have added that the timing is a little advanced when the engine is cold but this is set by the ECU under guidance from the state of the coolant temperature sensor. The ECU consults its "Cold Map" when the coolant is below 60C.

..and it does this via a control (electro/mechanical?) attached to and under the fuel pump? I can't remember if I read this is 5 degrees cam or crank rotation now

Cold adavance is beneficial as it takes longer to get the diesel (or veg) burning when everything is cold so starting everything a little earlier gets it burning just at the right time. In fact I think a little more advance being applied when burning veg may well be beneficial. The shame is, it cannot be adjusted on a semi-electronic pump.

Is the lack of adjustment limitation with the ECU or the fuel pump?

[Xaccers]

Mike, Cassy idles around 1100rpm on veg oil.

[MikeT]

Timing, a quick lesson Mike
(snip well-written lecture)
Hope that helps Mike.

Thanks for taking the time out to write that Jim. Although I do almost understand it, it's all the other variables that make my head spin. Not least having known only petrol engines until this year, I didn't fully twig that timing in a diesel is not an electrical issue providing a spark but one of fuel delivery - I'll get the hang of it one day, honest. Now you mention how cold fuel makes a difference to flame propagation I can see the relation to cold-start timing better.

[MikeT]

Mike,

another cause of high idle: I've just been looking at another pump and noticed the spring that keeps the fast idle lever in the slow position is broken and the natural tendency of the fast idle lever is to drift to the fast setting if the spring is not doing its job. The spring is a coil wrapped around the shaft and will be hard to spot with the pump in-situ but you should be able to feel if it is there or not by operating the lever by hand.

I made it my job to find that lever and moved it back (towards the drivers side with a screwdriver), that's when I noticed the drop in revs and smoother running but as it immediately returns to fast idle when released I assume the fast idle is on permanently as something is creating a positive pull on the lever.

[MikeT]

Mike, Cassy idles around 1100rpm on veg oil.

Hi Xac, sorry to read about your crank problems though having another car must offset the misery a little? EDIT Just noticed it's SORN

Mine was idling fast before I discovered veg oil though it didn't hunt then so I can't say it's the fuel alone causing the fast idle and another thing, although I notice the engine note change when air con is engaged, there is no visible difference to the rev counter reading.

[MikeT]

In your pump, timing is under control of the ECU totally. It looks at ....how far the throttle is open .....and sets the pump timing accordingly on the servo. It uses the injector lift sensor signal to confirm the timing.

I have the odd hesitation on acceleration and previously read this can be due to throttle sensor tracks giving poor readings. I'm now more convinced so will be looking to test that too. I Hope they can be cleaned up and fixed as it's more noticeable when the fuelling was increased. On a positive note, you've instructed me how to test the lift sensor and that's working fine, we know.

....The cable is attached to a vacuum diaphragm. When vacuum is present on one side of the diaphragm, the cable will pull and increase the idle.

This is what I suspect is permanently activated, given the screwdriver to lever test.

firstly disconnect the vacuum pipe to the diaphragm and feel with your finger for the presence of a vacuum. The idle may fall if the vacuum was present. Next, disconncet the electrical feed to the electrovalve and observe/feel for vacuum. You may have a duff diaphragm or a duff electrovalve allowing vacuum through all the time. You can feel vacuum from the pump on yor fingertip quite easily and then trace it through the system. Vacuum should be present at the fast-idle diaphragm when the engine is cold only.

Hope that helps Mike.

Yes, it's helped me immensely as more than one of the posts in the other threads I've read said a cure was to bypass the electrovalves and connect the vacuum from the camshaft directly to the cable diaphragm stating it doesn't hurt the cold running but cured/lowered the fast idle Hence my earlier statement believing fast idle was the default.

I'll post my findings once I've carried out those tests. Mucho gracias, Jim.

[CitroJim]

Is the lack of adjustment limitation with the ECU or the fuel pump?

It's the ECU Mike. The only way to change the timing would be to re-map the ECU, essentially give it a new set of look-up tables. I've never heard of anyone doing this although someone must to be able to provide a diesel turing service. They may do this by using a "piggy-back" ECU that looks at what the main ECU is supplying, intercepting it amd modifying it before passing it on to the pump servo.

Basically, the timing on the pump is set by sending a train of pules to the timing servo which, essentially, is like a fuel injector. This varies the pressure behind a spring-loaded piston, which, through a rack, rotates the HP Pump ring relative to the pump cam and thereby adjusting the point at which fuel is delivered; changing the timing.

A mechanical pump can have its timing adjusted by moving the pump body relative to its mountings on the engine. Slots are provided on the pump body for this. Although those same slots are there on the electronic pump, it'll make no difference as the ECU will just compenase and leave the timing precisely where the ECU thinks it should be. All you'll do is restrict the range of electronic adjustment slightly.

I have the odd hesitation on acceleration and previously read this can be due to throttle sensor tracks giving poor readings.

Yep, can be. It is only a pot (potentiometer) and may respond nicely to a spary of electronic contact cleaner. Some say you can move the pot relative to the throttle spindle so it works on a new section of track. I have no experience of this but it should work as the ECU should be able to compensate for the change in absolute position of the throttle pot and re-learn.

The ECU is, I believe, an adaptive type and will "learn" your driving habits. An occasional reset by leaving the battery disconnected for 10 minutes or so can be beneficial.

[MikeT]

My findings this morning from stone cold. Before starting, I checked the fast idle cable (that goes to the pump lever) and found it quite slack as expected. However, on starting, the cable remained slack

I then pulled the vacuum pipe off the cable diaphragm and could not sense any vacuum and the revs did not change, I even held a cigarette to the pipe and did not observe the smoke being sucked in.

Next, I tried moving the pump lever with a screwdriver and as before, pulling it towards the o/s reduced the revs but immediately on release, the revs went back up.

Just out of interest and to satisfy myself I was able to sense vaccum, I replaced the EGR vacuum pipe onto the electrovalve but again, there was no vacuum. So lastly, pulling the pipe off the camshaft component I immediately heard it sucking and could feel a strong suction at the outlet.

To summarise, vacuum is provided to both electrovalves but at idle, neither emit vacuum whether the engine is cold or warm.

[MikeT]

... It is only a pot (potentiometer) and may respond nicely to a spary of electronic contact cleaner.

I've had success in cleaning such pots before so it's worth a try and a good friend recently gave me a new can of cleaner.