 | Home > Puma (Tdci) > Eratic Idle especially when cold |
 
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| ROBBONTHEROCK Member Since: 23 Jun 2014 Location: Aberdeenshire Posts: 637   
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Thanks John. Cheers Andy |
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24th Feb 2017 12:01am |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233  
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Good news zsd-puma,
I find it very odd that a pressure relief valve would prevent a learn procedure completing. In theory the relief valve should only operate if the pressure exceeds a safe level, i.e. under fault conditions. Did you notice that the common rail pressure was being limited when that valve was fitted? Perhaps it was operating at a lower pressure than it should do hence preventing the pump from achieving the required pressure during the learn cycle? I'd love to understand exactly what the pilot learn procedure actually does and what values it stores in the ECU. I have tried to find out but it appears to be shrouded in mystery. "You are never to old to learn something new" |
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6th Mar 2017 10:19pm |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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This is from the Ford supplied workshop manual for the 2.4 Puma Engine
"Installed into the other end of the fuel rail is the fuel pressure relief valve. The fuel pressure relief valve monitors the fuel pressure in the fuel rail. If the fuel pressure becomes abnormally high, the fuel pressure relief valve opens to allow the excess fuel to return to the fuel tank and reduce the fuel pressure to the correct value. The fuel pressure relief valve is a serviceable part. When the fuel pressure relief valve has been triggered a diagnostic trouble code (DTC) will be stored in the PCM and the malfunction indicator lamp (MIL) will be illuminated. When the DTC has been triggered, a new fuel pressure relief valve must be installed." I note that Ford call the VCV the Fuel Metering Valve (FMV) and the manual describes that it normally operates fully open (full bore) but is only commanded to operate to restrict fuel flow to the low pressure inlet of the high pressure side of the fuel pump during deceleration. This makes sense to reduce pressure in the common rail when fuel is not required at the same volume. "You are never to old to learn something new" |
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6th Mar 2017 10:44pm |
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| zsd-puma Member Since: 09 Aug 2016 Location: Kent Posts: 2720  
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There is also a fuel pressure sensor on one end of the rail and a pressure relief valve on the other.
I suspect the pilot learn basically commands the pump to overcharge the rail to make the PRV open, when it opens the sudden drop in pressure gets picked up by the sensor. Going through the learn procedure it seems the engine is run at different speeds to calibrate the timing and fuel pressure. I think all it does with the valve is check what pressure it releases at, it's a fairly simple spring loaded pressure relief valve, so if it was a bit out of spec it would open too early or too late so fail the test. |
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6th Mar 2017 11:32pm |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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I would like to think that the PRV should not play any part in the Pilot Correction Learn process. The PRV is designed to dump fuel at extremes of pressure in a fault situation. The common rail acts like an accumulator in that the pressure keeps building up with fuel entering from the HP fuel pump and reduces when the injector opens and takes fuel into the cylinders. My understanding is the PRV is just there to act as a safety valve if the pump carries on supplying fuel and the injectors are not taking a sufficient amount to keep the pressure to a controlled upper value. My guess is that the engine is run at different rev values and the VCV is initially given a default value of pulse width control (set point) to reduce flow rate into the pump. The pressure in the rail is then measured (from the pressure sensor) and if the pressure does not meet the expected value it then adjusts the VCV control set point to achieve the desired common rail pressure thus learning the effective control input signal level to achieve the fuel flow control or common rail pressure. This is called the transfer function gain coefficient. As a control engineer this is what I would call open loop characterisation. The new set point values I would guess are then stored in the ECU.
This is my expected procedure but hopefully someone with greater understanding will correct me and put me in my place. "You are never to old to learn something new" |
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7th Mar 2017 12:09am |
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| zsd-puma Member Since: 09 Aug 2016 Location: Kent Posts: 2720
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That makes sense.
I think it basically just tests the PRV to ensure it's within spec, ie. if it opened too soon then the fuel pressure will be too low and if it opens too late it can cause damage. So i suspect it over pressurises it momentarily to check it's operation. Although it's set for a specific pressure, i suspect there is a manufacturing tollerance, and the pilot learn tests to see where abouts in that tollerance range it is for some reason. The rest of the pilot learn it probably just about calibrating the whole engine management system, to ensure perfect idle and counteract any minor quirks of a particular engine. I would think it self adapts over time anyway, but the pilot learn gives it a starting point. All i know for sure is the cheap PRV just failed the pilot learn right at the start of the test, whereas the genuine one carried on right to the end. |
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7th Mar 2017 1:04am |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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I'm not convinced the pilot learn attempts to over pressurise the common rail to make the PRV operste. I also have not read anywhere that suggests that after the pilot learn process the ECU carries on optimising the coefficients. I think the new values it learns are stored in flash memory and that is what it uses until a new Pilot learn cycle is repeated. I think the reason they call it a Pilot learn is because the VCV is like a Pilot valve in a proportional hydraulic control valve. So the pilot learn is just trying to characterise the VCV control function and not any other engine management components. I may try to hang an oscilloscope on the VCV control solenoid input and the common rail pressure sensor and see how it controls for different revs and during acceleration and deceleration. "You are never to old to learn something new"
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7th Mar 2017 1:38am |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 16879 
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Let us hope not, because the PRV is non-resettable, once it has opened, it must be replaced!
There is a rather crucial word missing either from the original quote, or more likely from the Ford manual (Ford manuals, like LR manuals, are shockingly poorly checked now). I have added it in bold in the quote above. Once the PRV has opened (or if it leaks, which is not unusual) the engine will not run optimally and VCV calibration cannot successfully be carried out. |
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7th Mar 2017 9:24am |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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Thanks Blackwolf. In a polite way I was trying to suggest that the pilot learn is unlikely to over pressurise the common rail to make the PRV operate. The Ford workshop manual does go on to state that a new PRV must be installed if it has operated or leaks. I think Fords idea of serviceable is that it's replaceable as a component in this throw away age! Anyway thanks for clarifying. "You are never to old to learn something new"
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7th Mar 2017 9:47am |
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| zsd-puma Member Since: 09 Aug 2016 Location: Kent Posts: 2720
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I think that's most dealerships idea of servicable part these days. Citroen's for example don't supply wheel bearings, you just get a complete hub with the bearing pre-fitted and bolt that on.
I was under the impression it was just a spring loaded valve, similar in principle to a rad cap. ie. pressure goes too high it opens, once it drops back down it closes again? Reading on the transit forums when they fail they come out in peices. So as i said my cheap one wouldn't pilot learn, that either means it was opening when it shouldn't or it wasn't opening when it should. I mean it's only got two states, so to fail the pilot learn it must be in the opposite state to what the ECU is expecting. |
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7th Mar 2017 9:14pm |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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Zsd -My guess is your ebay PRV was relieving when it was not expected to, ie. The former rather than the latter of your suggestions as we have concluded that the pilot learn cannot possibly induce a high pressure to cause an in specification PRV to relieve. "You are never to old to learn something new"
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8th Mar 2017 2:00am |
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| OleDave Member Since: 19 Oct 2013 Location: CARDIFF Posts: 175 
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Hi Guys, well after months of threatening to change the fuel vapour valve (SCV) the item finally pooped itself. so I arranged with my local garage who come highly reccomended for LR repairs to do the business, the new valve has now been fitted and a complete reset of the pump and valve carried out on their diagnostic equipment, on completion all seemed fine, on the way home after fitting the throttle response seemed much better and gear changes much smoother as the engine did dive not into eratic fits when you lifted off the pedal, all seemed well that is until I pulled up at the traffic lights!! the engine cut out!!! since then its done it a few times and while at idle its still is a bit eratic now and again.
When on the diagnostic equipment resetting the valve their equipment did flag up an egr problem? I am told its quite a common problem with the Ford 2.4 engine, but my question to you guys is if the egr valve has a fault can this also give eratic idle or cut out the engine? have looked at EGR prices and they are circa £150 so do not want to buy one if the problem still lies in maybe a fault in the new suction control valve, any idea guys? Regards Ole Dave |
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21st Sep 2017 1:02pm |
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| B4Lamb Member Since: 21 Mar 2015 Location: Abergavenny, Wales Posts: 233
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Hi OleDave,
If your egr is still doing the cleaning cycle when you stop the engine ie. The chugga chugga 4 times then it is probably ok and closing fully. I've never known the egr to cause erratic idle or cutting out. The easiest way to find out though is to disconnect the electrical connector from the egr. The Egr valve is spring loaded to the closed position so little or no exhaust gasses will be directed back into the inlet manifold while it is disconnected. The ECU will report one or two EGR fault codes after a few engine starts and the orange engine warning light will permanently illuminate when these fault codes are recorded. There will be no other noticeable effect while driving. If you still get your erratic idle or engine cutting out while the egr is disconnected then it's not the egr valve, simple. You will have to reset the fault codes when you reconnect the egr to extinguish the warning light. I use a cheapo OBD2 bluetooth plug in module purchased from ebay and run an app called Torque on my android tablet equally cheap to get full live engine data displays and for reading and resetting fault codes. I can highly recommend you do similar so you are not constantly tripping to the garage with their diagnostic tools. I'd be intested to here how well you get on. "You are never to old to learn something new" |
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21st Sep 2017 2:16pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 7687 
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very informative post. i have the erratic idle problem too. and EGR fault codes coming up.
If you disconnect the EGR electrically as suggested and its not closed fully would you not still get the erratic idle though? If its fully closed there shouldnt be a problem that i do get but what if its not. Mine does the 4 chukkas Poor/Erratic idle is listed in the WSM as possible EGR stuck open see ref to it on other post currently here. Cheers James 110 XS Utility 130 Puma Station wagon/camper (in the making) 90 Puma Hardtop |
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21st Sep 2017 3:21pm |
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