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  #21  
Old 14th August 2017, 09:46 PM
Mechcanico Lee Mechcanico Lee is offline
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Hello , rail pressure will fade pretty quickly, this is called pressure decay and is normal , yes there very often is a check valve or non return valve in the system but this is just to keep the low pressure primed up to the high pressure pump to stop ' drain back ' .
Actual fuel pressure will always follow the desired pressure or be pretty close throughout the load range .

300 bar at idle sounds about correct , 10 % duty cycle sounds about correct for the idle aswell , the inlet metering valve must be the ' fail to closed type ' so in its un powered state the valve is closed ....but it's not fully closed some fuel can bypass it and this would normally give a default pressure of 80 bar .

Now ,when the car does the non start instance what is the pressure ? .....around 80 bar ...... coincidence or is there something in this ??

I can see that the imv duty cycle is going higher trying to build the start up pressure of around 300 bar on the graph but after 11 seconds of cranking time the rail pressure has not risen .

So what could cause this ..... rail pressure loss due to spilling injectors , intank pump not running on cranking or not making pre pump pressure. Imv fault or valve not being driven for whatever reason .

Normally with excessive back spill the car struggles a lot from cold which originally we did not have , without a guage in low pressure line we cannot fully rule out low pressure feed .

At this point I have a duty cycle tool I can plug into the imv and I can take over manual control of the imv and drive it to where I want to and see what happens with the pressure ....you have to put an emulator in the loom plug for the imv so the ecm still thinks it's connected to the car .

Looking at rail pressure whilst in non start condition it does not even fluctuate, it's as though the valve is stuck .

If you could get someone else to do the cranking when it's in a non start condition and give that imv a clonk with a spanner see if anything happens .

Just try this test .... have rail pressure showing on vcds un plug imv and crank over and see what rail pressure shows .

It could be a faulty imv but without some of the proving test gear in place the diagnostic pathway tests are not conclusive , so it is more a calculated guess with this one .

Keep at it , your getting closer to a fix
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  #22  
Old 15th August 2017, 07:23 PM
erubus erubus is offline
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Thanks again. A couple more things to check. I tried the easy start thing yesterday but it didnt even cough - but - I was spraying it into a little gap in the airbox, with the filter in etc. I would really need to open the airbox and take the filter out and spray the stuff right down its throat I think. I didnt have tools to hand to try it.

I took some more data this morning just for comparison. One of the graphs is the first start this morning from cold. Everything goes nicely as it should.

The second graph is Full acceleration from 30 mph to ***mph. This is engine at full operating temperature. I am away out to the shop just now, so when I get back I will try the percussive repair technique (hitting it) on the IMV and I will go down to my workshop to get some tools to take one of the airboxes off and the air filter and I will try giving it a sniff of easy start and report back.

Its only been switched off for a couple of hours so definitiely wont start on its own, and probably wont with the jump leads attached as well, but i will try logging a scan just in case t does to see what it looks like.
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Last edited by erubus; 15th August 2017 at 07:32 PM.
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  #23  
Old 16th August 2017, 07:01 PM
erubus erubus is offline
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Here are a couple more graphs, Im not sure whether they come up with names or not on here so the smoother looking one is the A4 3.0TDi hot start for a comparison and the wobbly one is the A8 doing a hot start with Easy start. It cranked over for ages then just as it was about to stop if banged then ran away fine.

Whether it would have started on its own without the easystart is open to question as very occasionally it does. engine temp was down to 70ish on the gauge - I wasnt logging it on vcds though. It did pop and wheeze a couple of times as it burned the ether so I'm inclined to think it was that.

what I notice with the graph though, its cranks over steadily at 147 rpm, pressure is steady at 75.9 bar, rising to our old friend 82.8 bar at 12.08 seconds. at 12.96 seconds though, the revs jump to 210, pressure to 103.5 then bang it starts, i assume this is the easy start. At 13.27 seconds its doing 882 rpm and 243 bar and then runs pretty much exactly on the specified line.

Is the duty cycle of the IMV as measured by VCDS the actual opening of the valve or just what it is being told to do and not neccesarily what is actually happening? It seems odd to my brain that it can work perfectly when its stinking hot but as soon as its switched off and tried again it appears not to work. It seems logical to me that there is something else holding off the start process but I lack enough knowledge of the thing and I certainly defer to your frankly amazing expertise and understanding mechanicolee - especially when you dont have the thing in front of you.

I didnt do the hammer test or the plug out test as once it had surprised me by starting on theeasy start i didnt want to shut it down again. once the OH is back home i can move the car from the farmyard back into the drive. I will try both methods then and will also log a jump start as that is likely how it will start anyway.
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Last edited by erubus; 16th August 2017 at 07:25 PM.
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  #24  
Old 16th August 2017, 08:17 PM
erubus erubus is offline
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am i imagining both cars starting at c200 rpm and c100 bar rail pressure? or is it just wishful thinking that its just engine speed and the new starter will fix it and I'm wildly stabbing in the dark?
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  #25  
Old 16th August 2017, 08:43 PM
Joe2.0E Joe2.0E is offline
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When the new starter was put in my Q7, the difference in cranking speed was massive, I think you should fit the new starter and a decent battery before going any further.
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  #26  
Old 16th August 2017, 11:44 PM
erubus erubus is offline
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When i started it tonight it was absolutely battering down rain so i didnt bother plugging it in, just wanted it started and on the drive. tried it on its own steam, nothing. Tried jumping it, nothing. By this time I was soaking so took the top off of one of the airboxes and gave it it's fix. started on the button.

Following mechanicolee's diagnostic pathway from a few posts back it does look to me like an RPM/CPS sensor thing so changing the starter could work. I am basing this on one graph though. The difference in sound between the two engines cranking over is significant, even though the A4 is only a 3.0 rather than a 4.0 - theres 200W between the two starters so have to factor that in as well. Even my 40 year old Land Rover sounds like it turns over faster tbh.

I'm not good at diagnosing electronic issues, I'm a mechanical engineer, i do gears and pistons. I make my living mostly working with steam/steam raising plant. I'm enjoying the learning experience though, although I kinda wish I was learning on someone else's car haha
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  #27  
Old 18th August 2017, 06:14 PM
Mechcanico Lee Mechcanico Lee is offline
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Hello , been having a read up, the system is bosch cp 3 twin point control , with inlet metering valve in pump and drv (diesel regulating valve ) in end of rail .

My heads going a few ways and without proper test gear we can not prove some of the diagnostic pathways .

On the graph of the non start i can see that the pressure is sat at around 80 bar but the inlet metering valve duty is going up trying to get rail pressure to go higher but its not responding , this could be because of injector spills being excessive or poor high pressure pump or the drv in the rail is leaking off rail pressure or imv sticking , so without the bosck kit for capping off the drv or checking spills , or checking low side pressure as ive already said we cannot fully rule these things out .

I have spoken to a good diesel guy i know and he does say that 200rpms for cranking speed is around what you should be looking for and a starter that is taking to much current draws alot of the residual voltage down so all the ecms are starved of voltage which does not help with start up .
The wiff off easy start just gives the engine a cranking boost and gets the motor spinning faster helping the pump generate more pressure .

Now if we wanted to do cranking and battery checks a inductive amps clamp is a good tool , we could put it around the heavy starter cable and see how many amps are being pulled out and look at battery volts also .......if the amps goes really high we could say the starter is drawing to much current and this also would show that the battery is capable of delivering that current and the volts stays above 10 volts whilst under continuious cranking this is showing the batterys good , if the current draw was not that high but battery volts drop alot when under cranking we can say the battery is poor.

So like already said we could be looking at a bad starter ......but with these vehicles now doing a starter could be an epic task so with diagnostic pathway tests you do not need guess work ....just the correct kit ........test not guess ..... work with this mantra ...... " what if i fit x or y and it does not fix it ...then what ? "
As a repairer we cannot guess with customers money ....although most of the garages do just that ...... we call it " firing the parts cannon at it " and hoping you fix it ......... what happens when you have fitted a load of parts and its still the same .......then what ??

Going off on a tangent here , we have a 61 plate discovery 4 in just now 3L diesel, fault code for crankshaft sensor keeps flagging , ive scoped this to death to prove this fault ,as to do the job the body has to be removed from the chassis then engine pulled out to remove passenger side turbo to get at the crank sensor ( superb design eh ? ) £55 for the sensor , around £3000 labour so we have to be sure .............
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  #28  
Old 18th August 2017, 08:02 PM
erubus erubus is offline
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Thanks for that Mechanicolee, I really appreciate the work you're putting in to this problem. It's frustrating that I don't have the kit to do the proper tests on the injection side of things. I should be able to get a shot of an inductive current clamp from work to test the current draw. From memory the voltage from the battery drops to about 10.8v on continuous cranking. current draw, I have no idea though. I still havent had a look underneath to gauge what sort of mammoth task changing the starter will be anyway. Ive been holding off fitting it for the very reason you said. I could go to all the trouble - hours of labour and find that its a leaking valve inside the HP pump or something.

I shall get a shot of a current clamp when Im back at work on Monday and will test the current draw on a hot and a cold start and see if that gives me a better idea of the health of the starter. Are there different start protocols for a hot start and a cold start? ie would the car want to have a higher rpm to start from hot than it would from cold? maybe the hot wires and windings in the starter at 90 degrees increases the resistance enough to sap what power it does have. Certainly its hot starts are getting more laboured as time goes on, even the cold starts sometimes take a bit of cranking.

In your opinion,if you were me and you didnt have the equipment to properly test/prove/rule out the injection side and the starter tests showed a high current draw from the starter, would you think it reasonable to look into changing the starter? Cost isn't an issue as I'd be doing the work myself and I already have the motor but its going to be a big job, I'm definitely going to lose my temper with it several times and there are no gaurantees. I'd love to think that its just a case of swapping a sensor and everything will be fine but I have no belief that it will be the case.

Off on a similar tangent to your disco 4 story. My friend has a defender 90 with a TD5 engine. He was having similar problems although his just never started full stop. would just crank and crank till the battery was dead but would start perfectly with a bump start. He had it scanned and it was a sensor that supposedly detects wear in the crankshaft by picking up vibration. The front bearing on his starter motor had worn so that when it was cranking it vibrated making the sensor think the crank bearings were gone.

I like my old series three. it only has four fuses, which are mostly for the lights. Everything is completely mechanical and i can tell what needs fixed or adjusted just by ear. maybe I'm too old fashioned for such a complictaed car!
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  #29  
Old 18th August 2017, 11:01 PM
Mechcanico Lee Mechcanico Lee is offline
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If you can get an inductive amps clamp it will need to cover up to 800 /1000 amps and use a volt meter on the battery , give it a test when it's non starting and lets see what amps it's pulling out and see what the battery drops down to .
Also if it does take a load of current to turn the starter that high resistance in leads or earth points , earth straps from engine to body can cause slow cranking and high amps draw

I still think it's odd that it cannot make over 80 bar of rail pressure when it's non starting but normally sticking imv or drv show some kinds of issues when running or idling aswell ....hunting up and down and rail pressure uneven and jumping around and so on .

The start protocols from cold to warm are roughly the same , a little more fuel pressure and fast idle dialled in for cold start , injection begin time will be slightly different but not enough to cause non start conditions .

What can be quite different from cold to warm is the ability of the pump to make pressure , cold diesel is less viscous so it's easier to make higher pressures , same as engine oil ....there's lots more oil pressure when cold to when oils up to temp , so if the high pressure pump was down a little on efficiency and an injector or if the injectors where spilling a little to much this scenario supports why it can start from cold easier than warm , all theory's but sound feasible.

See if you can get that amps clamp and see what the starters pulling out , keep at it .
Lee
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  #30  
Old 19th August 2017, 11:10 AM
erubus erubus is offline
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i still have to try the test where I pull the plug on the IMV and log the rail pressure. Trouble is, the thing I thoguht was the IMV might be the DRV going by our description above. I have circled the thing I thought it was in the attached picture.

As with you, the thing that confuses me is that everything seems to run perfectly when its actually running, so a sticky IMV or DRV would presumably cause some disruption while running.

Is there any way of testing the efficiency of the pump, injector spill etc without specialist equipment? I did think of one possible redneck method of testing whether the fuel viscosity has an effect but I'm not sure how easy it would be, and thats to have a bucket of cold diesel and dump the tank pump in there when it is hot - I'd have a good three or four hours to get it out - and see if it started with nice cold fuel. An easier, if somewhat more redneck way would be to fill it with a gallon of veg oil next time the tanks nearly empty and try a hot start. Veg oil is considerably more viscous than diesel, and hot is much the same as cold diesel.

I'm also starting to wonder if the ticking noise at idle is actually a failing piston in the HP pump or something pump related. I had up till this pont assumed it was the 4.0tdi timing chain issue as it has all the hallmarks. spray can rattle on start up followed by rhythmic ticking that sounds like its coming from the region of cylinder 4. I have gone over the engine with a stethoscope trying to track down the origin of the noise and to be honest it doesnt sound like its coming from the pump, but just listening by ear it sounds like its coming from that area. With the stethoscope it does sound a bit more like chain slap than a tick. Maybe I'll try posting a video in another thread to get some more opinions.

gill
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