TDI hot start issues

[Mechcanico Lee]

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

[erubus]

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

[erubus]

Another thought. I could measure the amount of fuel spilling from the injectors by physically measuring the fuel return to the tank at idle. If it is know what the fuel return volume [i]should[i] be at idle then it would be simple enough to see whether it is spilling too much. Is this a piece of information that anyone has published though?

[Mechcanico Lee]

The item circled in the pic is a DRV ...it's part of the high pressure rail assembly, the IMV is on the high pressure pump .

To do a proving test on the pump ideally you need to either take control of imv duty or 'dead head ' the pump , cap off all injectors , disable drv , this is so you are taking away all the routes that the pressure can escape from , we use a pressure block here with an extra rail pressure sensor in it so we can confirm the cars own rail pressure sensor is reading correctly and also the block had a safety blow off valve that lets the pressure off when it gets to 2000 bar !!

Taking the submerged pump out of the tank sounds a big faff , plus the a8 has a saddle tank so it will have a suction jet assembly for tank level equalisation....to much to be messing with at this point .

You could do spills without any fancy gear , just check one injector at a time , a good one does not really spill so much and bad one squirts everywhere, you tube injector spills ...loads of vids showing you what it's about .

I think from what we know up to now with slow cranking over and lack of gear to rule out the other items I think chasing down the slow cranking is the best pathway for now , could you post up a vid of it cranking over so I can hear it cranking over .

[erubus]

I now have a new theory having had a couple of days without being able to get on the forum. My theory is that there are a combination of factors at play here. 1: I reckon the starter is weak 2: I think there may be one or more injectors spilling excessively - I have not yet tested this though and 3: despite all my previous protestations to the contrary, the battery aint great.

After opening the doors and boot but not switching anything on, connected up, the battery is reading around 12.2v on average, droping to about 10.4v when cranking. This is after having driven for 45 minutes coming home from work.

The old battery, which lives in the boot as a booster that will usually start it if the temp is below 65 degrees, sits at 12.66v but is only 800CCA and 850Ah. It still doesn't start the car.

I have found it surprisingly difficult to find an amps clamp that can do DC and is rated up to 1000A. I am going to try the electrical engineering technicians at work tomorrow and see if they have anything. Can get plenty of AC ones no problem!

When i attach the jump leads to the car, if I leave them for a minute with the DMM connected and wait for the voltage to rise to 12.88v or above, it starts every time. Anything below 12.88v and it just turns over regardless of which battery is connected.

Hopefully there should be a couple of vids attached to this post showing a cold start, hot running, switch off and restart attempt and a hot start attached o the jump leads.

Vids not uploading for some reason, just sticking on "sending request" will keep at it

[erubus]

Still don't appear to be able to get an amps clamp that does DC. I suppose i could test the resistance of the windings, and then check the volt drop when its starting and calculate the current drawn that way. I dont overly fancy being underneath the car while its on axle stands and getting started though. Even on the highest setting of my axle stands, I can only really squeeze in under the car. It would give me an idea of how big the job of changin the starter will be though.

Still cant upload vids soI have put them on youtube instead:

https://youtu.be/D4WgLlfe9oQ

https://youtu.be/y5YA4YxCN4g

https://youtu.be/EgPNA40_VBg

[ainarssems]

I suppose i could test the resistance of the windings, and then check the volt drop when its starting and calculate the current drawn that way.

It does not work like that, you will find that measuring winding resistance it will be pretty much zero. It's the inductive and to some degree capacitative resistance that make up total resistance when turning. And any way it will be very low if you consider that to get 400A with 12v you need total resistance of 0.03ohm. Yo cannot directly and precisely measure resistance this low with off the shelf multimeter.