Car needs an exorcist. Looking for advice on what the problem might be.


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Hello. I am posting here in hopes someone might have an idea of what is going on with my car. It currently cranks but doesn't start. I've been fighting with it since the start of April to get it off the driveway.

It is an 1.6TDI with 161k miles done.

It started at the beginning of April when the crankshaft sensor failed. I replaced the sensor with a new one. Car then started, we drove it up & down the drive and it seemed okay but after switching it on & off it popped a couple of errors.
P256300 - Turbocharger boost control position sensor; implausible - Permanent
00668 - +30 voltage, below lower regulation limit threshold - Intermittent

It then would start the engine, run it for approx 30 seconds, then switch off. Eventually it would struggle to even crank. And sometimes it sounded a bit weird on startup. You can see it on video here

I have then done the following.
-Replaced sender fuel pump in the tank
-Checked that there is no air in the system
-Checked the turbo actuator switch
-Checked turbo lines

The car continued having problems, so I replaced the fuel rail, after replacing said fuel rail, car will now not start at all. I even put back the old fuel rail in to see if something would change but its now in a state where it will not turn on.
I then have done the following
-Replaced the battery since the one it had was on its way out
-Quadruple checked that there is no air in the system, both in the hard lines and from and to the sender pump and the high pressure pump.
-Checked the mass air flow sensor
-Checked fuses
-Checked glow plugs and the wiring loom for it, while they don't turn on that is to be expected in a TDI engine during a warm day and the cables didn't look damaged.

Those original errors are now gone, they just poofed at some point, the computer doesn't see anything wrong with the car. Checked fuel rail pressure and its at optimal 280-300 BAR. Its worth noting the old fuel rail was displaying pressure at 420 BAR.

I am going to try running the engine tomorrow without the fuel filter, but after that I literally have no further ideas. One more thing I can think of is the car might not have ground to engine? Or somehow the injectors broke? The problem seems to be in the fact that the combustion just isn't happening for some reason.
Or maybe its still a problem with the crankshaft sensor. I was told by a friend that if the sensor don't match exactly OEM wise the car can throw a fit. And tbh I didn't check the numbers between them when fitting but they looked the same.

One last thing in case it matters, the car needs it head gasket doing, which I was planning to do in June. But I can't see how that is affecting it not starting right now.

Literally any ideas would be appreciated. Thank you.
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The running for 30 seconds maybe due to using the fuel in the lines, when used up, maybe thats when its dying, are you sure its actually getting fuel & its circulating back.


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I'd check if injectors are actually working and the valve in the fuel return from injectors. I had a similar problem, checked everything, in the end, relief valve failed and let the air get into the injectors. Replaced the fuel return lines and valve (it came as ine single unit on my v6 tdi), got injectors to shop to clean them up and get the air out, and everything worked.

Audi A4 B9 3.0 TDi quattro 200 kw


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The injectors are definitely getting fuel, as to whether they are firing properly I don't know as I haven't taken them out yet, really worried about that part so wanted to eliminate everything else beforehand.
I checked the fuel return valve and it is functioning as expected, fuel is definitely getting to and from injectors.


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Check the harness for chaffing there were a few common areas that used to rub and once the wires start breaking you got all sorts of issues I’ll see if I can dig out the old tpi with the check spots but if you start checking for signs of wear you can at least rule them out

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Check the harness for chaffing there were a few common areas that used to rub and once the wires start breaking you got all sorts of issues I’ll see if I can dig out the old tpi with the check spots but if you start checking for signs of wear you can at least rule them out

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Can't see any chaffing on it. There are a few places where the cable is exposed but the wires don't look damaged. Namely at the junction to the ECU and the harness from glow plugs, from which I am attaching a picture. I've now patched those up.

I also tried running the car with no fuel filter but no change.

Maybe it's the high pressure fuel pump? I am trying to look around to see how to check if it's good but struggling a bit.

What gets me about this is that it stopped turning on even for those 30 seconds after I fitted a new fuel rail. And I even fitted the old one back on and it still doesn't turn on. So surely it should be a problem in that region?

Also going to try to get a compression check done but getting a diesel tester isn't the easiest job.


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So, to update on this. I have taken the front end off again, and took out the new crankshaft sensor back out. I've now contacted a friend to get me an OEM part, maybe its the sensor after all.
Also checked the lambda sensor, thinking the engine might have trouble starting because the exhaust might be clogged (I have a head gasket to do and it currently leaks coolant into the exhaust system, I know that for a fact) and that same friend came over with a better PC and was able to look through more ECU data, all looks good. The high pressure fuel pump is very probably okay, the ECU doesn't see anything wrong with it, and it should if indeed it wasn't supplying enough pressure (ergo: ******)

Couldn't get a compression kit for diesel engines for a small amount of money. So I contacted a bunch of garages to see if they would lend me theirs for a small fee, all of them said no, even if I offered to leave my ID card, terrifyingly, one garage didn't know what I was talking about.
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Registered User
So, another update. The OEM crankshaft sensor arrived today and it has not fixed the problem. I am at wits end as to what this could be at this point.

Something has gone wrong when I replaced the fuel rail, because since then it doesn't start at all, even when I put the old rail back in. I thought it might have been the high pressure fuel pump but the computer is showing optimal 300 bar in the rail so it can't be.
Could it be the injectors are somehow screwed? Or maybe its the compression on the cylinders after all because of the faulty head gasket? Or maybe its the pressure regulating valve on the rail? I read that Audi insist I must use a new one if I am taking the rail out.
Here is a video of it trying to start, it should be said that it did crank much better, its just on this try on the video we just cranked it before so it was still a bit weak in the battery & starter

And also if you're wondering, the front end is loose because that is the only way to get to the crankshaft sensor, we used the following tool to get to it, made of a broken straight piece of the allen key in a size 7 socket on an extension.