I changed the HPFP on my 2011 Q7 3.0 TDI, now that is quite a job, especially when you didn't realise the new pumps have swapped the in and out. The car will start just and then dies. Lots of shake, but the big thing is a really loud high pitched squeal. The squeal stops when I disconnect any of the small vacuum lines.
I am looking for suggestions on what might cause this and how to troubleshoot.
Thanks

 

Have you scanned the car for fault codes w/VAGCOM scan tool? That's always your go-to starting point to see what the car can tell you.

It is a 'big job' to change the HPFP related to what has to come off car and be disconnected, and normally when the car doesn't run right after any big job in which lots of bits get unplugged it's related to a sensor, hose, etc, that you unplugged/removed, and/or possibly damaged or didn't reconnect properly or securely on the electricals.

Just in general, the symptom of a loud squeal related to vacuum pressure buildup is normally related to the PCV/oil separator failure, but you don't see this represented much in the TDI engine posts, and you haven't mentioned any other, related odd symptoms like oil level rising or abnormal oil consumption problems, etc., so we don't have anything to work with except the data you provide about your car...like why were you replacing the HPFP? If replacing the HPFP because it 'blew glitter' and imploded, then it will also contaminate the entire fuel system, to include your fuel injectors, etc., so the details matter. You'll kill your new HPFP, if the prior one blew glitter and you only swapped out the HPFP...that repair is much, much more involved when the fuel system gets contaminated with metal shavings. Hopefully that's not the case....is it?

The PCV part failure is extremely common in the gasoline engines, and when it fails the loud squeal noise is an indicator...pulling the oil filler cap off when it's squealing is nearly impossible, so that's a test. Since the squeal noise only began after you had everything apart it suggests that either there was a problem prior you were simply unaware of, or that you somehow created the problem condition during your reassembly process. Your car is also old enough that there would be a lot of brittle plastic and hose bits in/around the engine valley area you are working in, so you'd need to verify your work. I always take pics before and during the disassembly process to help me get car back together properly, and if there's a ton of confusion, I'll label some stuff too, as this is a complex sensor/hose array setup.

Post up any/all the details you have that may be relevant, including diagnostic scan data related to the engine.

 

@10Q7TDI_Prestige thanks for the quick response.
I changed the HPFP because the car was going into limp mode when accelerating harder / the engine was under load. The Audi dealership in Adelaide, Australia diagnosed the problem as the HPFP after some significant investigation, and then quoted more than the car was worth to replace it. I drove it home normally and set to work.
There were no other issues with the car,
I don't have a scanner, I'll need to go get one. I did test the oil cap for suction, there wasn't any, but it maybe because the car doesn't run long enough to create enough suction.

I agree with your suggestion that there are so many plastic bits that need to be touched or even accidentally knocked that I could have easily broken something. I've gone round and round the engine, disassembled, reassembled, but no change to the squeal. I am definitely missing something, but I don't know what.

 

IMHO, you should definitely, immediately, pull the screen trap out of the old HPFP and check it for metal shavings. If there are shavings present then all this other stuff is academic because the fuel system is now contaminated.

Now, if there is no signs of metal shavings in screen trap, then the root cause lies elsewhere...Audi dealers aren't the sharpest tool in the shed, but they do like to quote expensive parts replacements like they know exactly what they are doing. There are myriad items that can fail, like sensors, emissions system items, etc. that will cause your car to go into limp mode...the SCR /reductant system will do that if you got a bad sensor, or the EGR Cooler temp sensor, or EGR itself is bad, etc. I've not yet run across an honest, morally straight, Audi Service Advisor or a competent mechanic in any Audi dealer's I've been to, and they have lied to me to my face trying to get me to sign off on repair work that was not necessary, or making excuses as to why my car still leaked coolant after they 'fixed the coolant leak'. You can interpret that how you want, but an Audi dealership is the very last place I would ever take my Q7 TDI.

My mind is just a wee bit boggled that you would attempt such a heady DIY job and not have a VAGCOM diagnostic scan tool to support your diagnosing the car, clearing fault codes from everything you disconnected on it, doing any required adaptations related to HPFP replacement, performing fuel line/system purges to get all the air out, etc. etc. Certainly, there is the purely mechanical aspect of remove part/ fit new part, but on this car there's generally quite a bit more that has to be done in support of changing out a major component like that...using a scan tool with full access to the control modules, like the VCDS VAGCOM by Ross-Tech, etc. Just disconnecting the battery on this car creates massive trouble codes/faults internally, etc. Don't be fooled, this Q7 is full of computers (control modules), has more sensors than you can shake a stick at, and often times there are no direct fault codes from the underlying, but you do often get seemingly unrelated fault codes show up to help you out. Bottom line; it's a complex vehicle, so don't buy the cheapest VAGCOM, get the one that works.