996 Turbo - Boost Leak Test - Air coming straight out of the turbos

Having had the engine out and rebuilt, it’s all back in, but will not make any boost. I’ve done a leak test by taking the inter cooler pipes off the turbos, plugging one side, compressor on the other. It won’t hold any pressure at all. The air just comes out of the turbos where I’ve taken the pipe off. Also, if I do the test at the Y pipe, the air comes straight out of the inter coolers. I’ve also smoke tested it and can’t see any leaks in the vacuum lines., but some smoke will come out of the oil filler cap eventually. Clearly there is something not working correctly within the lines, check valves or diverted valves. When I built it all back up I was quite ***** about checking all vacuum connections and one way valves etc, but I must have done something wrong or put one of the the wrong way round.. Was hoping someone might have an idea of where the problem is most likely to be before I have to go through everything. Essentially the system cannot hold any pressure. Thanks in advance.

?? Maybe a picture would help? 1:take off intake pipes into turbo. 2:block and add air 3:see if it leaks Simple as that. Intercooler pipes stay on.

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So I've tried the leak test going in at the Y pipe, as below, and cant get it to hold any pressure. Air will still flow straight back out of the disconnected pipe circled red in the photo I borrowed below. Having delved a little deeper, I could hear the diverter valves honking when trying to pressurise the system. Now I tested these and they held a vacuum before fitting. I took them off, and sure enough, they hold a vacuum, but when I blocked the hole in the Y pipe where the F pipe attaches, I could get the system to hold pressure (whilst F pipe and the diverters were removed). So i suspect this means my diverter valves (or one of them) are leaking the other way under pressure. They should hold pressure right? Want to be sure before I replace. No visible cracks in the F pipe. Thoughts?

Please don't be offended here but are you sure you shouldn't be asking a specialist to complete this? Put everything back together, and test it from the turbo inlet as i showed in the picture above.

No idea why anyone would test like that, it's cutting out 1/2 of the boosted pipework, all the intercooler pipes and joints.

None taken. I'm pretty handy and like to learn. The above picture is taken from Markski Tuning website, he tests from both locations. I will be testing from the turbos as you suggested, but before, when testing from the bottom, I couldn't investigate anything at the top as it would hold no pressure at all. As said before, the air would just rush straight out again, so I needed to try and address that issue first. It looks like the DV's are causing this, but I just wanted reassurance that they should be holding pressure as well as vacuum before spending hundreds on replacements.

You sure you put the dump valves back in the right way round?

Pretty sure I did. I think it would be difficult to put them in the wrong way?

The dump valves are actually held shut by the boost pressure being applied to the small top pipe. If you have no pressure on the top pipe it only takes a couple of psi to open the dump valves as you've found out. If you did the test again but had say 20 psi connected to the top connections then they would stay shut upto at least 20 psi!

The top pipe connects to a diaphragm which has a larger area than the valve inlet. As the car comes on boost and the boost rises the pressure above pushing the valve shut is always higher than the force from below trying to push the valve open, so it stays shut. When you shut the throttle the electric change over valve switches the top connection from boost to vacuum and the dump valve opens very fast. It's surprising how few people actually understand how the dump valves work! There are many threads across car forums where someone has tested their valves finding they open at just a few psi then declared them dead as they know their car makes more than a couple of psi boost!!!

Thinking out loud here if you pressure test from the throttle body I'd of thought the pressure would flow through the change over valve to the top of the dump valves so they should stay shut. That makes me wonder if you've got the small bore lines to the top of the dump valves connected correctly. There is a pic of how they are connected I'll try and find it

Dump valve connections

Thanks Mac. That’s the diagram I used when I put it all back together, but now wondering whether there’s a check valve the wrong way round. It would be useful if there was a version of that diagram which shows the direction of airflow/vacuum.