17964/P1556 - Charge Pressure Control: Negative Deviation

**Jamiem** · 05-03-2011,10:00 PM

Drove the car this afternoon, all was good, then went out this evening to find I have no boost or torque until above 3000rpm, then its as normal. I've just scanned it and got 17964/P1556 - Charge Pressure Control: Negative Deviation. After doing some reading it looks likely to be the N75 or leaking hoses to/around it. Anyone got any other pointers before I go prodding tomorrow? I checked the MAP reading, and that was showing air pressure with engine off as 1020mb, and about the same with it running at idle. The maf was showing about 420-440mg, do these sound about right?

Thanks,
Jamie

**kenney** · 05-03-2011,10:32 PM

on idle yes

**martin1810** · 05-03-2011,11:17 PM

You must log in 3rd gear between about 1000 rpm and 4000rpm at full throttle for results to be meaningfull. MAF fault won't give low pressure fault code. Most likely fault is damaged vacuum hose or faulty turbo actuator. Faulty N75 is possible but they don't fail much. I guess you don't have xs black smoke or you would have said.

**Jamiem** · 05-03-2011,11:32 PM

No noticable black smoke no, but it was dark. I only have a lite copy of VCDS, so not sure if i can log anything? Will look tomorrow. I dont think its stuck turbo vanes, as it gets used "quite a bit" and like i said, it gives full power once abover 3000ish. I'm hoping its a split hose, but ill bet its the n75 of worst - thats just my luck with this car!

**martin1810** · 05-03-2011,11:50 PM

A damaged vacuum hose often struggles to give enough vacuum until higher rpm. So Turbo ok at 3000 ish suggests it could just be a leaking vacuum hose. You don't get low boost with stuck turbo vanes.

**Crasher** · 06-03-2011,12:05 AM

Take the vacuum hose off the servo pipe and suck on it. If you can pull air, find the leak. Common areas are where the plastic pipe passes through a metal clip on the air box and just as the plastic pipe comes out to the EGR valve under the coolant bottle.

**Jamiem** · 06-03-2011,04:44 PM

Just logged 2 runs from 1200ish rpm up to 4500rpm, log attached (ive had to change the extension from .csv to .doc to allow uploading).

It's almost certainly an air leak - the egr actuator only moves briefly when the engine is switched off; i'm sure i remember it holding closed for a few seconds before? I had a good look at the vacuum hoses, and found 2 things. The one going to the turbo actuator had a chaffed section, so I changed that out, and then I also found the T-piece at the back of the airbox had a disconnected hose. Confident I found it, I cleared the fault then took it for a run, but no different. I'll go look for some more air leaks, but in the mean time, any further ideas?

Thanks so far

**Crasher** · 06-03-2011,05:10 PM

Suck on the vacuum pipe!

**Jamiem** · 06-03-2011,06:08 PM

I've just had it all apart again, checked each hose individually by disconnecting each end and sucking it. Every hose held a vacuum when disconnected. I then removed the N75, and I'm pretty sure this is the problem. I believe that when switched off, there should be no air leakage through this? If i disconnect the large hose from one side of the N75, then suck through the turbo actuator hose, air is pulled through the other side. If i block it off with my thumb, it holds a vacuum; in fact if I disconnect the other hose on the T-piece with the accuator hose, then put the hose back on the actuator, i can pull a vacuum and open the actuator, so I know this is all good.

So the N75 is the most probable cause?