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Hi all, I hope I am in the right area for this, being newbie i’m sure someone will put me straight.
A while back I helped a friend's son purchase a 2k Caddy with the BLS engine. It went well enough except after a journey it would pump coolant out the overflow. After numerous tests i diagnosed head gasket and sure enough was putting compression from no.1 into the water jacket. Did the repair which included a camshaft kit and injector re-seal. Started and ran fine but could not get it up our driveway (generous slope) without stalling. On the flat it would go but needed 2 or 3 downshifts to get it up moderate hills. No codes showing.
Checked compression - all about 39b, checked and adjusted torsion to -0.5, checked operation of turbo actuator - ok, checked vac pressure - 25mb at reservoir, pressure tested charge air system found leaks at intercooler and turbo silencer, replaced both. Checked fuel lift pump - (marginal), tested tandem pump, found low pressure on load and leaky seals, re-sealed so pressure now 8b + at pump about 1b in return. Had turbo rebuilt. Checked exhaust for blockages - clear.
Bought VCDS, the turbo pressure seems a little low and even hesitant at low revs otherwise not sure what some parameters are telling me!
Still no codes.
So here I am thinking I am no good at this any more
Anyone got any suggestions? Burn it and claim the insurance not being an option.

Hi JK, welcome to the forum.
With VCDS, there's lots of parameters you can check.
I don't have a BLS engine to hand to plug into but first up injector idle stabilization is worthwhile checking/monitoring all the way from stone cold to 70° coolant temperature.
Then, actual V's specified boost, MAF and intake temperature on a run can give useful data.

Many thanx Doc will give it a go tommorrow.

Silly question especially to someone like you but was the oil system flushed out?

Check your air filter as well mate. No air in, no air out

Doc wrote: ↑Tue Mar 16, 2021 7:04 pm Hi JK, welcome to the forum.
With VCDS, there's lots of parameters you can check.
I don't have a BLS engine to hand to plug into but first up injector idle stabilization is worthwhile checking/monitoring all the way from stone cold to 70° coolant temperature.
Then, actual V's specified boost, MAF and intake temperature on a run can give useful data.

Hi Doc, thanks for your previous input. Since then i did what you suggested with the log runs. The boost run showed a terrible turbo lag. The actual would not reach the spec until 3k plus revs. I took it to a mapping centre near me as they had a dyno etc. They could find no problem with the turbo controls and sent me off (£114 lighter) saying it was a duff turbo. I contacted the people who overhauled the turbo who said it really cannot be as it was fully adjusted and testedok. They suggested an exhaust blockage. I have just done another run with the exhaust removed at the manifold. No change.
I have logged the fuelling as you suggested but am not sure what i should be looking for here. Any guidance would be much appreciated.

Have you got a table of boost request Vs actual for your run? N75 duty cycle on there too would be useful.

Yes i have one taken today. It has acc posn, engine speed, boost request and boost actual. No N75 though could do one with that as well if required. Since my first post i have changed the n75.
I have the fuelling one which looks a bit odd if that helps.

Post up what you've got.

Hi Doc, Ive replaced the exhaust and re-run the two tests. I have included the boost control duty cycle on the boost run. in each case the run was commenced at 25mph in 3rd gear and the revs maxed out at 3.2k.
Not sure how I go about getting this to you. I thought I could send you the csv files but not sure how.

Send the logs in an email, I'll graph them and post the results.

Hi Doc, did you get any sense out of those logs ? Cheers.

Hi all. Still struggling with this problem. I produced a log using vcds basic settings 04 group 11. This shows the n75 doing 10% off to 90% on. The boost pressure varies 969 to 1071 mb so about 102mb, all at 1400 rpm. So i guess this part of the system is working as it should?
I want now to test for maf failure. I thought to check the intake air temperature sensor integral to the maf but vcds shows a G42 and a G72 IAT. They show identical temperatures which increase with engine speed so neither can be the one in the maf. Can anyone help me id the maf IAT sensor on vcds?
Also does anyone have a performance curve (mass air to voltage output) i could use to prove the maf ok or otherwise?
Thanks in anticipation, and many thanks to Doc forhis guidance so far.

Hi all, so didnt get anywhere trying to test the maf other than testing continuity of cabling back to the ecu. The maf was reading and had variation across the rev range to a point so i guess it was working but i changed it for new one anyway to be certain. This did nothing to improve the performance issue. I am getting a p0299 code for underboost but assume this is due to the n75 never getting above 55%.
So question is, what else is there that determines the operation of the n75 that i can check and how? Had this problem for months now and have got nowhere despite having thrown the spares book at it. Any suggestions chaps?

Having looked at the table of boost request vs actual boost, I have to question whether there is something going on in your ECU map that is causing this N75 not to react.
To the best of my knowledge, the N75 duty cycle should react to boost (PID loop controlled if I'm not mistaken). I don't have a PD engine to log on at the moment but I believe the N75 duty cycle starts off low and as the actual boost gets closer to the requested boost the duty cycle should increase.
I can't say for sure if there is another influence on the N75 duty cycle but I don't believe there is.
If you clear the under boost fault and take it for a run with only part throttle, check does the N75 duty cycle adjust at all?