Overboost - No limp mode!

I've previously adjusted an actuator and stop screw on a turbo and had good success so I'm fairly well versed in this topic, however...

This one has me a little stumped, I picked up a cheap PD150 ARL MK4 last night as the chap I bought it from couldn't resolve the overboost issue.

Essentially, at 4k RPM the turbo hit 2.0 bar, 94% duty cycle.

I have yet to adjust the actuator myself but I'm told by the previous owner he's literally "Tried everything" including adjustment, MAP sensor replacement and such forth.

I did test the MAF which appeared to show expected vs actual airflow readings where quite close.

The turbo has had a new CHRA installed as the previous one blew up, seemingly due to the same overboosting issue.

What confuses me is why the car isn't entering LIMP mode?

The first thing I'm going to try is to run a basic setting on the N75 and check that the actuator is moving as it should and take it from there.

Does anyone have any suggestions?

I think that your plan of action will get you going in the right direction.

Unless you know that the CHRA was built by a highly reputable company I'd hold it suspect. (was it brand new, or just new to the car?)

From what I'm told, it was a new CHRA which he fitted into the old housing. Accordingly the old housing was free from damage etc.

I'm inclined to think that won't be the route cause of the issue, but won't rule it out yet.

What really stumps me is that the car is clearly overboosting, but not entering limp mode or throwing any codes, that's not something I thought could happen.

My wife's car overboosts and doesn't throw any codes: one day I'll look in to it- pretty sure it's sticking vanes. I run Ultragauges in both cars and I hit in the 18psi range in my car and in hers it hits over 22psi (which is probably why it managed to hit 258 ft-lbs on the dyno- only a Malone stage 2 tune, DLC520 nozzles). The cars are identically equipped/tuned.

This is a nice writeup on sticky vanes:

http://www.myturbodiesel.com/wiki/vnt-turbo-cleaning-disassembly-replacement-repair-stuck-vanes-mk4/

Ah I see, I'm fairly convinced it wont be the vanes, as again the previous owner did say they were cleaned when the CHRA was fitted.

My gut feeling is that perhaps there's an issue with the actuator / N75, holding vac fine and not being able to release it.

I will take a look when I get chance and see how it goes, I'm aware that sometimes the boost limit is mapped out on a tune to allow for the increase in boost, without triggering limp, but I'm surprised the ECU isn't triggering the limp on a standard tune

Time will tell! (I hope)

ColtTDI said:

Ah I see, I'm fairly convinced it wont be the vanes, as again the previous owner did say they were cleaned when the CHRA was fitted.

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The backing plates that the vanes ride on warp, which also results in sticking vanes. Do a search and you'll find this out. Unless someone knows about this then one can clean it all up and not know any better; put it back together assuming that it's all taken care of. Some folks have managed to resurface the plate, though I'm not sure how that was done (or how successful it was in the long-run). This is a big reason why it's often better to just break down and get a new turbo.

Update...

I ran a basic setting test on block 11 and watched the actuator, it moved up and down freely without any issue.

So I adjusted the actuator and made the length longer, although it already felt as long as it could be really. Physically pushing against the top of the VNT mechanism arm...

After driving it with a fully extended actuator arm, it was still boosting like hell...

The MAF appeared to be alright as far as readings went, but I swapped it over anyway

The car began to drive entirely different, very laggy and triggering LIMP, underboost which is exactly how I imagine it should be running with an actuator set like that.

Hopefully it's just a case of setting it all correctly again

what are the requested vs actual map readings ? can you post log along with the n75 duty cycle ?

Here's the log:

This is with the actuator fully extended to it's maximum length, in the hope that it would give the ECU more control... which clearly didn't work!




Here's with the MAF sensor unplugged and the actuator and stop screw set to factory specs, I discovered the MAF I swapped over was the incorrect part no, and or defunct. The car behaves the same with the MAF unplugged as it does with that sensor.

I've caved in, decided to swap the turbo over!

I've checked every avenue now to no avail, also had a GT1749VB turbo laying around which seems OK to use, no play at all but very slight impeller damage, looks to have ingested some particulates at some point.

I'm going to delete the EGR cooler whilst I'm at it, as I really cannot be bothered to put it back on, everything is very tight and irritating compared to the VE engines I'm use to!

The turbo is now off, it didn't take me long to realize what that issue was...

The VNT mechanism is jamming just before fully open, a single vain is sticking pretty firmly, other than that the turbo is in really good condition and I'm hoping a good clean of the VNT mech will resolve the issue

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ColtTDI said:

Ah I see, I'm fairly convinced it wont be the vanes, as again the previous owner did say they were cleaned when the CHRA was fitted.

Click to expand...

Either the PO didn't do a very good job cleaning or the backing plate is warped. Search around and you can find discussions about (possibly) rectifying this.