Idle dropping

CatmanV2

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Is it me, or does this look like the wategate is normally *open*....

s-l500.webp


C
 

Oneball

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11,883
Looks it, which I reckon leads you back to the dump valve being the cause of the weird MAF readings on overrun.
 

CatmanV2

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51,409
Looks it, which I reckon leads you back to the dump valve being the cause of the weird MAF readings on overrun.

From the workshop manual

Screenshot 2024-11-12 at 18.17.30.png

So *one* of the turbo wastegates appears to be not shutting when the engine starts (assumption based on vacuum being applied to wastegates on engine start) Vacuum testing tomorrow. Let's see if we can work out which one is working properly...

C
 

Oneball

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11,883
I’m not sure that’s correct. If the wastegate was faulty you wouldn’t get an increase in flow through the MAF on your overrun.

If one of the dump valves is faulty the pressure on overrun has nowhere to go as the throttle is closed so it goes the wrong way through the MAF. By disconnecting the actuation of the wastegate you’re reducing the efficiency of the turbo and thereby reducing the pressure so it may not be flowing back through the MAF and causing the spike in MAF readings on overrun.

I’m not sure that explains your symptoms 100% either.
 

CatmanV2

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51,409
I’m not sure that’s correct. If the wastegate was faulty you wouldn’t get an increase in flow through the MAF on your overrun.

If one of the dump valves is faulty the pressure on overrun has nowhere to go as the throttle is closed so it goes the wrong way through the MAF. By disconnecting the actuation of the wastegate you’re reducing the efficiency of the turbo and thereby reducing the pressure so it may not be flowing back through the MAF and causing the spike in MAF readings on overrun.

I’m not sure that explains your symptoms 100% either.

I really appreciate you sticking with me on this! Owe you a beer (or several!) Are you using the terms wastegate and dump valve interchangeably? I'm not sure where a dump may be on these, but happy to learn (been watching lots of videos!)

My plan is to try vacuum on each turbo in turn. I suspect bank 2 will increase its MAF readings as the wastegate closes (I wish I could actually see it. It *might* be possible I rather expect bank 1 will not see the same increase, and what's actually happening when I disconnect vacuum from both is not that bank1 is working correctly but rather that bank 2 is coming into line with bank 1 so we get a balance across both banks and a more stable idle.

My hypothesis is that applying vacuum on bank 1 will make no difference to the MAF readings, but will have a difference on bank 2.

No idea about the overrun and so on. At least I can *see* the wastegate on bank1 (sort of)

Or I could be (still) wrong :) We shall see, but I'm actually sort of enjoying it now!

C
 

Oneball

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11,883
Dump valve (blow off or bypass) is in the compressor side of a turbo. It releases pressure in the inlet tract when you lift off so you dont get pressure spikes in the inlet when the turbo is spinning quickly but the throttle is closed (technically a dump valve releases pressure to atmosphere where as yours is a bypass as it’s internal).

Wastegate is in the exhaust side of a turbo and limits maximum boost by controlling turbo speed by bleeding of exhaust off.
 

CatmanV2

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51,409
Thanks Not sure I have these have dump valves, but there *are* electronic bypass valves. I'm assuming this is peforming the same function?


Again, hilarious Scuderia description.

So *if* the dump valve was faulty, I *guess* even if the wastegate *is* closing correctly, we might get a reduction in airflow and screwy overrun?

Hopefully by applying vacuum to the wastegates I can assess if they are moving 'sensibly' and see any effect on the MAF readings. At least that should narrow it down to one side or the other?

C
 

Oneball

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Yep bypass valve performs the same function as a dump valve. It’s just internal. Which is how most OEM set ups are if you use a dump valve which bleeds to atmosphere you get the Max Power woosh noise when you lift off
 

CatmanV2

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51,409
Cool ta. Swhat I figured. I would have swapped that bypass if I could reach the damned thing. High on my list of possible issues a few days ago, and swappable from side to side to see if the issue follows. Will see what the vacuum test brings before I hit my card again!

C
 

CatmanV2

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51,409
Vacuum tester just arrived. Nipped out to the car and popped it on the wastegates

Both take and hold vacuum (well for at least 30 seconds, which I would assume is enough). I can see the actuator of bank 1 moving through a reasonable range of motion. Bank 2 has a different design so can't see it until I remove the heat shield. I must admit I was expecting bank 1 to not move / not hold vacuum.

I've got a day booked off on Friday so I can mess around, but it'll be interesting to see what happens to the MAF readings when I can apply (simply) vacuum to either bank independently.

C
 

CatmanV2

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51,409
Well wasn't *that* fun. :)

I am now supremely confident we know what the issue is, simply because it is the most expensive thing it could possibly have been. (Sometimes it pays to fire the parts cannon, I guess!)

After much cursing, swearing and dropping of tools, 5 out of the 6 heatshield bolts came off. Not that it does much good because on bank 3 you *still* can't see the wategate actuator, or the diverter valve.

With some advice from the US, got into bank one, and lo! The wategate pivot into the manifold is worn. So that's a replacement tubo, thankyou very much indeed!.

Well it happens, One is on its way. I can probably claw back a lot of my costs on this bundle of laughs so all's well. Or will be well. Or might be I guess :)

C
 

DLax69

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5,705
Well wasn't *that* fun. :)

I am now supremely confident we know what the issue is, simply because it is the most expensive thing it could possibly have been. (Sometimes it pays to fire the parts cannon, I guess!)

After much cursing, swearing and dropping of tools, 5 out of the 6 heatshield bolts came off. Not that it does much good because on bank 3 you *still* can't see the wategate actuator, or the diverter valve.

With some advice from the US, got into bank one, and lo! The wategate pivot into the manifold is worn. So that's a replacement tubo, thankyou very much indeed!.

Well it happens, One is on its way. I can probably claw back a lot of my costs on this bundle of laughs so all's well. Or will be well. Or might be I guess :)

C
Glad the mystery is solved; sorry it's not a less expensive lesson...
 

CatmanV2

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51,409
That will have been the rattle then Chris ?
How many miles has she done ?

93k. Oddly it doesn't rattle now (of course the wastegate isn't doing anything). The rattle we could hear was the actuator pivot clip fouling on the heatshield. I managed to get the clip off, and it no longer fouls. Every cloud and all that :)

I imagine, if I reconnected the wastegates, it *would* rattle. Might try for a giggles. Thing is it's rather more drivable with the wastegates off. This makes me wonder how much power I'm actually down!

C
 

Bebs

Member
Messages
3,891
Well wasn't *that* fun. :)

I am now supremely confident we know what the issue is, simply because it is the most expensive thing it could possibly have been. (Sometimes it pays to fire the parts cannon, I guess!)

After much cursing, swearing and dropping of tools, 5 out of the 6 heatshield bolts came off. Not that it does much good because on bank 3 you *still* can't see the wategate actuator, or the diverter valve.

With some advice from the US, got into bank one, and lo! The wategate pivot into the manifold is worn. So that's a replacement tubo, thankyou very much indeed!.

Well it happens, One is on its way. I can probably claw back a lot of my costs on this bundle of laughs so all's well. Or will be well. Or might be I guess :)

C
Good to hear you finally got to the buttom of the issue :)
 

Oneball

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11,883
Fingers crossed for you. Did you see if putting a vacuum on the wastegate affected the MAF readings?
 

CatmanV2

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51,409
Fingers crossed for you. Did you see if putting a vacuum on the wastegate affected the MAF readings?

I did. Short answer: Not really for Bank 1

Raw Vacuum
740​
Solenoid Vacuum
500​
Bank 1 VacuumBank 2 VacuumMAF 1MAF 2
0​
0​
8​
8​
1​
1​
7​
14​
1​
0​
10​
7​
0​
1​
6​
20​

This was 500mm HG static applied by my vacuum tester. I'm not convinced the the solenoid isn't, in fact, switching on and off in a duty cycle, but I don't have a scope to test it, and I'm not really sure it would tell me anything other than for curiosity. I could simply re-plumb stuff to use the car's vacuum rather than the tester, but this was quicker / simpler.

C