Boost rise then fall

[Bigsi15]

Quick one guys…

Whilst I'm building the other engine I'm getting the current car to Stage 0 and I'm having a bit of an odd problem. Current engine idle is fine runs fine doesn't smoke doesn't hesitate, however when I accelerate Either full throttle or mildly the boost will build to wherever it decides to sit depending on acceleration severity nice and quickly but will then drop slightly back and maintain that level. For example if I put my foot down, by about 3000 rpm the boost has built to around about 0.6 bar but will then drop back by about 0.1 bar and stay comfortably at that level through the Rev range. Wastegate is set to open around 4 psi and I have tried fiddling with this pressure a little to see if it makes any difference

Anyone got any ideas what this is? I put a differently tuned ECU in and it quite happily boosted to around one bar so the wastegate despite being stock seems able to hold itself closed so I'm not sure why the Boost seems to be cut back at stock. If I can I'll get a video and put a YouTube link to it as I don't think I'm explaining it very well!

Thanks all!
Si

[Bigsi15]

Oh forgot to mention it is a 1996 850 T5 running completely stock and returning on average mpg of about 26 to 28 on a roads and inner city driving

[Mart5in]

Quick one guys…

Whilst I'm building the other engine I'm getting the current car to Stage 0 and I'm having a bit of an odd problem. Current engine idle is fine runs fine doesn't smoke doesn't hesitate, however when I accelerate Either full throttle or mildly the boost will build to wherever it decides to sit depending on acceleration severity nice and quickly but will then drop slightly back and maintain that level. For example if I put my foot down, by about 3000 rpm the boost has built to around about 0.6 bar but will then drop back by about 0.1 bar and stay comfortably at that level through the Rev range. Wastegate is set to open around 4 psi and I have tried fiddling with this pressure a little to see if it makes any difference

Anyone got any ideas what this is? I put a differently tuned ECU in and it quite happily boosted to around one bar so the wastegate despite being stock seems able to hold itself closed so I'm not sure why the Boost seems to be cut back at stock. If I can I'll get a video and put a YouTube link to it as I don't think I'm explaining it very well!

Thanks all!
Si

With the standard ECU that sounds normal to me. That's similar to what mine was before I changed the ECU.
It would go to about just over 8Psi and drop to just under 8psi and sit there.

[LeeT5]

I agree. Thought boost would always be higher when initial acceleration due to compressor spooling up. Once it reaches max spool for the given throttle request, it will naturally fall away slightly.
Due to the 16T having practically no lag, the boost is very controllable using your right foot.

However, 0.1bar is only 1.4psi so you must have a boost leak somewhere or something is very wrong!

Have you tried unplugging the boost pressure solenoid and seeing what boost you get? Boost will be controlled by the wastegate and this is set by manufacturer at around 3-4psi. You should see 3-4psi on your gauge with the BPS disconnected. If not, you have a leak somewhere.

[Mart5in]

I've just had a thought!

The one way valve is probably failing. Is there oil misting around your oil filler cap or oil sitting around the head?
It's next to the throttle body on the manifold, there's 2 pipes. The upper pipe goes to this one-way valve.

Mine just failed and it did exactly what you describe. Boost would build to 10psi, then drop to 6psi and sit at 6psi. blocked that pipe temporary and boost sits and stays at 10psi

[LeeT5]

I've just had a thought!

The one way valve is probably failing. Is there oil misting around your oil filler cap or oil sitting around the head?
It's next to the throttle body on the manifold, there's 2 pipes. The upper pipe goes to this one-way valve.

Mine just failed and it did exactly what you describe. Boost would build to 10psi, then drop to 6psi and sit at 6psi. blocked that pipe temporary and boost sits and stays at 10psi

There are TWO one way valves and he's only getting 1.4psi not 6 or 10 or anything above 2!!!

Martin is right though. Replace both the one way valves first and then see how she goes. You'll probably only gain about 2 psi if they leak. Otherwise, your gonna need a smoke leak test.

[Bigsi15]

Thanks guys I'll definitely have to look into that as I wasn't aware of any one way valves. . Leet5 luckily it's not quite as bad as you mention it is only dropping by about 0.1 not down to 0.1 LOL! Mart5in this sounds quite plausible are you talking about the vacuum hoses or is there something else in there I missing? I've never noticed the valves other than the idle control valve?

Thanks for all your help guys

[LeeT5]

Thanks guys I'll definitely have to look into that as I wasn't aware of any one way valves. . Leet5 luckily it's not quite as bad as you mention it is only dropping by about 0.1 not down to 0.1 LOL! Mart5in this sounds quite plausible are you talking about the vacuum hoses or is there something else in there I missing? I've never noticed the valves other than the idle control valve?

Thanks for all your help guys

Clearly, I misunderstood your original post then? Anyway, here are the valves so you can easily find them and replace them.
Mr Tim Williams will concur....they are very often overlooked and never replaced.

Vacuum valve No1

30404

Vacuum valve No2

30405

30403

[Bigsi15]

That's no problem I probably didn't word it very clearly :) thanks for the pictures I know exactly what your on about now brilliant. Il give those a go and see if it helps!

Thanks again

[Markyb]

Mine arrived today in the expensive Volvo packaged bags.

[Bigsi15]

Markyb if yours were also for an 850 t5 would you mind posting the part number and price? Thanks buddy

[Markyb]

Mine were for a p2 R

[Bigsi15]

Wonder if there is much/any difference?

[Gold 'N' Brown]

So are both these present and in the same places on a Ph2 T5? Would they be recommended to be swapped out as part of Stage 0 maintenance?

[LeeT5]

So are both these present and in the same places on a Ph2 T5? Would they be recommended to be swapped out as part of Stage 0 maintenance?

Yes.

[960kg]

.......

[LeeT5]

.......

960KG, why have you changed your post or deleted it? I noted what you said and your wrong!
Have you realised your mistake or has admin edited your reply?

[Mart5in]

The valves shown in the pic are in different places on the 850 I think, those are ME7 pics?

Valves on my 850 were attached to the radiator fan shroud :)

[LeeT5]

The valves shown in the pic are in different places on the 850 I think, those are ME7 pics?

Valves on my 850 were attached to the radiator fan shroud :)

Yes, they are P2 R pictures. Exactly the same on a P2 T5.

I can't remember if they are in the same location on a P1, but I really posted the pictures so that the OP understood which valves I was referring to.

[960kg]

960KG, why have you changed your post or deleted it? I noted what you said and your wrong!
Have you realised your mistake or has admin edited your reply?

I deleted it myself as i then noticed that the hose the valve was on went onto your inlet manifold which was removed.

Not a very clear indication, however congratulations for reading it as that is why it was there....lol

[LeeT5]

I deleted it myself as i then noticed that the hose the valve was on went onto your inlet manifold which was removed.

Not a very clear indication, however congratulations for reading it as that is why it was there....lol

What on earth does that mean??

It's a perfectly clear indication of where the valves are on a P2 T5 or P2 R model. It's also totally obvious the inlet manifold is not present, so I'm really not a clue what your on about?

[jdavis]

does anyone have part numbers for these for an 850 please? description sounds like mine need renewing

[Bigsi15]

http://youtu.be/IfnqBBO84Lg

[Bigsi15]

Finally got a video up! Boost rises then falls back off then picks up a little again. Seems to reach different max pressures each time too. Will try the vacuum valves as soon as possible first

[jdavis]

Yeah mines does that occasionally.

[LeeT5]

Finally got a video up! Boost rises then falls back off then picks up a little again. Seems to reach different max pressures each time too. Will try the vacuum valves as soon as possible first

Looking at the video reminds me of when I had similar boost issues.

Some questions, if I may....

1. Have you had the codes read on the car and what are they?
2. When you accelerate hard and the boost does what it does in the video, if you come to a stop...does the car feel like it's going to stall slightly? Rather, do the revs 'DIP' below normal idle speed??
3. Are you using PTFE tubing anywhere from the vacuum side to the boost gauge?

If the answer to Q2 or Q3 is yes, then you almost certainly have a boost leak post MAF.
If you have a split in the PTFE tube then it will give the symptoms shown in the video. (check this using a MITYVAC)

Likely places for your boost leak will be:

MAF to Turbo pipe (very common and easy to spot - look for oil residue)
Split PTFE pipe to boost gauge
Inlet manifold gasket
Vac tree and associated rubber tubing/vac lines
Split IC
Compressor Bypass valve (spring weakens and rubber deteriorates with age)
Leaking Boost pressure sensor (uses a very small 'O' ring)

On visual inspection....any misty, oily residue around these areas indicates a boost leak.
Some leaks can be almost impossible to detect visually, which is why it's highly recommended to have your car smoke leak tested.

When I had mine done it revealed 4 boost leaks!!! 2 of which, I would have never found. The 5th remaining leak I found once I purchased a Mityvac, because the vacuum hose to the boost gauge was only leaking above 10 psi, therefore, not detected on the smoke leak test.

A smoke leak test will save you a lot of time and potentially money trying to diagnose a boost leak.

All main dealers have smoke leak test machines.

[Bigsi15]

Thanks leet5 this is very helpful and food for thought. I wouldn't be surprised if I have a boost leak somewhere tbh. Il try the vacuum check valves and give it a good look over.

I don't recall any issues with idle or revs dropping too low, codes were clear last time I checked and I'm not sure what ptfe tubing is lol but both the Volvo stock gauge and aftermarket gauge both show the same thing, as far as vac lines go they are almost all pretty new and seem in good shape other than the ones with those check valves in interestingly... Anyone know a good source for good vac lines for these cars and sizings? Might just do them all anyway

Thanks again