850 T5 300Hp - Loss Of Power At High Boost (Rumbling Hesitation)

[Boxman]

I've had this problem with my second 850 for a while:

96 850 T5

15G turbo

stock engine

300HP drop-in ECU tune (originally for 16T)

Uses oil (valve stem seals probably)

When I go full throttle, it pulls well to 3000RPM until the boost goes beyond halfway the white part of the turbo gauge. At that point, the car starts rumbling and most of the power is gone. It's a deep rumbling, in no way metallic (so no knocking), and I haven't been able to source the problem of this yet. If I want to pull as best as possible, I have to carefully apply my throttle to keep the boost before the halfway-white position.

I already:

- Replaced CBV

- Replaced all vacuum in the engine bay for silicone

- Replaced Turbo Control Valve

- Cleaned PTC

- Cleaned PCV system

- Replaced MAF with one lying around in my shop

- Changed spark plugs

- Changed rotor&cap with the working ones that came from my other engine

- Changed HT-leads with the working ones that came from my other engine

Does this sound familiar to anyone?

[Midnight Caller]

My suspicion is still knocking leading to timing pull. What's your MAF measurement at that boost level ? Is it over 800kg/h ?

[Boxman]

I doubt that at a theoretical 300HP my MAF runs out of breath, especially since it already happens at 3000RPM.

Say it is knocking, what could cause knocking other than boost and low octane fuel (as the boost isn't that high - doesn't even leave the white of the stock turbo gauge - and I run 95 ocaten fuel)?

[Midnight Caller]

Firstly I DOUBT you have 300WHP on a 16T, even with a tune. More like 200. Ever dyno-ed it ?

Secondly, I am maxing my MAF at 12PSI on a 16T with a mild tune, so yes, I am thinking your MAF is maxed out. And I have under 250HP at the crank.

Do you have a wideband and boost gauge installed ? Don't rely on the stock one, it may be showing balonies.

[Boxman]

I'm quite certain it doesn't give me 300BHP, because I can't floor it. I'm running a 15G - the ECU is tuned for a 16T. It's the drop-in RICA tune. I figured with a slightly smaller turbo it would just max out the turbo and lose the top-end power. My problems start already at 3000RPM.

I'll believe you on your word when you say you max out your MAF, though I severely doubt that at 250BHP you max it out at 3000RPM already. That would basically imply the MAF supplied by Volvo will be maxed out in the entire turbo-relevant RPM range for stock cars even - as stock these things come with 225BHP.

This is my temporary car while I'm working on my 400HP build, so I don't really intend to throw much money at it - don't want to get it dyno'd, don't see the need to, and I don't think the next set of owners is going to be interested in paying for a wideband-AFR they won't use. I just want this car to run properly. Besides, it's a drop-in tune that works for every single other stock Volvo 850 T5 except this one. That means something is faulty in this one and I'm hoping to find out what before I sell it to a friend of mine.

[Midnight Caller]

Did you check up for any codes ? Yes, it's true, I'm not maxing at 3000rpm, but I'm maxing it at around 4500rpm.

And 16T is a little bigger than 15G but the difference is insignificant to give you that behaviour.

And no, the MAF won't be maxed out on stock cars, because I am maxing it at 12PSI, and the T-5 runs at 9-10PSI. It's close, but not quite.

It's true, something IS faulty, and could be anything from a loose hose to a stucking injector or CBV diaphragm.

You should double-check everything. And see if you have any codes.

[Boxman]

I have yet to check for codes, need my brothers OBDII reader thing he made for that.

Question though, is the actual turbo pressure fed back to the ECU in some way, or could this be explained by:

- Tune was originally made to 'guess' values beyond the MAF range, thus expecting certain pressure

- Pressure is not met due to faulty turbo / faulty CBV membrane or weak spring
- Too rich condition occurs due to unexpected low pressure

[Midnight Caller]

No. Pressure is not monitored in any way.

Pedal sensor + RPM sensor + MAF sensor => LOAD

LOAD dictates opening of the TCV => Turbo pressure.

Look for the M4.4 tuning how-to for details.

Detonation occurs in too LEAN condition, not too RICH condition.

I think what you have there is not too LITTLE turbo pressure, but TOO MANY detonations leading to timing pull -> lower exhaust pressure -> lower turbo pressure.

[MrWinkey]

You say replaced your vacuum lines however does that mean your charge air lines as well? What do the intercooler rubber hoses look like? If your sure it's not a knocking noise and it's a deep rumbling with loss of boost and power it kinda sounds like faulty charge air hose or hoses that are weak creating a negative pressure feedback expanding and collasping and they will rumble cause the pig is starving for air. I'm not gonna debate theory or analogy on this just stating experience and possibly a fix.

[Midnight Caller]

Charge air lines don't rumble They go "PSHHHHHHHHHHHHHHT" under boost. They're not baloons

[MrWinkey]

Charge air lines don't rumble They go "PSHHHHHHHHHHHHHHT" under boost. They're not baloons

OK...You are probably correct and I'm an idiot

[Midnight Caller]

Oh come on now ))

How about we wait for mr. Boxman to pressure-test his sistem and then jump to conclusions

[Boxman]

Yeah, pressure test was on my list, though I have no major leaks - I checked all hoses individually and couldn't find anything. I have yet to make a DIY contraption to test the pressure. Weep hole in the intercooler may cost me some but I doubt that's the problem?

[Midnight Caller]

You can always plug the weep hole to pressure test. Also be sure to plug the dipstick tube, as you will also have a fake leak there through the PCV

I had leaks at :

1). CBV (only leaked over 9PSI)

2). Vacuum tree sealing ring at the intake manifold

3). Turbo intake hose (the one with MAF on it)

4). Leaking intercooler

5). Small hose between the vacuum tree and the idle control valve.

Hope this helps.

[volvoguy23]

Sounds like old or worn ignition wires honestly... What plugs/ wires you using?