Temporary lack of boost

[Laser411]

I had an incident yesterday where I went to accelerate with a BMW M5 and I noticed a lack of power. A few more pulls and it was apparent that there was no boost at all. Today, everything seems fine so far.

A little background:
-Bought used, has 130k miles
-No tune or mods as far as I can tell
-The car was fully warm and I had been driving normally for about 20-30 minutes prior
-While there was not very much power, it was VERY smooth.
-No check engine light or codes thrown.

I do have a PC based scan tool with some logging abilities. What parameters/PID's should I be monitoring?

Thanks in advance for any insight provided!

 

[krewat]

I'm going to throw out an idea: The bypass valve is sticking open.

 

[Laser411]

Sounds like a great guess! Where is that located? is there one for each turbo or just one total?

If there's one for each turbo, wouldn't there still be some boost and a really crappy running engine?

 

[krewat]

I'm not an expert on this stuff, I haven't had to work on any of my SHO's (yet). But I think there's two valves, and when they open, they bypass boost from after the turbo, back to before the turbo.

https://shoforum.com/index.php?thre...mosphere-with-factory-blow-off-valves.125509/

That thread is about venting to atmosphere, but you can get an idea of where the factory BoV's are from that.

Of course, it could be an intermittent boost leak too...

 

[Laser411]

Ohh, you meant the BOV, yeah I know where those are. I did the VTA mod at one point but reverted it because I couldn't get a good seal on the intake hose.

If that was stuck open, would there be any unusual noise?

 

[SM105K]

Ohh, you meant the BOV, yeah I know where those are. I did the VTA mod at one point but reverted it because I couldn't get a good seal on the intake hose.

If that was stuck open, would there be any unusual noise?

Yes pull the BOV's off and check the diaphragms on them.

 

[SHOdded]

Any codes in the PCM? Check the hoses on top of the intake manifold that connect to the solenoid. Ford leaves them unclamped, and they can/do come loose.

 

[Laser411]

How do I check the diaphragms?

There's no CEL but I have yet to check if there are hidden codes.

Theoretically, a loose hose should still be an issue today too though, right? I will check them when I get home in a few hours.

 

[krewat]

I would think even a torn diaphragm would be a permanent condition, and not intermittent. But again, I have no experience with these things beyond changing the oil - yet

 

[Laser411]

Yeah, this was a 20-30min drive with no issues and when I went to keep up with the BMW, it stayed in the low gears for a while (because it was slow, not overrevving or slow shifting). I thought the transmission was just not shifting at first until I did a second pull and noticed that it was just a slow smooth power curve seemingly without boost. It went all the way up to ~6k before shifting like it would normally would, just took much longer to get there.

It's exactly how I would imagine it would feel of you just unhooked the intake from the turbo. No abnormal sounds (possibly a lack of turbo whistle, but i did not pay close enough attention). It was also the smoothest shift I have ever felt under WOT, probably due to the lack of torque.

Today, it's driving like it never happened, maybe even slightly better.

 

[SHOdded]

Getting eyes on, and checking for codes, is your best bet at this ooint. When you look at the BOV, there is an orange diaphragm. If it is torn, it will be obvious. Boost leaks can start off as intermittent, and take a while to become glaringly obvious. Or a pipe could simply blow completely off and be very apparent immediately. Check the turbo piping for looseness/cracks on the underside. Telltale sign are oil and dirt accumulation near the clamps.

 

[Laser411]

Sure enough, a hose popped off. This part is actually the "bypass valve" whatever that actually does. It ran perfectly when not hot, better actually, but once i got more on the throttle it started to cut out and when I did a 4th-5th gear low rpm WOT you could feel it surge on and off.

TLDR; it's probably fixed (didn't take it back out yet) by this little hose that was disconnected. Picture attached.

 

[SHOdded]

 

[SM105K]

I have that same picture in my ownership thread....same thing happened to me.

 

[Shadow351]

I believe that line is part of the wastegate control system, which could absolutely cause a lack of boost.