Boost spiking on throttle off

[eyeballprawn]

I understand about spiking from spooling up too fast, but why would my boost be spiking on throttle off? I have an EBC installed and it will show a steady 10psi but then it spikes as high as 13psi when I let off the throttle at decent revs.

The car is an auto, and it's the same spike in both Tip (if I stay in the same gear) and Auto (if I lift off and let it change up a gear or two). The BOV is stock, could it be that it can't expel the extra boost properly?

The hosing for the EBC solenoid is all as short as can be, however the control unit in the cabin requires a vacuum hose to be run all the way to it. The unit is in the coin pocket that folds down near your right knee.

I've Tee'd off the the line going to the Fuel Pressure Solenoid for the control unit, the manual said to do it there. The unit is also where I'm reading these boost levels from.

Cheers.

[peter thomson]

Have you checked the BOV is working correctly

[eyeballprawn]

No I haven't had a chance to have a look at it yet sorry, I only just got back this evening from test driving. However, it does seem to make a release noise at the correct times but I believe the stock BOVs are pretty quiet anyway aren't they? How can I test it, just by removing and blowing air through it?

[orionn2o]

As you let off the throttle you close the butterfly...

What happens then? Lots of air rushing out of the turbo, big jump in pressure, then a quick fade away and tail off.

[Turbo_Steve]

Are you sure the release noise isn't the turbos stalling? Sounds awesome but not reccomended.

That would explain the pressure increase.

[Nick Mann]

Where are you monitoring the boost? If you are monitoring pre-butterfly, then the spike is the butterfly closing.

There is no way that I can see that the manifold pressure can increase when the throttle is closed, unless the BOV is not working.

[Davezj]

agree with nick here!

if boost is being monitored correctly, after the butterfly valve in the throttle body boost should not be able to increase when trottle is released.

[bradc]

What they all said ^ sounds like normal operation to me. It should only be a very brief spike when you lift off the throttle.

[eyeballprawn]

Boost is being measured after the throttle body, from the hose that goes to the Fuel Pressure Solenoid. So that's why I can't understand how it can still increase after the throttle is closed.

So if this is normal operation in a turbo car, does anyone care to humour me with some theory as to how it works?

My feeble mind sees it this way:

Letting off the throttle suddenly, the pistons rapidly slow down so the boost pressure in the intake backs up, however normally the BOV should release this. But if the throttle is snapped shut the higher, backed up boost is now stuck in the plenum with nowhere to go and just has to wait until the engine can pass it through.

Does that sound plausible?

[Davezj]

where exactly is the fuel pressure reg being feed from are you 110% sure it is from thr plenum chamber.

What side of the fuel pressure control solenoid valve are you connected to, as that could be you problem. i would imagine it should be the plenum side of the valve to read manifold pressure correctly and not the messed about with pressure that is controlling the fuel pressure regulator.

do you have one of the translated jap manuals if so then look at vol. 1 page 72, it has the pressure pipe work diagram on it and you can see what i mean, look at diamond shape 3 valve.

[eyeballprawn]

This is where I've Tee'd into for the EBC control unit/gauge.

[Davezj]

that looks like the right place to me, as i have my boost gauge on exactly the same point.
However i do not get a boost spike when i lift off the throttle. but my boost is stock about 7-8psi. i only get a spike when i initialy put my foot down at 3000rpm in 3rd, 4th, 5th. then after a few seconds it all settles done and boost is constant.
now we understand you have everything plumed in correctly and you are actully measureing the plenum pressure we can get to the diagnosis. but it is very important that we know what you know. so we are not making assumptions.

i will double check on my car tomorrow and do a 3000rpm in 3rd, 4th in tip mode. and get back to you but i am sure there is no spike on mine when i lift off the throttle.

the only way i can think of that the pressure can increas ing the plenum when the butterfly valve closes is there is some kind of blow back from the cylinders, which would be very bad, valve not sealing properly, etc. but you would expect to see that after every 2 revolutions of the engine when that cylinder fires again and you would see it at idle as well. if explosive gases were being forced back past the valves into the plenunm chamber at idle the boost pressure would surely show up. don't worry i am just thinking out loud.

[eyeballprawn]

Thanks for that. My EBC has a 'last boost' display that flashes when I let off the throttle. It's this that's showing the spike, however under power it holds nice and steady.

[Davezj]

does it spike when you initially put you foot down.

When you say last boost, does that mean it is the last maximum boost recoreded, if so then that will be the spike as you put your foot down as it starts to accelerate at about 3000rpm i say 300rpm as this is most likely to give you a boost spike when you put you foot down.

if not then what does it actully record. last boost what is that and how is it beneficial to recored it.

[eyeballprawn]

No I'm not getting spiking when I initially put my foot down, and if I did I'm able to tune it out by adjusting the START BOOST to spool up slower so that it doesn't overshoot.

From the manual:

LAST BOOST
This mode is used to display the last boost that was recorded every time the accelerator was released for 3 sec.

It's not worded very well, it means the last boost is flashed up for 3 seconds before returning to displaying the normal current reading.

There is also a PEAK boost recorded. If I reset this before a test run in a certain gear, say 3rd in Tiptronic mode, it will correlate with the LAST BOOST.

However.. there is also a GAIN function to help stop the boost falling off at high revs. Unfortunately the controller won't let me reduce this to zero, the lowest I can go is 5%.

GAIN
This mode is used to adjust the boost consistency. (Adjusts the sensitivity of the valve control).
- When boost spike occurs, lower the GAIN control.
- When the boost falls off at high RPM, or the boost increases as RPM increases, raise the GAIN value.

If anything my boost slightly drops through the revs, perhaps 0.5psi, but I would consider that pretty steady.

There's also a LIMITER function that will pull the boost back a certain percentage when exceeded, but since the throttle is shut it has no effect on the spikes I'm seeing.

[kc427]

I agree with you Dave, once the throttle is closed, and there is a spike, inside the plenum, there must be something to increase the pressure inside the plenum.

I am not sure, but what I can guess is, let's say, when acceleration hard, and suddenly shut the throttle, then, for example, if the BOV does not release the gas rapidly enough back to the intake(pre-turbo pipe), it could cause the turbo stalls. OK, let's assume this is the case, then, if the turbo stalls, ie, the turbine stalls, and the exhaust gas between cyclinder to the turbine has nowhere to go, then, what will happen? I don't know, as I am not a mechanical guy, but I just guess what it might happen.

that looks like the right place to me, as i have my boost gauge on exactly the same point.
However i do not get a boost spike when i lift off the throttle. but my boost is stock about 7-8psi. i only get a spike when i initialy put my foot down at 3000rpm in 3rd, 4th, 5th. then after a few seconds it all settles done and boost is constant.
now we understand you have everything plumed in correctly and you are actully measureing the plenum pressure we can get to the diagnosis. but it is very important that we know what you know. so we are not making assumptions.

i will double check on my car tomorrow and do a 3000rpm in 3rd, 4th in tip mode. and get back to you but i am sure there is no spike on mine when i lift off the throttle.

the only way i can think of that the pressure can increas ing the plenum when the butterfly valve closes is there is some kind of blow back from the cylinders, which would be very bad, valve not sealing properly, etc. but you would expect to see that after every 2 revolutions of the engine when that cylinder fires again and you would see it at idle as well. if explosive gases were being forced back past the valves into the plenunm chamber at idle the boost pressure would surely show up. don't worry i am just thinking out loud.

[kc427]

OFF TOPIC: Try not to have overshoot when initially step on the gas. I have used EvoScan to log the knock, and I saw there was as much as 3x knock sum counts when overshooting. In this case, the ECU will retard the ignition timing, and I guess you know what will happen if ignition retards. Finally, I had to adjust my EBC to avoid overshooting in order to have good result.

No I'm not getting spiking when I initially put my foot down, and if I did I'm able to tune it out by adjusting the START BOOST to spool up slower so that it doesn't overshoot.

From the manual:



It's not worded very well, it means the last boost is flashed up for 3 seconds before returning to displaying the normal current reading.

There is also a PEAK boost recorded. If I reset this before a test run in a certain gear, say 3rd in Tiptronic mode, it will correlate with the LAST BOOST.

However.. there is also a GAIN function to help stop the boost falling off at high revs. Unfortunately the controller won't let me reduce this to zero, the lowest I can go is 5%.



If anything my boost slightly drops through the revs, perhaps 0.5psi, but I would consider that pretty steady.

There's also a LIMITER function that will pull the boost back a certain percentage when exceeded, but since the throttle is shut it has no effect on the spikes I'm seeing.

[Davezj]

No I'm not getting spiking when I initially put my foot down, and if I did I'm able to tune it out by adjusting the START BOOST to spool up slower so that it doesn't overshoot.

From the manual:



It's not worded very well, it means the last boost is flashed up for 3 seconds before returning to displaying the normal current reading.

There is also a PEAK boost recorded. If I reset this before a test run in a certain gear, say 3rd in Tiptronic mode, it will correlate with the LAST BOOST.

However.. there is also a GAIN function to help stop the boost falling off at high revs. Unfortunately the controller won't let me reduce this to zero, the lowest I can go is 5%.



If anything my boost slightly drops through the revs, perhaps 0.5psi, but I would consider that pretty steady.

There's also a LIMITER function that will pull the boost back a certain percentage when exceeded, but since the throttle is shut it has no effect on the spikes I'm seeing.

i am not famillier with EBC but i would sugest fitting a standard mechanical boost gauge for a short time (tee it in from inside the cabin from the pipe the EBC is connected to) just to check this out in real time as a spike when you take your foot off the accelerator should be visable on this mechanical gauge.
From the wording in the manual it looks like it displaying the peak boost when you take you foot off the accelerator, which occured earlier on in the acceleration run. I know you it is not doing that but that is what it sounds like to is doing from the wording.
glad to see we are getting somewhere now.

[orionn2o]

Or take a short mobile vid of it happening.
Is it a greddy ebc?
With most ebcs, they ll show the max boost during that particular cycle as suggested above.
Without accurately plotting boost, its difficult to say whats happening.
When i take my foot off the gas, my boost appears to go sky high for a second. However it doesnt, cos it cant! Its just the gauge reading funny.

Ill upload a pic in a min

[orionn2o]