Drewmon,
How can you tell that your tensioner failed?

SB,
I actually didn't notice the boost drop untill I had my car dynoed and the muchanic pointed it out to me with the grapjh posted above. I verified it with my boost guage (I didn;t really pay attention to it before), the needle did indeed dip at around 2800rpms, then climb back up at around 3500rpms. So I don't think it is a software problem.

El_griton,
Yep, my boost is generally on the low side, specially after 2800rpms where my boost fell, then slowly rises. I'm actually trying to pinpoint if I have a boost leak or a tensioner problem. Please take note that my climate is extemely hot and humid, so that can bring my boost levels a bit down.

John

 

i'm in the same boat for the climate. last 4 days here in charleston the heat index has been over 110. and today was 120. not so great for power but I still see boost levels in the 18 psi range.

 

Symptoms can vary with tensioner failure, but with mine I noticed a metallic rattle from the belt side of the motor and when watching the belt go around one could observe erratic movement in the tensioner arm as it struggled to compensate for the extreme movement. The metallic rattle would dissipate after the car ran for ~5 - 10 minutes but the movement was still observable.

When accelerating alongside a wall to reflect the sound one could also hear kind of flapping noise. Mild, but there - maybe a result of the belt slapping slightly under acceleration?

The tensioner was shot on the bearing (which wobbled and made the noise), just about fell apart in our hands...

 

Does the 19% spin the charger to fast and how much boost should I see on my gage when In install it

 

not only does the 19% pulley overspin the SC but it is also spinning your water pump to fast. do a search on it and there will be tons of info.

my next step is electric water pump.

 

I'll check my tensioner according to the symptoms that you guys have observed. Hopefully its just my tensioner.

John

 

looks like the throtle is closing @ 2750 rpm . something is telling it to close at exactly 2750 and open fully later on hence boost drop ,

 

Hmm....that's another way to look at it. Has anyone with a giac remap and a 19% pulley observed this with they're boost? I'll have to check the AFR's, maybe it would show a similar movement in the graph.

John

 

they need to be able to pressurize your intake track and sniff for the leaks. The symptoms on your boost are perfectly consistant with a leak that doesn't show up till a certain boost. When it opens, air can flow out of a new path, so the pressure drops a bit before it starts to climb again. Something that seals the intake and looks for stuff that leaks into the intake track will never find it.

I'd start by wiretieing all the vacuum hose connections that you did for your boost guage etc. You might get lucky. Don't just look at the IC boots, take them off, clean out the oil (and there will be oil) and clean off the flanges before putting it all back together.

Good luck on this, but I'd bet that if it's not an easy one at your boost guage, then you'll need to find a shop that can do the pressure testing of your intake path....

ps, this isn't a software issue. Software won't change how the boost raises vs RPM, it will just change how the fuel is squirted and when it's fired. All this stuff is just based on fluid dynamics and how much air is moving through the engine.

Matt

 

Hmm....why didn't I think of that. I used to pressure test my waterlines for my watercrafts. I don't think there's a shop with that kind of equipment where I am, so I might do this myself. Where do you suggest I block off the airpath? The easiest would be at the throttle body but that would be too far upstream. I'm assuming the SC will leak air pressure back up to the throttle body, so some sort of plug should be used somewhere in between the IC and the SC. Any suggestions?

I will try checking my boost guage/vacuum lines though first. Thanks!!

John

 

then I'd just try really clamping down on all the connections with wire ties and hope to get lucky....

But don't dispair so soon on the equipement. Don't call standard service shops. Call race shops or places that deal with turbos apps. And if I'd have to guess, I'd think that the best place is the SC input, but it's a wierd shape. I don't know how much pressure the plastic TB-IC plenum can take, but you only have to test to about 4 pounds or something to get your leak. You'll have to pinch the crankcase vent lines.

Matt

 

I don't think belt slippage would cause the boost drop around 3000rpm. The belt would be more likely to slip at the top end. That said, your boost curve has a normal shape for the MCS, even though it might be overall a bit low for a 19%. The good Dr., among others, have posted similar findings:
https://www.northamericanmotoring.co...&highlight=HAI

I once suggested this boost drop is the source of the famous "flat spot", but it didn't garner much support.

 

I'd totally blanked on that! FWIW, I was running a HAI, pulley and maybe a bit more at the time, but my endyne head wasn't on yet..... Wonder if my VGS mods need some wireties?

The only thing that could expain this is the drive by wire programming, but I'd have to log throttle positions to do it. About item 100,004 on my to do list!

Matt

 

Arghh....this is confusing me. Can anyone here with a boost guaage confirm that there is a drop in boost at around 2750-3250rpms? But I do have pretty low boost numbers for a 19%, so I'm still going to check for leaks. I'm living in a tropical country so could humidity and high temps bring my boost this low?

John

 

The throttle is most likely cause of the boost anomaly,it looks as though it is not fully opening and shutting at around the 2750 rpm mark
The throttle is opened and closed by an electric motor,and controlled by a feedback loop consisting of the throttle pedal position (demand signal) and the position sensor in the throttle (feedback) there are in fact two feedback loops 1 for angular displacement of the throttle and one for a rate of change for the throttle
when you press the pedal to the floor you create a demand signal for the throttle to open -it opens until the sensor in the throttle says im now open and the throttle then stops
the speed the throttle opens at (rate feedback) looks at what is selected from the pedal (size of demand) against what position the throttle is already at (error signal) the ecu the clever little thing integrates this signal to control the speed.
either position sensor the pedal or the throttle could be duff ,and either can cause the throttle to not fully open-or close a bit after fully opening.
the ecu/map is far less likely to cause this although possible.
without a test set to read the throttle position it is tough to tell ,substitution would be the easiest way. Know any S owners you can borrow a throttle from? or a pedal assembly?
btw 19% is too much in IMHO ,the endless quest for more boost can lead to loss of torque up top ,quality air is better than chasing high boost numbers . I could be wrong ,I frequently am

 

I have both a peddal assembly and a stock TB I can lend for trouble shooting. PM me if you're interested. I don't want any money, just postage for the lending of the parts....

Let me know if you think it would help.

Matt

ps, if it's the pedal assembly that was at fault, you could just keep my old one. Let me have your shot one back for parts (I like to take things apart. Putting them together, not so much so....)

 

Screwsharged....hmm....so what your saying is that my TB's sensors must be shot. I am running an M7 AGS, there is a wiring harness that lengthens the TB harness so that it could go around the TB, maybe there is a bad connection somewhere along that harness.

Dr. O,
I'd gladly take your offer up......too bad I'm on the other side of the world. Thanks for the offer and the generosity though. Hopefully i can find someone to lend me a TB and pedal assembly. Is the pedal assembly difficult to remove?

I'm a bit disable right now so I can't really get into troubleshooting my car....soon I'll start to work on it. I'll keep you guys updated.

John

 

Yes it seems to me the throttle is closing,this could be either TB or pedal assy both easy to substitute but it can also be the throttle being signalled to close, ie a spurious signal from ecu-this can be due a false torque signal,or programme error ,software problem,abs/traction signal and so on
sorry I cant be more specific but it is hard without the car in front of you and having the diagnostic plugged in or a meter. so sustitution is the easiest way for you.
such a constant drop in pressure at the same point in rpms does not point to a boost leak but throttle closure (or different cam)
As a side issue I am not a believer in very small pulley sizes and crank pulley combos ,IMHO , the extra boost pressure generated at the high rpms has too many implications on the engine and charger,
A slightly lower boost pressure with your pulley may be a blessing in disguise ,your particular engine may have a better flow than most and although the boost looks down a bit compared to others ,the power will be up.

 

really easy to remove. It slides away from the exhuast tunnel, and there's a clip on it, I think....
There is a locking plug on the back side, you'll see it as the pedal assy slides out. The gas pedal is really just a wind-up spring and two potentiomoters (the redundancy is for safety).

Matt