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Boost Decay at Partial Throttle


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#1 JSwan

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Posted 14 August 2019 - 01:49 AM

May sound crazy or others may say what are you talking about dummy but, my car sat for about 8 years and I recently found time to get it running again.  Many "get it running again basics" done and a recent tune up has allowed more progressively spirited driving.  Now that i'm revving and boosting with confidence again, i noticed when i hit throttle to 50-75%, the boost will build pretty quick to 10-12 psi and then will begin to decay (down towards 5psi or so) as the RPMs build towards 5k or more with no change in my foot throttle.  Seems i remember with my stock car years ago, the boost would peak at 10 psi and stay there even at 50-75% throttle.  So i wonder if i have a problem or if this is normal and i just don't remember.  Could be my overall mod combo, not sure.   The car's power to throttle pedal feel is more like a carburator 4-barrel than a FI system, meaning that there is a more significant difference in power between 75% and 100% throttle.  My induction now is mostly stock with mostly bolt on replacements and i have a TEP two port external waste gate on the 14G TDO5H, which i wonder about the waste gate to some degree.  Just curious what other show for boost behavior at moderate throttle when compared to WOT or beyond cruising throttle?
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.





#2 speedyquest

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Posted 14 August 2019 - 07:30 AM

The first question to comes to mind for me is are you using the stock boost gauge for those numbers or do you have an aftermarket boost gauge? I ask because the stock gauge is really not accurate at all and is just an approximation based on a few things the ecu is seeing.

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#3 croquest87

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Posted 14 August 2019 - 10:32 AM

I am leaning more towards ignition and fuel issues. Most likely culprit dirty fuel system and crappie spray pattern on your injectors. I had a hell of a time with my 87 when I first bought it . Boost and general performance was absolute crap and gage was all over the place.

#4 JSwan

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Posted 14 August 2019 - 10:45 AM

I have a mechanical boost gauge i'm referencing, not just the stock gauge.  Also, a Halmeter A/F gauge tied into a BOSCH 3-wire O2 sensor (new).  Trilogy Injectors and Trilogy FPR.   Also, i have a BOV that blows off slightly even during off-boost normal gear shifts.  I don't see a drop in a rich condition on the A/F meter during moderate throttle boost decay i described, but i kind of suspected my injectors may be dirty because the car use to idle one bar Rich before it sat and now it idles Lean all the time and seems to have an occasional lean stumble at low throttle between 1st/2nd gear shifts.  I plan to change all the fuel filters i can access without dropping the fuel tank first and see where that goes but i feel like i might be chasing multiple problems as always with the car.
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#5 croquest87

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Posted 14 August 2019 - 11:45 AM

View PostJSwan, on 14 August 2019 - 10:45 AM, said:

I have a mechanical boost gauge i'm referencing, not just the stock gauge.  Also, a Halmeter A/F gauge tied into a BOSCH 3-wire O2 sensor (new).  Trilogy Injectors and Trilogy FPR.   Also, i have a BOV that blows off slightly even during off-boost normal gear shifts.  I don't see a drop in a rich condition on the A/F meter during moderate throttle boost decay i described, but i kind of suspected my injectors may be dirty because the car use to idle one bar Rich before it sat and now it idles Lean all the time and seems to have an occasional lean stumble at low throttle between 1st/2nd gear shifts.  I plan to change all the fuel filters i can access without dropping the fuel tank first and see where that goes but i feel like i might be chasing multiple problems as always with the car.
From tank to fuel injectors all needs to be in tip top shape. That includes seals and clips for the injectors and good connections. I see to many engines rebuilds  due to leaning out detonation and heat. 30 year old metal and new gasolines do not get along very well. This comes from my personal experience and mistakes made over the past  almost 2 1/2 decades owning these POS. Lol.  Start w basics.

Edited by croquest87, 14 August 2019 - 11:59 AM.


#6 JSwan

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Posted 14 August 2019 - 04:13 PM

i hear you croquest87!  injector clips are new, soldered and in good shape after i inspected.  Can i get to all the filters at the tank through the access port in the back hatch?  i know its tight but have not opened the can of worms yet:)
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#7 Turbo Cary

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Posted 14 August 2019 - 05:37 PM

Also check for cracks or exhaust leak. I have seen a few SQ manifolds crack which is why most stress relieve them. You may not hear the leak but anything bypassing the turbine housing prevents it from spooling.

#8 JSwan

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Posted 14 August 2019 - 06:59 PM

Found an exhaust leak where my down pipe meets the b-pipe.  Its far down the exhaust of course but is that a potential cause for any concern other than added noise and maybe the reason my O2 sensor trips often under light cruise throttle'/speed?  Also, i have one of Chad's headers with less than 8k miles on it and it looks good condition.
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#9 Turbo Cary

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Posted 14 August 2019 - 07:18 PM

View PostJSwan, on 14 August 2019 - 06:59 PM, said:

Found an exhaust leak where my down pipe meets the b-pipe.  Its far down the exhaust of course but is that a potential cause for any concern other than added noise and maybe the reason my O2 sensor trips often under light cruise throttle'/speed?  Also, i have one of Chad's headers with less than 8k miles on it and it looks good condition.

Only exhaust leaks pre turbo would prevent it from spoiling properly. If the header looks good I wouldn't worry about that too much but it is something to check. It could be the mounting surfaces or a backed out bolt/stud.

Like stated above a fuel related or ignition related concern can cause delayed boost or not enough boost.

The halmeter AF gauge is that a narrow band gauge? You said it is tied into a 3 wire sensor so it probably is. Look into an AEM gauge. They are usually around 150-160 dollars with sensor.

Edited by Turbo Cary, 14 August 2019 - 07:20 PM.


#10 croquest87

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Posted 14 August 2019 - 08:37 PM

View PostJSwan, on 14 August 2019 - 04:13 PM, said:

i hear you croquest87!  injector clips are new, soldered and in good shape after i inspected.  Can i get to all the filters at the tank through the access port in the back hatch?  i know its tight but have not opened the can of worms yet:)
Sure  you can. But those screws that hold the pick up tube/sending unit can be rusty and tight(sometimes they break) and hard to work with through that small opening.Please dont make the hole larger lol. Takes half hr to remove the tank (make sure its empty) and work on it in style on your bench.

As far as your manifold goes? If it was mine  it would come off  studds would be  fixed if broken.  After that l would  find the crack in it. Drill holes at the end of the crack/cracks. V groove from hole to hole. Put it on your grill on high for 1 hr  pull it and weld at 80 amps w good cast iron electrode( l can recommended you one). Let cool naturally  w no breeze or wind (you can place it in play sand if you want). Cut the webs  for stress relief  and make the surface flat/belt sand it flat . Install OEM metal gasket for best results. Re torque after couple of heat cycles.  Sounds like alot of work but it's very simple just takes some time and patience to make it all nice flat and tight.

Edited by croquest87, 14 August 2019 - 09:46 PM.


#11 JSwan

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Posted 14 August 2019 - 11:40 PM

Guys, thanks for all the insight here.  Let me back track slightly to my original question and i will phrase it another way...  in a correct running car, if you press the throttle to about 50%, a good running car will build full boost and hold it steady all the way to 5k+ rpm without pressing the accelerator any past 50%?
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#12 croquest87

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Posted 15 August 2019 - 06:48 AM

My 89 auto spins tires at around 5-6 psi once it hits 2nd gear holding pedal down and around 4000 rpms  stock everything . Moded trans valve body.

My 87 mpi  well that's a whole other animal  right there  it scares the S... out of me sometimes. Lol

I guess it all depends on alot of factors.

Edited by croquest87, 15 August 2019 - 06:50 AM.


#13 JSwan

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Posted 09 August 2020 - 01:49 AM

Been working slowly on this problem, But here is an update here boys.  Dropped fuel tank (interior looked good), replaced in-tank pickup filter, replaced fuel pump with Walbro, Replaced Engine bay filter, and added Fuel Pressure Gauge In-Line.  Got New Injectors on the way after the lean idle/cruising condition could not be fixed and accidently mounting the secondary in the primary port on the throttle body and found lean condition was fixed but hesitation at 2500 RPM persisted thus demonstrating the primary injector in that location was still problematic (problem followed the injector).  Noticed thus far (minus new injector install) the car runs good under stock boost but will not hold more than 12psi of boost.  Not going to go over that 12psi until i get the fuel system right, although still running real rich on the AF Meter.  Thinking primary injector is dumping fuel out rather than misting fuel like it should.  Looking forward on this problem because its never a single thing on this car.

Has anyone really had problems with too large of a spark plug gap where the turbo is blowing the plug out at high RPM/+stock boost? Not like i have a massive turbo here, but the misfire is always after a big boost dump or higher RPM >4800.  Any insight is much appreciated as i wait to replace the injectors.
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#14 obsolete

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Posted 09 August 2020 - 09:54 AM

View PostJSwan, on 09 August 2020 - 01:49 AM, said:

Has anyone really had problems with too large of a spark plug gap where the turbo is blowing the plug out at high RPM/+stock boost? Not like i have a massive turbo here, but the misfire is always after a big boost dump or higher RPM >4800.  Any insight is much appreciated as i wait to replace the injectors.
Yes, absolutely. My car came with crappy old Autolite plugs gapped at .040", and it would hiccup at full throttle/high RPM/10psi. New NGK 7031s gapped at .032", now I can leave my foot in it for as long as I want. Boost controller set to 12psi.

Edited by obsolete, 09 August 2020 - 09:54 AM.

Buy the ticket, take the ride.

#15 JSwan

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Posted 09 August 2020 - 11:56 PM

View Postobsolete, on 09 August 2020 - 09:54 AM, said:

Yes, absolutely. My car came with crappy old Autolite plugs gapped at .040", and it would hiccup at full throttle/high RPM/10psi. New NGK 7031s gapped at .032", now I can leave my foot in it for as long as I want. Boost controller set to 12psi.

Ah, good to know Obsolete.  I think mine are gapped at 36 or 38, so i will try closing them them down a bit after the new fuel injectors are installed if the problem persists.
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#16 Turbo Cary

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Posted 10 August 2020 - 05:08 AM

I dont know if you ever clarified that your A/F gauge is a narrow band or wideband sensor. If it is a narrow band gauge it will show super rich or lean one way or the other because it cant show you the full range of what the AF actually is. It may only read from 12:1 to 14:1.

Try the plug gapping first before you install the injectors. Let us know what plugs you're running and if there was any change after gapping.

#17 croquest87

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Posted 10 August 2020 - 11:02 AM

Leaking exhaust mani?

#18 JSwan

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Posted 11 August 2020 - 11:23 PM

View PostTurbo Cary, on 10 August 2020 - 05:08 AM, said:

I dont know if you ever clarified that your A/F gauge is a narrow band or wideband sensor. If it is a narrow band gauge it will show super rich or lean one way or the other because it cant show you the full range of what the AF actually is. It may only read from 12:1 to 14:1.

Try the plug gapping first before you install the injectors. Let us know what plugs you're running and if there was any change after gapping.

Its a Halmeter AF30 narrow band, doesn't give me the ratio just several dots that are progressive for lean (red), A=1 (amber) and rich (green).  Running NGK 7031 BUR7EA-11 with a Marnel Head.  I will work on re-gapping them and let you all know.

J
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#19 JSwan

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Posted 11 August 2020 - 11:36 PM

View Postcroquest87, on 10 August 2020 - 11:02 AM, said:

Leaking exhaust mani?

not going to rule that out.  is the only sign is extra noise from the area or is there testing i can perform?  every exhaust stud i can get a wrench on seems tight, but i didn't hunker down and put a whole lot of torque on them with things in the way, it was just a quick check tonight on a few.
1989 TSI SHP - Upgrades: Marnal NJV Head, Ported/Polished/Gasket Matched, SS OS Valves, Schneider HD Springs, 14G TD05H, 1GMAS, Trilogy Injectors, Trilogy FPR, Custom Header by Chad, BS Removal, 2.5" TEP Downpipe/B-Pipe/HKS Exhaust (no cats oops)/4"Tip, *DM/101 Hardpipes, HKS Type BOV, Walbro 255 Fuel Pump, SS Fuel Lines, BOSCH Red Coil, S&M Strut Bar, Poly Bushings, Polk Audio/Sherwood AMP/w/8" Sub, and Polished SHP Rims.  Working on Custom Interior, Brake Upgrades and Always on the Look for Hood Mods Including 83, Cowl, or Firebird Type.

#20 Turbo Cary

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Posted 12 August 2020 - 04:24 AM

View PostJSwan, on 11 August 2020 - 11:23 PM, said:



Its a Halmeter AF30 narrow band, doesn't give me the ratio just several dots that are progressive for lean (red), A=1 (amber) and rich (green).  Running NGK 7031 BUR7EA-11 with a Marnel Head.  I will work on re-gapping them and let you all know.

J

You really need to get an AEM or Innovate wideband gauge. The narrow band isnt telling you anything it's more for show.

For example on my old SQ I ran a narrow band gauge for a while. It always went "rich" and I thought things were fine. I put an upgraded turbo on and it still ran ok without issues. I was also tuning it with a MAFT. I finally got an AEM wideband and on the first pull I saw I was only hitting 12.8 at WOT! That's way too lean. The narrow band had been just showing rich.

Having all the information at hand leads to the best possible diagnosis available. Once I saw what my ECU was actually doing for fueling I corrected the MAFT, did another pull and it ran much better/safer.

Edited by Turbo Cary, 12 August 2020 - 05:10 AM.





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