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Discussion Starter · #1 ·
I've been dealing with this issue for over 6 months now and ready to hang myself. 3800 III L32 misfires incessantly with boost engaged and can't accelerate, problem worsens as its warmer. I think there's a telltale sign for those of you with more knowledge than me: Boost engages and misfires in reverse.
Today I unplugged the solenoid and tried a bypass two different ways but despite reading every thread on this forum on the issue, specific details on the bypass aren't given. First I just ran the tube straight to the BBV, with no change. Then I capped it (with tape I couldn't find the right cap) and left the BBV bottom open, again no change. I posted pics of this below.
Is this correct or do I need to do it differently?
Also, is there a way (without a tuner) to disable the SC and see if the car runs normally? That would narrow it down a lot but I can't find anything anywhere about doing that.
At this point I think it almost has to be the BBV itself but can't prove it without sitting under the hood with someone else driving.
View attachment 11164
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1998 GTP coupe. All stock and rust free. That will change soon.
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what year, how many miles, any other mods?
possibly clogged cat converter can cause misfires under slight throttle conditions.

you can remove the SC belt and see if boost is the issue. If not you'll have to start checking other things - is it a specific cylinder, etc.

also try unplugging the MAF sensor, it will stumble but should smooth out. it will throw a MAF code but see if that makes a difference.
 

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Discussion Starter · #3 ·
what year, how many miles, any other mods?
possibly clogged cat converter can cause misfires under slight throttle conditions.

you can remove the SC belt and see if boost is the issue. If not you'll have to start checking other things - is it a specific cylinder, etc.

also try unplugging the MAF sensor, it will stumble but should smooth out. it will throw a MAF code but see if that makes a difference.
Hi thanks, the cat's been gone for years. It literally rusted out and fell on the ground while driving 5 years ago so I down piped it. I swapped out MAF and MAP with no change. There's no specific cylinder and no misfire codes on the scanner. I'll try removing the belt that sounds good I guess.
OH and there are no other mods, all stock except cat. 200k miles. I replaced fuel filter and all the wires/plugs last year. The problem suddenly onset with no apparent reason but those repairs failed to fix it.
 

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Discussion Starter · #4 ·
Repairs done in attempt to fix this:
new fuel filter
new coil packs
new wires and plugs
swapped ICM, MAF and MAPs with newer junkyard parts with no change
disconnected boost solenoid and bypassed perhaps incorrectly
banged head against wall repeatedly with no change.
 

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See this previous post for info on vacuum lines (basically, leave it open at the bottom if you have the DTC for the solenoid turned off, or leave it connected to the solenoid if your PCM is stock):
I could get more nuanced, but that's the basics for that item.

You can have someone rev the engine a bit, and you should be able to watch the bypass plunger move up and down as vacuum is applied differently to the top of the valve.

This may not be your issue, from the sounds of it. BBV's aren't typically problems, and there's a variety of things that could impact a 3800 SC's health when it's been beaten on for many years without close attention.
 

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Discussion Starter · #6 ·
New info. I took off the belt and immediately had less misfire and more acceleration, however the misfire increased as the engine heated up, again only under load. So I can blame the load of the Sc but not the Sc itself. So frustrating.
 

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Check for vacuum leaks, compression test each cylinder, and look over your electrical harnesses to ensure no breaks/frays/etc.

Beyond that, scanning for data while driving is key. That can be done with a $15 OBDII Bluetooth scanner.
 

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Discussion Starter · #8 ·
I forgot to post that I did compression test and that checked out OK. The OBD is not giving any misfire codes at all no matter when I run it. Wiring harness and connections look fine.
 

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Codes and scanning live sensor data are 2 very different levels of information.
 

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Discussion Starter · #10 ·
What am I looking for live on the OBD? I have an Innova 3100 in the car right now.
 

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Not sure if it can, but monitoring Long & Short Term Fuel trim behavior, along with O2 mV values, MAP kPa, and realtime misfires by cylinder will give you some background info.

Those are about 20-30% of what I scan when tuning a car, along with some extras like a wideband O2, MAF hz, and others for a comprehensive tracking of the vehicle's behavior and feedback.
 

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Discussion Starter · #12 ·
Good news and bad news.
Good: After reading another thread somewhere on the issue, I took off all the metal heat shields from the the spark wires, reconnected the SC belt, and voila, reached full boost with no misfire while cool.
Bad : it's still boosting in reverse, and when it gets warm a few misfires still occur, but not nearly as many. I can now accelerate again warm or cold, but much better when cold.
Ugly: I don't think my scanner does live data btw, I can't find that option. I plan on getting a tuner next week.
 

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Jeez, how hard do you hit the pedal in reverse?!?

Sounds like an issue with grounding. I run the metal boots on all stock-style plug wires (including the turbo car I'm currently beating on), because it gets nuclear hot around there.
 

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Discussion Starter · #14 ·
I only hit it halfway in reverse for testing purposes and boost came on. I read in multiple places that boost is not supposed to engage in reverse at all, so I'm confused as to why it does, especially after disconnecting the solenoid.
 

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The solenoid is meant to fight the natural vacuum action that opening the throttle causes (for purposes of traction control, transmission abuse, and other types of fun GM doesn't want us to have), so whoever said that "tip" isn't too on top of it. Deleting the solenoid ensures there's nothing interfering with the direct reaction of the supercharger to your foot's demands.
 

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Discussion Starter · #16 ·
So what is the component telling it to not engage in reverse? Or are you saying that's misinformation?
 

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Your foot not pushing your pedal harder than a responsible driver goes in reverse. Just like in drive, the amount of throttle to get into boost is going to accelerate you decently enough that it's not smart/safe to screw around like that in reverse (unless you're trying out for Darwin Awards or doing reverse donuts in an open lot... We've all been 19 & dumb).
 

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Discussion Starter · #18 ·
I read several places including maybe here? this week that the boost gets completely disengaged in reverse, that's why I was testing it. Soon as I hit the gas in reverse the boost engages. Another 3800 expert on FB said that was odd. I did become notorious in high school for driving 3.7 miles home from high school in reverse in a 77 Suburban but that's another story. For now I was just testing it in an empty parking lot.
 

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The solenoid is for controlling how much abuse is applied to the boost, so piece all that together. Between the handful of posts you're reading, the information is there. I have a Freedom 500 to go get beer and chips to go watch!
 

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are you using dielectric grease in your plug boots?
 
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