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1998 Bonneville L67
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Discussion Starter · #1 ·
So a bit of backstory. This is a 98 Bonneville with an l67 swap. 3.4 pulley, 1.84 rockers, power log, ported supercharger and manifold, intense pcm.
Ever since I finished the swap a few months ago, I always felt like the car was slow. Got around to hooking up a vac/boost gauge which showed I was making 4 psi at WOT. I unhooked the line from the boost solenoid and plugged it to see what would happen. Now at WOT it's making ~10 psi but it won't go over 3500 rpm. It just bounces around there while shuddering. Now I hooked the line back to the solenoid and I can floor it to 6000 rpm but on 4 psi of boost.

Any ideas? Clogged cat maybe?
 

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Possibly, I wouldn't get on the pedal much until you have a scantool hooked up to monitor the engine sensors in real-time (bluetooth, or someone with HPTuners that's nearby to go for a ride, etc.).

Just end the vac line at the fuel regulator, and remove the short tubing that connects the solenoid to the bottom port of the bypass (bypass operates normally with just the connection to the top tree). The computer will think it's still connected for now, but it won't cause anything to be messed with. Again, I wouldn't really get on it to check boost levels while you're possibly diagnosing a bad cat or something else, unless you want to smash pistons into spark plugs and go engine shopping after a couple too many romps on a struggling engine.
 

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Discussion Starter · #3 ·
What sensors/readings should I be looking at? I just got hptuners working yesterday after dealing with their support for a week.
 

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Long and short term fuel trims, O2 cycling normally from ~200-800mV, MAF and MAP are fairly sensitive to throw a code, but get an idea of the Hz and kPa values you're seeing. Also see what the fuel injector pulse width msec are, which I'm still trying to find the right one in the HPT menu for my car. Knock retard degrees and rear O2 signal may also help, and you'll get more ideas as you dig in.
 

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1998 Bonneville L67
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Discussion Starter · #5 ·
Ltft is at 16 anytime I'm not on the gas. During light throttle or cruising its around -3 to 5. Heavy throttle sends it back up to 16. Rear o2 sensor is usually about .200v less than sensor 1. I'll take a closer look at the scans once I'm done work to see the other values and stuff. No kr ever popped up.
This was all done with the boost bypass solenoid connected in case it matters.
 

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Might have a vac leak somewhere, if the LTFT is having to add that much in many situations. Also, I'd clean the MAF filaments with a can of that specific cleaner from the store.
 

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Discussion Starter · #7 ·
Ya I'm gonna check the egr tube. All the vacuum lines are brand new with very tight fits and I've taken off the supercharger and resealed like 4 times so I'd be surprised if its leaking there.
The maf is brand new from when I did the engine swap but it wouldn't hurt to clean it I guess.
 

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Discussion Starter · #8 ·
Aaaand the egr tube has a crack in it. Must've broken when I swapped it from my l36 since I never any issues before the swap.
Kinda hard to think that it would cause a boost issue though? It does have a rough idle on startup sometimes so hopefully that's it. Once I get this sorted out we'll see how it responds.
 

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If that M90 is trying to draw in a bunch of extra air, it will pull from anywhere it can ahead of those rotors, which is where the EGR passage opens near the bypass port.

Any extra air getting in un-metered by the MAF sensor is going to make the car unhappy. You can take the opportunity to eliminate EGR, or just snag a junkyard one for fresh tubes.
 

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Discussion Starter · #10 ·
So egr is gone and both holes blocked by a piece of metal with rtv. On cold start idle was all over the place bouncing from 500 - 2000 rpm. Ltft was going from -10 to 16 during this. Once it was holding a steady idle and warmed up ltft was at 13.3. Light throttle puts it at -3.9, medium throttle at -13, heavy throttle at 4.7. Anytime i was cruising with no throttle ltft was usually between 8 and 10. With car stopped it goes to 13.
Does it take some time to relearn itself? The numbers are better than before but still not perfect. Boost was still maxed out at 4 psi with solenoid connected. Didn't have a chance to try with it disconnected since it started raining.
 

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I'd monitor STFT as well, since that's a more instantaneous reaction to O2 sensor info, and LTFT changes at each spot based on an average of the STFT's over time there.

You may have something else still letting some extra air in, dunno. Scan and keep an eye on MAP readings to be sure. Fuel trims will try to correct for any issues (within its range) plus any normal airflow diversion from the original MAF table (upgrades, etc.), based on differences from the original configuration of airflow from the factory.
 

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Discussion Starter · #12 ·
Map at idle is 25 kpa at 5k rpm it was at 60 kpa. Light throttle 32ish kpa. Does that sound right?
Stft under light throttle bounces between -3.9 and 3. For the most part stft is usually in the negatives. Really the only times it went +5-6 was when the o2 sensors would stop moving for a second or two. O2 1 would stay at .053v for a second then go to .800v for a second and go back to cycling up and down. Or it would just hold at ~.800v without holding at a low voltage and then start cycling. Is that normal?
Thanks for all your help so far.
 

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Discussion Starter · #13 ·
Just tried with boost solenoid disconnected. Still doesn't go past 3500 rpm
 

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STFT's should be cycling +/- a few % in normal operation, once the LTFT is settled into a happy spot (hopefully not at too big of a value).

Not sure what's causing the 3500 thing, and it's not due to the BBV, because I've revved out my old supercharged setup plenty without any belt on the blower, for giggles.

At this point, I'm not super certain what to suggest. Feel free to attach a spreadsheet of a normal driving log for folks to review. I wouldn't beat on it much until it's resolved, because there's obviously something unhappy & you don't want to junk a motor because your foot got itchy.
 

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Discussion Starter · #15 ·
Ya once I've got some time at the end of the week I'm gonna start poking around and see if I can figure something out. I blew my original motor so I'm definitely not interested in blowing this one too lol.
 

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Discussion Starter · #16 ·
Well I think I can rule out the cat. I took off the piece of metal blocking the egr port on the exhaust manifold and took it for a quick spin. Didn't go past 3500.
 

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EGR and cat have no connection, not sure of your logic here.
 

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Discussion Starter · #18 ·
If the cat was clogged then having the open port on the exhaust manifold would give it a way to get rid of excess pressure. I didn't do anything with the egr itself (already got rid of it).
But if nothing changed then my problem lies elsewhere...
 

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A 3/4" opening off one cylinder into your engine bay is not a suitable solution to a blocked 2.5-3" pipe downstream, and also not suggested for safety purposes. I get what you were going for, but there's better approaches.

You're jumping a little bit too quickly at random ideas and dismissing things without full comprehension, and I think you could have a solid learning and troubleshooting experience if you didn't take everything popping in your head as a certainty. I'm rooting for you to figure out what requires maintenance on that thing, so hopefully you get there. It's difficult to make suggestions with limited ability to diagnose based on internet comments, but we try.
 
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