1997 firebird 3800 high rpm stumble

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zophar

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Hey guys, its me again.
I wasn't going to start a new thread but this doesn't exactly pertain to my previous thread.

I got the crank sensor replaced and got the fire chicken running and starting good again. Also fixed a rubbed spot on the wiring harness leading to the cam position sensor.

My current problem has been around off and on since i got this car running a few months ago. Can't find a problem description that fits it exactly.

I haven't jumped through any big hoops to start tracking it down, because frankly the car has emptied my bank account a few times for one reason or another.
Here's the symptoms:
Under hard accel, the car pulls great to about 3k rpm, then it seems to fall on its face but will sometimes smooth out and start pulling again.
When it does that you can let up on the throttle and roll back into it and it may or may not pick up and run all the way to the shift point.
It seems to have the problem worse in 2nd or 3rd gear. 1st is so quick that it doesnt have time to stumble before it's time to hit 2nd.
But like I said it may or may not have a problem when you lay the hammer down.

Here's what I have checked out already.
Vacuum test showed good at 13, considering the valve lift and duration.
The Air/fuel gauge shows no change to the rich or lean side.
Listening to the exhaust and intake it doesn't start missing at all.

It's almost like when a timing advance goes out on an older style setup. but all that is handled through the ecm.
I plain on using a Live data tool to watch whats going on while I drive it, and possibly pinpoint what's going wrong by looking and a recorded run on a timeline.

Anybody ever have any experienceor know somebody who did?
I have no SES codes showing a light, or even a code hiding in the background.
The only code I have pulled off of it is an o2 preheater code, sensor 1 bank 2.

A little extra info here. It seems to have the problem worse after everything has warmed up to op temp. It still happens some from 160 degrees up to 180. from 180 up it seems to happen more steadily.

i know it is a lot to read through, but I like to be thorough. If you guys have any tips as to what could cause it and where to look on the live data I would appreciate it.

I'm an old school obd1 guy lol.
 
The air/fuel ratio is tested just at idle or you hooked it up to test while running the car?Have you checked the fuel pressure? Your comment about valve lift and duration would indicate a non-stock camshaft? How radical a cam did you install and have you done any other modifications to the engine or any other part of the drivetrain? Have you tried to correct that sensor issue?
 
The crank sensor is swapped out. I have an a/f gauge in the dash.
Original plan was too s/c the car after a little while.

The cam is a shade above stock and with the rocker arms it's got about .500 valve opening. Haven't checked fuel pressure in a while, but the pump is only 5 months old. That would almost explain a high rpm power loss, but not that you could let out and punch it, and it well pick up and run sometimes.
I might hook up the pressure gauge for giggles in the morning and watch it on the way to work. I'll report that back.
I can't remember what pressures should run. But it might be a possibility. Just figured it would show on the a/f gauge.
 
For reference the motor mods are:
9:1 pistons, powdered metal rods, cam and lifter combo to get maximum valve opening on stock upper end, headers, cat back exhaust, ported heads and gasket matched intake, swirl polished valves with 3 angle valve job, deleted balance shaft as of a month ago, new cam sensor button, high pressure modification to the oil pressure valve, ls7 lifters and matched length push rods for proper lifter preload. I think that covers the main stuff.
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Holy crap, just happened to think. I'm running the old catalytic converter.
Wouldn't a clogged cat change a/f when it started making too much back pressure.
If something doesn't show itself on the easy tests i may have a shop do a flow check on that cat.
 
A plugged up cat could be a concern. A simple way to check would be to unhook the exhaust just past the headers and tie the pipes out of the way. A quick test drive should tell you if that makes any difference.
 
Lol my neighbors wouldn't enjoy that too much.
I checked fuel pressure today while driving. it's holding right at 50 psi under heavy throttle, even when it hiccups.
Another thing to note. I let the exhaust cool down enough that the a/f gauge was registering full rich. Fired up and got into it. The thing pulled great until the gauge got hot enough to work. Which is a few hundred degrees.
It only took about 5 blocks to get it to that temp. Which is way faster than before. Anybody know the correct amount of back pressure for when i test it?
 
Ok. A few more simple tests today. Hooked up the scanner and drive while it was messing up under throttle i monitored sensors and voltages. The timing went nuts and the egr position fluttered open and shut a few times.
Then the notorious rotten egg smell filled the car. So i am going to do a cat test as soon as i remember where my vacuum gauge is.
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dont you have the engine with that wonderful "distributor" on the front of the engine which is known for failure, OPTISPARK?
 
Nope. I wish.
But like the title says, it's a 3800 sixer.
With the work done to it, it well keep up with most of the 350's around here.
It puta whooping on a tbi Chevrolet a couple days ago, until it got warm enough that this problem started happening.
If i can get this problem fixed and get a tune on it, i plan on doing a few dyno runs and see what it's putting down.
 
thats funny I read the title as "3800 high rpm stumble" meaning at 3,800rpm you had a stumble.
So you covered things when you built this such as valve spring coil bind? Lifters arent adjusted too tight to prevent pump up? Still only the O2 sensor code? The fuel injectors are the stock ones or high performance aftermarket? Any changes to the ECM?
 
Yeah, everything was checked out. No coil bind, and the lifters are dead in the middle of preload. It also doesn't have any problem revving out as long as the exhaust is cool.
Let it cool down again, tried it out. With engine temp at 195 and the exhaust cooled down enough for the a/f gauge reading full rich it pulled great. But where the gauge used to take a half mile to work accurately. It's hot enough to read within 3 blocks. When it hits full temp is when it starts bogging.
Plus after a long pull with it bogging out i got the dreaded rotten egg smell from the exhaust.
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Still havent tried running it with the converter disconnected? You could buy a test pipe for a few bucks to replace the converter. How about ignition module failure when the engine heats up?
 
Cut out the converter and test piped it. The converter is part of the y pipe so it had to be cut.
It runs out a lot better, not to mention it almost swapped ends on the road when i hit 2nd gear.
Still have a little stutter at around 3k rpm. But it will actually pull through it now.
Nothing shows out of the ordinary using a scan tool. And air fuel gauge doesn't move at all from its usual position when this happens.
 
It's not incredibly rich. And with the cat on it was actually running about 14:1.
With the cat off it seems to hang more at the 13:1 range. Still not pig rich.

But after some problem hunting, it turns out only to have this problem when the computer goes into closed loop.
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with a aftermarket cam at high rpm your duration could be too long and sucking out your fresh air through the exhaust valves,forced air fixes that prob..also could be having valve float,what # springs do you have...light cam should be at least 105
 
My springs have a seat pressure of 95 lbs.
After a ride around with HP tuners running we found the problem.
Under heavy throttle in closed loop the knock sensors are taking out nearly all the timing. As soon as we switched of closed loop it lost the stutter and ran great.
I remember a friend of mine having a problem like this with his Ls motor after installing a high lift cam.
This thing is going to end up taking a few psi to the manifold before it's over anyway, so a good tune will fix that.
We did bolt on a set of det cans to listen for any actual ping the only thing we picked up was the exhaust valves making a light ping every time each one shut. Double checked this with a stethoscope on each header tube.
The tune my ls buddy had done has a base of about 64 degrees timing and the kr is taking out up to 20 degrees under heavy throttle. And it's a tune from a pro ls tuner.
After some more hunting around i have found a few more cases of high lift cam setups having to desensitize the knock sensor.

All this cause i just couldnt leave the sixer alone.
But the problem is located now anyway.
Do we have any boosted 3800 guys on the forum?
 
Ok. This looks like the final update to this thread. I appreciate all the help guys.
After writing the first two tunes on the firebird last night, to fix the kr table among other things that were way out of the ordinary with GM's stock tune, it runs exceptional. No more issues.
The stock GM tables had a huge kr drop in the exact range that i was experiencing problems with..
From 2400 to 3000 the stock table had it removing 15 degrees for any knock sensor activity.
That combined with the stock tune only putting 14 degrees of timing in as an average in that rpm range, it was literally taking every bit of timing out.

Now we are going to start tinkering and see how much timing we can play with. After seeing several Hmm timing tables hitting 35 degrees or better on a standard tune, and a few more in the 40's. It's going to be interesting to see how much makes it run good before we hit a wall.
So far we have only put a gradual 30 in it and it had gone from spinning on take off, to flat out white smoking through 1st gear.

Once again i do appreciate the help. i will still be floating around the board of i see something that calls my name you guys will here from me again.
 
Well in the good old days of mechanical advance most of the time 36 degrees would be the magic number plus another 10 or so from the vacuum advance, Never to exceed a combination of 52.
 
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