2000 Chevy Malibu acceleration issue

not too sure where to start on this one

for a few years now I've had this issue where the vehicle would stumble or rather stutter or hesitate when accelerating.
first noticed the issue around four years ago. Called up a Craigslist mechanic who threw spark plugs and a tune-up at it in hopes of fixing the issue which kind of did but did not really. a lot of the little misfire jolts were gone but the main acceleration problem still exists.

fast forward years later, I've got my tools my own equipment and am interested in Diagnostics.

brief history on this as with most engines of this vintage it had an overheating issue when I first got it which at the time the vehicle was still covered under warranty. It was taken to a shop where they determined the intake gaskets needed to be replaced. I assume they didn't actually diagnose the problem but referred to a bulletin or perhaps just common knowledge about the gaskets and this engine. needless to say that did not fix the problem but replacing the thermostat did.

anyway, the car will hesitate when accelerating especially when I am trying to speed up or pass someone. that and overall acceleration from a dead stop is just pretty darn slow.

I plan on rebuilding the engine pretty soon because it has around 220k miles on it and this car has held me down through the roughest times. the car has some significant oil and coolant leaks. I am unsure if it is leaking internally but I have noticed droplets of coolant outside of the engine however no visible puddles have formed on the ground. I said that to wonder if head gasket problems can contribute to this issue.

I've also noticed rarely at times I'll see a very small puddle of fuel on top of the fuel pressure regulator.however I have not been able to get fuel to come out of the regulator or vacuum line hose that goes to it so I'm unsure if the regulator is truly faulty or not. fuel trims increase if that line is removed.

I am definitely running rich most of the time while he's there the car is idling or in Drive and by the looks of things bank 2 is the biggest problem.

so, here are two videos which may help some of the more experienced technicians on this site help diagnose what may be going on. just for the sake of clarity I alternated between different parameters in the graphing sections of my tool. these are the global ODB parameters available to my scan tool. also when connecting to the GM enhanced data I am getting a ton of intermittent trouble codes that are not setting off the check engine light. It's pretty unbelievable and I'm not too sure if this is simply a list of available troubleshooting codes or if this is exclusive to my car

DATA WHILE DRIVING (15 min)

DATA WHILE IDLE (4 min)

DTC VID LIST-

any guidance would be appreciated

So many codes!

Before i watched the clips you posted, this stuck out at me

I've also noticed rarely at times I'll see a very small puddle of fuel on top of the fuel pressure regulator.however I have not been able to get fuel to come out of the regulator or vacuum line hose that goes to it so I'm unsure if the regulator is truly faulty or not. fuel trims increase if that line is removed.

There should be no fuel, anywhere, ever. The presence of fuel is enough to warrant replacement of the pressure regulator in my eyes.
It seems like it's breathing fuel through that vacuum line based on your fuel trim observation. Fuel pressure should increase on average 10psi with the vacuum line disconnected. In theory, this should result in negative fuel trim.

The fuel trims going negative suggest that the regulator has become a fuel source for lack of a better term on my part. Hope that makes sense.

I think there is a very real possibility of a clogged catalytic converter as well. They don't like being fed a bunch of fuel, coupled with the potential for there being some kind of possible coolant ingestion, and it kind of matches your low power complaint. But at this point, that's more of an educated guess.

And the codes list! :blink: lol, are those all pending codes or are any of them hard codes?
With all the conflicting high and low voltage circuit codes, it almost seems like the PCM lost (or partially lost) power or ground at some point.

I don't know where to start with this one, but the fuel leak seems like as good a place as any if not purely for safety sake.

And I'm assuming this is 3.1l V6 Malibu? Looks like you got a hand full! I'll be glad to help as much as I can. I'm sure some of the other members will have some advice for you too.

Hey Donnyten!

First off, the enhanced code list. GM uses a (as far as I know) unique feature in their Codes section that will basically list out EVERY trouble code that hasn't completed testing at that point. It's really goofy, and downright confusing if you've never seen it before! ScannerDanner showed it in one of his videos, I'll have to track it down later... Basically, if it's not storing a code on the Global side, then you're good

There should be no fuel, anywhere, ever. The presence of fuel is enough to warrant replacement of the pressure regulator in my eyes.
It seems like it's breathing fuel through that vacuum line based on your fuel trim observation. Fuel pressure should increase on average 10psi with the vacuum line disconnected. In theory, this should result in negative fuel trim.

I'm with Noah on this one. It'd be reassuring to actually catch it leaking, but I think you've got all the evidence you need to justify replacement. This is probably not the only issue at work here, but it can't hurt.

Looking at your data captures, and I'm somewhat confused. It lists short and long term trims for B1 and... B3? That might make sense if they meant B2, but no engine that came with the 2000 Malibu has bank-to-bank fuel control :huh: In the North American market, anyway. What is the engine size?

The values associated with the trim PIDs are unusual, as well. At one point I saw 97% on a long term? No fuel control strategy is gonna allow for that much correction. It may be that those trim numbers are actually Global-specific PIDs related to the O2 sensor signals. I can grab some data off my car and explain these PIDs better later on.

I'm also thinking along the lines of a plugged catalytic converter, especially considering the misfire history and possible rich condition. Scan data could definitely help point us in the right direction. If you can, use the Enhanced data to capture some PIDs during a WOT acceleration through first and second gears. Here's a ScanShare link to a known good capture with the stuff we're interested in:

www.scanshare.io/share/PthKNw1SfUGJnZW7S...1,2,3,4,5,6,7,8,9,10

In case that doesn't work, we're looking for MAF, MAP, TPS, engine speed, Load, upstream O2 sensor, and fuel trims. Watching those PIDs on a WOT run will definitely expose a breathing problem if there is one.

This ScannerDanner video also explains the test we're doing, and what the results mean. The make/model and problem are different from yours, but the test itself is the same:

Tyler wrote: Hey Donnyten!

First off, the enhanced code list. GM uses a (as far as I know) unique feature in their Codes section that will basically list out EVERY trouble code that hasn't completed testing at that point. It's really goofy, and downright confusing if you've never seen it before! ScannerDanner showed it in one of his videos, I'll have to track it down later... Basically, if it's not storing a code on the Global side, then you're good

WOW! I remember hearing ScannerDanner say that in a video, completely spaced on it!

I also noticed the bank 3 designation and over 90% fuel trim, not sure what to make of that...

Noah wrote: WOW! I remember hearing ScannerDanner say that in a video, completely spaced on it!

I also noticed the bank 3 designation and over 90% fuel trim, not sure what to make of that...

Good! Was worried I imagined the whole thing :lol: Was it one of the RTC shop videos?

I'm pretty positive the fuel trim thing is a screwy Global PID. I'll get an example out of the Scion later today and post it up.

Thanks for the replies.

I will definitely be replacing the regulator soon, caught it leaking with my own eyes quite a bit a few times over the past few days. Think I will replace it when I do the gaskets on this engine along with a tune up

As far as the WOT throttle test, I have a few questions/ issues.

In the Enhanced functions on my scan tool * android phone with blue tooth adapter and car gauge pro*, I can only see fuel trims for bank 1. I have always gotten * NO DATA* for bank 2, and the banks are properly labeled as such * B1+ B2* in the enhanced functions, its only oddly mislabeled as Bank 3 ( bank 2) in the global parameters.

Also, There's a TPS parameter to view however its stuck at *0 %* no matter throttle position. My engine size is a 3.1 V6. I also thought the values for the parameters were odd, as I thought the max correction allowed is no more than 25 percent. Also, good to know about those codes.

I have access to maf freq, maf airflow- gms/s, map pressure, map voltage, engine speed, load, half of fuel trims and O2 values.

Also i'm not getting any O2 sensor data for bank 2, only bank 1, with an os2s2 value as well, i take this to be the downstream o2 sensor.


if I am ok with these parameters, Can I give it a shot? WOT in 1st and second gears....??

ALSO, why is it important that we perform this test during 1st and second?

Those can't be all the codes in your car. That looks like a list of available codes

,it's the man himself . whatup scanner danner.. good to know...

Hey sir, sorry for not getting back to you sooner!

Donnyten wrote: As far as the WOT throttle test, I have a few questions/ issues.

In the Enhanced functions on my scan tool * android phone with blue tooth adapter and car gauge pro*, I can only see fuel trims for bank 1. I have always gotten * NO DATA* for bank 2, and the banks are properly labeled as such * B1+ B2* in the enhanced functions, its only oddly mislabeled as Bank 3 ( bank 2) in the global parameters.

Also i'm not getting any O2 sensor data for bank 2, only bank 1, with an os2s2 value as well, i take this to be the downstream o2 sensor.

This is part of the reason I was somewhat confused earlier - this engine doesn't use bank-to-bank fuel control, so there shouldn't be any fuel trim data for bank 2 OR bank 3. By bank-to-bank, I mean that this engine doesn't have separate upstream O2 sensors for each bank of the engine. One upstream O2 for all six cylinders, one downstream O2, one short term and one long term fuel trim.

Also, There's a TPS parameter to view however its stuck at *0 %* no matter throttle position. My engine size is a 3.1 V6.

That's odd, too. No offense meant here, but I'm pretty suspicious of that Car Gauge Pro software now. I've heard good things about Torque, maybe give that a try?

I have access to maf freq, maf airflow- gms/s, map pressure, map voltage, engine speed, load, half of fuel trims and O2 values.

Perfect. The MAF frequency, MAP voltage, engine speed, Load, fuel trims and the upstream O2 would be ideal for the WOT test.

if I am ok with these parameters, Can I give it a shot? WOT in 1st and second gears....??

ALSO, why is it important that we perform this test during 1st and second?

Good to go Safely, of course. Doing it at WOT in first and second is important because it heavily loads the engine, asking a lot of the fuel system AND the engines breathing ability. The overall idea is to determine if a lack of fuel is causing the hesitation/poor acceleration, or a breathing problem. It's not a pinpoint test, but it will point us in the right direction for further diagnostics.

As an example, here's a shot of a known good '01 Silverado 5.3L doing the same test:

www.scanshare.io/share/PthKNw1SfUGJnZW7SEl4Og#0,1,2,4,5,6

Hey, thanks for responding. Are you absolutely sure there's only supposed to be 2 fuel trim values for a 3.1 V6? I thought that only applied to inline 4 and six cyls. I find it odd that i'm getting full readings in the global parameters * which i hear is best to default to* in most troubleshooting cases, and incomplete values in the enhanced parameters.


Or, is it that some V6/V8s feature separate bank controls?

Re: car gauge pro, My friend has auto engenuity so i'll connect his laptop and see if I can get the TPS to work correctly. Then again, he doesn't have the enhanced gm function on his auto eng

Re: TPS data- it works without any issues in global parameters, Not sure what the deal is with it just showing 0* in the enhanced functions.

I had Torque pro installed, but found car gauge pro to be a bit more intuitive for my liking due to the enhanced controls, and active component testing which sadly, doesnt really work on gauge pro, I can only test the evap solenoid. heh.

Tyler wrote: Hey sir, sorry for not getting back to you sooner!

Donnyten wrote: As far as the WOT throttle test, I have a few questions/ issues.

In the Enhanced functions on my scan tool * android phone with blue tooth adapter and car gauge pro*, I can only see fuel trims for bank 1. I have always gotten * NO DATA* for bank 2, and the banks are properly labeled as such * B1+ B2* in the enhanced functions, its only oddly mislabeled as Bank 3 ( bank 2) in the global parameters.

Also i'm not getting any O2 sensor data for bank 2, only bank 1, with an os2s2 value as well, i take this to be the downstream o2 sensor.

This is part of the reason I was somewhat confused earlier - this engine doesn't use bank-to-bank fuel control, so there shouldn't be any fuel trim data for bank 2 OR bank 3. By bank-to-bank, I mean that this engine doesn't have separate upstream O2 sensors for each bank of the engine. One upstream O2 for all six cylinders, one downstream O2, one short term and one long term fuel trim.

Also, There's a TPS parameter to view however its stuck at *0 %* no matter throttle position. My engine size is a 3.1 V6.

That's odd, too. No offense meant here, but I'm pretty suspicious of that Car Gauge Pro software now. I've heard good things about Torque, maybe give that a try?

I have access to maf freq, maf airflow- gms/s, map pressure, map voltage, engine speed, load, half of fuel trims and O2 values.

Perfect. The MAF frequency, MAP voltage, engine speed, Load, fuel trims and the upstream O2 would be ideal for the WOT test.

if I am ok with these parameters, Can I give it a shot? WOT in 1st and second gears....??

ALSO, why is it important that we perform this test during 1st and second?

Good to go Safely, of course. Doing it at WOT in first and second is important because it heavily loads the engine, asking a lot of the fuel system AND the engines breathing ability. The overall idea is to determine if a lack of fuel is causing the hesitation/poor acceleration, or a breathing problem. It's not a pinpoint test, but it will point us in the right direction for further diagnostics.

As an example, here's a shot of a known good '01 Silverado 5.3L doing the same test:

www.scanshare.io/share/PthKNw1SfUGJnZW7SEl4Og#0,1,2,4,5,6

Tyler's right. I missed it the first time I read through this, but those cars only have one cat, one upstream o2 and one downstream o2.
There's no way to have more than one stft and one ltft.

Yep, what Noah said, 100% sure there's only one upstream O2 on this engine (and the 3400 and 3800 of this generation, for that matter).

Here;s a really brief video showing the parameters during WOT ( or kinda WOT). I really didn't feel comfortable doing this test given the age and condition of the car. . Hopefully its enough. RPMS sky rocket but the car takes a second or two or 3 or four to respond it seems. The car does a lot of gurgling and stuttering, really struggling to get out of its way. Seems like its misfiring..

does your scanner show misfire counters?? you said it feels like its misfiring. just out of curiosity have you done a exhaust backpressure test yet??

thanks foir responding.

I did do a back pressure test the other day... according to my gauge the catalyst is fine.. I'll have to check up on the misfires counters. I know the app for my android does.. will check to see if scan master does

at the 24 second mark your at 4700 rpm's and the maf g/s is only at 95. seems low

Low? what would a recommended spec be? at this rpm?

are you doing a w.o.t test in a lower gear? or just in drive??

Yes I am. doing it fro ma dead stop at a stop light. switching from gears 1-2 as suggested ( not manually, but letting the car auto change gears of course)

.Hopefully its enough. RPMS sky rocket but the car takes a second or two or 3 or four to respond it seems. The car does a lot of gurgling and stuttering, really struggling to get out of its way

Uh oh! The delayed engagement sounds like a transmission problem.
Have you checked the trans fluid level and condition lately?
That could also account for a "no power" scenario. If the car is stuck in say 3rd gear on take off, or doesn't kick down under acceleration, it will just bog right down and not get out of its own way. A transmission slipping will have very poor acceleration as well. The RPM will continue to climb, but you won't be gaining any speed