I have a 2004 Monte Carlo SS with the naturally aspirated 3.8l engine. I recently replaced the engine with a low mileage long block from a Buick LeSabre. I reused the throttle body with all sensors and the car had a problem dying and low power. After replacing MANY parts, I have the car to where it will not die, but it has no power when driving. It's very similar to a poorly tuned carburetor. I have replaced, the throttle position sensor, idle air control valve, spark plugs, spark plug wires, catalytic converter, 02 sensors, crankshaft position sensor and camshaft position sensor. I did all of this when I swapped the new engine in to try and have a long, maintenance free experience. I was having problems with the 02 sensors and replaced them with Denso brand and that seemed to eliminate the stalling problem, but still has very little power. The car is sputtering like a bad carb tune.

In another group, people were saying I needed the ECM from the donor car that the engine came from, but again I used the original engines throttle body, plenum etc. Does anybody have any insight? Experienced a similar problem with their car? Thanks in advance.

Tim

 

I probably sound like a broken record on this, but its time to break out the scan tool. With all of these sensors you mentioned, there's a whole pile of diagnostic data at your fingertips to help get to the bottom of this.

If you're certain AFR is off - my first stop would be front O2 reading and fuel trims. Are the trims pegged one way or the other, especially when you get in the throttle and it feels like power is low?

If that's totally fine - then Id look into misfire counters and knock/timing levels. Is the engine getting full timing or is something causing it to pull a bunch? This can also play into the spark system - are the ICM / coil packs struggling.

Given you just replaced a bunch of sensors, my last stop would be looking through the scan data at each one to see if its reading as expected - especially the MAF. Did you buy an OEM quality MAF or a cheap parts store alternate brand? If you're revving the engine and MAF isn't climbing smoothly and proportionately, it could be a dud out of the box. As this is a primary driver of fueling, issues here can cause massive drivability problems.

One other thing to think about - does it act better or worse when in open loop (ice cold) vs closed? You can monitor which mode its in via scan tool - but that too can give clues if it behaves differently open vs closed.

If you still turn up no issues after that, I'd probably do a CASE learn since you replaced the crank position sensor.

 

I appreciate the response. I had a scanner on it and the 02 sensor readings fluctuated quite a bit. The car would not go into closed loop either so that is when I changed the 02 sensors and now the car doesn't die and goes into closed loop. I am not sure what a good A/F trim looks like or should be for this car.

The only thing that has me suspecting that is the car will idle perfectly and drive slowly without issue. It is when you "jump on it" it pops and sputters like a badly tuned carburetor. What would be an appropriate trim when the car is pegged? I will have to have my brother bring his laptop back over and see what we can figure out.

I had a duplicate ICM and coil packs, and I swapped them when the car was dying and it did not have any improvement with the swapped module or coils. I haven't tried them again since I got the car to stop dying though. I bought AC Delco for every sensor that I could. I will have to hook back up to it and see what the MAF readings look like.

I'm not sure if it is better or worse in open vs closed. I took the car to two different mechanics and they couldn't figure it out so I am stuck with my brother and I trying to figure it out. I will see if we can get the scanner back on there and see what we can figure out. I am not getting any misfire codes or anything. I also just checked the fuel injectors and fuel pressure. I have 50PSI on key on, then about 43psi when the car is running. It DOES bleed down slowly to about 30 PSI after the car is shut off. That was after about 1 hour. Not sure if those pressures are in a normal range.

Again, thank you very much for replying...

Tim

 

O2 fluctuation is very normal on narrow band sensors, especially on a brand new engine where it hasn't learned trims yet. The total measurement range is only about 14.0 to 15.5 across the entire 0-1 volt spread. So even once the trims dial in, you'll still see a good bit of oscillation around 14.7.

That's where the trims come into play. The O2 will always bounce around, but over time the car will learn what adjustments are needed to keep it around 14.7. So you normally end up with + or - a few points of trim for each fuel cell. What you want to look for is the trend and magnitude of the trims. If they're solid positive (or negative) double digits across the board- that points to an issue. Even more so if the long term trim maxes out in either direction.

So there is an acceleration enrichment and a performance enrichment mode when that happens - but Im worried the base fuel trims may be way out of whack (ie the car has enough adjustment range to keep it alive at idle, when you throw in heavy accelerator, it can't give enough to keep things OK).


Id still look at cylinder misfire counts. Some codes need several conditions met to set a light. Could be it just hasn't tripped the threshold yet. But would be nice to know if you've got virtually 0 misfires across the board, a whole bunch scattered, or if one cylinder is racking up a bunch - especially when the issue happens.

 

Ok, I will get the scanner back on it and see if we can look at the trims. If the trims are way out of whack, does that point to anything in particular? I know I have personally checked it for any vacuum leaks and so have the mechanics, but just the other day when I drove it I thought I could hear a faint "whistle" when pegging it. Could it be a leak in the upper plenum? I forgot to mention that I had re-gasketed the engine including everything on the lower and upper intakes. So we should be able to see the misfires in Tech 2?

Thanks again!

 

I believe key on engine off is 48-55, so you look to be in spec there. I don't recall engine running pressure, but that value doesn't seem alarming off the top of my head. Does it hold OK when you rev it? That would be alarming if pressure falls off dramatically with rpm - but you would also pick this up in your trims as it will be maxed on fuel adding adjustment.

I wouldnt be too worried about the bleed off. That seems to be pretty common with these cars as they age. If it immediately went to 0, an injector could be sticking a bit - but you'd also notice that in your fueling as you'd likely also have one cylinder running way wonky / misfiring at idle due to all of the added fuel.

Depends how much and which direction. Adding fuel tends to be a vacuum leak (air getting in post MAF). Taking away fuel could be something like a sticking injector. That is over generalizing though as there are several causes of each. A wonky MAF for example could cause both. Misfires also play havoc (especially multiple) as they dump in full cylinder loads of un-combusted air.



Anything's possible, lol. Seriously though - if your fuel trims come back adding a bunch of fuel, its probably worth a closer look. Could try flooding that area with some smoke to see if its being pulled in anywhere.

Yes, there is a misfire counter in the scan tool selection.

Since you have access to a Tech 2 - have you done a CASE learn? I know most people get away without it, even after playing with the crank / sensor - but it can cause some odd issues. Most don't have access to one, so they dont have the option - but if you do, I'd perform it while you're there.

 

The fuel pressure actually increases by several PSI with a quick press of the pedal. Under load (in drive with foot on brake) the fuel pressure barely dips 1-2psi. I had the injectors all off and tested with some electronic pulse tool where I was able to flush them with cleaner and see that they pulse correctly. The MAF is new, but I guess that doesn't necessarily rule it out. I just don't know what to look for exactly in the scan tool, so your input is very helpful.

I have not done a case learn. I'll see if we can figure out how to do that.

Thanks again!

 

That's a good sign! I've seen weak pumps before that seem OK at idle but absolutely tank when the engine needs more fuel, but its good you've ruled that one out.

They're finicky for some reason. Ive not seen new Delco bad out of the box, but I have seen bad from other brands out of the box. Given the supply chain issues the last few years, I suspect quality issues are worse across the board.

This can be somewhat tested without a scan tool though. If you unplug it before starting, the engine will run off the map sensor instead. So if it magically runs ok after turning off the car and unplugging the maf, that gives you a pretty clear giveaway at the issue.


Ill admit that your issues don't seem related to it, but official GM service procedure is to do it whenever the engine, pcm, crank sensor, etc is replaced. The vast majority of people don't do it unless they have a code for it as most basic scan tools can't - but I think its worth ruling out since it'll basically be free to you.

I believe it'll show up as CKP Variation Learn Procedure

 

Ok, so we were having problems with the Tech 2 connecting to the vehicle, but we were able to hook up with a different scan tool. We noticed that cylinders 3 and 6 had a bun h of misfires but not other cylinders did. I have extra coil packs around, so I swapped out that coil pack just to see what would happen since those cylinders share a coil pack. No change from the coil pack. Then we got all the way to your "unplug the MAF to use the MAP" suggestion and the car runs MUCH better! Nearly all of the popping and sputtering were gone. I just bought a new MAF sensor and will recheck for misfires after that is installed. Could the MAF cause those misfires by creating a poor AFR?

The 02 sensor reading is all over the place and constantly bounces between .1 and .9V. The fuel trim on bank 1 was always negative as well. Went from -3.9 up to around -19%. Not really sure what that means, but again the car ran pretty much normal with the MAF sensor disconnected. I really appreciate you taking the time and getting me closer to resolved than two mechanics did!

 

Was it continuing to rack up new misfires or were they all historical?

Being too far out of whack on AFR can certainly do that - but its puzzling that its only on 2 specific cylinders. Generally if AFR is getting to the fringe, you'll start seeing it everywhere.

Its also suspicious that its happening on cylinders that share a coil pack (although I know you swapped it with no result). Not sure what to make of that.

The hard part is if its actively misfiring, thatll skew fueling (as you're dumping a bunch of raw o2 into the exhaust). In this case, the O2 is oscillating around stoich and the trims aren't maxed (ie if the pcm saw it was still too rich, it could pull more fuel from the base map to correct- but its not as it sees its able to achieve stoich). So that makes me think we need to focus on their misfires next.

Since the pcm is seeing over fueling and there aren't a lot of causes there - Id probably try swapping injector locations. Stick injectors 3 and 6 somewhere else and see if the misfires move.

That sounds about right. The AFR range that represents is very tight. Its concerning when the O2 sensor pegs to one side or the other.


Short or long term trims? Negative 19 is really high and is close to tripping a code (I think -22 is the trip point). Thats pulling an awful lot of fuel - we just need to figure out why.