Cutting out/surging during acceleration

Mooseman

Mooseman

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Last weekend, I replaced my catalytic converter and also eliminated the resonator. My cat was pretty plugged and immediately saw an increase in power and MPG. Since yesterday, I noticed that when accelerating, especially at WOT in 1st gear, the engine cuts out at certain points. It will accelerate, cut out for 1/4 second, accelerate again, cut out again, accelerate again and then shift into 2nd gear. It does it also at moderate acceleration but not as bad but I do notice it. Light acceleration might do it but is almost not noticeable. No codes coming up.

Some history:
efans (fan codes removed)
PCM4less tune
Replaced variable camshaft phaser 2 months ago (the one on the timing chain)
Cleaned TB and intake manifold at same time as phaser replacement
New GM CPAS 6 months ago
New plugs, fuel filter last July when engine was replaced

Some possibilities that are popping into my head:
bad gas causing knock and PCM is momentarily retarding the ignition (I do occasionally hear some ping on acceleration)
low fuel pressure (I've had this before on my 4.3L Jimmy but caused constant loss of power on acceleration)
PCM is having hard time with new found exhaust flow?
Borked PCM tune?

Reason I say a possible bad tune is because the truck always did this after the tune. Was more like a "blip" rather than an actual rpm drop. This blip disappeared right after I replaced the cat but now is back and worse.

Any insight would be appreciated.
 
My first guess would also be fuel. Try some higher octane - different brand. Do you have access to a scan tool that can store data to look at knock retard after a run?
 
I have an ELM327 that I'm currently using with an OBDII app on an older Windows Mobile phone. Not sure if it records timing but will check on it. I might have to borrow my son's Android phone to use with Torque or check for something to use with the laptop.
 
Since you've improved air flow through the engine, computer may be trying to run it on "older" parameters and is having a few "hiccups" during hard accel.

I'd try disconnecting the battery for an hour (and might as well do another throttle body cleaning during the wait), and allow the PCM to go back to factory defaults again, just like when the cat was new.

Make sure air filter is clean.
 
Mooseman said:
Last weekend, I replaced my catalytic converter and also eliminated the resonator. My cat was pretty plugged and immediately saw an increase in power and MPG. Since yesterday, I noticed that when accelerating, especially at WOT in 1st gear, the engine cuts out at certain points. It will accelerate, cut out for 1/4 second, accelerate again, cut out again, accelerate again and then shift into 2nd gear. It does it also at moderate acceleration but not as bad but I do notice it. Light acceleration might do it but is almost not noticeable. No codes coming up.

Some history:
efans (fan codes removed)
PCM4less tune
Replaced variable camshaft phaser 2 months ago (the one on the timing chain)
Cleaned TB and intake manifold at same time as phaser replacement
New GM CPAS 6 months ago
New plugs, fuel filter last July when engine was replaced

Some possibilities that are popping into my head:
bad gas causing knock and PCM is momentarily retarding the ignition (I do occasionally hear some ping on acceleration)
low fuel pressure (I've had this before on my 4.3L Jimmy but caused constant loss of power on acceleration)
PCM is having hard time with new found exhaust flow?
Borked PCM tune?

Reason I say a possible bad tune is because the truck always did this after the tune. Was more like a "blip" rather than an actual rpm drop. This blip disappeared right after I replaced the cat but now is back and worse.

Any insight would be appreciated.
Click to expand...

First thing is first. Does the hesitation only happen in open or closed loop operation or both?
 
I think it happens in both but I don't floor it or accelerate hard until it's warmed up. I still have to disconnect the battery to reset the PCM.
 
Mooseman said:
I think it happens in both but I don't floor it or accelerate hard until it's warmed up. I still have to disconnect the battery to reset the PCM.
Click to expand...

I would take a look at your fuel pressure, fuel pressure regulator, LTFT and STFT readings. You could have a lean, rich or misfire condition. Check your misfire data if possible.
 
This morning will be my troubleshooting time with all the above suggestions and then I'm heading down to O'burg NY to pick up some packages, which will be be a good test drive.

I'll report back with results later. Thanks all!
 
Disconnected the battery and cleaned the TB again. No change. I recorded this video to show exactly what it does

[video=youtube;GAWNiqYo4Do]http://www.youtube.com/watch?v=GAWNiqYo4Do[/video]

I also recorded the OBD data and attached them here. Don't know if anyone can use the .dat file so I converted it to .csv. If anyone can decipher this, it would be appreciated.
 
I was always told an when engine dies slowly, it's a fuel problem.

If it is an instant die, like turning the key OFF, it's an electrical problem.

Watching that video, it looks more like an electrical problem.

Like the key was turned OFF and back ON again. :twocents:
 
It was the gas. Filled it with 93 octane, problem gone. But it begs the question why would it be knocking so much with the 87 octane that it would cut the engine? I'm still thinking that the tune was possibly done wrong and set for running hi octane. I'll experiment some more with different brands and octanes and see how it goes. I'll fire off an email to PCM4Less to see if they have a record of what was programmed.

There's no way I will run premium. It's expensive enough running it on regular! :mad:
 
Mooseman said:
It was the gas. Filled it with 93 octane, problem gone. But it begs the question why would it be knocking so much with the 87 octane that it would cut the engine? I'm still thinking that the tune was possibly done wrong and set for running hi octane. I'll experiment some more with different brands and octanes and see how it goes. I'll fire off an email to PCM4Less to see if they have a record of what was programmed.

There's no way I will run premium. It's expensive enough running it on regular! :mad:
Click to expand...

Well when you are doing a tune you are telling the engine that it can advance more which means you need higher octane fuel to prevent preignition. That's one of the reasons why I won't be doing a tune to my I6.
 
CaptainXL said:
Well when you are doing a tune you are telling the engine that it can advance more which means you need higher octane fuel to prevent preignition. That's one of the reasons why I won't be doing a tune to my I6.
Click to expand...

PCM4Less offers a bunch of options and they have a specific selection for fuel type. If you choose regular it will work perfectly fine with it. I have never had a problem with my P4L tune in the years I've had it.
 
Mooseman said:
...why would it be knocking so much with the 87 octane that it would cut the engine? ....
Click to expand...
Preignition can come from hot spots in the combustion chamber that cause dieseling. Carbon buildup comes immediately to mind. Try a Seafoam treatment or two.
 
khill said:
PCM4Less offers a bunch of options and they have a specific selection for fuel type. If you choose regular it will work perfectly fine with it. I have never had a problem with my P4L tune in the years I've had it.
Click to expand...

Well I am just saying that their website FAQ states that their tunes are done with high octane fuel in mind and should be used.
 
I pulled up my original order and I did request for regular 87 octane fuel. I sent an email asking to check their records. You never know, we're all human and stuff can happen.

Good idea on the Seafoam. With a plugged up cat for so long, who knows what that could have done.
 
Update: I'm 90% certain it's my fuel pump. I Seafoamed it. Great smoke show and poured about half a can in the tank that was half full. It started doing it again even though it was 93 octane. Thought that the Seafoam was causing it. So I burned off that gas and filled it with 91 octane, problem gone again. Then as that tank was going down below half and especially 1/4, it started doing it again. Today, I filled it with 87 octane, problem gone again. So definitely not an octane issue but something hinky with the fuel pump. I'll be shopping for a fuel pump either at RockAuto or Amazon, either an ACDelco or Delphi.
 
CaptainXL said:
Try swapping fuel pump relay with another.
Click to expand...

Good suggestion but I doubt it would do anything. It has consistently done this as the fuel level got below 1/2. It's like there is a leak in either the fuel pickup or pump itself and as the level goes down, it exposes this leak and starts sucking air. I'll swap it anyway just to eliminate that possibility (however slim :cool:)

- - - Updated - - -

CaptainXL said:
Try swapping fuel pump relay with another.
Click to expand...

Good suggestion but I doubt it would do anything. It has consistently done this as the fuel level got below 1/2. It's like there is a leak in either the fuel pickup or pump itself and as the level goes down, it exposes this leak and starts sucking air. I'll swap it anyway just to eliminate that possibility (however slim :cool:)
 
Mooseman said:
Good suggestion but I doubt it would do anything. It has consistently done this as the fuel level got below 1/2. It's like there is a leak in either the fuel pickup or pump itself and as the level goes down, it exposes this leak and starts sucking air. I'll swap it anyway just to eliminate that possibility (however slim :cool:)

- - - Updated - - -



Good suggestion but I doubt it would do anything. It has consistently done this as the fuel level got below 1/2. It's like there is a leak in either the fuel pickup or pump itself and as the level goes down, it exposes this leak and starts sucking air. I'll swap it anyway just to eliminate that possibility (however slim :cool:)
Click to expand...

The pump sits at the bottom of the tank. So nothing to leak. Check fuel pressure.
 
I know you replaced the fuel filter last july but possible that it has a blockage or defect. Check fuel pressure first and go from there.
 
I had similar problems when my pump died last year. It acted like it was out of gas - step on the throttle and nothing would happen. Pull over, put it in park, sit for 30 seconds, and it would be fine.

It only did it when the car had been running for 15 minutes or so and the fuel level was about 1/4 tank or so.

Replacing the pump solved the problem.

My WAG is that the pump was overheating somehow.
 
Sparky said:
I know you replaced the fuel filter last july but possible that it has a blockage or defect. Check fuel pressure first and go from there.
Click to expand...

Just too much of a coincidence that when it's at 1/4 tank it does it every time , similar to strat's problem. Anyway fuel pump is on order. Just have to find the time to do it.
 
Oooops! Sorry about that. Was doing a lot of work on it at that time and it slipped my mind.:redface:

I replaced the pump with a new Delphi and the problem is solved. Over a month now and all's well.

You having a similar issue?
 
Mooseman said:
Oooops! Sorry about that. Was doing a lot of work on it at that time and it slipped my mind.:redface:

I replaced the pump with a new Delphi and the problem is solved. Over a month now and all's well.

You having a similar issue?
Click to expand...

I do have a slight hesitation in the higher RPM's > 4000 and power is just not there. Feels like the truck is ever-so slightly bucking. Also I need to floor it just to force a downshift to accelerate. Putting around town feels like I am towing something. The fuel pressure checks out ok. Wondering if the volume is not there. I took a few plugs out yesterday and the porcelain insulator was bright white which to me would indicate a lean mixture at open loop when floored. Thoughts?
 
CaptainXL said:
....Also I need to floor it just to force a downshift to accelerate.......
Click to expand...
Do you have a tune in it?

Getting rid of most of the torque management fixes this problem.....trans shifts much sooner, and no need to mash the gas to get it to do so (unless you're trying to pass someone quickly, etc).
 
MacMan said:
Do you have a tune in it?

Getting rid of most of the torque management fixes this problem.....trans shifts much sooner, and no need to mash the gas to get it to do so (unless you're trying to pass someone quickly, etc).
Click to expand...

No tune. But! I found some more evidence today that something is wrong. I did some trial runs with my voy at a dead stop flooring it to get up to 60 today. The results are not good as far as what I know the XL's 0-60 times should be. Every run I did it came out to about 13-14 seconds to get up to 60 when it should be 9 seconds. I used a horsepower calculator I found on the web that uses your weight and 0-60 times and I had to lower the horsepower rating to match my times. As it turned out I only have 140 HP at the flywheel. So something is not right. Also I found my max speed only got to 95mph before the engine started puttering. I think Envoys are supposed to get up to 110 if I am not mistaken?
 
I am going to take out the O2 and see what we see. Engine performance seems to have gotten worse than the last time I brought it in to the exhaust shop and they ruled it out. I can't imagine anything else, its got to be the problem. But I suppose it could be a bad fuel pump as well not delivering enough volume.
 
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