My Suburban has 170,000 miles on it. Recently I had a mechanic replace the AFM lifters and program it. I now have an issue where it stutters and will die if I can't accelerate through the stuttering. It seems to only happen when the fuel tank is between full and 3/4 tank and a cold start and have driven down the road about 3 miles.
I have replaced the throttle body, MAF, and MAP sensors. I thought it was fixed but when the fuel level got to the mentioned point, it stuttered. Please help diagnose this issue. I have been running a Top Scan Pro on it trying to figure it out but I haven't had any luck yet. I am not sure what system to monitor.

A lot of this depends on what the mechanic programmed it with. Did you replace the AFM lifters with non-AFM lifters and then program it to delete AFM?

Is the programming custom outside of the AFM part?

Is this work under warranty such that the mechanic needs to fix it? . . . and possibly us messing with it might void the mechanic'* warranty?

I'd monitor the oxygen sensors and fuel trims across an event like this.

The AFM were replaced with non AFM'*. The program was also done. I paid to have this done so I am not familiar with all the inner workings of it.
I have had it back at the mechanic that did the work a couple times. The problem is, he is busy and doesn't have a lot of time to spend on it. I have left it for a couple weeks and when he did drive it, it did it once for him but he didn't know what it could be. He is leaning to the crankshaft sensor. I don't want to keep throwing parts at the problem. We have been working with him on it. He said the grounds are all good. I don't feel like us working on it would void the AFM work.

There is a ticking sound from under the hood that isn't constant. This is new since the delete. I have mentioned it to the mechanic and since it isn't a constant, he said it can't be the lifters. It is heard more so inside the vehicle when it'* a cold start than outside. We haven't figured that out either. Not sure if they could be connected.

I have monitored the oxygen sensors and on a regular drive and they are good. The fuel trim does get wonky when it happens. I have recorded that.

I wish more systems could be monitored at once with my scanner but I can only monitor 1 at a time.

I am tempted to put gas in it and get it back to 3/4 to see if it does it again.

I was able to attach a video showing the fuel trim and O2 sensors when it had an episode.

Thanks for any help.

The file appears to be corrupt, I can't play it with several players including VLP which plays just about anything.

If you don't know that the crank position sensor has ever been changed in 177,000 miles, it'* a good idea to go ahead and to it. It'* cheap and relatively easy.

If it wasn't doing this before the work, the mechanic really needs to fix it. The mechanic is the expert, and he committed to doing this job and having it come out correctly. It'* too bad that he doesn't think he has time, but your car was in line before any other work came in the door. What you might run into is we mess with it and then while your mechanic is fixing it anyways, we will have added variables to his equation. Maybe that will make it take longer and he'll support it. Maybe he says "I don't know what to look at now that stuff has been changed". Either way, it takes longer for you.

Anyways, if you still want to dig into it more:


"Ticking" instead of "knocking" tells me that it has a problem with the top end. Your mechanic just replaced lifters in the top end, and replacing them required removing pushrods and rockers as well. Are all of the pushrods straight? Are all of the rockers in good condition? Are they adjusted properly? Did he pre-soak all of the lifters? What is the condition of the camshaft?

Points back at the work that was done.

This is incorrect. This can be the lifters. Is oil pressure low? Is one or more of them defective? Is there some bit of debris clogging something? It'* too easy to say "The result of my work is perfect" instead of saying "There is a chance that something I did and/or a part I installed may be the issue with the new noise that appeared in the exact place that I did major surgery immediately after I did that major surgery."

So when oil is cold and doesn't flow as well, there are noises coming from the area filled with parts that were just installed and that require good oil pressure in order to work properly and not make noises?

I think it could be connected.

Good news.

This would be good to see, and/or at least have described in more detail.

Beats paying $2,000.00 for a scanner, right? We can work with this.

So it absolutely does not do it while under 3/4 tank? That tells me the fuel pump is going bad.

Yes, it'* normally in that range.

That'* disappointing the video won't play. I changed it to a format that was accepted too. I am attaching a screenshot of the crank and camshaft readings from today when it had an episode. Not sure if this tells anything.

Yes, it'* normally in that range.

That'* disappointing the video won't play. I changed it to a format that was accepted too. I am attaching a screenshot of the crank and camshaft readings from today when it had an episode. Not sure if this tells anything.
https://cimg8.ibsrv.net/gimg/www.gmf...362862ddbe.png

Another picture of fuel trim when it died. This all happens when idling or when taking off.

Strikes me as a vacuum leak, or maybe a fuel pressure issue but probably a vacuum leak.

Since it only happens when fuel is 3/4+ full, I almost wonder if it isn't something weird like a evap line connect wrong or an evap check valve not working right or installed backwards.

Just had an idea: Check your purge valve. If it is sitting there open and disconnected, the stumbling-over-3/4-tank thing totally makes sense to me.

It is on top of the intake on the driver'* side. Right where your mechanic was doing lots of stuff. Could just be an innocent mistake forgetting to connect it or maybe it is sad about having been removed then reinstalled for the first time in 177,000 miles.

We replaced all the evap parts in the past year. We had to blow lines out multiple times because of the carbon pieces. The first time I had an episode, we had it towed to the mechanic. He did replace an evap line that connects to the engine. The connector had disintegrated. We thought that repair would fix the issue but it didn't. I haven't checked to make sure all the lines are connected good. I can check that.
Did you see the picture of the crank and camshaft graphs? Just wondering if that gives any hints.