'07 Suburban rough idle

Hoping all the excellent minds around here would engage in some speculation on a truck that threw me for a loop. :blink:

2007 Suburban (LC9) comes in with a complaint of a rough idle. I get in and the symptom is evident right away. It's tough to describe - somewhere between the feeling of a trashed engine mount, a low idle, and an unbalanced engine. Less like a roughness, more like a strong vibration. Instantly goes away off idle. Repeatable at all engine temperatures. Putting it in Drive/Reverse has little impact.

No codes. Scan data on the test drive is lovely, aside from a occasional low idle when coming to a stop. The TPS readings are too high at idle, so I figure I'm on track for a dirty throttle body. Clean it and relearn, numbers are better, but the vibration remains. Then I lift the engine up off the mounts with a block of wood and a floor jack. Still no help.

I check misfire counts - no history, but the current counters aren't working, either. As in, it's not being monitored. For whatever reason, I decide on a relative compression test, and hit paydirt. Sync on #1 plug wire:



I advised further testing for a mechanical issue, and the customer declined. Bought recently, so no history.

My question is, is it possible to have a timing issue on this engine with no code? :huh: And would that account for the misfire monitor? A plugged cat it out, IMO, as the O2's and the VE at WOT are good. Or, can anyone think of another reason why one bank would have a compression issue?

Tyler wrote: , can anyone think of another reason why one bank would have a compression issue?

I'm not familiar with those engines but could be cam timing out one tooth on that side?

Sorry, didn't specify, these are OHV engines. :blush: Only one cam, so a timing issue should screw with both banks...

I'm thinking about this.

This is a good question.

But, I looked up the timing and found it to be 1 8 7 2 6 5 4 3. If this is correct, did I miss it? You should either have two high compression cylinders or two low compression cylinders next to each other. Right?

But, it looks like you're alternating. Weird.

I'm still thinking... LOL

I was typing in my post as you were entering your second post.

Are those the cylinders that can be deactivated?
1 4 6 7

You guys are too sharp. :lol: I'm an idiot.

As I was replying to Paul, I was going over the firing order in my head. It doesn't alternate between banks, as GeekDIY pointed out. The only way you get this waveform is from an AFM issue. :silly:

Which now makes total sense, in light of the fact that I noted that the top of the VLOM had some kind of hose port on the top, which had a plastic cap on it. The factory ones don't have that, so this had to be a replacement of some kind.

I'm now wondering if this engine has had some kind of AFM delete (hardware and software) . That'd explain the VLOM and the lack of misfire counters.

Many thanks to everyone for pointing out what's right in front of my face! :lol:

Yup! It's been deleted. See below:

sdparts.com/i-24048327-sdpc-sdafmd53-dod...kit-for-ls-5-3l.html

I dunno if this is the exact kit used, but the VLOM looks very similar.

Now I know how to spot these for next time.

This happens to me all the time.

Without wondering about things, it is really hard to get better. In addition, this discussion helped others, which is what this forum is all about.

So, I know what you did here. You were testing us all. LOL

Have a good one!

So..

Does this mean the delete kit was not properly installed?

GeekDIYMechanic wrote: So, I know what you did here. You were testing us all. LOL

Have a good one!

:lol: Well if that's true, you guys dominated!

Does this mean the delete kit was not properly installed?

I guess? I was reading about that delete kit I posted, and it specifically mentioned this:

SDPC wrote: *Note: This kit is intended for use with a non-DOD/AFM camshaft (aftermarket or factory). This kit does not include the camshaft. Use of a factory DOD/AFM camshaft with this kit will result in engine misfires. Also, computer tuning IS required to complete this modification. You must have DOD/AFM disabled in the PCM to prevent the ECM from setting codes.

I don't know what the difference is between AFM and non-AFM camshafts is? Different lift/duration for the AFM cylinders would be my guess. But it would seem that someone installed the kit, THEN found out it needed the cam, too.

How much you wanna bet this is the reason the truck got sold? :silly:

EDIT: More Googling. Found this blog post:

www.onallcylinders.com/2017/11/30/how-to...t-afm-on-gm-engines/

When installing an AFM delete kit:

You MUST upgrade to a non-AFM camshaft because the cam lobes in the four AFM cylinders are ground differently. The engine will have 25 less pounds of compression in those four cylinders which can result in a misfire code.

That's a winner! :woohoo:

Are you sure this isn't a test sir?
Lol

Noah wrote: Are you sure this isn't a test sir?
Lol

I am Tyler's complete lack of waveform analysis skills. :lol:

Update! Did some more digging, and I believe this one has definitely been programmed for a delete kit. :lol:

I grabbed the Calibration ID's and the Calibration Verification Numbers from the vehicle for comparison to what GM has listed at TIS2Web . This is what the vehicle shows:





Now to what TIS2Web shows as correct for this vehicle:



All the calibrations and CVN's match perfectly until you get down to the 'Engine Diagnostic' section. Highlighted for clarity:



The calibrations match, but the CVN's don't. :silly: