2011 GMC Acadia 3.6L - High Fuel Trims, High Voltage on Knock Sensors, Intermittent No-Crank, No DTCs

Hello,

I am hoping for some guidance with pinpointing the source of the following problems.  I was asked to look at this vehicle after a number of random no-cranks and complaint of running poor on cold starts. The symptoms all started at the same time to the best of my knowledge. 

Symptoms:
* Random No-Crank. It will typically start cranking after a few tries but has taken a lot of tries on occasion. No loss of gear selection indicator (seems to be common on these vehicles)
* Very rough running on startup, just while in open loop
* Once in closed loop the idle smooths out but Fuel Trims start climbing immediately. Total FT on bank2 gets to the upper 50's while bank1 gets to the upper 40's 
* Calculated engine load is in the 30's-40's when idling and nears 100% cruising on the highway
* Both knock sensors are at around 3.8volts at idle and climb as high as 32volts when revving to 2500'ish RPM in park. I also see this while driving. Voltage increases as RPM increases. Timing is pulled back by about 8 degrees when the voltage climbs. 
* Exhaust tips are blackened/sooty
* No DTCs are setting

Troubleshooting done so far:
* Cleaned battery cables and posts. Ultimately replaced a weak battery.
* Cleaned up G110 ground as well as G302 
* Performed smoke test - No leaks found
* Replaced MAF with the OE part - No change
* Tried junkyard ECM but the original is back in now - No change with junkyard part

I have an Autel MP808BT Scantool and a 2 channel Picoscope. Please help me with the logical steps to resolving this. I hate throwing parts at a problem. The MAF and ECU were under the guidance of another mechanic i was working with. The battery was needed. 

I'll be back at the vehicle tomorrow after work and will update with anything new.

Thanks,
Ryan

* Very rough running on startup, just while in open loop
* Once in closed loop the idle smooths out but Fuel Trims start climbing immediately. Total FT on bank2 gets to the upper 50's while bank1 gets to the upper 40's

What is your MAF g/s value at idle.
Once in CLOSED LOOP, what is your Total FT, in PARK, at 2500 RPM?
What is your Fuel Pressure?

Hi Chad,

Thanks for taking a look at this.
The MAF measures 3.52g/s
Total FT @2500RPM in Park B1=28    B2=45
Fuel pressure fluctuates between 519 and 664psi at idle and about 560psi at 2000rpm

Ryan

I didn't see the PID in the GM specific OBD data but in the genericOBD I see the low pressure fuel is only at 47psi at idle. That seems low but I have to get home. I'll check the spec when I am back here tomorrow.

Also I have not had any no-crank situations since fixing the G110 ground. It's still too early to be sure but that one may no longer be an issue.

I am really surprised there are no trouble codes with those trim values.
Is there a flex fuel percentage pid?

Yeah me too. This is not a flex fuel vehicle but I will check for the PID this evening.

Thanks,
Ryan

I just noticed I gave idle FT and stated they were at 2500RPM

The correct totals @2500rpm are B1=15 B2=26

Random No-Crank. It will typically start cranking after a few tries but has taken a lot of tries on occasion. No loss of gear selection indicator (seems to be common on these vehicles)

If you're confident in the battery and connections, then I'd strongly suspect a failing starter. Very common on this engine. How many miles on this Acadia? You can have a look from the underside, just past the front catalytic converter. If you see a GM sticker on the starter, it's very likely original. 

If you want to catch it with a scope test, you can watch battery voltage during cranking (or not cranking). Getting to the starter itself isn't really an option, but you can watch it at the negative jump post by the right strut tower, and the B+ stud on the underhood fuse box.

Here's an example from a '12 Acadia with an intermittent no crank. Voltage in green, starter current in red. If you don't have a high current probe, don't sweat it. The voltage trace will work well enough.

 

We're looking for the circled 'noise'. That's current arcing across a connection, in this case the starter solenoid contacts. Next is the same starter when it decided not to crank:

 

The rule of thumb I've learned is three spikes. Three or fewer is normal. More than that is suspect.

I just noticed I gave idle FT and stated they were at 2500RPM

The correct totals @2500rpm are B1=15 B2=26

Still behaves like a vacuum leak. I'd try taking the vapor line off the purge valve, cap the purge valve and recheck your trims. Not unusual for these to stick wide open and cause skewed trims.

Good information, thank you! I do have high and low amp clamps. I actually just recently picked them up. Yes I would like more hands on time with the scope so I will definitely check out the starter as suggested.

It is a high mileage vehicle. Well over 200k and has not had a starter replacement as far as I know. The battery was very weak and just replaced. The terminals I cleaned before replacing the battery.

I'll be at the vehicle a little later this evening. I plan to look up the proper fuel pressure spec and get fuel pressure from a gauge instead of relying on scantool data. I will try your suggestion with the purge valve tonight as well.

Thank you for your time,
Ryan

There is no flex fuel PID.

Disconnecting the purge valve line and plugging the valve did not make a difference.

So much for gauge fuel pressure. It does not look like this vehicle has a service port. I did read in the service manual to command on the fuel pump KOEO and look for 50-100PSI. I did so and had 63psi which immediately drops to 61psi when the pump switches off. Pressure then holds steady.

I found the new battery drawn down just since last night. I guess tomorrow I will try to find a parasitic drain.

Ryan