08 GMC Yukon - Lean
I have this 08 Yukon that came in for the complaint of a high idle. It had P0171 P0174 and P0455 (large evap leak) present. The vehicle runs fine and I cannot duplicate the high idle complaint. When sitting at idle in Park the fuel trims are slightly positive (around +8), but when I put it in gear (reverse or drive) the fuel trims go way positive (both banks about +35 total trim). If I idle it up the fuel trims go back to about +8 total trim suggesting a vacuum leak. 205k miles.
Blocked off Purge valve and checked for leaking valve.
Injector balance test- they all dropped the same.
Sprayed intake with brake clean (no change)
Fuel pressure = 46psi - within spec for this non-flex vehicle with a FPCM.
Ethanol tested fuel sample= about 10%
Cleaned MAF with no change.
Found cracked PCV elbow on left valve cover, replaced it and reset fuel trims, no change.
Found negative battery terminal loose, tightened, no change.
Checked for PCM calibration updates, found none that would apply.
Why do the fuel trims go way positive when I put the vehicle in drive and just sit there? I can sit there and idle in park for as long as I want and they stay within 10%. I have scan data that I will post later after I go through it.
Check the power brake booster for vacuum leak.
Only thing different your doing is putting your foot on the brake. I’d take a good look at the brake booster leaking.
Your right, sure sounds like a vacuum leak, can you smoke intake for leaks?
So is this engine a non flex fuel engine? can you check fuel trims at idle , part throttle, and under load or at least compare idle to 2500 rpms and see difference in park and in gear and post data.
Fuel trims should go down at high RPM if it is a vacuum leak. What's you MAF at idle and in drive?
You 100% have a vacuum/unmetered air leak, as you raise the RPM trims go down. If you had a fueling issue this would not happen. This vehicle is reliant on a MAF so an unmeasured vacuum leak would cause a fuel trim problem on both banks like you have. When you open the throttle and raise RPM, you are bringing the intake down close to atmospheric pressure which is why fuel trims go back to…
+1 on just making sure engine torque in gear doesn't open up a post MAF leak as opposed to a booster. Sure sounds like it's the booster unit though.
I don't know why I didn't think of the booster as being a possibility, but I will block it off first thing in the morning and see what the trims do. If it is not leaking I will record trims as suggested.
When i check fuel pressure on mitchel it calls for 50-60 psi with engine not running. what does fuel pressure do while you put it in gear or even drive?
Harm, There are two fuel pressure specs for this vehicle, with and without a fuel pump control module. For the vehicles with the fpcm the pressure spec is lower, which is the vehicle I have. My fuel pressure stays at 46psi even in drive. Thank you for the input.
Closed with fix: The brake booster was leaking. Thank you all for the help!