Ignition timing retarding under load/ hesitation
Thread Starter
TECH Resident
Joined: Mar 2012
Posts: 859
Likes: 402
From: Texas
Ignition timing retarding under load/ hesitation I'm dealing with a 2011 Silverado with an LMG 5.3, 207K miles.
It was brought to me for a bad hesitation under load. Truck came in with NO DTCs.
When driving the truck and giving it more than about 1/4 throttle, it feels almost like clogged catalytic converters; like the engine wants to rev up, but can't. (They aren't clogged, by the way; I checked.)
It is NOT misfiring at this time.
While viewing live data on the scanner, I saw that at those times, ignition timing will retard to anywhere from 0 degrees ATDC to -10 degrees.
If you ease off the gas pedal, it goes back to BTDC numbers.
Truck is stock from what I can tell. All sensors seem to be reading normally, fuel pressure is good. Has new AC Delco plugs gapped to specification.
I disconnected the knock sensors (thinking that they might have been too sensitive), and there was no change.
Has anyone ever seen this before, and if so, what was the cause?
If not, does anyone have any suggestions on what to check next?
It was brought to me for a bad hesitation under load. Truck came in with NO DTCs.
When driving the truck and giving it more than about 1/4 throttle, it feels almost like clogged catalytic converters; like the engine wants to rev up, but can't. (They aren't clogged, by the way; I checked.)
It is NOT misfiring at this time.
While viewing live data on the scanner, I saw that at those times, ignition timing will retard to anywhere from 0 degrees ATDC to -10 degrees.
If you ease off the gas pedal, it goes back to BTDC numbers.
Truck is stock from what I can tell. All sensors seem to be reading normally, fuel pressure is good. Has new AC Delco plugs gapped to specification.
I disconnected the knock sensors (thinking that they might have been too sensitive), and there was no change.
Has anyone ever seen this before, and if so, what was the cause?
If not, does anyone have any suggestions on what to check next?
TECH Fanatic
Joined: Feb 2015
Posts: 1,080
Likes: 178
Look at STFT and MAP during power loss. Map will drop at cruise if exhaust is restricted. What pressure measurement are you getting pre and post cat, idle and 2500rpm? May not be cats, but muffler etc down stream. Had a Hyundai that the muffler restricted and overheated/melted the secondary downstream cat.
Thread Starter
TECH Resident
Joined: Mar 2012
Posts: 859
Likes: 402
From: Texas
STFT was within reasonable limits, about -5% or so, never going over 10%.
Exhaust restriction is NOT the issue; I disconnected both collectors and let the pipes hang open, test drove it, and the problem remains.
Pressure measurement PRE cats is ZERO psi.
Oh, and for what it's worth, the guy got impatient and took the truck this morning. I'd still appreciate any input that anyone wants to give.
Exhaust restriction is NOT the issue; I disconnected both collectors and let the pipes hang open, test drove it, and the problem remains.
Pressure measurement PRE cats is ZERO psi.
Oh, and for what it's worth, the guy got impatient and took the truck this morning. I'd still appreciate any input that anyone wants to give.
Last edited by dixiebandit69; Jun 18, 2021 at 06:33 PM.
