Hello,
New member to this forum, thx for letting me in. I have been lucky enough to get help from other people on other forums about different cars over the years because usually the people in it know a hell of a lot more than I do about the car in question, so if you guys might give it your consideration, I'll try that here. Thanks in advance for any suggestions
I have an 05 Chevrolet Z71 5.3L gas with about 200k on it that has a problem I can't get a handle on. It gets about 12mpg
under normal conditions and has low power intermittently and has these situations where it will spit, backfire and only
be capable of about 10mph for a few minutes leaving you stranded. It will eventually come out of it and run almost normally.
At first, I thought it was a vacuum leak because it feels just like that. I have a lower end scanner (Topdon) and when it
does this, I notices 2 things:
1. It has hundreds of misfires when this is happening
2. It is in open loop mode. When I command it to go into closed loop mode with the scanner, it clears up pretty quickly.

So, what would cause this engine to revert to open loop mode after it has already been warmed up and run? Is it just
temperature that the ECM uses to switch it? I am thinking about changing out both temp sensors because I think there
are 2 , one for the gauge and one for the ECM. But, my scanner says it is getting a temp reading that jives.
It does have a low coolant signal through the scanner, but has plenty of coolant. I have that tank and sensor on order.
Here is what I have replaced and done to it to diagnose. All with no change.

CODES:
P0171 Fuel trim lean BK 1
P0174 Fuel trim lean BK 2
P0300 misfire
P0327 Knock sensor 1 low freq
P0332 Knock sensor 2 low freq

REPLACED:
All 4 O2 sensors (bosch)
crank position sensor (walker)
Spark Plugs (bosch)
Injectors (AC Delco)
TPS inside throttle body
Intake Manifold Gaskets
Ignition Coils (AC Delco)
Gas tank Evap canister and Solenoid

PARTS SWAPPED FROM OTHER RUNNING TRUCK
ECM (no change)
Throttle petal position sensor (no change)
Throttle body (no change)
Ign Coil Wires (no change)

WORK DONE
Refreshed all known grounds to engine, frame and body
Added new preemptive grounds to all of the above
Verified each coil ground, power and low signal
Verified the connections at the ECM connector

As you can see, I am basically stumped, unless it is an intermittent problem with the connection to the ECM or an intermittent
problem with Both ECM s.
Please ask me any questions you want to and any suggestions would be appreciated. Thanks for your consideration.

 

have you poped off both valve covers and had someone turn it over as you watch the valve train?

 

Hello, thanks for the reply.
I have not done that yet. But I have thought about doing a comp test just to verify that all that is working. It would have to be an intermittent problem with the valves where they work correctly sometimes and other times malfunction. When it runs good, the valve train would have to be functioning correctly for it to have that amount of power (when it is working correctly). But I am gonna add the comp test to my to do list just to eliminate that as a possibility.
I did change out the spark plugs with AC Delco platinum that Chem_man recommended and it did have a slight improvement.....I guess the 5.3 is finicky about the plugs it uses, and I have a new throttle petal sensor on order because I am getting some fluctuating readings between the desired and actual throttle angle, and I have already replaced the throttle position sensor.
Thanks for the idea, I will verify the valves are working correctly just to take that possibility off the table

 

do a cam/crank relearn.