New to this forum, I am stumped and wondered if anyone has any ideas.Thanks in advance

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 posision 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 (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.

Love the truck, but I'm getting sick of this situation
Please ask me any questions you want to and any suggestions would be appreciated. Thanks for your consideration.

 

Two things of concern right off the starting line.

One, the Bosch spark plugs. These engines are finicky and often do not run well on non-OEM recommended spark plugs. The OEM recommended plug is the ACDelco 41-962 double platinum plugs. My recommendation is to get rid of the Bosch plugs and install ACDelco 41-962 plugs. The Bosch plugs could well be the cause of the P0300 DTC.

Two, you changed the crankshaft position sensor. When crankshaft sensors are replaced, what is called a CASE relearn must be performed in order to reprogram the ECM with the new crankshaft position sensor and camshaft position sensor position variances. Not performing the CASE relearn can cause driveability problems.

You mention that you have a "...lower end scanner...". Do you know if it can show misfires by cylinder?

 

Chem_man,
Thanks for the reply. I did not know these 5.3s were that finicky about their plugs. Got a set of ACDelco 41-962 plugs on order THX.
I did not do a relearn for the CPS, I will get into that and see if the scanner I have can perform that function. If it "goes off the reservation" intermittently, that could certainly cause the backfiring.
Yes, the scanner does show misfires by cylinder. They are mostly 4, 3 and 8 in that order most to least. When I changed the coils around before I replaced them, the misfire did move, but did not follow the coil that were servicing the previous misfire cylinders, it was a random pattern. I need to check that again, because I just installed the coils.
THX

 

OK on being able to see which cylinders have the most misfires. I would look at the electrical connectors to the ignition coils on those cylinders carefully to make sure that all the wires going into them are intact. Make sure they are well secured into the backside of the connector and that none are "feeble" so to speak,

I know when I replaced the knock sensors on our 1999 Silverado, one of the wires for the #5 ignition cylinder broke off where it entered the connector. What surprised me was that the wire was not really stressed to the point of breaking relative to the other wires. Must have been an issue of aging and heat.

Good Luck!

 

Chem_man,
Yea, I think more and more that I'm looking at intermittent wiring problems. I will have the new plugs here by the weekend. I still haven't found out if my Amazonscanner will do the CPS relearn, worst case scenerio, I'll take it to a shop and pay them to do that. I will work on these things.
THX

 

It takes a professional level (Snap-On, Bosh, GM Tech 2, etc) type bidirectional scanner to do a CASE relearn procedure.

Also, be sure to follow the wiring back into the harness to make sure the wiring is not compromised (rubbing against the engine block, worn insulation, loose connectors, etc.)

Good Luck with the hunt!