Diagnostic help needed - SOLVED

[Tbruz]

2004 Tahoe 5.3 Gas ( "T" in VIN) with 253K miles.
We are the original owners so all previous repair history is known.
I can drive the vehicle all day long, cross country If I choose to, as long as I stay under 80MPH.
If I go over 80 MPH I get a check engine light that reports back to a random misfire code.
If I wait overnight the check engine light will be off next time I start the vehicle and will not come back- unless I again drive over 80MPH.
I've gone weeks/months driving local roads, highways and even off road(all under 80MPH) with no CEL.
One excursion to 80MPH or over that and the CEL is flashing and then latches on to permanent.

So far I've replaced plugs, plug wires and coils-tired of throwing parts at this.
FYI, Injectors are original as are the intake and head gaskets
I currently do not have a scanner capable of live misfire data or fuel injector testing.
Advice and direction on next steps appreciated.
Thanks in advance.
Tbruz

 

[NickTransmissions]

2004 Tahoe 5.3 with 253K miles.
We are the original owners so all previous repair history is known.
I can drive the vehicle all day long, cross country If I choose to, as long as I stay under 80MPH.
If I go over 80 MPH I get a check engine light that reports back to a random misfire code.
If I wait overnight the check engine light will be off next time I start the vehicle and will not come back- unless I again drive over 80MPH.
I've gone weeks/months driving local roads, highways and even off road(all under 80MPH) with no CEL.
One excursion to 80MPH or over that and the CEL is flashing and then latches on to permanent.

So far I've replaced plugs, plug wires and coils-tired of throwing parts at this.
FYI, Injectors are original as are the intake and head gaskets
I currently do not have a scanner capable of live misfire data or fuel injector testing.
Advice and direction on next steps appreciated.
Thanks in advance.
Tbruz

Stick a vacuum gauge on it to see if it reveals anything. You can google "reading an automotive vacuum gauge" or something similar to help with interpretation.

Also, most auto parts stores sell one or two scan tool models that will read live data. Its def worth the investment vs not having anything if you do any sort of work on your own vehicles (or want to start).

 

[Fless]

As @NickTransmissions recommended, a vacuum test would be helpful. Is this the L59 Flex Fuel engine? (what is the 8th character of your VIN?)

We could start with seeing what the short- and long-term fuel trims are on both banks, just under normal (<80 mph) driving. And over 80, if you can do that. It would be best to have one person drive it while another reads the scanner.

Also fuel pressure could be affecting the misfires; that would likely require using a fuel pressure gauge on the fuel rail, and having a long enough hose to tape it to the windshield while you drive over 80 mph.

Is the air filter clean? Has the MAF been cleaned lately?

One other thing is to run a Crankshaft Position Correlation; it's fast and easy with a capable scanner.

 

[OR VietVet]

What brand fuel do you use? What @NickTransmissions and @Fless said is good recommendations but I am curious, because you did not say, do you lose power or experience a misfire condition when the CEL comes on? I ask about the fuel because I wonder if the injectors are partially clogged. Have you ran cleaner in the fuel system? Maybe some Techron or Seafoam.

 

[Fless]

What brand fuel do you use? What @NickTransmissions and @Fless said is good recommendations but I am curious, because you did not say, do you lose power or experience a misfire condition when the CEL comes on? I ask about the fuel because I wonder if the injectors are partially clogged. Have you ran cleaner in the fuel system? Maybe some Techron or Seafoam.

That's kind of where I was going with the fuel trims and to see if it's an L59 so he could try to run a few tanks of E85 if available. Injector balance is another concern but getting them clean might be helpful if they're dirty.

 

[OR VietVet]

8th digit "Z" is Flex Fuel and 8th digit "T" is Gas.

 

[Tbruz]

8th digit is a "T" so its gas and not a flex fuel engine.
No loss of power or stuttering at 80MPH when the CEL comes on.
Fuel used is usually 87 octane and I have tried running Techron cleaner thru on a couple of tankful's.
I do have a vacuum gauge and a fuel pressure gauge so Ill give that a shot tomorrow and report back.
Thanks
Tbruz

 

[OR VietVet]

That fuel gauge would be especially helpful with a long enough hose to run out under the back of the hood and tape to w/s and see what the cold start up pressure is, warm pressure and driving pressure and pressure at 80 mph and above.

 

[Doubeleive]

I would check the grounds on the injector and coil harness, maybe even patch one in for testing purposes
also I would live data monitor these misfires to try and pinpoint where the issue is happening, for instance left bank or right bank
that's the direction I would go first.

 

[Mean_Green]

I haven't exceeded 80 in probably 40 years and don't see any reason to ever do so again.