I thought about that. Then I thought about the time I dropped my mom off at her local mechanic, he's been in business like 40 years, he jokingly told me not to bring the Tahoe There, he doesn't work on Hybrids(I think he was joking, maybe)

Since the dealer is apprehensive about servicing the Tahoe, I would be really worried about someone unfamiliar with it's quirks.

I sure there are plenty of competent independent mechanics that could do it, I just don't know of any. Even the guy that services my collectable cars won't work on the Tahoe(he's terrified of the orange wires-lol)

 

the service tech of these GM hybrids can make a living going indie. maybe hillbilly too? I can buy a gas station and an auto shop on the side?

 

the good thing about the GM hybrid is it can go 100k miles without any servicing at all.

 

Afraid of the orange wires? How much did these Ludites mourn the loss of Tetra Ethyl Lead in the fuel? Or are they afraid to admit it's just another Small Block under the hood?

 

HH - I did my plugs and wires last week. Everything ran fine for a while, then on a 1 hour drive I noticed a loss of power and the engine ran rough. I made it home, and a couple days later on a short drive it happened again. On two other occasions I got the flashing Check Engine line indicating a misfire. Any code I found (stored or pending) only indicated random cylinder misfire.

My first thought was that plug 8 didn't seat properly, but I pulled it and reinstalled it and it seems fine. I ran it again any seemed fine until it got hot (oil temp 180) then it was running rough and even "Knocking" (?).

As I watched the gauges during above, I noticed the ignition timing at idle was about 27 or so. I recall before changing the plugs and wires it was about -1 or -2. I also noticed that the timing was slowly decreasing from 28 to 27 to 26 to 25, then it would start running rough, then back up to 26. Occasionally it would start to creep down, then go up when it was running rough.

BIG QUESTION 1: Does the truck need to go through some type of relearn cycle after changing plugs / wires? I could guess that new plugs and wires could deliver a faster spark than old ones, requiring the relearn?

BIG QUESTION 2: If not the previous solution, what is more likely: a problem with the plugs, or a problem with the wires (or some other problem)? Is there any "gauge" or value I can look at to point me towards which cylinder is causing the problem?


Thanks

Jt

 

No learn is ever necessary after plugs or wires. The GM service scan tool Tech 2 can pull out the Freeze Frames from the P0300 and look at the individual cylinder misfire counts.
That is how I would go looking for the problem cylinder. That said there is a oil screen/filter under the back end of the intake manifold area. It screens the oil for the V8/V4 lifter hydraulic control. If plugging up it can cause trouoble.

 

Thank you sir. I will check the freeze frame data. Oddly, after I posted I got a sensor stuck rich code and subsequently a Bank 2 Sensor 1 high voltage code.

I rarely think there are two problems at the same time, and surely a failing pre-cat sensor could cause some "searching" which I had interpreted as "learning". Perhaps major changes (i.e. ignition timing changing from -1 / -2 to +27 / +25) would cause an errant ignition which would appear as knocking, misfire, rough idling, and poor performance.

On the other hand, the correct diagnosis can often be found in recent history. In this case the recent history would be the plug / wire replacement. I would think that even the worst case scenario (a cross threaded plug), provided it is in tight, would still spark. The more likely scenario would be a wire with poor contact, which has already been checked and cleared. In either case, statistics would probably lie in a single cylinder problem with two cylinders combining to make up the vast majority of cases. In this case, the issue could be further clarified by whether or not it happens in V4 mode (which would correlate to cylinders 2,4,6,7 if I am not mistaken.

Anyway, looking back further in the history, I think I changed the plugs / wires due to a misfire / rough idle condition in the first place. I suspect that would point to the O2 sensor.

If you have any thoughts, I would always love to hear your thought process.




Thanks again,

Jt

 

The O2 sensor does not have enough ability to change A/F enough to cause a misfire.

 

I think the freeze frame data as well as the system monitor point to Cylinder 8

Cylinder Freeze Frame System Monitor
1 0 0
2 4 21
3 0 0
4 5 20
5 0 0
6 0 0
7 0 0
8 15 134


Thanks for the tip HH!!!

So armed with that information, and being that cylinder 8 is by far the most difficult to get to, my question is this.... I had double checked it after I started having trouble. I pulled the plug and re-inserted it. it's not the smoothest thread, but it is in there fairly tight. If it was cross threaded, but tight, could it still cause a misfire? In that case I could retap the threads. On the other hand, is there an easy way to test a coil pack? or should I just swap it with another and track the misfiring cylinder?

Any other suggestions for diagnosis?




Thanks again, as always,

Jt

 

Crossed threads are bad but won't cause misfire. A damaged plug wire will.

Swapping coil and wire is a valid strategy.