Back again with 99 Tahoe troubles

[Stripermike]

Hey all, hope everyone's having a good holiday season, and thanks to all of you who helped out with my previous issues on this truck. Here's the latest -
since correcting the fuel injector issue I've been driving the truck regularly for work and around town. It's been running pretty good, but under certain conditions, like accelerating up long inclines, after the truck shifts into the upper gears I lose power, the engine starts running rough, and the SES light flashes. The truck eventually gains some power back and the light stops flashing. Code says random cylinder misfire.
I replaced the plugs, assuming they must be tired after the fuel injector situation, but it hasn't fixed the issue. Wires seem fairly new.

 

[lesterl]

Coil, cap, rotor.... Would be great to know what your fuel trims were doing when the light was going nuts..... Have you cleaned the MAF senor? Check Fuel pressure?

Actually how did you have the timing set for the dist?

 

[rtarchery]

I had the same problem. The shroud sticker said 60 on spark plug gap. Read somewhere that this was wrong and 45 would work. I tried it and no more stumbling up an incline and gained mileage.

 

[lesterl]

If 60 was too large on the vortec, the coil could be weak or cap and rotor could be at its end.....

I had a 35 thou sticker on my 95 (TBI) and opened them up to 50 with the installation of an Accell super coil and it run good......

Left the Vortec in my 98 Sierra at 60 when I put the Accell in....

 

[rtarchery]

I have done quite a bit of research on plug gap, here is one of many explanations I have found. It works for me, I have more power,better mileage,smooth idle,no stumbling on incline and I use NGK.





GM had a drive-ability problem caused by the big gaps with the Vortecs in 96. Sloppy idle, pinging, etc. The solution was to tighten the gaps down. The large gaps put a tremendous load on the entire ignition system. They issued a document some time ago to tighten the gaps down to .040", long before iridium plugs came out. I don't have the actual document, but a friend of mine who is very much in the know shared this info with me. Apparently this info was distributed through GMs "Techline", not a formal TSB. Which is why it doesn't show up on AllData. If you recall, GM tried big gaps in the seventies, it didn't work then either.

The coating, whether it's copper, platinum, iridium, doesn't change the physics of sparking that gap under the extreme pressure of the combustion chamber.

You can run whatever gap you like, I have mine at .040. I will tell you from first hand experience that when I had the Bosch+4 in my truck, with their huge gaps, I thought my motor was toast. Sloppy idle, overheating, no power, non stop pinging. I put in the AC Delcos gapped at .040 and it felt like a brand new truck. I only changed the plugs and had that difference.

 

[lesterl]

I have never had issue with 60 tho.... now on the other hand Bosch plugs are crap, the +4s are even bigger pieces of crap.

I have gone to NGKs and like them as an OEM alternative. I like AC Delco/ Delphi stuff the best tho....