Ok fellas, help a girl out PLEASE! I have a 99 Blazer, 4.3, 2×2, 4 door (Yes that matters 🤷*♀️) I deliver mail for USPS in this truck. She is VERY solid and all in all runs like a dream. My route is 80 miles and just over half is dirt roads. I am rough on the ole girl but take good care of her as well. I'm constantly tweaking and tightening things from the beating I put her through lol So my question and problem is.... Recently I did a tune up (plugs, wires, cap/button) NOW she's misfiring. #6 is the culprit so I replaced that plug hoping it was fouled but still hesitating and missing. I'm SO over hearing "Well it could be, or it could be.... No!! I'm not just stabbing in the dark and being a parts changer. I'd love some solid advice!! I also replaced the water temp censor (located just behind drivers front tire) and yesterday she was skipping and my temp gauge fell flat then right back to normal. Did this twice within a matter of a couple mins. I know these trucks are notorious for wiring issues. I'm just completely lost on why she would start acting up / flipping out after only regular maintenance. occasionally if my check engine light does come on, it's always just a O2 sensor which every vehicle I have ever owned says it needs a O2 sensor replaced, I have never replaced them lol I would very much appreciate any advice or ideas.




Thx, Melissa

(Almost postal)

 

I'm not a professional mechanic, but I can tell you what I know and it's free.

A couple of things ...

You're right that it could also be an electrical gremlin. Inspect as much of the wiring harness as you can. Look for spots where moisture might have gotten in, or the jacket is rubbed through, wires hanging out etc. Same for the O2 sensors and temp sensor, ECM etc. Check the ground connections. Check the ECM and fuse blocks for moisture ingress. GM Upfitters is an excellent resource for wiring schematics and part location drawings:
https://www.gmupfitter.com/pages/body-builder-manuals



How much on the odo? The 4.3's are notorious for blowing gaskets. Cyclinder #6 is at the rear I believe, and that's where the problem is the worst because that part of the block gets the hottest. Inspect around the intake manifold, and the cyclinder head for signs of coolant leakage. Here's what the gasket on my 4.3 looked like when it failed at 150k. The surfaces were very pitted.







A couple of other really easy things to check are the fuel pressure. Should be 50 or 60 PSI I believe. And compression test. You just unscrew a plug and put a gauge in it.