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Service engine light blinks while going uphill

Need help
I have a 2000 GMC Yukon Denali with my Service engine soon light blinking at me when I go up hills. It also shakes and has a smell when I drive up hills. I had it scanned it said Misfire cylinder #4. So I had the Spark plugs and wires replaced, fuel injectors replaced, distributor cap and button replaced. And yet I am still having the same problem happening. Me and my husband and our friend who is mechanic are stuck and can’t think of what is wrong with it. Please help

WAG, I would be looking at fuel pressure checks, or fuel pressure regulator if applicable and oil level.

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Oil pressure is fine no leak and it’s not over heating

Compression test on all 8 cylinders, fuel pressure test at idle and driving up the hill. Hook up a vacuum gauge as well.

Does the engine use oil? How many miles to a quart? What code, exactly, did you see? P0304?

You replaced a bunch of parts hoping the problem would be fixed but you never checked the basic mechanical condition of the engine and fuel supply.

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Agreed; a compression check is in order. Since the spark plugs were out that would and should have been the time to check compression especially since the truck is 20 years old with ? miles on it?

You could remove the No. 4 plug only and check the compression on that cylinder only. It matters not. If one cylinder is down odds are the others are not far behind and major engine work is needed.
Your mechanic friend should have considered this option.

A friend of mine has a 2003 Chevrolet truck with the 4.3L V6. It has the exact same problem. Whenever you drive the truck uphill and/or have a load, the Check Engine light comes on, and it feels like you’re pulling a load of rocks or something. Friend has spent all kinds of money chasing all sorts of repairs, and it never seems to stay fixed.

You have my sympathy.