1999 Gmc sierra 1500 4.3 misfire

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Aug 4, 2021
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Cannot figure out misfire from a 1999 gmc sierra 1500 4.3. Truck starts and idles great cold start but aftet 5 minute warm up and going on a cruise it studders. I first got cylinder 5 code and then cylinder 3 then multiple cylinder. The maf, iac, tps, fuel pump, fuel filter, pressure regulator, spark plugs, wires, cap and rotor and chip have all been replaced within a year. With good quality ac Delco parts for the most part. Also did a leak down test on fuel system a month ago (how i found regulator leaking) and after passed smog and has never ran better. I did find a hole on a vacuum line from the smog pump to the plenum and replaced it and would appear that solved it and drove quite a ways before it started misfiring again.i cannot find anymore air leaks any ideas? Also could i have messed up spark plugs from test driving on misfire?
 
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After warming up, does it misfire mostly at idle, or when driving, or both?

If you look at live data like fuel trims, you'll probably find it is running real lean. A common reason for that is an air leak in the intake system anywhere after the MAF.
 

Jesseyourcalipers

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After warming up, does it misfire mostly at idle, or when driving, or both?

If you look at live data like fuel trims, you'll probably find it is running real lean. A common reason for that is an air leak in the intake system anywhere after the MAF.
Thats the thing thats driving me crazy.... it has always had a lean but always drove really good and one of the fuel trims is off and it would barely scrape by smog inspection here in CA and the hose i found that had a hole fixed the fuel trim alot. its spuddering both at idle and accelerating
 
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change out your entire distributer on the 4.3L Vortec. This way my problem on my 2001 Silverado!! Check out this video!

 

Jesseyourcalipers

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change out your entire distributer on the 4.3L Vortec. This way my problem on my 2001 Silverado!! Check out this video!


If it were the distributor teeth wouldn't it misfire all the time? And i dont always get the multiple cylinders misfiring. Sometimes it is sometimes its cylinder 5 or just 3
 
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If it were the distributor teeth wouldn't it misfire all the time? And i dont always get the multiple cylinders misfiring. Sometimes it is sometimes its cylinder 5 or just 3
No - it was random multiple misfires depending on the engine rpm and where the rotor would line up for firing. In the video, the guy also states random misfires. Also when this happens the cam sensor kind of freaks out too!
 

Jesseyourcalipers

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No - it was random multiple misfires depending on the engine rpm and where the rotor would line up for firing. In the video, the guy also states random misfires. Also when this happens the cam sensor kind of freaks out too!
But how does the guy know it was the distributor and not just the rotor that comes with the distributor assembly? Im also lean on fuel trim. The vac line i replaced fixed it for a good while but the trim still off. And also the engine is swapped and is newer year not alot of miles like these other cars theyre talking about
 
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