Vortec 7400 slight miss at idle

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4,601
Location
Kuwait
The engine had an obvious miss at idle earlier, which would come and go at higher engine speeds, and yesterday I changed out the spark plugs (ACDelco iridiums) and wires (ACDelco Premium silicones). Whilst I was at it, I did some other maintenance as well - Genuine GM belt, ACDelco fuel filter, ACDelco Supreme DOT 3 brake fluid, transmission drain and refill with a Genuine GM filter and ACDelco Dexron VI. Also dumped in a bottle of Lubrizol combustion chamber/fuel injector cleaner. Afterwards, the brakes were MUCH better, transmission shifted MUCH smoother and the miss at idle was was less noticeable. There is some engine vibration at idle (mounts are good), which disappears at higher engine speeds and it drove like a dream. Fast forward to today, and the engine vibration is very noticeable at all RPMs. I was thinking about changing out the distributor cap and rotor, and thought of doing a compression test, in case it's a valve (although I somehow doubt that). Any ideas on what else could be the culprit? confused
 
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8,643
Location
Texas
Do the cap and rotor for sure. You already did the plugs and wires so do this too. They are probably the same age as the wires were. Also check for vacuum leaks..especially from the small line to the accumulator ball.
 
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2,488
Location
Pennsylvania
97s are quite prone to this from the teeth at the bottom of the distributor shaft wearing into razor thin pieces of metal. Symptoms are as you describe. Many sites on Chevy trucks discuss this in detail.
 

Falcon_LS

Thread starter
Messages
4,601
Location
Kuwait
Thanks for all the input. thumbsup Checked the wires for arching - negative. Checked for vacuum leaks - negative. TB (was already clean)/MAF cleaned and PCM reset - slight improvement. Checked distributor - noticed quite a bit of play. eek Would have never thought of that! Thanks guys, BITOG saves the day once again. laugh So just to confirm, I'm looking to replace the distributor shaft gear kit in this case:
 
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5,651
Location
Iowa
Did you actually check the gear, because some play is normal. Also this is DEFINITELY a part where you need to use O.E.M. parts, but judging by your original post, going/getting O.E.M. isn't an issue.
 

wtd

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2,761
Location
southwest Mo.
I have a 98 chevy truck with the 7.4L and mine has had everything replaced that you replaced along with the distributor. I would also suggesting replacing the dist. cap & rotor as the way the cap routes the wires causes internal burning. If you have a lot of play in the distributor, I would replace it instead of just the gear. I would also suggest getting one of the billet aluminum distributors seen on ebay as they are better than the stock plastic bodied one. I had to have mine replaced because the cap hold down ears on mine were cracked down the side and would not hold the cap on anymore. These engines are also known for the fuel injectors going bad which also can cause a misfire so that may be your issue if you replace the cap and rotor and you still have misfires. My truck still to this day idles rough even with all new ignition components but it runs smooth down the road. Good luck. Wayne
 
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