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Old 09-29-2018, 08:18 PM   #1
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Join Date: Sep 2018
Location: San Dimas
Posts: 6
98 Chevy 1500 5.7 PO305 code

I have had a PO305 code come and go for about 6 mo. I realized early on that the fault only happened if I let the engine idle 4-5 min. on a cold start. Now it's time to have a smog test and code is more frequent now. The engine does sound and feel like there is a miss @ idle, but it does not sound anything like a dead cylinder miss, just a noticeable dip in what I hear and a slight vibration in the steering wheel. Now some checks i have done:
compression 155 psi ( i did not check other cylinders)
fuel pressure 60 psi engine off / 55 psi running
#5 spark plug has less than 10k miles and appears normal compared to others
swapped #5 plug with #1 - no change in code
replaced #5 plug - no change
replaced #5 wire - no change
replaced spider box recently - not AC Delco but Autozone GP Sorenson
visual check on distributor cap / rotor - appears normal
air filter / fuel filter / pcv valve - new
check for vacuum leaks w/ carb. cleaner spray @ gaskets - none

PO 305 is the only code thrown by computer
truck / engine has 190,000 miles
Performed a valve job @ 125K.
Accelerates great after spider box R&R
Engine does use about 1 1/2 qts oil between change @ 5K
No noticeable smoke or smelly fumes.
Except for the slight rough idle and the code, engine responds well.
I use an Actron OBD II scanner that has freeze frame capabilities.
Short Term & Long Term Fuel Trims #3 & #4 are wacky.
#3 Short Term 80.4%
#3 Long Term -54.6%
#4 Short Term -47.9%
#4 Long Term 28.9%

My next move will be to remove the valve cover and check #5 valve springs.
With so many of these trucks having been on the road, hoping someone else
has endured this and figured it out.
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Old 09-30-2018, 08:19 AM   #2
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Join Date: Jan 2006
Location: Smithfield, VA
Posts: 1,359
Distributor cap

I had mine start running like crap out of the blue. Got a new Delco cap that was even worse. Went to a third cap and all has been fine ever since.

Don't assume these crap caps are good just because the contacts look good. If you notice inside the plastic, there are runs or traces where they have to put the contacts on the sides. Internally there can be not exactly shorts but it can fire between those traces.

I have one I can send you if you don't believe it. So you should replace the cap.

One other thing you should replace but I don't think it is the cause of your #5 misfire. That is the coil. They are considered a tune up item.
99 Z71 (98 body style)
Pride and joy
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Old 10-01-2018, 09:27 AM   #3
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Join Date: Sep 2018
Location: San Dimas
Posts: 6
Re: 98 Chevy 1500 5.7 PO305 code

Speedy, thanks for the input, have seen this warning on other Chevy boards also. Have a Delco on the way from RockAuto
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