'98 Silverado idles rough - The 1947 - Present Chevrolet & GMC Truck Message Board Network

emeribd · 06-28-2004, 12:29 PM

i have a '98 chevy silverado w/ a 305 motor. for the last six months or so if i let it idle, it runs rough & the service light comes on. a local parts store has a code reader & checked it for me. the code was "P0300" - random / multiple cylinder misfire. the truck has 77,000 miles so he recommended i give it a tune up. i have since changed plugs, plug wires, distributor cap, & rotor. none of this seemed to help. the strange thing is that over 1000 rpm it seems to run fine. any ideas?

Z16 · 06-28-2004, 10:37 PM

Are you using a fuel injector cleaner regularly? I find I know when my 98 needs cleaner, it tends to idle crappy. I prefer Lucas Upper Cylinder Lubricant and Fuel Injector Cleaner. It works quick and well, and is priced right. The other one that works well is the Redline stuff, but it's more money, and takes a little longer to work. I've tried them all, cause i work at a parts store, and I always go back to the Lucas. I would start with a couple bottles thru a couple tankfuls, and see what that does.

emeribd · 06-30-2004, 01:34 PM

about every 4th fill-up i add either STP gas treatment or GUNK fuel injector cleaner i get from the gas station. maybe i need to do a heavier clean up. i'll try the LUCAS additive & see if that helps. i just figured it was something more than dirty injectors since the service engine light keeps coming on. thanks for offering advice!

fuzzypuppy · 07-16-2004, 04:39 AM

I had a pretty much the same problem, rough at idle and at times would surge at idle, but would run great on the highway, EGR was my problem, for 1 it had come a little loose plus it needed cleaning, just my 2 cents worth.

STEVE A WILBURN · 07-16-2004, 07:02 AM

I Use The Lucas Also On My 1988 And Clean The Injectors Right Up. I Also Use The Lucas Transmission Fix 168,000 On My Tran And Shift Like New.

sbc350gearhead · 07-21-2004, 10:18 PM

Check the EGR and the IAC.

99-2door · 07-23-2004, 10:43 AM

Check for vacuum leaks also

Cubdriver · 07-24-2004, 02:59 PM

it's probably the oxygen sensor

33Chev · 07-25-2004, 10:26 PM

Someone mentioned the IAC valve...(idle air control), which makes sense...just may need the tip cleaned and seated back into the valve.

How about checking the (throttle plate)...just spray some carb cleaner on the throttle plate where the air enters the throat...it may not be seating properly.

Rickracer · 08-01-2004, 12:07 PM

Replace the coil. IAC will not cause misfire, only low or high idle speeds. EGR leak(s) can cause misfire. Cleaning the throttle body may help. For those of you using fuel additives, it might be cheaper to just use good name brand fuels.

06-28-2004, 12:29 PM	#1
emeribd Registered User Join Date: Jun 2004 Location: Central Illinois Posts: 7	'98 Silverado idles rough i have a '98 chevy silverado w/ a 305 motor. for the last six months or so if i let it idle, it runs rough & the service light comes on. a local parts store has a code reader & checked it for me. the code was "P0300" - random / multiple cylinder misfire. the truck has 77,000 miles so he recommended i give it a tune up. i have since changed plugs, plug wires, distributor cap, & rotor. none of this seemed to help. the strange thing is that over 1000 rpm it seems to run fine. any ideas?

06-28-2004, 10:37 PM	#2
Z16 Registered User Join Date: Mar 2001 Location: Medicine Hat, Alberta, Canada Posts: 1,508	Are you using a fuel injector cleaner regularly? I find I know when my 98 needs cleaner, it tends to idle crappy. I prefer Lucas Upper Cylinder Lubricant and Fuel Injector Cleaner. It works quick and well, and is priced right. The other one that works well is the Redline stuff, but it's more money, and takes a little longer to work. I've tried them all, cause i work at a parts store, and I always go back to the Lucas. I would start with a couple bottles thru a couple tankfuls, and see what that does. __________________ 98 GMC Sierra Ex Cab, cowl hood, Euro LED lights, 30 bar billet, Whipple Supercharged 350 Vortec: Forged flattops, E-Tec aluminum heads, Comp Cam, Scat crank and rods, Granatelli Mass air flow, Headers, Flowmaster exhaust , Complete MSD Ignition, Custom Tuning, intake spacer , ported throttle body, AC Delco port injection upgrade, Roller Rockers, Power Pulleys. Gear Vendors Overdrive. Soon to hit a dyno. 2/4 drop(for now). 95 Chevy S-10 Ex Cab 4.3 5 Speed 2/3 drop, Euro lights, billet grill, More to come. The Daily Driver

06-30-2004, 01:34 PM	#3
emeribd Registered User Join Date: Jun 2004 Location: Central Illinois Posts: 7	about every 4th fill-up i add either STP gas treatment or GUNK fuel injector cleaner i get from the gas station. maybe i need to do a heavier clean up. i'll try the LUCAS additive & see if that helps. i just figured it was something more than dirty injectors since the service engine light keeps coming on. thanks for offering advice! __________________ Brian '84 1/2 ton Silverado 2WD 305 180,000+ miles '03 1/2 ton Silverado Z71 4WD 5.3L 65,000+ miles

07-16-2004, 07:02 AM	#5
STEVE A WILBURN Registered User Join Date: Feb 2004 Location: TONEY ALA Posts: 307	I Use The Lucas Also On My 1988 And Clean The Injectors Right Up. I Also Use The Lucas Transmission Fix 168,000 On My Tran And Shift Like New. __________________ JUST HAPPY TO BE ALIVE.

07-23-2004, 10:43 AM	#7
99-2door Registered User Join Date: Oct 2003 Location: Broken Bow, OK Posts: 1,385	Check for vacuum leaks also __________________ 95 K3500 SRW 5.7L Vortec 4L80e 4.10 G80 2001 GMC C3 6.0 AWD, Volant, 3/5 drop TBSS intake, HP tuners, pullies, 05 efans, 05 front brakes, etc. 2003 Tahoe Z71 Silverado front clip

07-16-2004, 04:39 AM	#4
fuzzypuppy Registered User Join Date: Jul 2004 Location: So, cal. Posts: 19	I had a pretty much the same problem, rough at idle and at times would surge at idle, but would run great on the highway, EGR was my problem, for 1 it had come a little loose plus it needed cleaning, just my 2 cents worth.

07-21-2004, 10:18 PM	#6
sbc350gearhead Registered User Join Date: Mar 2003 Location: Columbus Ohio Posts: 1,413	Check the EGR and the IAC.

07-24-2004, 02:59 PM	#8
Cubdriver Registered User Join Date: Jul 2004 Location: Alaska Posts: 2	it's probably the oxygen sensor

07-25-2004, 10:26 PM	#9
33Chev Registered User Join Date: Jul 2004 Location: Buffalo, NY Posts: 38	Someone mentioned the IAC valve...(idle air control), which makes sense...just may need the tip cleaned and seated back into the valve. How about checking the (throttle plate)...just spray some carb cleaner on the throttle plate where the air enters the throat...it may not be seating properly.

08-01-2004, 12:07 PM	#10
Rickracer Registered User Join Date: Aug 2004 Location: Kissimmee Fl. Posts: 7	Replace the coil. IAC will not cause misfire, only low or high idle speeds. EGR leak(s) can cause misfire. Cleaning the throttle body may help. For those of you using fuel additives, it might be cheaper to just use good name brand fuels. __________________ Rickracer I am an ASE certified Master tech with 28 years experience



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