Bought a 1998 K1500 Silverado Z71 with about 167K on her and the first time I had it on the interstate it seemed fine, but the first long hill the SES light started flashing and the whole truck started shaking. P0305 Cylinder 5 misfire. Just thought it was a bad spark plug or something, so not knowing the vehicles history I changed out all the ignition stuff, new rotor button, distributor, wires, and plugs. Re-routed wires for separation. Still starts shuddering at throws P0305 at speeds over 55mph while pulling hills, worse shaking if towing. Mechanic told me it might be a sticking exhaust valve. We've checked valve lift and that was good, valves are very clean for this year, everything good. Checked compression in each cylinder, all 8 reading the same at around 180psi on a brand new gauge. Decided this couldn't be a damaged valve or ring. Pulled intake and found an intake leak, installed new gaskets and sealant to eliminated that as a potential issue. Still threw the P0305 code immediately on test drive. Checked grounds, cleaned, all good. No change. Checked fuel pressure and did a bleed down test, all good there. I had just put a new sending unit, fuel pump and filter on. Cleaned Mass air flow, new air filter, no change. Took a test drive while my mechanic friend watched the scanner and it was retarding the timing advance under acceleration instead of advancing it. Prompted us to replace the knock sensor. That was the first positive change in drivability. The misfire and shaking is less pronounced, it's not knocking drinks out of cupholders anymore, but still throws p0305 on hills. Decided it had to be a bad spider injector. Replaced that last night and it's the newer MFI style, much nicer, and the old one looked pretty fried on cylinder 5 when we removed it. Was confident we found the issue. Nope! Took it down the interstate and while the timing advance and all the numbers on the scanner appeared to be good, the first steep hill and she started to lose power and was popping out of overdrive a lot and then the dreaded P0305 again. I'm just at a loss here. Anyone know what's going on I'm so tired of throwing money and time into this. It's an incredible looking truck for the year I'd love to be able to enjoy it and be able to tow with it occasionally. Right now all it's good for is sub-45mph around town stuff.
1998 5.7L Vortec Persistent P0305 misfire
justins1998
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Son of a gun. Thank you for updating the post.
I thought that was a 99+ thing.
Noted, and seriously thank you for updating the post.
I thought that was a 99+ thing.
Noted, and seriously thank you for updating the post.
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You ever figure it out man? Did the Sea Foam help?
I have identical issues as you except its cylinder #3/#4 and a P0300 for random missfire.
New parts include: Metal distributor, cap/rotor, platinum plugs, NKG wires, Coil pack, Ignition control module, MAF, Crank Sensor and recalibrate, Fuel pump a few years ago "ITS LOUD"
Fuel filter recently, Reman Spyder injector a few years ago as well as intake manifold gasket, compression tests good, never checked valves yet. Cats gutted, new 02 sensors, tested fuel pressure.
Vac tests and lots of scans, always cylinder 3 and 4 missfires, mostly #3. I DRIVE THE SHIT OUT OF IT and it doesnt help hahaha.
Loses power on the same hills, always starts out good then starts losing speed, downshift, downshift, downshift, starts shaking, flashing CEL, have to slow down to 80kmh for it to stop shaking and then its good until I get on it again and try to maintain speeds above 100kmh on big hills and summits.
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I did have to have the heads rebuilt. sticking valve. lots of carbon buildup on the valve stems, under load they get hot and swell and stick with that extra carbon on them. Head rebuild fixed it. Recommend you don't buy new head or Ebay heads. take your originals to have them rebuilt. Don't throw parts at it, let a good mechanic do a road test with a vacuum gauge hooked up. I sunk close to $6K into chasing my problem. Don't do what I did. change out your spider injectors out to the newer style MFI system while you are in there.
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Stop throwing parts at them guys and just have the heads rebuilt. It DID fix my truck finally and I should have started there and not wasted the extra dough. Although not an easy job to shade tree, and expensive at the shop, I think this is the root cause of many "mysterious" misfires you can't fix with electronic parts. Dirty, sticky valves will not be helped much if at all by seafoam and treatments etc... and driving the piss out of it only made it worse cause it got hot.
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unfortunately out of all of the 4 different mechanics, none of them suggested it could be the heads... Well looks like I know what Ill be doing.
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I didn't rebuild my heads, mechanics said there wasn't a reason to except maybe to clean all the carbon and gunk off them and remove carbon from the combustion chamber. Try a water de-carbon, look it up before trying and use the Seam foam oil/fuel treatment in the crank case, pour the rest in the gas tank and drive it for 300-500km, then do an oil change. Makes a hell of difference
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I am having the exact same problem with my 1998 Silverado with 214k miles. I have been fighting this for two years. New plugs, wires, cap, rotor, MAS, MAP, spider injector, all AC Delco parts, good compression and still starts running terrible at 70 in OD. Shakes to high heaven and if you stay on it will eventually die and stall. Will start right back up. I have tried sea foam and seemed to help a little. Will set a 306 miss fire or 300 miss fire, has set a MAP and MAS codes in the past. Runs great until you get to around 65 and it goes into OD and the faster you go the worse it gets. I think I will load it up with a big charge of seafoam and give that another shot, then consider head work.
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you find a fix? Just had valve springs done with no luck. Still have to ask the mechanic if he actually pulled the head and removed all the carbon build up or not
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144 Posts
Misfire Cylinder 5 unsolved
I have a 2012 GMC Sierra with 5.3L V8. I started to get a misfire while idling. I plugged in a code reader and got a p0305 code. Misfire on Cylinder 5. I changed plug wires, plugs, coil pack #5, injector #5. When I pulled the connector for each injector I got a difference except for #5. No...
www.silveradosierra.com
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Just letting anyone that's interested know, I had my lifters replaced and that helped the truck gain a ton of lost power but still had the misfires/shaking at highway speeds but just faintly.
I asked the mechanic what he thought it was and he had no idea. Asked him what it tested for fuel pressure and he said 53psi..... From reading on these forums its pretty well known that 60psi is the bare minimum for these trucks or they run like crap. Replaced the pump and problem is 100% fixed. Ran a full can of intake sea-foam to help clean out cylinder #3 as it apparently has had a collapsed lifter for over 4 years and other weak lifters that 6 plus other mechanics couldn't troubleshoot... Anywayssss, ran a whole can down the intake and took it for a good drive, smoked like a son bitch, ran noticeably better. Just to be safe put two cans of sea foam with a fresh full tank of gas, put 150ml in the crank case to help clean up all the carbon build up from weak lifters, a week later and the truck continues to perform even better. No more losing O/D from engine limping itself along and no more shakes on the highway, even with 2500lbs on my trailer.
Thanks for everyone who helped, cant always trust a mechanic to figure it out but it helps to have a good one on your side, good luck to the rest of you p0300 chasers
I asked the mechanic what he thought it was and he had no idea. Asked him what it tested for fuel pressure and he said 53psi..... From reading on these forums its pretty well known that 60psi is the bare minimum for these trucks or they run like crap. Replaced the pump and problem is 100% fixed. Ran a full can of intake sea-foam to help clean out cylinder #3 as it apparently has had a collapsed lifter for over 4 years and other weak lifters that 6 plus other mechanics couldn't troubleshoot... Anywayssss, ran a whole can down the intake and took it for a good drive, smoked like a son bitch, ran noticeably better. Just to be safe put two cans of sea foam with a fresh full tank of gas, put 150ml in the crank case to help clean up all the carbon build up from weak lifters, a week later and the truck continues to perform even better. No more losing O/D from engine limping itself along and no more shakes on the highway, even with 2500lbs on my trailer.
Thanks for everyone who helped, cant always trust a mechanic to figure it out but it helps to have a good one on your side, good luck to the rest of you p0300 chasers
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2006 6.0 p0300 misfire but runs smooth
I've got a CEL I'm having a hard time tracking down. ~30,000 miles, but nearly 1600 hours on the engine (Average 19mph) - this was a plow/salt truck at a ship yard. It was always beat hard and put away wet... and they never would have fixed any problems that didn't prevent it from moving. If...
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Just so you know, the problems fixed. So you can stop posting random vids2006 6.0 p0300 misfire but runs smooth
I've got a CEL I'm having a hard time tracking down. ~30,000 miles, but nearly 1600 hours on the engine (Average 19mph) - this was a plow/salt truck at a ship yard. It was always beat hard and put away wet... and they never would have fixed any problems that didn't prevent it from moving. If...
Variety, 1999 Silverado, 2003 Yukon, 2007 Escalade plus owned many others now sold
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I am working on a friends 1999 Silverado K1500 160,000 miles on it, Misfires on cylinder #3. He has gone though all the steps many of you have reported. New Spider, Wires, Plugs, Cap, Rotor and much more. Still had #3 misfire. He loves this truck, has put a ton of money into it, new Paint, new front end rebuild. If it needed it he did it. He kept going back to shops trying to fix this misfire. He dumped thousands of dollars into it before he called me for help. I am not an expert but have 40 years in the industry, I was ASE certified in Engine Repair , Engine Performance and all 5 areas of Engine Machine so GOLD certified ASE but with no prior Vortec experience except for one truck, I come to the forum to see what I should know before working on a set up I do not know.
He supplied paperwork showing this truck had recently had a compression check on #3 and it had 165 PSI so I did not double check that. I did determine that the lower intake manifold was leaking a tiny bit of coolant and had a slight vacuum leak right at the #1 and #3 hold down bolts. I pulled the intake, the gasket was broken at the front from being over torqued by one of the shops he paid dearly, but he has been chasing the #3 miss since 2019. With the lower manifold gasket repaired, upper manifold gaskets replaced and checked for straightness the engine fired up beautifully, zero misfires on any cylinder. Fuel Trims were adjusting back to normal within 5% and getting better. Ran it for a good hour. On initial Test drive it feels great, transmission shifting really smooth and then bang, #3 misfires come back and it forms a dead miss on #3.
When we got home I went back to basics, took my own compression check, Leak Down Test, found near zero compression #3, leak down was over 90% leak at 100 PSI. ????? I know this cylinder was running perfectly just 30 minutes ago? Took a vacuum check and it had 18.5 inches at idle with one dead cylinder. That seemed impossible to me. I Pulled the valve cover, with the leak down tester hooked up and pushing 100PSI into #3, Rocker arms removed I still had 90% or more leak on 3. I took a pry bar and pulled up the exhaust valve because my leak was into the exhaust and not the intake. By pulling up on the #3 exhaust valve the cylinder sealed up and my leak down was now only 5%. If I tapped on the exhaust valve with a ball pen hammer the leak down jumps back up to more than 90% leak and the valve does not snap closed after the tap. With the exhaust valve pried up a little and the leak reduced to under 5% I could tap the intake and hear the valve snap closed after each tap. With 100 PSI in the cylinder the valve springs are no longer in play, the air pressure will hold the valves firmly in place.
I did some GM bulletin checks and GM claims the valve spring issues they had did not start till 2002 and ran through 2007 I think? 1999 did not come up for valve spring failures.
Pulled both heads, really stunk having to do this 2 days after removing the intake manifold for the gasket repair. The sticking exhaust valve did have carbon build up on the stem keeping it from closing. I think the Build up has just been getting worse since 2019 when the misfires started, truck only goes 2-3,000 miles a year. Driver never pushes it to blow out the exhaust he pampers it everywhere he drives.
A buddy owns the machine shop I use, we have been friends 40 years and I trust his opinion completely. He says he can't count all the vortec engines he has fixed for random misfires caused by the sticky exhaust valves. It just slowly builds up till it becomes a factor. My guess as to the cause would be a little oil getting past the valve stem seal and burning to the surface of the hot valve caused the build up.
When the valve sticks open you have a dead miss, when it starts moving again you might have a perfectly good running engine making this issue very hard to find.
In my buddy's case initially it only happened while pulling his trailer over a long hill, then it slowly progressed till it was happening as soon as the engine hit full temperature. We have the Heads in being rebuilt right now, won't have them back till late next week but I am 100% sure this is the problem.
Maybe the Sea Foam treatment could have cleaned up the valve stems but I really doubt it, the build up on the valve stems felt rock hard to me.
I did advise my buddy to Drive it Like You Stole it once in a while to blow out the carbon. He laughed and said he will try. I think all the low RPM putting around did not help him keep the valves clean.
Hope this helps another owner get fixed up.
June 1, 2025 UPDATE:
Follow up from my Feb 24 post. The 99 Vortec Silverado engine is still running perfectly after the valve job. It was a carbon build up issue on the valves causing it to hang the valve open. That one is fixed!
He supplied paperwork showing this truck had recently had a compression check on #3 and it had 165 PSI so I did not double check that. I did determine that the lower intake manifold was leaking a tiny bit of coolant and had a slight vacuum leak right at the #1 and #3 hold down bolts. I pulled the intake, the gasket was broken at the front from being over torqued by one of the shops he paid dearly, but he has been chasing the #3 miss since 2019. With the lower manifold gasket repaired, upper manifold gaskets replaced and checked for straightness the engine fired up beautifully, zero misfires on any cylinder. Fuel Trims were adjusting back to normal within 5% and getting better. Ran it for a good hour. On initial Test drive it feels great, transmission shifting really smooth and then bang, #3 misfires come back and it forms a dead miss on #3.
When we got home I went back to basics, took my own compression check, Leak Down Test, found near zero compression #3, leak down was over 90% leak at 100 PSI. ????? I know this cylinder was running perfectly just 30 minutes ago? Took a vacuum check and it had 18.5 inches at idle with one dead cylinder. That seemed impossible to me. I Pulled the valve cover, with the leak down tester hooked up and pushing 100PSI into #3, Rocker arms removed I still had 90% or more leak on 3. I took a pry bar and pulled up the exhaust valve because my leak was into the exhaust and not the intake. By pulling up on the #3 exhaust valve the cylinder sealed up and my leak down was now only 5%. If I tapped on the exhaust valve with a ball pen hammer the leak down jumps back up to more than 90% leak and the valve does not snap closed after the tap. With the exhaust valve pried up a little and the leak reduced to under 5% I could tap the intake and hear the valve snap closed after each tap. With 100 PSI in the cylinder the valve springs are no longer in play, the air pressure will hold the valves firmly in place.
I did some GM bulletin checks and GM claims the valve spring issues they had did not start till 2002 and ran through 2007 I think? 1999 did not come up for valve spring failures.
Pulled both heads, really stunk having to do this 2 days after removing the intake manifold for the gasket repair. The sticking exhaust valve did have carbon build up on the stem keeping it from closing. I think the Build up has just been getting worse since 2019 when the misfires started, truck only goes 2-3,000 miles a year. Driver never pushes it to blow out the exhaust he pampers it everywhere he drives.
A buddy owns the machine shop I use, we have been friends 40 years and I trust his opinion completely. He says he can't count all the vortec engines he has fixed for random misfires caused by the sticky exhaust valves. It just slowly builds up till it becomes a factor. My guess as to the cause would be a little oil getting past the valve stem seal and burning to the surface of the hot valve caused the build up.
When the valve sticks open you have a dead miss, when it starts moving again you might have a perfectly good running engine making this issue very hard to find.
In my buddy's case initially it only happened while pulling his trailer over a long hill, then it slowly progressed till it was happening as soon as the engine hit full temperature. We have the Heads in being rebuilt right now, won't have them back till late next week but I am 100% sure this is the problem.
Maybe the Sea Foam treatment could have cleaned up the valve stems but I really doubt it, the build up on the valve stems felt rock hard to me.
I did advise my buddy to Drive it Like You Stole it once in a while to blow out the carbon. He laughed and said he will try. I think all the low RPM putting around did not help him keep the valves clean.
Hope this helps another owner get fixed up.
June 1, 2025 UPDATE:
Follow up from my Feb 24 post. The 99 Vortec Silverado engine is still running perfectly after the valve job. It was a carbon build up issue on the valves causing it to hang the valve open. That one is fixed!
I have a 1998 GMC Sierra K1500 5.7L Vortec with 272,000 miles on it. I love my truck. I have gone around and around on this "misfire under load" for many years. Finally, I checked the timing chain by removing the distributor cap and slightly moving the crankshaft back and forth a little bit. When you move the crankshaft you should see IMMEDIATE movement of the rotor on the distributor. In my case, I could move the crank probably 2-3 degrees before the rotor moved. So, I went after the timing chain and gears. Not a really tough job, just took a few hours to do. When I got to it there was a LOT of slack in the timing chain. Probably 1/2 to 3/4" of play side to side. I bought the truck with 18,000 miles on it. I never replaced the timing gears and chain in all those 24 years or so. So, I replaced the gears and chain and voila! First drive...NO MORE MISS under load. Timing chain and gears! I hope this helps someone.
I noticed the same thing with my 97 Chevy 5.7 vortec c1500 I've replaced the distributor cam sensor crank sensor throttle positioning sensor air position sensor clean the mass air flow sensor all wires are good new new spark plugs new distributor cap rotor haven't touched the spider injectors yet but I came to a conclusion on the downshifting of the transmission I have experience with the TCC I just recently went out and bought one thinking the torque converter solenoid is not kicking it out causing overdrive to downshift in and out I will check that as I'm going to do mine and to give it a good transmission filter and fluid change even flush the transmission out I will let you know how it does on mine I'm pretty positive the torque converter staying kicked in it's actually dragging the motor down and losing power and wants to shut it down whenever you come to a stop, idling goes up when I stop then eventually goes down to low this has led me to believe that the torque converter is staying engaged and for a simple $10 part is worth a try and plus you'll have new transmission fluid and filter into your transmission
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