Silverado running bad after seafoam

I have a 2001 Silverado with a 4.8. It sat for a few years but only has 126000 miles it was smoking but ran decent once I got it running so I seafoamed it through the brake booster vacuum line thinking that might clear it out and it would stop smoking, I took it on a test drive after the seafoam, and it barely made it out of my driveway it would die when you let of the gas, and just didn’t want to do anything in general, fast forward a few months it was still smoking and running bad never cleared up, I found out why it was smoking the other day. Bad valve stem seals. I put a quart of marvel
Mystery oil in it and drove it 4 miles, died after about 2, and I had to keep giving it gas everytime I let off it was like I just killed it. Havnt driven it since then but when it warms up it won’t run without your foot on the gas, it is still smoking because from what I’ve heard the marvel expands the seals and it can’t do that unless you drive it. If I give it gas while sitting the smoke clears up a lot. What can I do about it running bad though so I am able to drive it?

The gas may be bad.

Did you remove the gas cap and smell the gas before running it?


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I worded this wrong, the truck was a family member of mine and he started it twice a week but it hasn’t been driven since 2020, the gas was new gas it only sat without being started for a month when the pcm fuse blew

How did you interduce the Seafoam into the large vacuum hose??? meaning how did you regulate the amount going into the vacuum hose… Depending on the amount introduced, you could have fouled spark plugs or worse damaged the engine (valves)…

I would pull the spark plugs and see what they look like… Run a compression test while the plugs are out…

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You must have a check engine light on? If so, you really need to read the codes. (My guess is at the very least some misfiring codes), but quite possible also signs of running rich.

You should also at least have a look at the spark plugs. Even if they were ok before, they’re likely getting all gunked up.

Just went slow with it and did 1/4 can, when I took it out it started sounding like it had blown up so I turned around, got back in my driveway, it died in my driveway so I killed it and started it the next day it never had the clattering sound again, plugs were replaced a couple weeks ago, they are probably fouled out now, but it still didn’t run right when they were new never stopped missing

Has codes for misfire, and 02 sensors. There are no 02 sensors anymore I unplugged them since they were bad. I did have black liquid that would just run out my tailpipe, but since I unplugged them the black stuff is going away

The smoke in the mirror picture, was it before the Seafoam or after using it??

After I will post a picture of before the seafoam in a minute, I seafoamed it back in November but it never really started smoking more, the smoke dies down when it runs for a while, or when you drive it but I just can’t keep it running

This was the 2nd time it had been started and the first time I saw smoke

My guess, the smoke out the tailpipe is just gunk deposited on the inside of the tail-pipe during the long period of only starting and idling the vehicle in the driveway. I’m thinking that will go away on its own once you get the truck back on the road and the engine running properly. The engine however will never run properly until you configure it back to the way it came from the factory, with all the o2 sensors.

Besides that, you’ll just have to do the normal maintenance stuff every car owner must do.
I’d start with

  • spark plugs, engine air filter, and fuel filter replaced on schedule
  • engine oil and filter replaced on schedule
  • visible inspection shows no gunk is clogging up the mechanisms under the valve covers
  • verify the egr function is working correctly
  • verify the complete path from the engine air intake to the tailpipe is leak free

As far as I know, the 4.8L is a V8. That gives us 9 possible misfire codes - P0300 thru P0308. P0301 is cyl 1. P0302 is cyl 2, and so on. P0300 is random/multiple. Having the P0300 is very different from having only codes that specify specific cylinders. So just saying “misfire code” isn’t enough. Which specific cylinders? Or all/random?

What does the smoke smell like? Sweet? Or more acrid?

Pull your vacuum line off of the fuel pressure regulator and see if there’s any evidence of gasoline in the line.

P0300 is the code, I found out why it was smoking, like I said it was a valve stem seal, and marvel mystery oil is fixing it, but I am not able to run the truck long enough to get it to stop smoking, smoke looks white when it first starts, then the more it dies down it turns blue until it is completely gone. I will post a picture of it blue if I have one

Get the side of your car out of that mirror! lol

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Tell me about it I left my windows down one day and that was the worst choice I have ever made

Sorry if there’s any confusion with who I am replying to. I can’t figure out how to quote on here

How, exactly, did you find out that it was bad valve stem seals? At that age, and after sitting, I wouldn’t doubt it. But for it to be running as badly as it is and smoking constantly, I’m just doubting it. All 8 cylinders with oil just constantly pouring in that badly? I think not.

You said that “giving it gas” helps keep it running sometimes and clears up the smoke.

But “giving it gas” is just an old saying. When you hit the throttle pedal, you’re actually giving it air. (I.e. what has always been called the “gas pedal” is really an “air pedal” - it opens the throttle plate letting more air in. In the “old days” of carburetors, letting more air in was also sucking more gas thru the carb which I’m guessing (?) is why it got to be called the “gas pedal.” Old timers can correct me or clarify on that).

Let’s say you had a leaky fuel pressure regulator. The engine vacuum will be sucking excess raw fuel into the intake thru the regulator. You’ll have WAY TOO MUCH fuel in this case. The thing will smoke and run like sh…izzle if at all. Good chance it would also throw a P0300. This is what you described. Giving it more air (i.e. hitting the throttle) will help it run / smooth out / smoke less. Which is also what you described.

Start it up and let it choke / smoke for a while and possibly stall. Then check the vacuum line to the FPR for signs of fuel. I’m not saying I’m right, but it surely does fit. I can’t diagnose this by internet. But checking it will cost you nothing but about 2 minutes of time. And if that it the issue, it’s an easy and cheap thing that will save you a lot of further tail chasing.

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I talked to this guy I know, used to be a mechanic about 20 years ago, he told me his dodge did the exact same thing, and told me to run marvel in it. I ran the marvel then drove it about 2 miles then it died, got it started back up, just couldn’t take my foot off the pedal. Put a total of 4.3 miles on it, and there was no smoke after about a mile of driving in high rpm. Then the other day I had it running outside idled for a few minutes then I let it sit at about 3000 rpm a few times, it would blow a huge cloud of smoke out then fade to blue, then after repeating that there was no smoke. If I knew how to put a video on here I could show how it dies and it running without smoke