Hi all, hoping this sounds like a fairly easy fix as the truck still runs okay. For info, it is a 1994 K1500 5.7 V8. The truck has 205,000 miles and has issues every now and then but still runs okay. I've had roughly similar issues a couple years ago, but the knocking seems to be new.

Obviously, when I start the truck on cold days, the RPM of the idle starts at about 1,000, and drops to about 750 after it warms up. Here's the mystery: sometimes upon initial start up, the engine runs smoothly, sometimes it does not. But most of the time it starts out running okay, and then when the RPM drops there is a pretty prominent ticking/knocking sound and the truck runs rough. Interestingly enough, I can still drive it, as it I've noticed the second I get it to second gear, it runs smoothly and stops making sounds from that point on throughout the drive, outside of a slightly sluggish takeoff. Then after sitting for a few hours, it will go through the same process. Runs rough at idle and makes sounds, then drives up the block and runs okay for the rest of the drive including at idle and whatever speed. It is only rough upon the first start if it has sat undriven for 3-6 hours. For example, if I left home with it rough, drove up the hill and its fine after that, I could then drive 10 miles to the grocery store, spend an hour in there, and it still and runs fine like there's no issue... But the next morning, I will start it and it will be very rough in the driveway until I take it out again.

I have replaced all spark plugs to try and help this, but no better. Ran seafoam about a day ago but it hasn't run its full course yet, had a full tank. Changed oil last week. In the past I replaced the PCV valve when I had the sluggish takeoffs and that seemed to help that for a long time. Tomorrow I will clean the throttle body and maybe check and clean the Idle air control sensor. I also remember replacing a single spark plug wire for a reason probably similar to this (though i don't recall the exact reason) and have the rest of the set in the house. Maybe I should try that, too.

I'm really hoping if I run seafoam through a couple tanks of gas, that will do it. Kinda wonder if a fuel injector is gummed up. I hope because I haven't treated it in a couple years and one of the cylinders is burning oil (I've had and known about this issue for years but not about to take apart the engine and it does run pretty good for what it is), so I know it has to be pretty rough as far as deposits go. Shoulda kept up on treatment cause it liked it when I did it often, then I kinda just let it go cause it was fine for a long time.

Any other suggestions of easy fixes that a layman could try? Or things you think I've mentioned that you think probably aren't gonna help it? Thanks for your thoughts.

For info, it is a 1994 K1500 5.7 V8

 

Marginal fuel pressure can result in stuff like that. https://www.chevytalk.org/fusionbb/s...php?tid/253427
link on how to check
These engines have no feedback on fuel pressure and ECM just assumes it’s ok and when marginal a lot of strange, intermittent stuff can happen. When starting to hunt down problems like you describe you want to be sure of basics first, timing and fuel pressure, and then go from there.
good luck!

 

Check what you posted fuel pressure etc, I doubt that's it. If you have a cylinder fouling a plug, probably a valve seal, and once warm, runs better that's probably your issue.

 

Thanks for the reply. I don't have the means to check fuel pressure at the moment. I did suspect the previous issues I had a couple years ago could stem from the fuel pump, but I had no interest in trying that. I did change the fuel filter though, and it ran great for about 2-3 years, despite the fouled cylinder, like you almost wouldn't even notice it. Its strange. It drives fine, almost good to great. It just idles really rough as of now. But I can drive it around the block and it'll be good as new. Speaking of which, I should probably start it up now and take it for a quick spin.

 

Okay, so I cleaned and inspected the throttle body, and the injectors are spitting out fuel perfectly. So I am to believe it is not the fuel system, and the sea foam will not help the issue at hand (but it can't hurt the overall health of the truck).

Like I said earlier, I replaced all the spark plugs to try and fix the issue, but it did not work

My next try at hand is to replace the spark plug wires

 

So strange, but I'm not complaining. Since I ran the seafoam, I've about run through the full tank of the gas, and it has gradually gotten better. Barely even ticking anymore and not running so rough. Not perfect yet, but I like how it is improving each day, leading me to believe that as the seafoam runs through, whatever the issue is has gotten better. Only other thing I did was physically clean the throttle body and it really wasn't looking bad to begin with. Probably going to run seafoam again for the issue, and then continue using a can of seafoam each time I fill up. Stuff seems to work great! Only other thing I plan on doing is replacing the PCV valve to try and help with the takeoffs. It seemed to help that issue last time and it's been a while.

Now that the issue seems to be getting better from the seafoam, any thoughts as to what it was? It didn't really get noticabley better after I cleaned the injectors/throttle body, but the engine has been running significantly better each time I've driven it since and the issue seems to almost be resolved (knocks on wood).

Other thing I forgot to mention was that these issues started a few weeks ago with the truck wouldn't start on its own. After getting a jump to start it, the clock on the console reset to 1:00 every time. I would let it run at rough idle for several minutes, the clock would run normally. Take it for a drive and would run and restart fine. Let it sit over night, wouldn't start, and the clock reset to 1:00. Got a new battery and fixed the starting issue, but then would still run rough, but the clock no longer resets. But now it starts great. Before the new battery on cold days of approximately a low of 20's it would sound like a diesel trying start in 0 degrees. Now after the battery it no longer is reluctant to start, now just runs rough sometimes, and it seems as the seafoam is helping that gradually improve.

Such a strange set of symptoms he's had the last few weeks, but we seem to be in good working order now.