Slight Misfire - '02 Sierra - 4.8l

LegndLarry33 · December 15, 2008

Hi All - I'm new here - struggling with a slight misfire on my '02 Sierra (117k mi)...

Misfire started as a P0300 that would come occasionally, and then clear itself usually within a couple days...

Then it started going on and off every 3 starts like clockwork for a couple weeks (3 starts on, 3 starts off, 3 starts on, etc)...

Then it stopped going off...

I had replaced the plugs somewhat recently with Bosch 2+s and heard that might cause it, so I just changed them out for AC Delco 41985s... and then I manually cleared the code... felt the same, and sure enough, it came back on with a P0301...

FYI, the bosch plugs looked pretty bad (running rich), but not terrible, and they all looked the same... cylinder 1 didn't look any better or worse than the rest of them...

Since it seems to be giving me a P0301 now, I tried swapping the #1 and #5 coils, but the code came back as P0301, so that's not it...

I only really notice the misfire if the RPMs are low and I give it some gas, i.e. if I'm going up a hill on the highway (5th gear)... even then, it's not like the truck is bucking or missing hard (the passenger wouldnt even notice anything is wrong), but thats when I'll see the light start blinking, and it doesn't have the power it used to...

Any help would be greatly appreciated - I'm about ready to give it to my mechanic, but I'd really like to avoid that...

pennzoil29 · December 15, 2008

Welcome to the forum. A blinking SES light means, "DO NOT DRIVE". Did you replace the wires when you changed the plugs? Just an idea.

LegndLarry33 · December 16, 2008

Welcome to the forum. A blinking SES light means, "DO NOT DRIVE". Did you replace the wires when you changed the plugs? Just an idea.

Thanks, yeah I realize a blinking light is urgent, but it wont start blinking unless I push it hard with the RPMs low... it really isn't missing bad and idles fine, but obviously I do want to get it fixed asap... I did change the plug wires (AC Delco) when I put the bosch plugs in a while ago...

I have a K&N FIPK (previous owner), so I wasn't sure if this type of a problem was indicative of a faulty MAF sensor, as it seems to be affecting all cylinders the same, but I don't get any other codes...?

Also, I'm uploading a couple pictures of one of the plugs... they all look the same....

Thanks again for the help

gnet158 · December 17, 2008

What about the fuel system? Has that been cleaned too? Run 3/4th of a can of Seafoam thru the break boost, and the rest in the tank. And check the MAF for any gunk. It sounds like it’s either the plugs or wires. You didn’t gap your plugs did you?

LegndLarry33 · December 17, 2008

What about the fuel system? Has that been cleaned too? Run 3/4th of a can of Seafoam thru the break boost, and the rest in the tank. And check the MAF for any gunk. It sounds like it's either the plugs or wires. You didn't gap your plugs did you?

Thanks - Thats what I thought too, especially when I hear about the trouble some people have had with bosch plugs, but I changed those out (didnt change the .040 gap on the AC Delcos), and the wires are basically new (AC Ds)...

I did get some bad gas a while back, but I ran some fuel system cleaner through it and it seemed to clear it up... that being said, I'll definitely get some seafoam tomorrow... little confused about the "3/4th thru the break boost" though? Is that somehow connected to the fuel system? Where exactly would I put it?

I cleaned the MAF with some contact cleaner when the light first started cycling on and off... looked pretty clean when i put it back in...

gnet158 · December 17, 2008

I don’t have pictures so I’ll do my best to explain how to add seaform to your brake boost. The reason you add it to your break boost is to clean out the carbon buildup inside your engine. Make sure to change your oil after you seafoam your truck.

The break boost is right infront of your steering wheel in the engine bay. It’s a round looking thing.

After you’ve found the break boost, start your engine

There is an L shaped hose that connects to the actual break boost.

Disconnect the hose, you’ll hear a loud sucking noise after you disconnect it.

Now tilt the can of seafoam to the side, then bring the hose to the can of seafoam and have it suck up the seafoam SLOWLY. If it takes up too much too fast you’ll stall your engine. Not a big deal but your neighbors will hate you.

You’ll hear the engine start to stall if you give it too much. Keep feeding seaform until about ¾ of the can is gone.

Then shut off your engine. You’ll see some white smoke coming from the tail pipe, this is normal.

Pour the rest in your gas tank.

Wait about 30 mintues.

Start your truck up and haul @$$ down the street as your truck will smoke like there’s no tomorrow. It will keep smoking for about 15 minutes.

LegndLarry33 · December 18, 2008

I don't have pictures so I'll do my best to explain how to add seaform to your brake boost. The reason you add it to your break boost is to clean out the carbon buildup inside your engine. Make sure to change your oil after you seafoam your truck.

The break boost is right infront of your steering wheel in the engine bay. It's a round looking thing.

After you've found the break boost, start your engine

There is an L shaped hose that connects to the actual break boost.

Disconnect the hose, you'll hear a loud sucking noise after you disconnect it.

Now tilt the can of seafoam to the side, then bring the hose to the can of seafoam and have it suck up the seafoam SLOWLY. If it takes up too much too fast you'll stall your engine. Not a big deal but your neighbors will hate you.

You'll hear the engine start to stall if you give it too much. Keep feeding seaform until about ¾ of the can is gone.

Then shut off your engine. You'll see some white smoke coming from the tail pipe, this is normal.

Pour the rest in your gas tank.

Wait about 30 mintues.

Start your truck up and haul @$ down the street as your truck will smoke like there's no tomorrow. It will keep smoking for about 15 minutes.

Great - thanks again

I probably won't get to it for a couple days, but I'll report back on how it goes. Got to take it to a buddy's place because, based on previous experiences, I don't think the management here at my apartment complex would take to kindly to the smoke show...

gnet158 · December 18, 2008

No problem, good luck, and enjoy the smoke show.

LegndLarry33 · December 23, 2008

No problem, good luck, and enjoy the smoke show.

Alright, I'm back. All went well with the seafoam, pretty good amount of smoke, but unfortunately, the SES light will still blink when I get on the highway or try to accelerate in 5th gear (low RPM)...

The ride home just now (~45 min) was pretty interesting... blinked a lot when I first got on the highway and it continued to blink whenever I accelerated... then after getting stuck in traffic for ~15 min, it didn't do it again the whole way home... could be the temperature I guess (20' F), and the block was still warming up?

The hard part with all of this is that the truck isn't running bad. The only symptom is a slight lack of power at low RPM... the truck never bucks or misses hard, and the only code it will give is the P0300...

Made an appointment with the mechanic for Jan, 5 but hoping I can figure it out before then...

LegndLarry33 · January 8, 2009

Figured I would bring some closure to this because I hate when these threads just die...

I ended up breaking down and dropping it off at the chevy dealer... #1 fuel injector was bad, and the whole FI system needed to be flushed...

When they called and said what it was, it all started to make sense... previously, if I cleared the code and then plugged it in right when the light came on, I would get the P0301, but if I let it run for a while after the light came on, I would eventually get the P0300... so the #1 was the worst, but the rest were missing from time to time...

I ran two bottles of FI cleaner through it a while back, but I guess it needed a professional cleaning...

anyways, its running great now... well worth the $500