Oil on spark plugs - misfiring p0300

[donjetman]

Your engine issue could be counterfeit spark plugs? Or clogged injector?

I may have recently encountered one of these. I have a 1951 Chevy pickup truck with a low mileage 5.3L engine out of a 2008 GM pickup. It only has 2200 miles since truck was completed 10 yrs ago.
Lately it started missing when under load and threw engine code P0308 (#8 mis-fire). Checked header tube temperature with IR temp gun. #8 was half the temp of the rest. I put in a good used spark plug, poured a 20 oz can of Seafoam into the gas tank that only had about 4 gals of fuel in it, and it was fixed. Eventually I changed the rest the spark plugs, and added 5 more gals of fuel. I have driven it 100+ miles, and its running perfect.
That #8 spark plug looks strange. It looks polished (silver) or one side of the porcelain? Here's some pics of both sides:

 

[FAUST]

What vehicle is this?

As far as I know, spark plugs don't go through the the valve cover on any Tahoe/Yukon etc, like they do on the hemi or a Nissan VQ etc.

That’s what I am learning. 99% of videos are smaller 4 cylinder engines.

This is a 2007 5.3 Tahoe.

 

[FAUST]

Top is the original (bad design) valve cover w/130k miles.
Bottom is new updated design.
View attachment 415030

Perfect - I’ll be checking this next weekend. I was able to find this yesterday. If this is the updated valve cover, I’ll be working to fine out of the injectors are the issue.

Some work ahead. I’ll report back once I’m through the work.

Thank you all!

 

[mikez71]

Your engine issue could be counterfeit spark plugs? Or clogged injector?

I may have recently encountered one of these. I have a 1951 Chevy pickup truck with a low mileage 5.3L engine out of a 2008 GM pickup. It only has 2200 miles since truck was completed 10 yrs ago.
Lately it started missing when under load and threw engine code P0308 (#8 mis-fire). Checked header tube temperature with IR temp gun. #8 was half the temp of the rest. I put in a good used spark plug, poured a 20 oz can of Seafoam into the gas tank that only had about 4 gals of fuel in it, and it was fixed. Eventually I changed the rest the spark plugs, and added 5 more gals of fuel. I have driven it 100+ miles, and its running perfect.
That #8 spark plug looks strange. It looks polished (silver) or one side of the porcelain? Here's some pics of both sides:

View attachment 415038

Is that a crack under the tip?

 

[donjetman]

Is that a crack under the tip?

?
Myself (old hobby mechanic) and a couple of pro mechanic friends of mine have looked at it and haven't a clue as to why, what, etc.

 

[Larryjb]

That’s what I am learning. 99% of videos are smaller 4 cylinder engines.

This is a 2007 5.3 Tahoe.

OK, good that is cleared up then. There is no such thing as a plug tube seal on these. The oil is on the threads for one of the reasons others have suggested here.

I'd try the simplest first: pop the PVC off from the valve cover to make sure there is suction while the vehicle is running. If there is no suction, you probably need to replace the PVC valve which is easy.

 

[donjetman]

OK, good that is cleared up then. There is no such thing as a plug tube seal on these. The oil is on the threads for one of the reasons others have suggested here.

I'd try the simplest first: pop the PVC off from the valve cover to make sure there is suction while the vehicle is running. If there is no suction, you probably need to replace the PVC valve which is easy.

On the 07 and later engine they don't have the ole PVC valve.
The complete valve cover is the PVC.

 

[Fless]

On the 07 and later engine they don't have the ole PVC valve.
The complete valve cover is the PVC.

On my '04 L59 I have no proof that my valve cover is OE, but there's no indication that it's not. The complete valve cover is the PCV, even back then.

 

[Rocket Man]

Some engines have rubber seals where the spark plug tube meets the valve cover. I thought this engine would have those to replace - I’m finding oil on the threads of the plugs.

Sorry my original response was not specific.

That’s a 4 banger item. There’s no plug tubes on any v8 ever made with the possible exception of a Chrysler hemi.

 

[Larryjb]

On the 07 and later engine they don't have the ole PVC valve.
The complete valve cover is the PVC.

There is still a hose that connects the valve cover to the intake. I'd pop that hose off and check for suction.

 

[FAUST]

On it! I’ll do that today and see where I’m at!

Also - I have found what y’all are saying about the seals. Thank you all for confirming!

I hope it’s an easy fix - PCV - I’ll let y’all know what I find. If not the easy, I’ll go down the list.

Thanks again!

 

[FAUST]

On it! I’ll do that today and see where I’m at!

Also - I have found what y’all are saying about the seals. Thank you all for confirming!

I hope it’s an easy fix - PCV - I’ll let y’all know what I find. If not the easy, I’ll go down the list.

Thanks again!

PCV checked - vacuum there. Now I need to work through the list of fixes - I’ll start by checking the valve cover and make sure it’s the latest.

Then I’ll work on injectors.

Symptoms I’ve been observing - the truck doesn’t have any odd sounds or anything while running - the main issue I have is the shuttering feeling at low RPM when accelerating and when decelerating - it’s very obvious when coming to a stop at low RPM and when climbing a hill at low RPM.

Could it be something with the 4WD or something with the wheel hub?

I’m at a loss and not in a position to pay a shop for diagnostic again (3x - replaced o2 sensor and found nothing else).

Again - any ideas or help is appreciated!

 

[j91z28d1]

it's been my experience, not with these trucks but just engines in general, if they only miss under load, especially low rpm load it's normally weak spark for some reason. fouled or glazed plugs, bad cold or wire, low compression and could be fuel injector, but I've never actually had just one bad fuel injector go bad before, but I'm sure it does happen and I always check.

low rpm high load is when it's felt the most because of the longer time between firing. higher rpm miss feels smoother. you'll hear a lot of times people say it clears out when it's run higher rpm. it's just how it is.. miss is more noticeable at low rpm and more load. like say taking off in 3rd gear instead of 1st gear.

 

[Fless]

I don't see it mentioned but may have missed it. How clean is the throttle body and/or how long ago was it cleaned?

 

[Rocket Man]

Are you sure it’s not torque converter shudder? What trans do you have? I know the 6l80’s have that issue.

 

[strutaeng]

You need to invest in a diagnostic tool that reads live data (and gives specific misfire count per cylinder) and spend some time on YT learning how to diagnose a misfire to find this.

An injector balance test is also going to be helpful.

 

[FAUST]

I don't see it mentioned but may have missed it. How clean is the throttle body and/or how long ago was it cleaned?

Throttle body was cleaned less than a year ago - cleaned meaning I removed the throttle body and cleaned it front to back.

 

[FAUST]

Are you sure it’s not torque converter shudder? What trans do you have? I know the 6l80’s have that issue.

I am not sure what trans is in this truck - I will assume it’s stock - 2007 Tahoe LTZ. 5.3L.

I’ll look at how to diagnose that.

 

[FAUST]

You need to invest in a diagnostic tool that reads live data (and gives specific misfire count per cylinder) and spend some time on YT learning how to diagnose a misfire to find this.

An injector balance test is also going to be helpful.

I’ve seen mention of a specific tool for these rigs - do you have a suggestion?

I agree with you 100% - before I became engulfed in this process I spent $100 on blue driver scan tool - it has the ability to see misfires on each cylinder but it’s not live.

I’m trying to do what I can without spending a ton of money on “fixes” that aren’t fixes.

I’m sure y’all have been here before - 3 kids in private school, Mtg, extra curricular, etc. just not a lot of expendable cash.

This forum has been helpful, I’ve learned a ton about the engine and engines in general - for that I am grateful.

Thanks for continued support and brainstorming to help a new wrench turner along.

 

[FAUST]

Throttle body was cleaned less than a year ago - cleaned meaning I removed the throttle body and cleaned it front to back.

Some before and after pics.