1998 yukon Mystery p0300 and rough idle still

[OilBurner2003]

It kind of looks like unmetered air in that it’s trending down in short term and cutting fuel (negative short trim = slowing injector pulse times) but it’s able to manage the situation and is still within that 10% range.

At idle, the MAF tells how much air is coming in, the computer injects so much fuel to make the ratio right but then the O2 sensor says it got too much air at the end of it all. It goes back and adds more fuel (positive short trim) to get back to the right 14.7 to 1 ratio.

Now at higher idle, the O2 is saying you got a little too much fuel so cut it back a smidge (negative trims). Where is that increased air volume coming from then?

Intake manifold gasket? Maybe? You did a smoke test on it and didn’t see anything externally so…maybe and internal leak? As before, you’re within that 10% range so, without the misfires, most would probably not think too much about the trim numbers.

What does your MAF read at idle? 4.6 g/s is GM spec, 5-7 is probably fine.

Checking for vacuum at the dipstick may be the next step. How much vacuum is ok? I’ve read that 1-3 in. Hg. is normal but have no GM spec to back that up.

Or if you want a mess, pull the oil drain plug and see if you have coolant coming out of it. If there’s enough in there, it’ll be a little chocolate milk trail within the oil stream starting at the bottom of the plug hole. When I had two cracked heads and a hydro locked piston, the coolant accumulation was obvious and maybe 1/16” thick in the oil stream. For a barely leaking intake gasket, I couldn’t say.

Sorry, seems like a parts cannon is approaching. Maybe others can chime in to help get a more definitive diagnosis.

Hey, I've been working on this truck for a year come January, no need to stop now! I will go see what the MAF sensor is reading at Idle.

Edit: Here’s the mass airflow reading at 760 RMP, idle

 

[east302]

That wouldn’t give me any heartburn, I think mine usually read about the same.

Was looking to see if it was reporting a real oddball number. A really low number would imply that there was a leak between it and the throttle body. Then the total intake air would be drawn (split) through both the MAF and from a leak, the MAF then reporting less.

 

[OilBurner2003]

That wouldn’t give me any heartburn, I think mine usually read about the same.

Was looking to see if it was reporting so real oddball number.

I’m starting to think I should just do the lower intake gaskets and then go from there. It seems no matter where I’m told to look, air or vacuum of some sort needs to be ruled out. Based upon my own research it really should be done anyway. I’ve heard tons of issues being fixed because the intake was leaking inside and not being detected on a smoke test.

What are your thoughts on replacing the intake gaskets to at least rule it out if it doesn’t fix my issue? I’m starting to believe this may very will be the culprit. They are 11 years old.

 

[east302]

At this point, yeah it’d be on my list. You have something impacting multiple cylinders and the history of the gaskets is an unknown.

Tip - mark the distributor shaft at the intake, mark the starting and ending points of the rotor as it is pulled. Align the shaft mark, reinstall starting at the ending rotor point and it’ll seat at the original starting point. Word salad aside, don’t touch the crank

 

[east302]

Grab the PDF manuals for a 98 here:

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

It’ll have the torque sequencing for the intake and the dimensions for the RTV bead at the china walls. Felpro sells a nice set that has everything, including the intake bolts and o-rings for the fuel lines. I’ve always just used that and never had an issue.



Alldatadiy.com is a more mobile option and easier to use. It has the exact same information as the GM manual. Essentially, they digitized the manual and made it so much easier to use. Subscriptions are around $50/year per VIN. That’s always been my preference.

 

[OilBurner2003]

At this point, yeah it’d be on my list. You have something impacting multiple cylinders and the history of the gaskets is an unknown.

Tip - mark the distributor shaft at the intake, mark the starting and ending points of the rotor as it is pulled. Align the shaft mark, reinstall starting at the ending rotor point and it’ll seat at the original starting point. Word salad aside, don’t touch the crank

Roger, I was actually just about to mark it! I suppose I’ll start sometime within the next couple days if I feel up to it, maybe I should do that test by plugging up the oil fill and the other things and see if there’s a vaccum in the dipstick, just because.

 

[OilBurner2003]

Grab the PDF manuals for a 98 here:

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

It’ll have the torque sequencing for the intake and the dimensions for the RTV bead at the china walls. Felpro sells a nice set that has everything, including the intake bolts and o-rings for the fuel lines. I’ve always just used that and never had an issue.

View attachment 386544

Alldatadiy.com is a more mobile option and easier to use. It has the exact same information as the GM manual. Essentially, they digitized the manual and made it so much easier to use. Subscriptions are around $50/year per VIN. That’s always been my preference.

Are you usually supposed to change the intake bolts? I have the set of gaskets and that's it so far.

 

[east302]

It probably falls into a “good practice” sort of thing. They’ll need a thread sealant on them since some go into the lifter valley. It probably has the GM number for the sealant in the manual that you cross check with permatex, etc. part numbers.

 

[OilBurner2003]

It probably falls into a “good practice” sort of thing. They’ll need a thread sealant on them since some go into the lifter valley. It probably has the GM number for the sealant in the manual that you cross check with permatex, etc. part numbers.

Roger, thank you! I’ll do some research.

 

[east302]

12345382, looks like blue (medium) threadlock