1998 yukon Mystery p0300 and rough idle still

[OilBurner2003]

Not sure what the banks 3 and 4 are really reading from, but they’re so out of whack that I’d ignore them. Banks 1 and 2 look like more valid numbers and the ones to see.

It’s adding a little more fuel on bank 1 (driver side cylinders 1357) than on bank 2 (both are positive short trims) but when added to the long terms it looks reasonable. I think the target is having one bank’s short plus long term equaling 10% or less. Oxygen sensors are probably fine, then, but it wouldn’t hurt to see if their voltages are switching (about 0.1 to 0.9 volt)

Others can confirm, but nothing stands out as a red flag on those.

I’ve read that you can sort-of confirm an internal intake gasket leak by plugging the oil cap, PCV and breather holes in the valve covers. Crank it and see if you’re getting a large vacuum at the oil dipstick. Never tried it - mine have always leaked externally by the time I saw that something was wrong.

I have the O2 voltage I think on the other picture, Not sure if that helps with anything, or if I even selected the right ones lol.

 

[east302]

Hit the gas and see how the trims respond.

You’ll have four O2 sensors, I couldn’t see which was up or downstream in your screenshot since it truncated the description.

Downstream sensors should be relatively flat - they just monitor catalytic converter efficiency. If they mimic the upstream sensor then the converter isn’t doing anything. Quick check is to swap the downstream sensors.

 

[OilBurner2003]

Hit the gas and see how the trims respond.

You’ll have four O2 sensors, I couldn’t see which was up or downstream in your screenshot since it truncated the description.

Downstream sensors should be relatively flat - they just monitor catalytic converter efficiency. If they mimic the upstream sensor then the converter isn’t doing anything. Quick check is to swap the downstream sensors.

Roger, I will go do that now. Is there a specific RPM i should put it at?

 

[east302]

All of this needs to be read when engine is warmed up (closed loop) by the way.

 

[Fless]

And the downstream sensors don't heat up all that much at idle, so you may need to drive it a bit or raise the rpms (1,500 or 2,000?) to see better activity. Maybe the downstream O2 heater isn't working or is weak. Like @east302 noted, you could swap downstream sensors side to side and see what happens.

 

[OilBurner2003]

And the downstream sensors don't heat up all that much at idle, so you may need to drive it a bit or raise the rpms (1,500 or 2,000?) to see better activity. Maybe the downstream O2 heater isn't working or is weak. Like @east302 noted, you could swap downstream sensors side to side and see what happens.

I'll let it warm up for 15, just to be sure. Will check back in when it's warmed up.

 

[OilBurner2003]

All of this needs to be read when engine is warmed up (closed loop) by the way.

Warming up, back in 15.

 

[east302]

Roger, I will go do that now. Is there a specific RPM i should put it at?

Just up to 2k or something, the idea is to see if it goes off the charts or returns more to normal. But you’re pretty much normal now so it’s probably a moot point.

Unmetered air (vacuum leak) at closed throttle (idle) would show as a higher positive short term trim as the computer adds fuel to compensate. It should be less obvious at open throttle as you’d be bringing in more air. Your trims would trend toward normal (zero) then.

 

[OilBurner2003]

Just up to 2k or something, the idea is to see if it goes off the charts or returns more to normal. But you’re pretty much normal now so it’s probably a moot point.

Unmetered air (vacuum leak) at closed throttle (idle) would show as a higher positive short term trim as the computer adds fuel to compensate. It should be less obvious at open throttle as you’d be bringing in more air. Your trims would trend toward normal (zero) then.

Taken while rpm was within 2-2300. Coolant temperature is 175.

 

[east302]

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.