I've got one for ya

[Masterd]

What would possible cause my truck to idle rough/miss in park, if I had the oil cap off?

I took it off...and the engine started to miss intermittently, and when I put it back on, no more problem. This was in park...if I put it in gear, it wasnt an issue.

 

[boggs1227]

pcv valve related issue / egr ?

 

[Masterd]

Care to elaborate?

 

[James909]

I'm not sure if your 93 is the same, but on my 92 the oil fill is on a line that is connected to the intake tube. Why it's like this, I don't know. But I would assume that the truck was missing because when you took off the oil cap you are creating a pretty big vacuum leak.

Edit: I didn't see that putting it in gear fixes the problem. I don't know what may cause this.

 

[boggs1227]

just thinking that maybe once the engine is seeing a lower pressure with the cap off in park then maybe the egr system is haveing a problem with this .on mine i replaced the egr valve and the dpfe sensor.just now reading in my haynes book and found that the transmission range sensor affects many other systems ,idle spped ,torque converter lock up , and also affects the egr system .

maybe thats why it's doing it only in park and not drive .

edit ; oh yea that to what james 909 sid about that ...makes sence now could have a affect also if the tube that leads to the intake tube was seeing low pressure ....sorry did not think about that

 

[Masterd]

I think it was the fact that it was causing a fair sized vaccum leak.

Not enough of one to cause constant issues, but I did unplug that hose coming off the intake and covered /uncovered the hole with my finger, and the symptoms seemed the same.

Thanks.

 

[boggs1227]

then it could still be the egr system or range sensor i mentioned.

 

[Masterd]

Ill have to look into the egr.

 

[Alan_V]

As mentioned, the most likely cause is PCV. As oil breaks down, its vapors have to go somewhere. As those vapors are hydrocarbons (HC), you can't vent them to the atmosphere. Sooooo, way back when, the Granatelli Bros. (yes those Granetelli's) developed the first PCV valve to vent those vapors at a controlled rate into the intake system, where they would be burned with engine combustion. If you have the cap off on an engine with a properly working PCV, you have a vacuum leak. The fact that yours is as noticable as you describe may be due to the mileage on the vehicle making it weaker in other areas or needing attention in other intake/ignition/induction areas.

You don't drive with the cap off normally so if that's the only time it's noticable, don't sweat it.

 

[Masterd]

Yeah shes got 285k on her.

It wasnt constantly rough, it was more of an "idles smooth for a while, acts like its going to stall, then corrects itself"