1999 4.0 OHV, 134k miles. Idles rough, not really bad, but rough, not smooth. Truck has died a handful of times when coming up to a stop. It always fires back up, though. Seems to run rich out the tail pipe. The AC condenser kicks on and off about every 10-15 seconds when the AC or defrost is on, and every time it kicks on the idle goes up 3-400 rpms to compensate. It seems to run great under power when driving around, though. I checked it and it has codes p0172 and p0175, bank 1 and bank 2 rich. I replaced the IACV (had lots of what looked like dark oil blowby in it), the maf sensor, and the air filter. It ran great, smooth, rpms only fluctuated about 70 when AC pump would kick on, barely noticeable, let it run for 10 minutes. Restarted the truck and it's back to it's old ways, RPMs are up at idle, go up about 3-400 when condenser kicks on, idles very rough now, and rpms stay up around 1500 when you take your foot off the gas when driving around, but go down to about 1000 when stopped in Drive. I sprayed brake cleaner around the intake manifold and on the driver's side the idle goes up about 100rpms momentarily when sprayed in two spots. Is it as simple as a vacuum leak at the intake manifold gasket this whole time, or is that small amount of increased rpms an issue, but not enough to cause all this? I can fix stuff easily, but having a hard time with the actual diagnosis of this problem. Please help!

 

if the idle goes up with some kind of flammable fluid sprayed over the motor, you have a vacuum leak. try and pinpoint it.

 

If I have a slight vacuum leak at the intake to the intake manifold will it cause a rich condition, though, or a lean condition?

 

yes, any vacuum leak can change things.

 

Update on solved issue, since most people don't come back and update when they figure out their fix. I was digging around deeper (I'm usually left working on this thing in the dark with a headlamp), and found out the PCV breather hose/crank case breather hose that goes from the back of the driver's side valve cover up to the side of the plastic intake manifold was not hooked up at the valve cover side, and the hose is the incorrect hose and had no clamps on it. I hooked it back up and put house clamps on both sides to keep it in place and it runs like a champ. Ordered the correct hose and I'll swap it out when it gets here. Geez, so simple after all of that.

 

Update on solved issue, since most people don't come back and update when they figure out their fix. I was digging around deeper (I'm usually left working on this thing in the dark with a headlamp), and found out the PCV breather hose/crank case breather hose that goes from the back of the driver's side valve cover up to the side of the plastic intake manifold was not hooked up at the valve cover side, and the hose is the incorrect hose and had no clamps on it. I hooked it back up and put house clamps on both sides to keep it in place and it runs like a champ. Ordered the correct hose and I'll swap it out when it gets here. Geez, so simple after all of that.