1999 Ram 1500 4X4 5.9 - 148,000 miles. OK, so I finally got to replacing my heads due to the #6 exhaust valve seat being blistered. It was a PITA to get the job done, but it is. While it was down, I did the Hughes fix for the plenum, which only had a slight amount of oil in it. I also replaced the DC, Rotor, Plug wires and plugs, while I was at it. Of course, I replaced the 180 degree thermostat with a 196, all of the hoses, and gaskets with a Felpro top end gasket kit. I also replaced the head bolts, and intake bolts.

There was a problem when I first started it, with the MAP sensor (high reading). I pulled the connector, cleaned it, and that issue when away. But, now it "feels" weak. It runs smooth, idles smooth, runs down the highway well, but feels OFF. Like it's got a smaller engine or something. I've had a CEL twice for Misfire on cylinder 8, but never really noticed any misfires. Also, the fuel economy is lower than it was before the fix. I'm having to push the gas pedal down further slightly, and that ECO meter is reading two MPG lower than I used to see. I don't know where that meter gets its info, and it was ALWAYS off (reading 17.7 mpg when actual gas mileage was about 15 mpg). It's also idling slightly faster than it did. I'm getting about 15" of vacuum at idle, and it is steady. It's idling at about 700 rpms when it used to be 650. (I'm not nit picking...I'm sharing data.) I also noticed when I put it into drive, the RPMs jump for a second. Anyway, any advice or ideas would be appreciated.

 

You have a vacuum leak somewhere. Should be closer to 20" at idle. Maybe you missed a vacuum plug when you put it back together?

 

Well, as I remember it, there are three vacuum ports (all manifold vacuum) on the intake. Two large ports on the left side (brake booster and PCV from right side of engine), and one small port on the right side, that goes into a mess of vacuum hoses, check valves, elbows, etc. The only remaining vacuum port is in the air cleaner for access to the left side PCV. I replaced all of the bad vacuum plugs and elbows. I don't remember any ported vacuum ports on this car...nor would I expect any, since we no longer use vacuum modulators for shifting, or vacuum advances on distributors. I thought I remembered that the vacuum should be 20-22" at idle, but my diagnostic experience is VERY dated. I also sprayed carb cleaner all around the intake, those ports, and the TB/intake junction, with no difference. I wonder if the exhaust has become partially plugged? In fact, when I removed the right side exhaust manifold, so create extra room, the flange was very loose on one side...very loose in fact. I wonder if a previous owner noticed this and loosened it on purpose. What say you?

 

I know the number of ports on the passenger side of the manifold changed over time. Usually, there are two, or even three ports there, and the unused ports are simply capped off. PCV valve switched sides over the years as well.... May try disabling PCV, (pull the hoses, plug the holes in the valve covers with tape) take off the oil cap, and run the engine at idle. Should be a breeze coming OUT of the oil fill. If you have vacuum there, then the manifold didn't seal somewhere.

 

Thanks. I'll do that once I get back to the truck this evening.

 

Weird again. The truck just got better, all on its own. I drove 120 miles home last night. It ran perfectly, and the gas mileage is improved over the pre-head replacement. Idle is perfect and stable at 650. Maybe it took awhile to get the crap out of the engine and exhaust? It DID run for over a couple months with a bad exhaust valve seat, with the injector still operating...Oh well. I'll take it. Thanks HeyYou!

 

Gotta love the self-fixing truck.