I drive about 60 miles round trip for work everyday. In the mornings my truck seems to idle fine, nothing too out of the ordinary. Once I get to work and the engine has been running for some time the idle gets a little rougher, but nothing bad. It seems like in hot weather, and after a long trip, the idle gets rough. I'll pull in at home, and it shakes the truck pretty good. When it acts this way the rpm gauge moves up and down by 100-200. On occasions it will drop below 500 and then pick itself back up. Also there's a loud tapping sound coming from the engine bay. It gets louder back towards the distributor. Almost like it's coming from the transmission. The transmission was rebuilt about 10k miles ago. Today, after my trip back, I decided to throw my vacuum gauge on the brake booster hose going in the intake. It read about 17 inches at idle. The needle was only moving a hair. Blipping the throttle it goes down and raises back up to 21, then back to normal. At around 2000 rpm the vacuum will raise slowly by an inch or two. The only code is P0455 for a large evap leak. I did the plenum repair two weeks ago. I posted about a month ago concerning a coolant leak being a head gasket issue, turns out it was the water pump bolt dripping coolant that was fixed with RTV(thanks HeyYou). But to further my knowledge of engines, I went ahead and took the head off and checked for warpage per the service manual. All checked out, and I put on a new head gasket, new head bolts, and everything else. I haven't had any problems with overheating, coolant loss, or any misfire codes, so I'm assuming it should all be good. As soon as I find the time I was going to run a compression test so I know for sure.

Can y'all think of anything that's causing my idle problem?

 

When did the problem start? After the head gasket or plenum?

I would start with a tune up. Plugs, wires, air & fuel filter, clean or replace the IAC (idle air control), and replace the O2 sensors. Don't use Bosch.

 

For the loud tapping noise: If it only occurs when the PCM is in closed loop mode, then run the thing until it gets to talking to you, then locate the evap purge solenoid ("duty cycle solenoid" in the factory service manual) and just put a fingertip against the top of it. If the tap you feel is in time with the tap you hear, it's likely just a Dodge Ram being a Dodge Ram. You can unplug the electrical connector from the thing, and if the tapping stops you know it's the solenoid talking. Plug it back in afterward, and reset any codes its absence from the circuit might have generated.

Since we're in that neighborhood anyway: That little valve connects the charcoal filled vapor canisters to the intake manifold, so a gross evap leak is also an intermittent vacuum leak, and that can foul up your idle quality. But it will never happen at cold start because the purge valve isn't enabled yet, which seems kinda sorta like what you've said about your idle quality, too. Maybe it's just too much supposition and coincidence, but it feels a lot like something to look into.

 

I appreciate y'all pointing me in the right direction. I can get a bit carried away with trying to diagnose these things, so I like to turn to the experts.

The idle problem has been going on long before the head gasket and intake fix. If I remember correctly, I replaced the ignition coil because it was shorting. The wires, plugs, cap and rotor, along with the belt. It had a little more get up, but the idle problem quickly came back. The ignition coil is a valucraft. The wires are duralast 7mm. The cap and rotor are duralast gold. At the time I think I bought the gold's because they came together. IAC was black as night so I just swapped it for a master pro. The spark plugs are the champion coppers. Back 02 sensor was replaced a few months ago. Front hasn't. I'm not sure what brand the back 02 sensor is because I took it to a mechanic to see if he could help diagnose it. I'll have to look into it, but I think he did use Bosch. I had a good amount of oil in the intake before, so I'll definitely take a look at them.
I'll take a look at the solenoid after work, and the charcoal canister this weekend. I also have one more question while I'm at it. This truck has a clean title, and neither the paint or the frame are messed up, but the vents and the underside of the dash looks like someone gave it the haymaker. The dash is actually being held on by a lot of epoxy done by the previous owner. They probably messed up everything behind the dash in every way possible. In the middle of doing all these repairs a little black cable labeled 12V chassis ground cable fell down and is hanging by the hood release. The end of the cable is clean cut, not like it was pulled off with any exposed wire. Is this something to be concerned about?

 

Have an aftermarket radio? Or anything similar? Might be a wire that was never connected.....

 

There is an aftermarket radio. There was only one cable left over when I installed it but it wasn't a ground wire. I'll try and trace the cable back today

 

It's difficult to tell, but the tap coming from the engine is about the same tempo as the tap coming from the solenoid. As long as it isn't an internal engine problem, I'll live with it. A convenient cold front came through, but it should warm up by the weekend so I'll take a look into the rest then. Thanks for the help.

 

I got a chance to look into the hanging cable today. It's a Bosch 30 to 40 amp 5 pin relay. I took the cover over the electrical off and it looks like the black ground wire along with a yellow wire were just left over from whatever the relay goes too. The wires go up into the steering wheel. I'll try and hunt down that evap leak later and try to take a look at the front o2 sesnor.

 

Do you also have an aftermarket alarm? That could be for a horn-honk setup for an alarm.

Aftermarket running boards with lights, and they're not working?

Aftermarket fogs or driving lights that may have been removed?

 

I haven't personally, but I bought it used so I can't say for sure. It also turns out the rectangular black thing I saw hanging down under my truck is the front evap canister. A couple of the studs came out, and it has a couple of large cracks in it. I don't imagine this is the typical part for autozone to carry. Could I pull it off another truck or should I turn to the dealer?
EDIT: Parts being ordered, should get it tomorrow morning.