Hey all,
So I’ve been dealing with this issue for a loooong time and have worked at it a bit each time I have the time to do so but I finally want to nip this thing in the bud. As the title suggests, my engine tends to have a rough idle 30-60 seconds after startup (mostly on a cold startup but sometimes when already at temp) wherein the tachometer will go all the way down to 200-300 RPM before revving up to ~1500 RPM. If I depress on the gas pedal before allowing the engine to rev up and get it up to that same ~1500 RPM the idle will resume at the more typical ~600-700 RPM range. I’m only getting about 13 MPG combined highway/city with the AC on about half of that time, and while I know these things aren’t really known for their efficiency, that feels a bit lower than it should be.
In addition to this I’ve also had intermittent P0135 and P0141 codes. What initiated this post was them popping back up again after about six months of no issues. I was winterizing my truck and noticed a small coolant leak from the thermostat housing on the intake manifold and had to take the alternator off to access it, and after reinstalling and restarting the vehicle the codes appeared.
I’ve read quite a few of the older posts regarding this issue and still don’t really know which way to proceed. Below I’ll list the other suggestions I read about and tested or verified:

1. TCP Sensor - OEM, no codes
   
2. Crankshaft Position Sensor - OEM (I believe), no codes
   
3. IAC Valve - OEM, no codes, checked it and cleaned it even though it wasn’t dirty
   
4. MAP Sensor - this is the one sensor I believe isn’t OEM, not sure if people have had issues with these or not
   
5. Throttle body - checked and cleaned
   
6. Intake manifold - Hughes plenum kit installed, peered in and it doesn’t appear to have any oil inside, so I doubt there’s a plenum leak
   
7. O2 Sensors - all O2 sensors are new, OEM units, I also have no codes for them and have never used seafoam/engine cleaner which I’ve read can foul the sensors
   
   1. Verified voltage/ground at both connection to PCM and connection to sensor
      
8. Fuel sync - I set the fuel sync with a Snapon scanner I had at the time
   
9. Fuel pressure - tested with a rental gauge and it was within specification
   
10. Vacuum - checked all vacuum lines, no visible cracks or leaks
    
11. PCV valve - checked valve to make sure it wasn’t clogged, it’s fine
    

The only thing I can think of would be the following:

1. My cruise control does not work, I think this is because the servo unit is bad and the bellows do not work. Is it possible there’s a vacuum leak there internally within the bellows?
   
2. I cannot verify if my MAP sensor is indeed OEM or not, and based on my emails it appears that it is a cheap-o aftermarket unit. Could this be causing the issue as well?
   
3. Bad PCM? The unit I have appears to have been previously flashed/tampered with
   
   1. Additionally, when I bought the truck the catalytic converters had been cored out (unbeknownst to me) and the vehicle didn’t have any engine codes relating to emissions. Perhaps the previous owner flashed the PCM with a version of software that ignores the O2, and now that I have a complete emissions system it’s bugging out?
      

I really just want to get this thing figured out, I’ve poured so much time and money into this truck and want it to be finally complete. Thanks!

 

P0135 O2 Sensor 1/1 Heater Failure
P0141 O2 Sensor 1/2 Heater Failure
You have two 02 codes, So now you have a cat and 02's correct? You need to check that you have 12V+ at the heater wire on the 02's. If not you have to find out why. You should have power there anytime the engine is running. If you don't know what the heater wires are have a look here.

 

My apooogies, I typed that up at work the other day before I discovered the code issue.

Yes now I have a cat and o2’s, the CA emissions setup with 4 sensors and 3 cats to be exact. I believe I verified the voltage back in the day about a year and a half ago when I first got these intermittent codes popping up. I’ll do it again this weekend but out of curiosity do the heater wires also have 12v on them with the key in the on position or only when the engine is actually on?

 

They will probably have power for about 3 seconds at initial key on, but, after that, only if the PCM sees the engine turning.

 

So I got the O2's tested and they had about 12V with the engine on. I plugged everything back in and restarted the truck and now I magically have no codes? I've driven the truck about 120 miles since then and there aren't any pending codes so I'm not sure how or why they went away, but I still experience the rough idle after 30-60 seconds or so.

Any other suggestions?

 

May have just been poor connections.....

How old are you plugs/wires/cap/rotor?

 

My plugs, wires, rotor, and cap were all purchased in 2022 and have only about 2000 miles on them. I snagged the new rotor/cap because the plastic around one of the retaining screws was completely cracked off and wouldn't seal 100%. Not sure if any of this stuff could be the issue as they're all OEM or better?

 

Iridium plugs...... These engines really don't like the rare-earth fellers...... Swap 'em out for some standard copper plugs.

 

The engineering of this truck never ceases to amaze me lol. Do you think this could really be the issue? I'll get some ordered pronto.

 

Also, any substantial difference between the OE recommended 0.040" gap and say 0.044"?