Funny that this thread should resurface recently. I've been working on my most recent acquisition (93 Sonoma VIN W) so it will pass Washington emissions. The problem? High hydrocarbons at idle. It idles rough. The first thing I did was to run some Seafoam through the PCV hose into the intake. That resulted in thirty minutes of blue smoke billowing out of the exhaust. I followed that with new cap/rotor/plugs/wires–all AC-Delco–and an oil and filter change.

The second thing I did was to remove the EGR and check for carbon. From the looks of it the EGR has been replaced relatively recently. It looks very new. There was no carbon holding the pintle open but I cleaned the EGR passages as thoroughly as possible. I used Berryman's B-12 Chemtool and a very narrow bottle brush I found at the grocery store. It's the perfect diameter for the passages.

The idle was still rough. I put my scanner on the truck and found what I think is the most likely cause: lazy oxygen sensor. At idle the truck kept dropping out of closed loop mode. The oxygen sensor wasn't oscillating as it should. Instead it remained in the 700-800 mV range. I'll change the sensor in the morning and see if it has the intended effect.

Thorough diagnosis and testing is always the best path. There are many things which can cause a rough idle. It's important to follow a clear, logical troubleshooting path which will test only one variable at a time.

 

if its going into open loop its probably and open thermostat not allowing it to warm up to proper operating temp causing it to run rich, i had this same problem and a new thermostat fixed the problem

 

It well could be a thermostat problem. I replaced the O2 sensor in the morning and it idled much better afterward. The new sensor is oscillating properly. I also replaced the temperature sender for the gauge because it was reading low compared to what the coolant temperature sensor was showing. Currently the cooling system is drained. The heater core is leaking and I flushed the system before removing the core. I'll finish replacing the core in the morning and get some decent coolant in it. Then I'll run it up to temperature and see if the thermostat is opening properly.

Since the O2 sensor was a known problem I definitely wanted to replace that before moving on to the next thing.

 

let me know if the temp sensor changed your readings because my gauge shows a quarter to 100 under which looks like 180 or so but the scanner reads 195

 

My coolant temp gauge now reads correctly. Also, the needle on the gauge was bent. I discovered that when I replaced all the burned out bulbs in the gauge cluster. I also lost a 1/4" 10mm socket behind the dash when I replaced the heater core. After two hours of searching I finally gave up and decided to buy a new socket.

Interestingly enough I found another problem today. The injector assembly is leaking. Somehow I never got the memo about leaking spiders. That would explain the toasted O2 sensor and carboned-up engine. The leak was new one for me. One of the injector lines had rubbed a hole into the nut kit. I got new injectors, nut kit, and plenum gasket. I'll put it together in the morning.

 

They are designed to open at higher, freeway speeds to recirc the ehaust gases for smog control, and after they become plugged from the tiny bits of carbon, the valve sticks and throws a code, results in rough idle. I cleaned mine, reinstalled it and then left it unplugged. It will keep a SES light on the dash, but it will run great. With 300,00 on mine I can live with a light!

 

The truck passed emissions today but it still occasionally idles rough, particularly when in gear. I may look into it more in the future but I'm getting ready to take a trip to Idaho with my wife next weekend in her Jimmy so I'm not in a hurry to do anything more with my truck.