Truck doesn't idle properly?

[mccoyblues]

My truck refuses to idle smoothly. I replaced the cap, rotor, plugs, and wires, o2 sensor, map sensor, egr valve, fuel filter, etc and the thing still shakes idling in drive, and goes up and down in rpms in a endless loop while in park. While you are moving, it runs great, hauls behind, but leaves a cloud of black smoke behind you, and the engine light comes on if you stay at between 50mph and 60 for more than 20 seconds. Truck is a 91 S10 with the 4.3. What does this sound like?
Also getting bad gas mileage, I only get 200 miles out of a 18.5 gallon tank. Which is mostly highway driving by the way.
It doesn't burn oil. I put in 5 quarts of Mobil 1 5w30 2000 miles ago and it is still on the full mark, and it just now started turning brown.
Guess I'll just junk the truck tomorrow morning for $250 and cut my losses. Even the mechanics I took it to can't figure this out. Shame, only has 87k on it too with a straight body.

 

[scrubbag]

Sounds like you are running rich. The IAC will cause it to 'hunt" if it is out of adjustment.

I had the same throttle body I think you are probably running on your S10, and I installed it on my AMC Eagle 258 engine. 4.2 ltr. Almost same ltr. It's a two barrel throttle body, right?

I was having a problem with vacuum leak and didn't know it, so I messed with everything else. I had the TPS set to high on voltage, and it was causing the engine to surge, to hunt.
You see, the TPS Throttle Position Sensor, sense the position of the throttle, of course. So if it is off, it will cause the engine to run lean, or rich, depending on the position of the senser.

So I learned that they should run from around .45 volts (less then half a volt) when idling, to full open at around 5 volts (that is 5 not . 5). As the throttle valve is opened, the signal increases and the computer uses this to add gas to the engine. So it goes by voltage. So if the voltage is say, .67 instead of .45, it would be giving the wrong signal and the valve would still be in the same place, so the computer will add a bit more gas then needed. The O2 sensor then sees the added rich mixture and tries to get the computer to cut back. So, it swings.
And of course, as the throttle valve is opened further, the computer will add more gas then needed, because the voltage is wrong. At all speeds, it will be wrong.
So, you need a volt meter to set it .
I cant remember exactly how or what wires to test, to I refer you to this page.

http://www.ehow.com/how_4489318_troubleshoot-tps-sensor.html


I still think the smoke is running way to rich. So work towards that end.

One thing to think about. If your coolant sensor is bad, it will not allow the computer to put the other sensors into service, they will stay in bypass mode, and this will be "rich" mode.

When a car is started up cold, the coolant temp sensor tells the computer to bypass the othe sensor, like the o2, and MAF. So, it runs rich, and for a purpose. Gas will not burn to good when cold and therefore, it takes more of it, during cold engine periods. So, it runs rich during this time. And then, the coolant temp sensor tells the computer when the engine is warmed up and the other sensors go to work.
But if for some reason the computer does not go into closed run mode, it will continue to put in too much gas, and the 02 sensor is not in service to stop it. This designed so that if there are problems with sensors, say while on a trip, the car will run well enough to get you home, but a little rich.

Good luck.

 

[chris]

On this engine i would check my fuel pressure first and make sure it's in spec. Black smoke and poor fuel economy is for sure caused by the truck over fueling. 4.3 is well know for leaking fuel pressure regulator, this is located under the intake manifold. They cost about $1000 bucks for the part. If you suspect the fuel pressure regulator and your fuel pressure is on spec then clamp off the fuel return line and get someone to cylce the key to build pressure, now also clamp the fuel inlet line so that both the fuel return and pressure line are pinched off. If you see the fuel pressure drop withing 5min then you most likely have a leak in the fuel pressure regulator. A shop would have charged you $100 for this information. Good Luck