I have a 1995 S10 blazer 4.3 vortec engine with 175,000 miles. At times the idle runs a little rough and hard to start at times. Accelarates fine and runs good at highway speeds. Check engine light came on and had the code read at autozone and it said running rich. Changed the plugs and wires not to long ago and the injectors were replaced about 40000 miles ago. Could it be the o2 sensors?

 

Well, before I'd go replacing the O2 sensor, I would grab a fuel pressure gauge and see what happens to your fuel pressure after you turn the truck off. If it bleeds down over time, you have a fuel pressure leak somewhere in the system. The hard start and rich condition could both be a result of a fuel leak, and the rich condition would lead me to believe that it is leaking inside the intake manifold.

Is it more difficult to start the longer it sits? If you let the fuel pump prime up (turn the key to the ON position and let it sit there for 10-15 seconds before starting) does it start right up?

A rich condition could be caused by a faulty O2 sensor, but typically a failing O2 sensor would have triggered an actual O2 sensor code as well...

You stated that the injectors were replaced. What exactly was replaced? The CPI spyder? Did that include a new fuel pressure regulator? New o-rings? and was the nut-kit (internal fuel lines in the intake) also replaced?

 

Thanks for responding to my question. The engine is more difficult to start when I let it sit for like 5 or 10 minutes after running it up to operating temp. I will try the fuel pump prime to see what happens. I replaced the o2 sensor about 3 years ago because it triggered the sensor code and at accelaration it was bogging down. I'm not getting any of those symptoms now so it is unlikely it is an o2 sensor. I'm nor sure but I think I just replaced the spyder. Just for the part it cost me around 400 dollars. On the way to the auto store today to get some fuel injector cleaner the truck ran great, but I know it will not last long.

 

The reason why I asked about the parts that were replaced is because typically when replacing the spyder, the 'nut-kit' is replaced as well to ensure that there is not a problem with the internal plastic fuel lines. It is possible that these lines have started to go bad and are leaking now. You could check for leaks in the fuel system by removing the upper intake plenum (gasket may be required) and pressurizing the fuel system. Look for any drips around on the internal lines, the spyder itself, and the fuel pressure regulator.

If you really wanted to make sure that pulling the intake off is the right place to go, I think you can rent a fuel pressure gauge from Autozone or Advanced Auto (or just buy one) and see if the fuel pressure slowly (or quickly) drops after the key is shut off, you know you have a leak. Now it is possible that this is caused by a leaking check valve in the fuel pump assembly, but due to the rich condition, I think you'll find that it is in the intake manifold itself.

 

Out here Advanced Auto won't rent a fuel pressure gage, and the guys at autozone didn't know what one was when I asked if they had one. If you've got $50 to burn you can get your own though.

There is a way to test o2 sensors as well as just about every other sensor in your truck with a multimeter and a Haynes or Chilton manual that includes the specs for the sensors. I've got a Haynes manual for my Saturn and it's saved me a few hundred dollars of guessing which part might be bad. I'd test before blindly picking parts to replace other than stuff like plugs, wires, cap and rotor that probabally need to be replaced anyway.

 

How's your gas mileage? The reason I ask is that I was having the same problem with my '96...and it turned out to be the fuel injection assembly (spider). I put in a new one and I'm running like a mad-man!

 

As far as fuel pressure goes. There is no sensor that you can test to with a multimeter. In this case, a pressure gauge is necessary. So you either drop the $37 at Advanced for a gauge or go to a mechanic to have your fuel pressure tested.

 

Sure fuel pressure will need a gage, I was referring to sensors like the MAP sensor or CPS sensor, o2 sensors, TPS sensor or anything else that has wires coming out of it. There should be a voltage drop or resistance that can be measured for every sensor to confirm its proper fuction. Of course you can't use a multimeter to test your tire pressure or brake rotors, you need the appropriate gages for those jobs. I was really trying to suggest that the OP test the o2 sensors before replacing them blindly.