I have a 1995 Z28 I bought a few weeks ago. The car is running rich and the idle gets rough as it get to operating tempature. Also once it is to operating tempature if I shut it off it will not start unless I wait 5 minutes. I used freescan to check numbers and it appears that the tempature reading the computer is gettingmatches up with the gauge in the dash, so I guess it's not a bad tempature sensor. Wanted to know what the readings are suppose to be for the oxygen sensors? Also what should it be for the map sensor? Any help would be appreciated since I am just trying to learn about these cars. Oh, also I don't know how long you need to let freescan run but I let it go for like 15-20 mins and didn't get any codes.

 

Do you have any codes from the pcm? A SES light?

For a good expalnation of what the readings mean and what they should be, go to Injuneer's web page: http://www.injuneer.com/ScanMast.html


Runing rich can be caused by a lot of issues. The first thing to look for is an exhaust leak near the sensors. If you don't find anything, then try disconnecting an O2 sensor and see if it leans out and runs better. You might also wan to check the fuel pressure and the injectors themselves for leaky operation...

 

No, the SES light is not on when running. It comes on when I turn the key on then goes right back off.

 

First thing I would look at when running rich is the air filter. Then go from there. You need to be sure the scanning equipment you are using can recognize a fault. Force a fault and see if the scanner picks it up. for example, a good thing to do would be unplug an O2 and let it warm up. Sometime later it will throw a code. But in the mean time when it gets warm it will stay in open loop. See if it runs better while you're wating for the code to show up. If it runs better, it means you have a closed loop control issue.

 

Not a bad thought, i guess, but not a significant issue with the advent of mass air flow fuel injection. The pcm should only be fueling for the air it gets and fueling that correctly.

A clogged air filter can certainly be a cause of an engine being down on power...

 

Well it will only swing so far in either pulling or adding fuel. One thing that some folks do is cut the bottom of the air box out. Then the filter can get soaked. The PCM is not going to be able to compensate for a filter that is soaked. Not to mention a lot of folks don't check the filter near as often as they should.

 

I don't think we're saying the same thing. The PCM will only fuel the air it thinks it's getting. If the filter is blocked, then far less air gets through and the pcm only adds fuel to the air that does. It shouldn't add more than it needs in this scenario....and there is no 'swing' in this case. It's not trying to compensate for a lean or rich reading, it's fueling for less air. Either way, it's no big deal to check and clean/replace the filter

 

Tell you what wet your air filter and see how it runs.

 

OK, put that isn't the point....amd it won't run richer or leaner if I do.

This isn't like an unmetered leak...the PCM will apply 1 pound of fuel for every 14,7 pounds of air....it just won't be getting enough air.

 

Ok you two, it's not the filter. I just wanted to know what readings I should be getting from the o2 sensors.

 

Did you read the link that bw hunter posted? That tells you what many of your sensors should be reading, including the O2 sensors.

Did you just let the engine run in the driveway for 15-20 minutes? That won't uncover some the possible active codes, because the engine has to be operating under load/real world driving conditions for some of the diagnostics to run. If you have a recent SES light, or a recent code (not all codes turn on the SES light), they should be "stored" adn should appear when the scanner connects.

 

I don't know a lot, but seems like there would be a minimal amount of fuel that could be sent to the motor, which is what speedy is saying I think. If your adding the less amount of air to the motor than the fuel can compensate for then your AFR will drop from the 14.7 to a lower number, which in turn would be a rich mixture. Although, seems to me if it were the filter then it would run like crap all of the time.

My car was doing the same thing and turned out to be a leaking exhaust manifold, that caused cat to stop up also. Does it sound like it has a leak?

 

I don't hear any exhaust leaks from the headers.

 

The MAF sensor reads the mass air flow, the PCM divides that number by 14.7 to calculate the mass fuel flow. Then it sets the injector pulse width to deliver that amount of fuel. Where you get into trouble is at idle, where a very low air flow may require a smaller pulse width than the PCM is programmed to deliver. In that case, its going to over fuel the engine, run rich and probably tend to stall.

Under load, since the dirty air filter is producing significant pressure loss, the throttle blades need to be held open further to compensate for the lower air pressure. Now you've screwed up any calculation the PCM makes that is based on TPS. Not going to be a dramatic difference, but its not going to be running the same as it was.

 

Do you have any MAF readings? Correct me if I'm wrong but it think it should be between 6-9 kgps or gps, Injuneer will know but I can't remember the units. That could eliminate the filter being the problem......