Not sure Let's go at it again and this time, let it warm up to operating temp, go rough and pull out the sensor plug. If it smooths out, might be the MAF since that is no longer going in closed loop and just running off the programming in the ECM

 

It tells you that parts directly related to engine operation are fine...and that it's a sensor problem causing your issue. Your car operates in "open loop" until hot enough for O2 sensor feedback. During open loop, tables provide information on how to run...which is what you are seeing when you unplug something like the MAF to force the ECM to error AND STAY in open loop.

In closed loop, the ECM uses input from the MAF, O2 sensor, temp sensor, and MAP sensor to determine how well that table data is "set". IOW, it makes on-the-fly adjustments to run even better. When a sensor is "bad", it could be commanding the engine to run way too rich or way too lean. If the temp sensor thinks it's still cold, it will add fuel. If the MAF is reading less air (than actual), it might not be adding enough fuel...since it doesn't know how much air is really going in. If the O2 sensor is bad, it usually (reads low and) thinks it's lean and adds too much fuel.

When the ECM finally "notices" a sensor is bad, a fault is set/stored. The computer often goes into error mode -- ignoring sensor input where (as stated), it runs on pre-determined "table" data...which is good enough to keep it running...and even run pretty well under most situations. Doesn't mean it's going to burn all fuel efficiently, get great mileage, or be ideal.

 

Does that still apply since he started the car and unplugged it for the test today but the other day, he ran it and it was fine till it went into closed loop? IOW, if he didn't simulate the condition of closed loop, would it matter?

 

Ok,
Thanks so much for the information guys. This is how I understand it.
Disconnecting the MAF will cause the car to run in open loop. This will stop the ECM from checking sensors and will just run off the tables. This should tell me the injectors or other mechanical items are working and most likely points to a sensor that is bad.
Is this correct?

 

This is where I have the issue. You ran the car for 20 miles before it acted up. By then has moved from open loop (cold) to closed loop (hot).

When you did the test, you started it up (presumably cold) and pulled the sensor and it ran rough and then smoothed out. As I am understanding it.

I guess what I am wondering is whether the motor was in closed loop when you did the test since it was in closed loop when you had the issue. Does it matter either way? I'm not sure. I was just trying to duplicate the situation in which it occurred. I have speed density so I wasn't able to see if that matters.

 

Generally if the O2 goes bad, it does not set any code until its really really bad. Gas mileage heading south is the first telltale sign.

When the MAF goes it usually sets one or two codes on the way. Tapping it while the engine is running with a screwdriver sometimes works to see if its messed up, but unplugging it while the engine is in closed loop will make the engine run a little better since that throws it into open loop where it reads tables and the Coolant sensor. Doing this would not eliminate the MAF or O2 as the problem if both were causing issues.

I would guesstimate that your O2 is bad, fairly inexpensive, and its not a hard thing to swap out. The car is probably due one anyway. Change this, clear the codes, and see how the car acts. If it continues then you'll need a Helm Manual and check out the MAF circuit.

 

You are correct, I started the car, let it idle and it was fine, unplugged the MAF and it almost died then returned to idle. Most likely it was open loop the whole time, but not sure why it almost died before it straightened itself out.

 

I just went down, started the car and let it run until it started idling bad. I shut it down, unplugged the MAF and started it back up. The service engine soon light came on and it started running fine. What do you guys think?

 

SES light was because the MAF was not giving any value the ECM understood. Probably the MAF. Know anyone nearby that has a known good one to test

 

Nope. I am going to change the O2 sensor tomorrow night or saturday, then go from there. Next will be the MAF I guess.
Thanks for the insight.

 

Also check your timing and fuel pressure.

To check pressure, hook uptthe gauge, turn your key to run position and don't crank See what pressure you get, walk away and see what happens. It should drop a little Too fast means that you have issues. Tape the gauge to the windshield and do a WOT run in gear on an empty stretch of road. Tells us if the pump still can keep up with the demand

 

Are you sure you didn't get some bad gas?

 

All you did was remove the MAF from operation forcing it into open loop. You did not prove that the MAF was bad.

 

Yeah, I understand that now. Basically, it runs fine in the open loop which if I understand it correctly should point to a bad sensor rather than bad fuel, bad fuel pump, plugged injectors, etc? Right?

 

Correct.

 

All,
It ended up being the MAF. Thanks for all the help.

 

Guys, Just wanted to say that I was able to hook up a Snap-on MT2500 Scanner last night and check for any and all other issues. My car showed nothing wrong with engine sensors and my buddy (mechanic) went through all the screens and even showed me how to use the scanner and how vacuum leaks etc effected it. But, he was not sure if he could read why I have the abs light on and said he would check into that. But man, that is an awesome tool to have. I am so glad that everything seems fine with the motor, but any suggestions as far as reading the ABS problem?

 

ABS requires a different tool to scan it for faults, and good luck finding a replacement part if its one of the relays behind your seat.

 

I found an abs unit from a 1987 C4 on craigslist, but I am not sure if an 87 is compatible with an 89 unit.

 

Also, with the help of you guys, this vette is turning out great. It had the potential to be a problem child.