1993 Pontiac Firebird PLEASE Help

  • 5.7L
  • V8
  • RWD
  • 153 MILES
I have a highly modified trans am, it has an LT1 v8, cammed, and beefed up internals, not exactly sure what all it has, I'm having an idle issue. It will not idle with the MAP sensor plugged in, unplug the map sensor and it runs great. They replaced the sensor and it didn't do anything. I don't know what's wrong and I need help.
Do you
have the same problem?
Monday, October 13th, 2014 AT 12:20 PM

1 Reply

Your best bet is to contact the person who performed these modifications to find out if it ever ran right. The days are long gone when we could do things like this. Now, the computers are programmed to know how to calculate fuel metering based on a number of factors. When you change the lift and duration of the camshaft, you change the volume of air as well as how evenly or smoothly it flows. You'll have lower vacuum too and that's what the MAP sensor measures.

I can share some details that might help you come up with a plan for diagnosis. Chrysler is the only manufacturer that has been able to make an engine run right with just a MAP sensor. It has the biggest say in the fuel metering calculations. They are sensitive enough that we could use them to measure engine speed because each gulp of air a piston takes makes a pulse of a little extra vacuum. We don't actually do that, but we cold. Chrysler actually bought their first batch of MAP sensors from GM, and of course they had a huge failure rate. Before the engine is cranked, the MAP sensor's reading represents barometric pressure.

GM and every other manufacturer uses a mass air flow sensor for its main fuel calculation. Be sure there are no leaks or gaps in the tube between that sensor and the throttle body. Any air that sneaks in unmeasured won't get any fuel to go with it. The MAP sensor is still used to measure barometric pressure but on most engines it is used as a backup in case there's a problem with the mass air flow sensor. The engine may not run perfectly, but it will run.

When the Engine Computer detects a problem with a circuit, (not necessarily the sensor in that circuit), it sets a diagnostic fault code. If the problem it detected could adversely affect emissions, it turns on the Check Engine light. Also, since it knows it can't rely on that sensor's readings, it ignores them and "injects" an approximate value to run on.

Let me back up a minute. All sensors that put out a varying signal like this are fed with 5.0 volts and a ground wire that will have 0.2 volts on it. The acceptable range of output signal voltage is between 0.5 and 4.5 volts. (Those numbers are for discussing electrical theory. In practice, no two sensors are alike, and you could find a range of 0.4 to 4.3,or something like that). The point is, it will never hit 0.0 or 5.0 volts. A throttle position sensor has mechanical stops to limit its range to those values. A mass air flow sensor or MAP sensor has its values limited by its internal circuitry. A sensor can fail, but it's much more common to find a wiring problem to be the cause of the fault code. It's when the signal voltage falls outside that 0.5 to 4.5 volt range that a fault code is set.

If you do not have the Check Engine light on and there's no fault code in the Engine Computer, it means the computer is happy with the signal voltages it's seeing from the sensors, but that doesn't mean those voltages are right. As long as the MAP voltage stays within the 0.5 to 4.5 volt range, no fault code will be set. (There are other things that can set other codes for that sensor, but that's not part of my sad story). If your MAP sensor is connected to the intake manifold by a vacuum hose, a lose, dry-rotted, or cracked hose will cause the sensor to see lower than actual vacuum. Low vacuum occurs as the throttle blade is opened, meaning acceleration, and that means the need for more fuel. A vacuum leak anywhere will cause a lean condition because since the air sneaking in wasn't measured, the computer won't command the correct amount of fuel. On Chryslers that vacuum leak results in lower vacuum and more fuel so the air sneaking in gets the extra fuel, but that's because they only use the MAP sensor. Your car uses the mass air flow sensor and that isn't affected by a vacuum leak. What I mean is the sensor and its signal aren't affected by a vacuum leak. Engine performance is.

It's hard to say what your system is doing since it has been modified. On some cars there was a separate MAP sensor that was mounted on the firewall and was not connected to the intake manifold. It measured strictly barometric pressure all the time. I don't know what would happen to engine performance if it was disconnected after the engine started running.

With your MAP sensor, it sounds like the computer is receiving an acceptable signal voltage that may be wrong, and by unplugging it, the computer learns that it can't trust those values so it picks approximate values to run on based on other sensor signals and operating conditions. It still should be using the mass air flow sensor for its fuel calculations, so it sounds like it is ignoring that too.

Start by reading the fault codes. You can do that yourself by going here:


The next step involves connecting a scanner so you can view live data. You'll have to look at the readings from the other sensors to see what is incorrect. Most sensors are "input" sensors that send signals to the computer for calculating fuel needs and ignition timing. Other sensors are "output" sensors or actuators that are run by the computer or tell the computer how the engine performed in response to the things the computer told it to do. Mainly that is the oxygen sensor.
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Monday, October 13th, 2014 AT 7:14 PM

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