I have a 1991 Camaro RS with the 5.0 TBI. I bought the car, and due to a problem with the steering column, it sat for about 16 months until I could get time to fix it, which I recently did. During the down time, the battery totally died, so I replaced it. When I tried to start it, it backfired, stumbled, and refused to stay running. In the course of checking things out, I found that the distributor had a bad shaft bearing, so I put a new distributor in. I have also found out that if I unplug the MAP sensor and the TPS, the car will run perfectly at ~ 1,500 RPM, but the throttle has no effect other than to allow too much air into the engine (no extra fuel injected to account for throttle position change). If I plug in the TPS and the MAP, the car will start, but immediately miss and die unless I pump the throttle rapidly to barely keep it running. The TPS voltage changes with throttle movement, and there are good voltage readings at the MAP. I'm thinking the ECM has gone south. Any guesses?

 

You likely have a problem with the MAP. Of course, the hose connecting the sensor to the intake manifold needs to be intact, free from cracks, leaks, splits, or kinks. The electrical connector needs to be clean and tightly seated.

The MAP sensor can be checked with a vacuum source and a digital voltmeter. The sensor should output a 0-5VDC signal at the 'B' terminal. The output voltage is based on absolute pressure at the hose connection. The sensor should output full voltage (5.0VDC) when there is one BAR (barometer, or atmosphere) of pressure applied. This means there is basically no vacuum (0" Hg) and the Absolute Pressure is at 14.7 PSIA.

When the Absolute Pressure decreases (due to vacuum) the voltage output on the 'B' terminal should decrease as well. Test the sensor down to about 20" Hg of vacuum, or 0.33 BAR (4.9 PSIA), where the output voltage should be aroud 0.8-1.2 VDC. The voltage should vary in direct proportion to the Absolute Pressure, or amount of vacuum applied. If the MAP output "jumps" or "sticks" at a given point, the sensor bridge circuit or the mechanical diaphragm is failing. Since there is no good way to repair a MAP sensor, replacement would be indicated.



Compare the voltage readings to the table:

 

Doesn't look like my first attempt at replying worked

The MAP voltages were exactly what your table shows with no snags or stalls in travel. The vacuum line is in perfect condition. Also, the TPS voltages are right on with what I've seen posted elsewhere. Like I said before, I have to unplug both of these sensors to get the car started. As soon as I plug one in, the car acts like someone shut off the gas, and it won't stay running.

Any other leads? The MAP was a good one, but a dead end one.

Thanks,

Larry

 

Shoot, forgot added info.........I replaced the distributor because the old one was bad. Also replaced the plugs, wires, dist. cap and rotor.

 

Have you tested the MAT and coolant temperature sensors? A CTS which is just barely within range will not set a DTC but really screw over the fuel mixture, especially on a S/D system.

I'm just guessin now, but these are easy and cost-free tests that will verify an important sensor's operation.

 

Vader, I checked the CTS at our ambient air temperature of 95 degrees (isn't Las Vegas wonderful!), and it read 2.8 volts. The MAT reads 4.8 volts, again in a static environment since I can't keep the darn thing running. Do you have any numbers for voltages for these sensors? The car won't throw any codes when I start it and try to keep it running, and it still backfires with the sensors connected. Do you think I should be checking voltages @ the ECM? How about O2 sensors in this mix? I <briefly> had throttle control when I unplugged the CTS -- for about 20 seconds, then it went back to not running and backfiring. Have looked at EVERY vacuum connection several times, and have found no leaks or broken wires.

 

Slider,

I don't know the internal resistance of the ECM inputs, so I can't do the conversions. I only know the voltage and resistance of the sensors. I'd suggest checking the resistance and comparing them to the table:



Yeah, Las Vegas must be nasty. It was only 94° here yesterday, but we did have 82% humidity. That was great until the tornadoes moved through...

 

vader did you take that pic or is it just somethin ya found cause that looks f'in crazy