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Q: fuel pressure and map hose

asked by on

i have a 1993 cheyenne v8 tbi, and only runs good at part thorttle with the map hose disconnected, as soon i go full thorttle with it, starts stalling... if i put the hose on the map sensor and restart, the truck start poor and as soon i put a gear on it, the truck stops.

the mechanic said can be the pressure regulator, because the fuel pressure goes up when they plug the fuel return pipe

My car has 155000 miles.
My car has an automatic transmission.

A: Hello. The first thing that should be done ...

Hello. The first thing that should be done is to retrieve codes. This is an OBD I system, so it is more difficult than the newer OBD II systems. Lacking any codes or scan tool data, I will have to explain a few details about the MAP (manifold absolute pressure) sensor.

Disconnecting the MAP sensor hose will result in the PCM believing the intake is in a high pressure condition. This would be WOT (wide open throttle). Vacuum drops and pressure rises in the intake when the throttle is opened excessively. When the PCM sees this, it will add as much fuel as it can assuming you want to accelerate. So with the MAP hose disconnected the computer will be adding fuel. It's important to note, the TPS (throttle position sensor) is used to confirm the throttle has indeed been opened. If the PCM sees a low vacuum condition via the map and the throttle is not opened beyond idle, it knows something else is wrong. Such as a vacuum leak or a fuel supply issue of some sort.

The question is, do you have vacuum leak or a lack of fuel? Plugging the fuel return pipe will indeed increase fuel pressure, and I would expect the motor to run better. But this doesn't necessarily mean the fuel pressure regulator is the problem. A weak fuel pump can also do this when the return hose is plugged. Disconnecting the vacuum hose to the fuel pressure regulator is a better test, but not the last test in determining what is happening. You should see a 4 - 10 psi increase in fuel pressure when you disconnect the vacuum hose at the regulator. Normal fuel pressure with the motor off and the key on should be 9-13 psi.

Checking for a vacuum leak is done by listening for hissing noises, which is the easiest method, using a smoke machine which is the most accurate method or using a chemical spray, sprayed lightly in suspected vacuum leak areas. If when you spray in an area the motor responds with an increase idle, you may have vacuum leak in that area. Be careful the over spray doesn't go into the throttle body. This can be confusing when using this method.

A few things to note, check the fuel filter for clogging, make sure the hose to the MAP is not cracked, plugged or a source of a vacuum leak. Check to see if the TPS is working and check the camshaft timing. The easiest way to do this is by taking compression tests of each cylinder. If one or more is significantly lower than the rest, camshaft timing should be suspected.

If you need help a certified technician from YourMechanic can come to your car's location to verify the cause of the loss of power and perform any needed repairs.

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