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.

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. fuel pressure and map hose

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car starts and turn back off immediately

Hi. This car will not stay running if the anti theft light is blinking. This means that the car has shut down the fuel to the fuel injectors so the car does not run preventing someone from stealing it if...

Q: Black smoke and rich fuel the codes P0191 & P0452 and short trim bank 1 & 2. Both at -28.9

Hello. You have 2 separate problems here. The P0191 (fuel rail pressure sensor circuit problem) may just be a symptom - it is reporting suspect fuel pressure on the rail. The fuel pressure on the rail is controlled by the...

Q: Motor idles fine, when you give it gas it bogged down, already changed the plugs,wires,distributor cap and rotor

Hi there, thanks for writing in about your Chevrolet C1500. It's highly likely the mass airflow sensor is bad. To verify, a scan tool should be connected and observe voltage changes in the mass airflow sensor readings. Same for oxygen...

Related articles

How Long Does a Distributor O Ring Last?
The distributor is part of the ignition system in your vehicle and its purpose is to route high voltage from the ignition coil to the spark plug. The spark plug then...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.