1999 GMC Jimmy 4.3 2wd automatic 214,000 miles.
It would not start when hot or cold without using starting fluid. It would accelerate if using light throttle pressure but if you try to accelerate a little more agressivly it would bog down and loose RPMs.

I replaced the fuel filter and had 50 psi at the outlet. Then with the engine running had 50-52 psi at the test port. While driving the pressure stayed the same and did not drop when the engine bogs down.

I cleaned the Mass airflow sensor also. Now it usually will start without starting fluid. Fuel pump was replaced but not sure how long ago. I believe fuel pressure spec is 60 psi. However a few years ago my daughters blazer had a fuel pump go bad and pressure was about 50 psi. A new pump fixed it but the pressure with the new pump was 5 psi lower than the old pump??? Since the pressure does not drop when the engine bogs is it probable that the fuel pump is ok.

I checked the frequency at the MAF sensor. It did rise as the rpms went up. There was a little fluctuation but not much.
Is it worth testing the MAP sensor?

Any help would be appreciated.

 

A quick and easy way to test the MAF sensor is just by unplugging it. If it does run better when it is unplugged then you need a new sensor. I tried cleaning my brothers but that didn't help, we had to buy a brand new one. My brothers blazer was having similar issues when it was cold, also does the truck have a check engine light on?

 

Thanks. I did unplug the MAF. It seemed a bit more responsive on off idle acceleration but the bogging was still there. I forgot to mention that the check engine light is not on.

In the past it had bogging/driveability issues and unplugging the MAF cured it as a test. Then cleaning the MAF fixed it.

 

Without a doubt you have a fuel delivery problem, and you're just about done diagnosing the problem.

If your pressure check at the filter was done by ending all fuel flow at the pressure tester, your maximum pump output pressure is about 1/2 of what it needs to be. Must be 73psi to 108psi.

Key on, engine off, fuel pump running, the pressure reading at the service port must be 60psi to 66psi. It must remain above 55psi for at least 10 minutes after the pump shuts off.

Next step is unplug the module connector. When you initially turn the ignition to the RUN position, the relay will activate the pump circuit for ~2 seconds and shut off. This is commonly referred to as "pump prime". The reading on the grey wire during pump prime must show battery voltage. Next check resistance on both black wires at the connector. Must be less than 5 ohms to ground on each wire. If the electrical checks ok, the pump needs replacement.

 

Yes pressure reading at the filter outlet went straight to the gauge.
Pressure was the samje at the service port with engine running or not. If this was a fuel problem shouldn't I see a pressure drop when the engine bogs as if it is not getting enough fuel? Are you talking about the module connector at the fuel pump? If so can I access it without droppingthe tank?

I know it is supposed to have 60 psi + at the test port but I do not understand why (see above post) my daughters blazer had about 45 psi after a new pump was installed and it ran fine for years.

 

Chances are the pump is bad, you just need to make sure it's getting voltage and ground at the fuel pump module connector, and yes, probably will involve dropping the tank to get at it, unless you have real skinny hands, and long skinny arms. Look at the bright side, tank has to come down to replace the pump anyway

As for not noticing a drop when it bogs: Your max output is 50psi, we already know that from your test. That pressure reading is a "dead head" reading. Which means there is a maximum resistance load on the output of the pump. If the pump is working properly, dead head pressure regulates somewhere between 73psi and 108psi by means of an internal bypass in the pump. 50psi isn't enough to operate the bypass valve. What all this means is that the fuel pressure regulator is not regulating either. It should bypass anything above whatever it's set for, (60psi to 66psi). All available fuel pressure is being used to make, or try to make, the engine run. When the engine is running, the load on the pump is reduced from the injectors opening and when the engine bogs down, the PCM decreases the "open time" for the injectors, pressure builds, injectors open, and the cycle continues. Basically the PCM is reacting to the insufficient fuel and in a sense, acting as a fuel pressure regulator.

As for your daughters 45psi.... If you have the original injectors, (1996 or newer) each injector has a "spider line" from the injector to a poppet valve at the nozzle. When the injector opens, it pressurizes the spider line, if pressure in the line is below ~43psi, the poppet will not squirt fuel. At 45psi, I can gaurantee the engine will not start on its own. A shot of carb spray etc would be necessary to get it running, and even that's questionable at 45psi. If this engine starts on it own, you need a new pressure tester, it's not accurate

Edit: If her Blazer is a 1995 or older, it has a different fuel delivery system with different specs.

 

Fuel pressure regulator in the intake manafold try that

 

The pump output isn't enough to check the regulator, must be at least 66psi.