so my brothers 97 blazer check engine light came on and had code p0300 and ran ok, but slightly rough. has 117,000 miles. original everything. so i did plugs, cap, rotor. cleared the code and it ran perfect, for 5 miles. then making a turn it clicked and started running rough, now it barely runs, and the codes in it are
p0300- multiple misfire
p0108 - map sensor high voltage

i hooked up a fuel pressure gauge to the port on the back of the manifold. got 55 psi and then when it started running really really rough it jumped to 62 psi

installed all the old parts i took off and that didnt change anything.

nothing is blocking the port on the map sensor

i tested the map sensor voltage and got 5.02v (iirc, it was like 2 weeks ago)

all i can think of now is pulling the manifold and checking out the injectors.

anyone have any suggestions for me??

it will run ok for like 15 seconds if it sits for a few days, but still has a miss to it. and then after 15 seconds it starts running terrible and knocking a bit

 

Check fuel pressure and leakdown as follows: Key on, engine off, fuel pump running, pressure must be 60psi to 66psi. Pressure must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results.

 

i had the key on engine off for 10-15 minutes with the guage hooked up after it started running like ****. it stayed at 62 psi the whole time

 

The engine should not be running at all to check fuel pressure or leakdown.

When you initially turn the ignition to the RUN position, (don't start the engine) the pump will activate for ~ 2 seconds and shut off. Check the fuel pressure when the pump is running, (this is regulated fuel pressure). Turn the ignition to the OFF position. Wait 10 minutes, and without touching the ignition, read the pressure on the tester, (ignition must remain OFF during the 10 minutes). Post both the regulated pressure and the pressure after 10 minutes.

 

it went to ~58 psi and stayed there for 10 min

 

In post #3 you had 62psi... how did you get that reading? 62psi is acceptable. Now you get 58psi, which is unacceptable.

 

with the engine off it stays in the high 50's

when the car sits for a few days it runs kinda ok for 15 seconds, then starts to run really bad.

for the 15 or so seconds its running ok the fuel pressure stays essentially the same as what it is when you initially turn the key to on and it pressurizes. 55-58 or so. once it starts to run really really rough the pressure jumps to low 60's

 

Engine running fuel pressure means absolutely nothing with this system. Both regulated fuel pressure and leakdown pressure readings are taken with the engine off.

Key on, engine off, fuel pump running, regulated pressure needs to be 60psi to 66psi. The highest possible pressure reading at the service port is achieved with the pump running and the engine off.

All we need to know is fuel pressure while the pump is running, (engine off) and 10 minutes after the pump shuts off.

 

i did that and it was in the high 50's. never made it to 60, but it didnt drop at all

 

Next step is check maximum pump output pressure and leakdown at the fuel filter. This will tell if the pump is able to supply adequate pressure, and hold it. If it fails one or both tests, the pump needs replacement. If it passes both tests, the most likely problem is the fuel pressure regulator.

You'll need one of these: Dorman - OE Solutions Steel Body Fuel Line Connector 3/8 Inch 800-121: You= This will connect to the fuel filter, then use fuel injection rated rubber line and connect it to a gauge like this: Fuelab 71501 Fuel Pressure Gauge, 0-120 PSI - Speedway Motors, America's Oldest Speed Shop

All fuel pressure and flow must end at the gauge, with no leaks. Make sure the fuel filter is not restricted. Activate the pump and the pressure must be 73psi to 108psi, while the pump is running. It must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results.