I have a 99' 4.3 that has a code PO300. A few weekd ago I was driving at 55 mph. I went to pass another vehicle and shen I came out of the passing gear my engine began to stumble and had had very low power. I limped home. I drove it the following week in which in the mornings it would stumble slightly and have hesitation on ecceleration unless I was heavy on the accelerator then it would do fine, except the check engine light would come on. By the afternoon the stumble and hesitation would disappear on acceleration and the light would go off, untill the next morning and it would start all over again. I had a fuel gauge problem before this which was it was not giving true readings. It would bounce from full to 1/4 full while I was driving around. I replaced the fuel filter and the pump due to the sending unit not functioning. Then I replaced the plugs , wires, ditributor cap, rotor, air filter and cleaned the MAF. I still have the same code coming up, light coming on with the same symptoms in the morning only to disappear in the afternoon. What would be the next course of action I should now take to remedy this PO300 code?

 

I have a 99 blazer and it would "stumble" to. And like you by afternoon it was ok. I took it to a mechanic who told me it was the catalyst converter and an O2 sensor. Without fixing these I could not pass inspection, since it had some code, not sure which though. Hopefully this helps somewhat.

My check engine light would come on to then turn off also.

 

My cat is only 6 months old but I am not sure about the o2. I guess I will have these tested. Thanks for the help.

 

My mom bought a 95 wouldn't pass emissions. I can't remeber now if it had an engine light on or not.
But it did have a miss. The previous owners had put a new cat on it, can't remeber what else and sold it.
I did plugs, wires, cap/rotor, front O2 sensor, air filter, pcv valve. Still had a miss.
No check engine light.
Took off the upper intake tried to start it. didn't start but allowed me to fine the return line leaking in the intake. New supply and return line and it runs fine now.
The injector lines are also known to leak as they are "plastic" if you overheat or just with age they deteriorate.

 

The 95 is a different design. The 99 doesnt have plastic fuel lines in the intake. But it is probably a leaking fuel pressure regulator or spider injector. They are sold as 1 unit and would recommend replacing the whole thing. It usually throws a missfire code, an O2 too rich code, and possibly catalyst below efficiency. Because its running WAY too rich.

 

I just finished replacing the fuel regulator and TPS and now it runs really poor. I am going to replace the injector unit tomorrow. I hope this works.

 

Some aftermarket regulators are adjustable. You need to have a pressure gauge on it and adjust it accordingly

 

What is the difference between a CSEFI unit and a CMFI? Are they interchangable? The auto parts house has the CMFI only and my blazer has the CSEFI.
How do you adjust the fuel regulator using a pressure guage?

 

The CMFI is the original fuel injector design that really has problems, you do not want it. Yes it would likely fit and work for awhile...but get the CSEFI unit. Now I may have this backwards...but I believe the CSEFI is the improved design....just am not 100% sure of my facts this morning without any coffee!

Before you replace it, remove the air plenumn and then apply power to the fuel pump lead and see if you can find a leak. The fuel pump lead is an unconnected orange wire with a connector somewhere above the left fender wheel. Just apply +12 volts to it and the fuel pump will run that way you can check for leaks. Do not start the engine with the air plenum off.

 

FPR is not adjustable on a 99 4.3L, nor do i know of any aftermarket unit for these trucks that is... not sure where that came from.

The MPFI is an upgraded version of the SCPI spider that is on these engines. It uses six individual multec 2 injectors instead of the troublesome poppet ends. GM made the switch in 2002, now most parts store / dealers will not even sell you a SCPI setup, they only carry the MPFI spiders. Buy it with confidence, ive used them on both my trucks.

EDIT: not sure where you got the acronyms your using, but with 4.3L's and fuel injection it can only be one of these options...

TBI = Throttle Body Injection

CPI = Central Port Injection

SCPI = Sequential Central Port Injection

MPFI = Multiport Fuel Injection

Not trying to be a jerk just trying to avoid confusion