Can anyone recommend how to fix my problem(s) systematically without replacing stuff unnecessarily?

I want to resolve any serious issues before Tues when I have a long trip planned. My 96 Blazer with 135K seems to be running good, but I'm getting the codes and maybe got used to a rough idle. First I got a P0300 which was on and off a few times. After maybe the third time the P0300 appeared, I also got the P0420 code when my ODB scan tool finally completed the catalyst check. Not long after that, I filled up the tank and the SES light went off. Before filling up again, it went back on with both codes reappearing, first the P0300 and later the P0420. Somewhere along the way, I got a P0101, too, unfortunately I didn't keep track of exactly when. All three codes and the SES light remained on during the next tank fill, but the SES went off after my last fill up. The codes remained. During the last day or so, the SES went on, then off, and now it's on again with the same three codes. My catalytic converter has a rattle.

I edited this post after the SES went off again. I also deleted the codes in case they were history only. I left further questions about setting and clearing codes for another post.

 

Does SES mean the same as MIL referred to in the code diagnostics? Does the order of the codes displayed by my OBD scan tool indicate which came first or are they just in numerical order? I have an Actron CP9125 PocketScan reader.

 

MIL=Malfunction Indicator Light. SES=Service Engine Soon. CEL=Check Engine Light. They all mean the same thing.

DTC's, (Diagnostic Trouble Codes) are displayed in the order they were set. They must be diagnosed and repaired, one at a time, starting with the first one, before moving on to the next DTC. The first DTC can cause subsequent DTC's to set falsely.

P0300 is almost always caused by a fault in secondary ignition, (Plugs, wires, cap, rotor, & ignition coil). Low fuel pressure, worn distributor bushings or drive gear and an incorrect camshaft retard adjustment can also set it.

The P0101 is related to the MAF, (Mass Air Flow) sensor, which can also be set by low fuel pressure.

Might be best to start with a fuel pressure and leakdown test. When you initially turn the ignition to the RUN position, the fuel pump will activate for ~2 seconds and then shut off. Fuel pressure must be checked while the pump is running, and the engine is OFF. Pressure must be 60psi to 66psi. Pressure must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results.

 

Thank you, Captain. This tells me where to start. I'll get the fuel pressure and leakdown tester on my way home from church. Meanwhile I'll be praying for the case where my repairs turn out to be minor.

My scan tool does not report the DTCs in order they were set. I ordered another that will give me "freeze frame data" just in case I need that to troubleshoot the misfire.

 

I did the fuel test. The pressure went up to 60 immediately, then dropped back to a steady 53 for a few seconds. Unfortunately it dropped steadily down to 20 or so in about a minute. Nowhere close to 55 or above for 10 minutes.

What's next? I see from other posts this may be fuel regulator or injectors involved. Hopefully not a new pump.

Curious why no DTC has set after several trips?

 

There's a problem with fuel delivery. 60psi is borderline low and the leakdown is excessive. Next step is determine where the problems are. They could be in the tank, or they could be in the plenum, or one in each location. The outlet line at the fuel filter has a plastic quick connect fitting, remove it from the filter and do some "creative plumbing". You will need to modify the pressure tester so it connects directly to the fuel filter outlet. All pressure and flow must end at the pressure tester. With the tester connected, and the fuel pump activated, pressure must be 73psi to 108psi, and it must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results, they will tell where the problems are.

 

If I have a disconnect it wasn't quick or I didn't find it. I removed the input to the filter and used a connector in the kit to block one end. That allowed me to isolate the pump. I think that is what the Captain ordered. The pressure went over 100 for a second then immediately stopped at 93. For about 5-10 minutes it got down to 80 and held steady there for at least 15 minutes. Good news I hope?

 

Sorry, my mistake, 96 has steel fittings on each end of the filter. You did fine! The maximum pump output pressure and the leakdown rate are both good, which means the pump is good, and the leakdown and low pressure problem is in the plenum. Next step is check the fuel pressure regulator for an internal leak. Connect the tester to the service port and cycle the key a couple of times to bleed the air through. Next, cycle the pump and block off the return line immediately when the pump shuts off. Watch for leakdown on the tester, pressure must remain above 55psi for at least 10 minutes. If leakdown passes with the return line blocked, the regulator is leaking internally and needs to be replaced.

If leakdown still fails, the black plastic plenum must be removed to pinpoint the leak. After the plenum is removed, look for signs of fuel leakage. Usually the area is "washed" to a golden color, the black carbon crud is normal. If there are no visible signs of leakage, the fuel lines must be reconnected to the fuel metering block. The poppets need to be carefully removed, activate the fuel pump and watch for leaks at each poppet. It can also leak at the metering block, or an external leak from the fuel pressure regulator. Post your results.

 

Is there any easy way to block the return? The coupling near the firewall will be difficult and I'm leary of going near the tank because I busted a connector before switching tanks.

 

Follow the lines from the tank forward and look for a flexable line. There's usually a flex line near the firewall where they transition from the body to the engine. You can use hose crimping pliers like these: KD Tools Hose Pinch-Off Pliers 1-1/4 in. OD Capacity - Tools - Mechanics & Auto Tools - Automotive Specialty Tools