* Moved from Engine/Transmission Forum*

My 1996 Blazer (2dr, 4.3 V6, 4x4, Automatic) runs rough at lower RPMs (from about 1800-2300) if it's been sitting for more than a day. The longer it sits, the worse it is, and after being driven for a while the issue becomes less noticeable, and will completely go away after an hour or so of driving. I occasionally get a P0300 Random Misfire code.

So far I've tried:

Fuel Injector Cleaner
New Cap/Rotor/Plugs/Wires
EGR Valve
Sea Foam
Gas line water remover
New Fuel Filter

To me it sounds like it could be clogged poppet valves, and the reason the problem goes away with some driving is the valves unclogging as more fuel goes through them. If so, what all do I need? I assume one of these: http://www.amazon.com/Delphi-FJ10565...dp/B001J63ILW/ and the upper plenum gaskets. Anything else?

 

Well, you didn't actually move the thread since you can't, only staff members can do that. You wound up creating a double post, so I deleted your original thread.

I would check the fuel system to see if there are any pressure irregularities showing up that might lead you to a solution. 96+ Fuel System Diagnosis

 

Yeah, sorry about that. I apparently can't delete a post either, that's why I reported the other one. I'll pick up a fuel pressure gauge from the auto parts store and let you know what I find out. Thanks a lot!

 

OK, I have a fuel pressure gauge now, but I have a few questions about doing the test:

1.Should I let the car sit a while before connecting the gauge to the test valve? Seems like a good idea to keep gas from spraying out and making a mess/fire.

2.The gauge's owner's manual says to use teflon tape on the connections. I assume that meant the connection between the gauge and the hose that screws into it. Should teflon tape also be used on the test valve on the car?

3.Also, I don't have a scan tool that can toggle a fuel pump on and off, is it OK to just do that by turning the ignition switch on and off?

 

1. Not necessary. Just wrap a shop towel or two around the schraeder valve area to catch any spray.

2. Not necessary.

3. You can.

 

i had the exact same problem and i replaced the fuel injectors after having it checked and it has been a year and no problems since.

 

injectors rarely go bad.

 

I agree, I can't tell you the last injector I saw go bad, it's the poppets that get clogged. The CSFI to MFI conversion relocates the injectors to the intake port and does away with the poppet valves completely. Basically upgrades the engine from a "W" code to an "X". Huge improvement in all around performance.

 

OK, test results are in:

Key on, Engine off the gauge read 61 PSI, after the pump turned off this settled down to 60 PSI. After waiting 10 minutes it had fallen to 57 PSI.

Bled down to 12 PSI, after waiting 10 minutes it was still at 12 PSI (and may have increased slightly).

Started engine, at idle the gauge was reading 54 PSI. Throttling hard and suddenly (as per step 6 on the diagnostic chart) the gauge read 62 PSI. While the engine was running rough, the gauge wasn't jumping around, it was staying constant at 55 PSI. After turning the engine off, the pressure was at 51 PSI.

Those pressures seem slightly low, but they were consistent, and I'm not sure how well-calibrated the Harbor Freight gauge is.

 

KOEO at 61psi is acceptable, holding at 57psi after 10 minutes is fine. The drop to 12psi is most likely the check valve in the fuel pump not sealing properly. The pump must be isolated from the system after pressurizing to determine exactly what is causing the leakdown. The leakdown is probably NOT causing your driveability problem. If you're using an aftermarket distributor cap, that would be something to check along with restricted distributor vents, worn distributor gear and/or bushings and incorrect camshaft retard.