Engine miss fire at idle
 

Hello, new to this site. Hope i can get some help with at problem Im having.

The truck is a 1995 GMC sonoma. It has 104000 miles on it 4.3 motor. Recently it has started to run rough at idle. I did the basic tune up hoping it would solve the problem, plugs, wires, etc... Still has a rough idle problem. (When i pulled the plugs they were black and smelled of gas.) I also cleaned the throttle position sensor. The truck still ran poor.


I next moved on to the fuel pressure regulator, and CPI. The only wash out I could see was under the regulator, and when you keyed on the engine, there was an excessive amount of fuel coming from the bottom of the regulator. And when I took off the intake plenum, intake gasket was broken between the ports in the front of the motor. Hoping this was the problem, replaced the regulator, (no more fuel leak after i keyed on the motor) and the gasket.

The truck still is running rough at idle up to about 1800 rpm. above that it runs good. I then read somewhere that the if you used a different brand of plug than ACdelco then they wouldn't work. I had used autolite plugs the first time. I then changed them to copper core acdelco plugs,(when i pulled the autolites they were black and smelled of fuel) gaped to .045, fired up the truck and it idled really well for about 10 seconds and started to miss again.

I'm at a loss as to what to try next, any help would be great.

Thank you
Jon

Check fuel pressure and leakdown at the service port next to the distributor. Most auto parts stores rent/loan the tester for free. They also sell them for ~$40. When you initially turn the ignition to the run position, (don't start the engine) the fuel pump is activated for ~2 seconds and then shuts off. The pressure must tested while the pump is activated. Key on, engine off, fuel pump running, pressure must be 55psi to 61psi. It must remain above 50psi for at least 10 minutes after the pump shuts off. Post your readings.

What cap said... also.... Check and clean the EGR valve.

okay, I will try this next

I just got done testing it, after ten minutes the pressure dropped to 47psi

Check post #2 again. What is the fuel pressure with the pump running, engine off?

After initial priming the fuel pressure is at 57 psi.

Regulated pressure at 57psi is fine. Leakdown to 47psi is slightly excessive, and technically it fails, (shouldn't drop below 50psi). Any one or more of several things can cause that. To tell for sure if the problem is in the plenum or in the tank, you need to check fuel pressure and leakdown at the outlet of the fuel filter. All pressure and flow must end at the tester, (no fuel allowed to the engine).

If the plugs were fuel fouled/wet, chances are that the leak is in the plenum: leaky injector, leaky spider line, leaky poppet valve, leaky fuel lines, leaky regulator.

I'm kind of leaning to the spider as the problem. I agree if the plugs are fouled with in the time of installing them and only starting the truck a few times, it would make sense that the poppets are leaking.

I will test the fuel pressure at the filter to be sure though.

If pressure holds above 50psi at the pump, the leak is in the plenum. If not, the leak is in the tank.