#4 plug fouling / very rich

[elpiar]

I observed weird behavior with my rough idle having to do with #4 cylinder.

My #4 plug is always very rich / black all the way to the ground strap and keep fouling. If I unplug the wires during the rough idle, nothing changed. If I unplug #4 injector harness, the idle becomes smooth?

Help me to troubleshoot this, please.

I got new coils, plug wires, and spark plugs, I checked that I got very good spark by connecting a spark plug to the wires while the engine is running. Compression is good.

[480sx]

You need to replace number 4 injector.

[C-Kwik]

You definitely have some kind of a leak. A bad injector O-ring can have similar effects.

[elpiar]

DOes this explain why i have trouble starting when it's warmed up (or after short use)?

[ghx407]

You definetly have a bad o-ring or a stuck-open injector. It gets hard to start shortly after use because the #4 cylinder is already flooded, and gets even worse when the fuel pump pressurizes the fuel lines. If you unplug that injector before you start your car, it'll start easier. Fix this as fast as you can because all the excess fuel might trickle down past the piston rings, thinning your oil and cylinder wall lubrication. Changing your oil is not a bad idea once you get the leak fixed.

[elpiar]

If I unplug the #4 injector, the idle becomes smooth, so maybe it's leaking inside the injector instead of the O-ring?

I just need to know whether I need to just buy an o-ring or replace the injector altogether.

[ghx407]

It most probably is the o-ring. If the injector was leaking from the inside, or stuck open, there would be no difference in the idle when you unplugged it. The idle improves when you unplug the injector because there is less flooding since the injector remains closed.

[elpiar]

It most probably is the o-ring. If the injector was leaking from the inside, or stuck open, there would be no difference in the idle when you unplugged it. The idle improves when you unplug the injector because there is less flooding since the injector remains closed.

I thought if the o-ring is shot, then fuel will flood to the manifold regardless whether I pull out the injectors or not, because it's not leaking through the injector? Can you help explain with simple diagrams?

[WDRacing]

Just pull the injector, swap the orings and then we won't have to speculate.

[ghx407]

Just pull the injector, swap the orings and then we won't have to speculate.

I agree. Make sure that the o-ring seating surfaces on the injector and rail are flat and smooth; and use some lube.

[elpiar]

I'll do that sometime this week. thanks for the suggestions

[480sx]

IMO its easier and safer to take the whole fuel rail off and find out whats going on. If you take the fuel rail off, put down like 10 paper towels under the injectors and prime the fuel pump. See if its leaking fuel. If its not, you need a new injector.

The other idea here is to get a fuel pressure gage, and prime your fuel pump. Watch your gage and check to see if it drops reasonably fast. Your FP should stay within +-5 for 5 min.

I thought if the o-ring is shot, then fuel will flood to the manifold regardless whether I pull out the injectors or not, because it's not leaking through the injector? Can you help explain with simple diagrams?

Not exactly correct. When you pull the injector clip off and your idle stablizes, that could mean that the ammount of fuel leaking out of the rail, or the injector is just enough to let the car idle well. When you plug it back in, the car wants to run really rich because its now spraying fuel, AND leaking into the cyl.

[elpiar]

Ok. UPDATE:

I bought the injector o-rings, pull them off the rail, (such a PITA, how do you guys did it?). Before the o-ring replacement, I prime the pump and yes, it's dripping fuel considerably. After the o-ring replacement, the drip stops. But now it's actually worse. My idle is much rougher, and during cruise and acceleration it's very rich. After warmed up, my AFR is 10.

Two questions:

1. When you install the injectors, there is no "snap" feeling right? They are just seated in the rail and tighten using the caps?2. The cap has this little metal washer on top. What does it do?3. Is my injector stuck open? How come it's worse now than before?How do you test if my injector is stuck open? I tested the resistance and it's 11 ohms.

[WDRacing]

If the injector clicks when the motor is running then it's opening and closing like normal. Did you use alot of vaseline or petrolatum on the install? I got rid of my sidefeeds because I think the entire design is garbage. The Project Silvia top feed fuel rail is under 30 bucks...I'd buy it and upgrade injectors. But that's just me.

WD

[elpiar]

If I do convert to top feed which injector do you suggest (one that is compatible with that fuel rail)

[WDRacing]

If I do convert to top feed which injector do you suggest (one that is compatible with that fuel rail)

Well, the injector size depends on how much boost you want to run and your power goals. The 42lb injectors you inquired about will be good for beyond 15 psi without hitting 100% duty cycle. I had the same size injector on my S14 and ran 15 psi as a daily low boost setting.

With new injectors and the fuel rail you'll need an aftermarket fuel pressure regulator and injector clips for the new top feeds. Bosche makes a vacuum referenced fully adjustable FPR for under $60. Check http://www.summitracing.com for the clips and the FPR.

WD