Sudden Running Lean Issue

[chaseincats]

Hi guys,

I'm running lean all of a sudden and not sure why.

It is worst when hot-starting with a very lumpy idle that stabilizes after a minute or so but it is weak throughout the rev range (unless flooring the pedal).  The idle stabilizes the longer it sits but is definitely misfiring (or not firing) when hot started.  The idle is very lumpy but also smells very rich.

New plugs with ~2 hours run time on them.  Injectors about 2 years old.  Any idea on the below plug images?

Edited February 1 by chaseincats

[Zed Head]

The hot start issue is very common.  But it usually clears up within a minute and the engine runs normally.  Your plugs are too new to tell much.

[chaseincats]

4 minutes ago, Zed Head said:

The hot start issue is very common.  But it usually clears up within a minute and the engine runs normally.  Your plugs are too new to tell much.

It's weird this would just start all of a sudden though.  The car started running really lean on the freeway out of the blue, I had to get off the freeway and richen it a tooth on the AFM because I was concerned about potential engine damage if I didn't (it read 18 on my AirFuel gauge all of a sudden and started to misfire).

Cylinder 3's spark plug looked very rich compared to the others.

Edited February 2 by chaseincats

[Zed Head]

Do you know what fuel pressure was?  Could be something simple like a clogged fuel pickup tube in the tank.

 

[chaseincats]

1 minute ago, Zed Head said:

Do you know what fuel pressure was?  Could be something simple like a clogged fuel pickup tube in the tank.

 

coincidentally I just got off the phone with oreillys to confirm they had a fuel pressure test kit - will get back to you with an answer on that tonight

[chaseincats]

@Zed Head Fuel pressure looks good - 38 at cranking.

[Yarb]

IMO Install a fuel pressure gauge between the filter and metal fuel line. You can monitor pressure at any given time to give you clues moving forward.

[chaseincats]

17 hours ago, Yarb said:

IMO Install a fuel pressure gauge between the filter and metal fuel line. You can monitor pressure at any given time to give you clues moving forward.

Update: the gauge is saying 32psi idling.  When I tap the gas and hold it, it will go up to 38 but drop back down to 32.  Isn't idle PSI supposed to be 36?

If I unplug the vacuum line to the pressure regulator it will sit at 40psi when idling.  Gas is not coming out of the pressure regulator with the hose unplugged.

Edited February 2 by chaseincats

[Yarb]

These systems are very old and antiquated. IMO the pressure will regulate. As long as you are within the range I wouldn’t worry about it. Did you install the gauge permanently in the system where you can monitor at all stages and heat ranges?

[chaseincats]

1 minute ago, Yarb said:

These systems are very old and antiquated. IMO the pressure will regulate. As long as you are within the range I wouldn’t worry about it. Did you install the gauge permanently in the system where you can monitor at all stages and heat ranges?

I haven't since its just a hose gauge from oreilly's but I did start the car this time and let it run to temperature and watched the gauge as the car warmed up and at a variety of different rpms.  The car never went above 32 when idling at any heat range if that helps.

[Yarb]

The regulator is bypassing the fuel back to the tank. The motor doesn’t control the pressure so to speak. There’s multiple parameters regarding fuel pressure.

[siteunseen]

When I was trying to figure out my lean running '77 I ran a fuel hose off a T to my driver's wiper blade, put the gauge on and zip-tied it to the window wiper and went for a drive. Mine was losing pressure while driving I discovered (couldn't see the engine mounted gauge while driving) so I ask an '80's Nissan Certified mechanic about it. He clamped off the return line which increased the fuel pressure on the rail and it ran much better. Told me to, basically, throw new parts in. Well after a lot of reading on this lovely forum I did the CTS tweak with a potentiometer from Radio Shack, $5 volume knob. Runs great when I drive it. 

This rainy day long read was my "ah ha!' moment.

This was the fix.

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html