Hello everyone, first time poster, long time lurker.

I have a 1999 Ford Ranger 3.0 FFV that is giving me fits.

The problem started on the off ramp for the freeway and the engine just started shaking at idle. Like a switch was flipped. I really thought it was going to die at idle but it continued to function and a few miles later the problem dropped off completely. Over the next week it would come and go. And I mean when it happened it was instant like a switch was flipped.

Now its a permanent problem and wont go away. Each time I drive it, I end up with a P0171 P0174 lean on both sides. It has run worse and worse since the initial problem. When it first started it had power at high RPM but now its really bogging. Ive never had a fuel pressure issue that only happened at low rpms. Fuel Filter is 5000 miles old and so are the plugs and wires.

I can pull the wires at the coil and number 3 and 4 have no effect when removed. So I put a coil in it thinking the 3 / 4 was out. No change. This past weekend I switched the plugs and wires around and I see that it must not be the plugs or wires because the 3 / 4 issue sticks around. I can pull both from the coil and no change in engine run. All others result in huge miss.

I have run a few tanks of E85 in the truck recently and thought that may be the cause so I filled it up with 87. That hasn't seemed to change anything.

I can pull the IAC plug and the engine almost dies so I suspect no vac leak since it would happen like a switch was flipped. I have cleaned the MAS and it didnt help. Compression is strong and the heads were re-surfaced 5000 miles ago and everything on the top end looks good. All the spark checks out good.

I'm really confused as to what this may be. So my next approach was to rent a fuel pressure gauge and check that. But like I mentioned earlier, every car I have ever had an issue with fuel pressure with it was a problem at high rpm. And I cant seem to see what 3 and 4 have in common since they are on different sides of the engine other than the coil.

I have a scanner coming today that can look at live data.

Does anyone have any good suggestions on what may be going on here? I think the next logical thing to check is fuel pressure.

 

Welcome to the forum

The P0171 and P0174 codes are cause by misfires
Lean and Rich can be confusing because we always think of these being related to Fuel, its not in OBD2 terms
Lean is too much Oxygen in exhaust, Rich is too little Oxygen in exhaust, nothing to do with fuel
When a cylinder misfires no Oxygen is burned/used so it all gets dumped into exhaust, so too much Oxygen and Lean Code

3 and 4 are Matched Pairs, they are both at TDC at the same time
As are 1/5 and 2/6

Ford uses a Waste Spark system, first spark system ever invented, 1890's
There are only 3 coils in the coil pack, not 6
So what 3 and 4 have in common are.....they use the same coil, because they are a match pair, 1/5 uses another coil and 2/6 the other one
Pulling spark plug wires was OK on distributors but with Waste spark you are disabling TWO cylinders when one wire is pulled, just FYI
Better to use an old timing light to see if a wire is pulsing/sparking

There are 4 wires on the coil pack, one 12volt wire(shared by the 3 coils) and 3 ground wires, one for each coil
Check the connector to make sure all 3 ground wires look OK
While its a long shot the computer can fail and not fire one of the coils, very very rare, and it should cause a code
You could disable fuel, pull fuel pump relay, so get a No Start, then use a 12v test light on each coils ground wire to see if its pulsing when cranking engine over

Yes, fuel pressure would be a long shot, 1998 and up Rangers use/need 55-65psi running pressure

Cam sensors can cause running issues with 3.0l, but should set a code