Confusing intermittent problem, 94 Fordzda Branger - vapor lock?

About three times in the last year, my B4000 has a hiccup.  It cranks and starts and runs rough and barely idles at about 500 rpm.  I can usually get through it with modest throttle or just waiting a bit.  It will run rough for 30 seconds or so, then kicks in with a misfire on one cylinder for a few seconds, then runs great.  Fuel pressure tests great, right in the middle of the spec.

The other day I pulled into a restaurant for dinner.  Came out, started, sputtered, died.  Crank but no run.  Occasional fire on a couple cylinders but not enough to keep it running.  I started poking around under the hood.  Pulled a plug wire and got good spark, but when I pushed the schraeder valve on the fuel rail I got vapor/air for the first couple seconds.  I cycled the key a few times, got more vapor/fuel.  Once I got mostly fuel, I tried to start it but still no-go.  Had it towed to my buddy's shop.  Three days and she couldn't duplicate the problem.  She used it for parts runs to get it up to temp, tried it cold, tested pressure.  No issues.

I took it back and used it yesterday and it acted up one more time (sputtering idle for a bit)  OBD1, so no CEL.

Ideas?

Gas has a low boiling point so the point of maintaining fuel pressure even after the pump stops running is to use that pressure to keep the boiling point of the fuel above the temps of the hot engine after shutoff to avoid hard restarts. An older vehicle like that probably uses a return style fuel system. Once the pump turns off there are 3 major items that retain that fuel pressure and keep the fuel in the rail in a liquid state: a check valve in the pump, the fuel pressure regulator, and the fuel injectors themselves. It's also possible to have fuel line/hose and injector O-ring leaks.  

If you're lucky enough to have rubber fuel hoses on both the supply and return lines in your engine bay, it's pretty easy to narrow down which things are leaking. First you'd put your gauge on, turn the car off and verify that it does indeed drop pressure rapidly once the pump stops. If that's the case you can gently use some pliers or use some actual hose-pinching pliers to squeeze the supply and return hoses shut one at a time and see what changes. If you clamp the supply line and pressure stops dropping, it's most likely a fuel pump check valve leaking, or possibly a leak between the pump and the engine bay (could even be internal to the tank, like a leaking hose between the pump and the hanger assembly).  If you pinch the return line and the pressure stops dropping, the regulator is what's leaking. Flow through the regulator should completely stop when the pump turns off. If you crimp BOTH the supply and return lines and it still leaks, you probably have one or more leaking fuel injectors, which can also contribute to hard start by flooding cylinders until you've cranked enough air through the engine to dry out (i.e. un-short-to-ground) the spark plug on that cylinder and get a spark across it again.  

Will the problem occur on a cold start first thing in the morning?  Or only on warm/hot restart?

I'm inclined to suspect leaky injectors. 

Problem has only happened 3-4 times, but it has happened on a hot day after running and first startup in the morning at 50 degrees.

I dug out the paper from my buddy's shop to look at her test results.

KOEO x1 = 35psi
KOEO x2 = 38psi
KOEO x3 (and subsequent times) 38psi.

Maintained ~38 psi for 30 min.

Running idle = steady 32psi
Running fast idle/throttle sweeps = 38 - 42psi

I don't think she checked pressure the next morning before key-on, but it started right away.

All of those are above the middle of the range for factory spec.  So... assuming the pump is working the same all the time (doesn't have a dead spot or intermittent short/low voltage) there shouldn't be a fuel pump issue. It is definitely not a source voltage issue, but I haven't crawled under to check voltage near the tank or tank ground. It's my daily driver, so yanking the bed isn't an option right now.

Leaking injector would explain things IF I was losing pressure, but I don't think I am, at least not any of the times pressure was tested.  Regarding a leak, I don't think there is a leak.  Certainly not a liquid fuel out leak, and as long as there is pressure in the system it wouldn't let air in.  About the only thing I can think that might cause this is a leak in the pickup in the tank - something after the pump but before it leaves the tank.  It could be draining back in the tank allowing a bubble up the line... but even then (since it is a return system) I would expect the 20+ key cycles I tried should have purged any air out of the rails.

Next step, pull the plugs?  Leaky injector would show up as a rich plug, right?

It might, but if the intake valve is closed on that cylinder when it stops and you don't crank it before removing plug it might also just puddle up on top of the valve. 

It doesn't look like you were losing pressure at the time of the test, though you didn't say when that was and that doesn't mean the test captured what's happening during the extremely intermittent issue. What you described sounds exactly like a leaky injector dumping all your fuel pressure into one cylinder leading to long crank and misfire on that cylinder upon restart. The wrench in that idea is you said it has also happened cold. Even if the injector leaked out all the pressure the previous evening, all the heat would be gone and you'd have a pretty quick start, even if it did misfire on one cylinder for a few seconds. 

The times I've had problems with my 4.0 right after starting were solved by replacing the IAC.

It nay not be it, but give your IAC a light smack the next time it does it before anything else. If it goes away, the IAC is gummed up and needs cleaned/replaced

Antihero said:

The times I've had problems with my 4.0 right after starting were solved by replacing the IAC.

 

It nay not be it, but give your IAC a light smack the next time it does it before anything else. If it goes away, the IAC is gummed up and needs cleaned/replaced

I did whack it... not that it would have definitely fixed it.

I would think if its an IAC issue, the problem would "go away" with throttle input effectively bypassing the IAC.

Vigo said:

It might, but if the intake valve is closed on that cylinder when it stops and you don't crank it before removing plug it might also just puddle up on top of the valve. 

It doesn't look like you were losing pressure at the time of the test, though you didn't say when that was and that doesn't mean the test captured what's happening during the extremely intermittent issue. What you described sounds exactly like a leaky injector dumping all your fuel pressure into one cylinder leading to long crank and misfire on that cylinder upon restart. The wrench in that idea is you said it has also happened cold. Even if the injector leaked out all the pressure the previous evening, all the heat would be gone and you'd have a pretty quick start, even if it did misfire on one cylinder for a few seconds. 

Will an injector leak intermittently?  Her pressure tests all showed normal, but if the problem comes and goes, that could be the issue.  I would think though, that a leaking injector wouldn't cause the long periods of stumble and sputter.  I could imagine a misfire on one cylinder for a bit, but it chugs and puffs for 30 seconds or so.  During that time, it will either magically fix itself, or it will die and leave me sitting (based on the one time it let me sit)

I would think that if one injector was leaking, it would cause a misfire on that cylinder, but the fuel pump should almost instantly supply the rest of the injectors with pressure leaving one cylinder to misfire for a bit, not a whole engine sputter.

Metal fuel tank?

Possible that you’re picking up junk off the bottom of the tank, and it’s clogging the fuel pump pickup momentarily? I had an 80s F150 that would do that. Took me a while to figure it out. 

Curtis said:

Antihero said:

The times I've had problems with my 4.0 right after starting were solved by replacing the IAC.

 

It nay not be it, but give your IAC a light smack the next time it does it before anything else. If it goes away, the IAC is gummed up and needs cleaned/replaced

I did whack it... not that it would have definitely fixed it.

I would think if its an IAC issue, the problem would "go away" with throttle input effectively bypassing the IAC.

So did I, but IAC seemed to fix my problem on my 91 ranger with a similar issue.

It might have been a coincidence, but since the IAC was $9 on Amazon and it takes 2 minutes to replace I gave it a shot and it's been fine ever since. The IAC was really gummed up too

Only way an injector would cause that type of misfire was if it was pretty much stuck open, not just 'leaking'.  You can flood a cylinder at idle, but get much off idle and even way too much fuel is still not enough to cause a cylinder to stop firing like a dead miss. I don't know exactly what the number is but on my 2009 Challenge car I had an electrical issue causing it to go super rich and the engine still ran ok until <9:1 AFR. IF fuel was causing that you'd probably have a fuel odor or a pulsing puff of fuel smoke/haze from the tailpipe during the misfire. It's possible it's just not fuel related.  But, if it has a wasted spark ignition system and it's only misfiring on ONE cylinder, it is more likely to be fuel than ignition since you would generally drop 2 cyls at a time. You can have a shorted/fouled plug cause one cyl to drop and the other not, or the ignition coil shorting through it's case so that the loop doesn't necessarily go through the other plug wire. 

If you have no idea what cyl is misfiring you could probably figure it out with a temp gun, with the hard part being getting the problem to happen in the first place. You would just let it start and run and then shut it down as soon as the misfire cleared up. Then you could temp gun each exhaust runner and find the coldest one. Could also pull the plugs but depending on where you are when the issue occurs that might not be practical. 

It must be wasted spark.  The coil is a single block with three pairs of sections.

Good ideas on the exhaust temps.

I think I'll pull the plugs (more of a pain than it sounds like) and see if one is obviously gapped differently, browner, blacker, or otherwise different.  The coil, wires, and plugs looked like they had been done the day before I bought it, but that isn't any guarantee that they're good, especially with Ford ignition stuff.  One thing I learned in my time running shops is that you just call the Ford parts counter for ignition parts.  They're not that much more expensive and aftermarket stuff isn't really accurate enough to satisfy something about Ford's picky electronics.

Pulling injectors isn't the easiest either.  The fuel rails are retained by being sandwiched between the upper and lower parts of the intake.  Not terrible, but not a 1 hour job.

Maybe I'll try an IAC, pull the plugs and read them, and test wire resistance while moving them to look for intermittent breaks.  Then I'll feel like I'm doing something.

Curtis said:

I think I'll pull the plugs (more of a pain than it sounds like)

A friend of mine has a Ranger with the V6 (don't recall which one). The plug procedure sounded like a PITA (through the wheel well) but really wasn't.

I didn't see here whether you have changed plugs before or not - either way just do it. Plugs are cheap.

Plugs/wire/coil were apparently done as a tune-up immediately before I bought it.  They look new, but warrant a check.

Actually, 5 of the 6 are not terrible.  The passenger rear is tucked in behind the A/C evap box, the accumulator, and a half mile of hoses and wiring.  Not looking forward to that one.

Have i mentioned you could just buy  this?   It's BEAUTIFUL.

Curtis said:

Plugs/wire/coil were apparently done as a tune-up immediately before I bought it.  They look new, but warrant a check.

Actually, 5 of the 6 are not terrible.  The passenger rear is tucked in behind the A/C evap box, the accumulator, and a half mile of hoses and wiring.  Not looking forward to that one.

I don't remember it being too bad the last time I changed mine, it has been years though

Does that have a MAF sensor? On newer ford stuff cleaning the MAF sensor is always step one for any engine wonkiness. Also check the plumbing between the maf and the throttle body for any openings causing unmetered air to get in.

It does have a MAF... or at least what looks to be a MAF.  I can't imagine an IAT sensor being 3" in diameter with 4 wires.

The intake on these is so convoluted.  A homebrew air intake is in the works.  It goes across the front, down beside where it picks up hot air from the exhaust manifold, then into a box which sources air through a tiny hole in the rad support.

3" PVC and a cone are in order.

Since it had no fuel pressure the one time you were able to do some troubleshooting while it acted up I'd focus on that.  The system should be able to build pressure even with an injector stuck wide open so I think the problem is on the supply side.  Statistically it's most likely the fuel pump but it could be pretty much anything electrical related to the pump.

If it were mine, I'd put a pump in it.

I certainly don't disagree, and I'm all for throwing parts at a problem, especially as cheap as they are for this truck.

Just trying to avoid uninstalling the bed on a whim.

For now I'll check the topside things and I have my fuel pressure gauge with me in the truck.  If it acts up again, I'll put the gauge on it while it's being a douche.