Specific engine issue 4.0 OHV 1994


Tater Tot

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I have a 94 ranger 4.0 2wd. Long time issue. Replaced many sensors. Timing issue? Vacuum leak? Ugh please help.

~112k miles

Symptoms, all in order:

Engine cold start: RPM’s always low at idle when cold. Always starts. Engine starts up fine, revs high for a few seconds and immediately drops idle very low, no tach but engine and truck visibly shakes, seems like it could die at any moment but never does. Exhaust sounds like a putter patter, Engine doesn’t rev up when cold at idle, even worse when a/c is on. Air idle valve is new. TPS, ECT, thermostat, spark plugs and wires, PCV, fuel injectors, upper intake gasket, o2 sensors, crankshaft position sensor, all <2 years old. MAF is 3 years old. Fuel rail pressure is 29 psi. Engine computer battles to regulate rpm, letting idle for a while, sometimes the engine rpm will shoot up randomly, but it tends to just idle super low mostly.

Taking off driving: I have decently loud exhaust, engine misfires while accelerating (muffled misfiring, not bangs or pops), when hot or cold (used to be intermittent misfire, now constant problem whether hot or cold). I live in FL so temp is never that cold.

Still driving: Seems like the engine won’t climb rpms, misfiring (not loud, chokes misfiring). Transmission short shifts, can’t climb rpms.

In 3rd gear, the gear seems long enough, once I reach a certain rpm, the engine springs to life like nothing is wrong. It’s misfires, has no power and won’t go anywhere even when floored and when it hits a certain rpm and everything seems normal and dandy. Sometimes I can’t hit a high enough rpm before it shifts, it seems like, and I can’t accelerate out of the problem. Trans maybe 30-35k miles on it. Transmission seems to work fine.

It’s done this most of the time I’ve had it, 3.5 years. Gotten way worse with my first full summer in hot hot FL.

Cleared all codes today, when misfiring after clearing codes while driving, the only code was 177 (rich left side).

Did check test with Innova for obd-1 immediately after driving. Only codes then were 136 and 172, lean right and lean left.

What the heck is going on?

Vacuum leak- if it is a vacuum leak, it’s not from vacuum hoses. I disconnected and plugged all vacuum lines, unless it’s the fuel pressure regulator leaking (new vacuum line to it). Seems doubtful, the regulator seems fine. Lower intake leak? Upper intake gasket is new. No audible hissing or other vacuum noises. Doesn’t seem to be oil or coolant in gas. Exhaust is clean, no smoke or vapor or anything. No clogged cats.

Why does my engine come to life at a certain rpm like nothing is wrong? Like you would think the engine could die at any moment, never does, then I hit a certain rpm and everything is just as good as it could be.

I’ve noticed as well when I get to 35-45 mph, before it was struggling all the time, the engine has a power jump that starts at higher rpms. Timing issue? PCM - CamshaftPS - motorcraft EDIS 6 (module behind front driver headlight basically).

Why would my truck not be able to accelerate and misfire, even when floored, then seemingly hit a certain RPM, and run amazingly?
 


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Tater Tot

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I took my air filter off (hard pipe aftermarket, conical filter). I was able to choke the engine out, noticed no vacuum leaks or sound.

The edis-6 system seems to use crank sensor for ignition timing. The crank sensor is pretty new.

The fuel injection seems to run off the camshaft position sensor. I’m suspecting that sensor, with the truck running lean and all of the sudden gaining power out of nowhere. I think it has to do with spinning- maybe not enough voltage unless the engine is spinning fast enough. Causing lean condition because of it messing up the injector pulsation- injectors not opening for long enough?
 

rusty ol ranger

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I would think if it was a lean issue it would get worse the more RPMs youre tryin to spin.

Personally....id check compression before i went any further.
 

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I have a compression gauge on the way. I don’t understand why it jumps up in power after a certain rpm.

If I was to test compression with the engine cold, what psi should I see? Idk how I’m going to manage checking compression after getting the engine hot. Not with the passenger bank, at least.
 

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Reads like a fuel delivery issue to me. Filter, regulator, or pump. I'd to a fuel pressure test key on engine off, then running through the rpm ranges. Then key off again to test for leakdown. Check the fpr vac line for fuel. Do a full battery of vacuum tests as well.
 

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Already done all of that. Fuel pressure is 29-34 psi, spikes up when revved up. Fuel pressure takes a while to decrease in the rail.

No fuel in vacuum line to pressure regulator.

Fuel filter isn’t going to cause problems at low rpm and not at high rpm.

That’s why I’m looking at PCM, injector pulsation timing and duration, whatever sensor control the fuel injection IF THERE WAS ACTUALLY ANY INFO ON IT. There’s not even info on it from the Haynes manual for my 94.

It’s soooo much work to even look at the camshaft positioning sensor. Does anyone know what that sensor actually does? The edis system seems to work off the crank sensor. So does the fuel injectors work off the cam sensor? Why am I not throwing any other codes than lean? If it was lower intake, why don’t I hear whistling? Why would it all of the sudden spring to life at higher rpms like a switch was flipped?

Wtf is going on.
 

Tater Tot

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I haven’t tested fuel pressure in the tank, and the fuel rail pressure indicates that’s not a problem. I know 29 psi is one psi below normal but it spikes up when I give it throttle and that wouldn’t affect low rpm.
 

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Done vacuum tests, vacuum (if idle stays up) is like 19 Mercury. It stays there, goes to 20 or so when revved. Drops down low when it idles low, but when it’s idling high, the vacuum seems alright. Like when I rev the engine, the vacuum is fine.
 

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I disconnect and plugged all vacuum lines besides the fuel pressure regulator and it still did the exact same horribleness idle sputters.
 

rusty ol ranger

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Have you tested the TPS?
 

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TPS is is like 1.5 years old.
 

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All the spark plugs look lean, very low buildup on them, parts of them are still white white.
 

rusty ol ranger

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All the spark plugs look lean, very low buildup on them, parts of them are still white white.
Maybe you got some plugged injectors/fuel rail then....

Disable the ignition and crank it. Pull the plugs 1 by 1 and see which ones are dry.
 

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I’m a noob; this is the first motor I’ve ever worked on, outside of audio related work. I just can’t find the diagnosis. I’ve thrown parts at it, you know how that story goes. Nobody has been able to find it. And I can’t afford much with my health condition. It sucks ?
Maybe you got some plugged injectors/fuel rail then....

Disable the ignition and crank it. Pull the plugs 1 by 1 and see which ones are dry.
All of the plus are dry, every single one. Pulled them twice in the past two weeks, including today for the compression test.

Fuel injectors are about a year old, brand new ones from Standard motor products. I highly doubt any of them are clogged, especially when the plugs all look fairly uniform.
 


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