1994 4.0 OHV idle headaches

Derbyu02 · Oct 6, 2018

Ok guys I've been reading alot on this forum, looking for an answer to my particular problem with no avail. As soon as you start the truck it dies...at least four times if you continue to start it without giving it gas to stay running. Once it gets to an idle I put it in gear...dies. then I drive it..& it smells like fuel & has no power, foot to the floor does nothing. After about 45secs of this it'll clear out & run like a raped ape. Get to the destination, shut it off...start it back up & the same thing happens. I've changed damn near every part I can think of. Please help!!

RonD · Oct 6, 2018

Welcome to TRS

When diagnosing a fuel injected engine problem, one thing to keep in the back of your mind is that they do not have a "gas pedal", they have an "air pedal"
Carbs had an accelerator pump so when you pushed down on the gas pedal, extra gasoline was actually being squirted into the engine.
Fuel injection doesn't have this, when you press down on the gas pedal you get more AIR, not more fuel, computer adds more fuel but it is not instant, and pumping pedal does nothing but change air flow, nothing to do with fuel.

On the engine is the Fuel Pressure Regulator(FPR)
It is behind the alternator on 4.0l OHV Rangers
It has the Fuel Return Hose connected to it, AND a small Vacuum hose
Check that small vacuum hose for gasoline
If FPR leaks, raw fuel is sucked into the engine, flooding it out, so opening the "air pedal" would balance out the extra fuel being sucked in.

Leaking injector
All fuel injection computers have "Clear Flooded Engine" routine
Turn key on
Press "gas pedal" down to the floor and hold it down
This tells computer to Shut OFF fuel injectors, 0RPMs and WOT(wide open throttle)

Crank engine over, it should NOT start, it should not Fire at all, no fuel

If it starts or fires then fuel is leaking in

As soon as you release gas pedal injectors will start, or if RPMs get to 400 computer will exit Clear Flooded Engine

I use this every morning when starting my high mile '94 4.0l, I like to get oil pump circulating oil first, so I crank engine for 5 to 10 second and then release gas pedal for startup

Derbyu02 · Oct 7, 2018

Thanks! I'll give it a try & report back.

Derbyu02 · Oct 8, 2018

Ok, so I tried that this morning. It did not start with the pedal down, released pedal & it fired right up...and died. Got it running. put it in gear & it dies again. 30 seconds later it runs like a top. Up until then it runs rich & stumbles.

RonD · Oct 8, 2018

Check FPR vacuum hose.

But it reads like a computer issue now
Computer runs cold engine from pre-set factory air:fuel mix tables in its memory, this is called Open Loop operation
When engine/coolant temp gets to about 140degF, 3 minutes or so, computer starts to use the two O2 sensors as feedback to adjust air:fuel mix on the fly, this is called Closed Loop operation.

Having BOTH O2 sensors fail at the same time would be extremely unlikely, and if one was failing then you would get a CEL(Check engine light)

Does the CEL work?
It should come on with Key on, and then go off after engine is started
If bulb is burned out then you could have codes that will help point to a area to check

But if computer is the problem it may not set any codes, because it doesn't "know" there is any problem.

MAF(mass air flow) sensor is a possibility but that usually just causes stumbling and bogging down, when it is "off a little".
Computer "knows" its running a 4 LITER engine so "knows" how much air should be flowing past the MAF sensor at any giver RPM, so if MAF sensor is "off" by too much it would set a code.
You can try unplugging MAF sensor's 4 wire connector and then try to start the engine.
CEL should come on within minute or so because MAF is unplugged
Computer will also stay in Open Loop even after warm up

Derbyu02 · Oct 9, 2018

FPR & hose are new. MAF is also new. CEL does work & comes on once in awhile then goes off. Both o2 sensors are new as well. I recently changed the coolant temp sensor & switch & also the charcoal canister solenoid.

Derbyu02 · Oct 9, 2018

I also unplugged the IAC (also new) while running & there was no change in RPM.

RonD · Oct 9, 2018

Derbyu02 said:
I also unplugged the IAC (also new) while running & there was no change in RPM.

Then there is a problem

Could be a vacuum leak but check the Anti-diesel screw first
On the throttle cable linkage there is a screw that "looks like" an idle screw, but we all know fuel injection can't use an idle screw, no idler Jets, so it is the "anti-diesel" screw.
With engine warmed up and idling, unplug IAC Valve
Turn the anti-diesel screw counter clockwise(unscrew) and see if RPMs start to drop, if so continue to turn until engine is about to stall, it's set
Plug IAC valve back in and computer should reset idle, it will take a few days for computer to relearn IAC Valves new settings for idle.

IF turning the anti-diesel screw doesn't cause RPMs to drop, turn it back to where it was and start looking for your vacuum leak, you have one

Anti-diesel screw is also the fine tuning adjustment for the TPS(throttle position sensor) at Closed throttle.
TPS should have slotted holes so you can adjust it there, but some didn't or range was limited.
TPS needs to send computer under 1volt when throttle is closed, .69-.99 is spec, and 4.5v, or higher volts, at WOT(wide open throttle), this is a Learned sensor
Center wire on the TPS is where you read that voltage, use a sewing needle to pierce the wire, key on, to read voltage.
Since the Anti-diesel screw adjusts the throttle plate it also adjusts the TPS minimum voltage

Derbyu02 · Oct 10, 2018

Ok. The anti-diesel screw adjustment did not affect RPMs. What vacuum leak would cause my particular problem?

Derbyu02 · Oct 20, 2018

Ok guys just a thought...I believe there is an exhaust leak right at the manifold & Y pipe. The Y flange & the manifold don't exactly line up so it's kinda iffy. Could that cause it to think it's running lean & give it more fuel? It runs fine after a minute or so.

Derbyu02 · Nov 12, 2018

Yay? Nay?

Bgunner · Nov 12, 2018

If there is a leak there it could. Any exhaust leak before the O2 sensor/s will give you issues.

Derbyu02 · Nov 12, 2018

Yay or nay?

Bgunner · Nov 13, 2018

Look, Any exhaust leak before the sensors will cause the O2 sensors to not read properly. Check the doughnut gaskets at the exhaust manifolds for a leak. If it is leaking fix it then reassess the issue after. The y pipe will not fit exactly square on the exhaust manifold this is why the doughnut gasket is shaped like a doughnut but coned. The cone shape allows for the pipe to seal even thought it is not dead on with the manifolds.

Have someone hold a rag on the end of the exhaust while you search for a leak, this will make a leak more pronounced and easier to find.

None of us can say for sure what the issue is because we can not test to see, all we can do is try to point you in the right direction with the info you give us.

Derbyu02 · Nov 13, 2018

Sorry about the double post, must have had bad service. Thanks for the info, I'll try that!

1994 4.0 OHV idle headaches

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