'93 Ranger 3.0L V6 Inconsistent Idle issue (quite the head scratcher!)

[diehlak]

Hello! First post here on TRS. First off just want to let you all know I appreciate the existence of this site and would like to be a valuable addition to its knowledge base. So let me know how I can phrase my questions more clearly and also answer others questions in a way that is most beneficial and time efficient. Now to the engine problem!

Truck doesn't start until 3rd or 4th crank
Once engine ignites RPMs shoot to 2000 or so, after 5-10 seconds, falls to 700ish RPMs and dies if throttle is not pushed.
Once engine is warm (30-60 seconds I'm assuming) idle evens out and will not die unless neglected for a while.
With engine running and TPS disconnected, engine idle speed does not change.
With engine running and IAC disconnected, engine idle speed does not change.
With engine running and MAF sensor disconnected (1.5"x1.25" grey box connected to air filter box?) engine stalls.
Fuel pump makes noise for 3 secs or so with KOEO and there is fuel pressure although I haven't measured how much.
Purchased truck 1.5 months ago. Previous owner replaced: Fuel pump, Fuel Filter, Starter, Battery, Spark Plugs.

I have a few theories, based off my limited mechanical knowledge and the different forums I have read, of what could be causing this kind of condition:
Vacuum leak; recommendations of where to start looking and how to test all vacuum lines/gaskets in a foolproof way would be awesome. Also any places that tend to leak/break on this year of truck and engine would be useful.
PCV valve potentially being stuck open and letting excess air into the engine.
Idle Air Bypass Valve? not sure what it does but I heard it could contribute to a similar condition.
Throttle plate was bent somehow, not a great sign, but I bent it back and now there is a (seemingly) good seal around the edge of it in the throttle body.
Gaskets on the air intake hose are very oily and broken.
Air filter box is partially broken with potential for some air to leak in around seal.

Thank you for any and all help, I hope to return the favor as soon as possible!

Merry Christmas from Central Oregon!

 

[rusty ol ranger]

Idle air bypass valve=Idle air control.

When you unplug that idle should drop or stall. Unplugging the TPS at idle wont do anything but piss the ECM off, and unplugging the MAP should stall it.

Clean/Replace the IAC (idle air control) and ill about gurandamntee youll be ready to rock.

 

[diehlak]

Previous to this I had cleaned and checked voltage on IAC and TPS with KOEO, I am not 100% sure I did the test correctly, but I followed someone's instructions on the internet closely and I seemed to get accurate results.

IAC seems clean, no carbon or buildup of any kind that I can see, and it seems that I can move the valve easily. Is it worth it to just replace parts even if they seem to be working both mechanically and electronically? (at least when put in a test)

Just tested the idle again; took off IAC completely and covered up the holes with tape, plugged the TPS back in, removed intake tube completely.
**Results** Engine starts up easier? (2nd crank it fires right up, once the initial ignition has happened it will start on the 1st crank)
Revs up to 1700 rpms. will die after 20ish seconds, or drop down to very low rpms (500 or so) then die after 5 seconds or so.

 

[rusty ol ranger]

Previous to this I had cleaned and checked voltage on IAC and TPS with KOEO, I am not 100% sure I did the test correctly, but I followed someone's instructions on the internet closely and I seemed to get accurate results.

IAC seems clean, no carbon or buildup of any kind that I can see, and it seems that I can move the valve easily. Is it worth it to just replace parts even if they seem to be working both mechanically and electronically? (at least when put in a test)

Just tested the idle again; took off IAC completely and covered up the holes with tape, plugged the TPS back in, removed intake tube completely.
**Results** Engine starts up easier? (2nd crank it fires right up, once the initial ignition has happened it will start on the 1st crank)
Revs up to 1700 rpms. will die after 20ish seconds, or drop down to very low rpms (500 or so) then die after 5 seconds or so.

@RonD

 

[RonD]

Welcome to TRS

Missed this one back in 2020, a year ago, lol

IAC Valve is controlled by the engine computer(PCM), since fuel injection doesn't have "idler Jets" like a carb, they can't use "idle screws" so they use the IAC Valve instead so computer can control idle RPMs and calculate correct fuel mix for idle
When you turn on the key the PCM will open up the IAC Valve all the way
So when engine start it should REV high
Then computer starts to close IAC valve to set idle RPMs base on engine temp(see ECT sensor below), PCM has "target idle RPMs" in its memory based on temperature of engine
Engine temp under 100deg idle should be 1,100rpms
At 130deg 900rpm
Over 150deg engine is warmed up and should be 650-700rpms with manual transmission(autos are 100 higher)

After engine is fully warmed up and idling, unplug the IAC Valves 2 wire connector
RPMs should drop to 500 or so or engine may even stall, either is GOOD, no vacuum leaks
If idle is high and doesn't drop then there is an air leak, leave IAC Valve unplugged and start unplugging vacuum line s and plug port with finger to see if you can find the leak
You can spray soapy water around upper and lower intake, it won't catch fire, lol, and engine should stumble when it sucks in soapy water, and in any case you end up with a cleaner engine

There are only 2 brands of IAC Valve that work on the Rangers, Motorcraft, of course, and Hitachi brand
Rangers used a solenoid type valve, other brands are combo "stepper"/solenoid, so can act up with Ford PCMs

PCM with IAC Valve combination is VERY accurate, +/- 4rpm for warm idle, and its fast to respond, so stalling or high idles are using something else, IAC Valves can fail of course, but hard to make that call until other things are checked

There is a screw on the throttle linkage that "looks like" and idle screw but its not, its called the anti-diesel screw(but works just like an idle screw, lol), if idle doesn't drop with IAC Valve unplugged try turning this screw counter-clockwise(unscrew), if idle starts to drop then keep going until engine is barely running
If idle doesn't change then put it back where it was
Find the vacuum leak then retest this screw

ECT(engine coolant temp) sensor used only by PCM, uses 5volts
There is also an Engine temp SENDER, used by dash board temp gauge, uses 12volts
They look similar but ECT sensor will have 2 wires, sender just 1 wire in 1993

ECT sensor is used to set Choke Mode, all gasoline engines need to be "choked" on cold start
Choked mean Rich fuel mix, high idle and advance spark
PCM does all that based on ECT sensor
So if engine runs poorly on cold start or is hard to start, I would change the ECT sensor on speculation, they can be tested but are not that expensive to waste time on
Google: Ford ect sensor voltage graph
You can use an OHM meter and boiling water to test

If your ECT sensor is not working correctly it can through off PCM at any running temp, and cause hard starts with No Choke, ot too much Choke on warm restarts



TPS(throttle position sensor) doesn't control idle, its used for a few things but not idle
It "tells" PCM when driver changes gas pedal position so PCM can add extra fuel instantly for acceleration, there would be a delay/hesitation if only the MAF sensor(or MAP sensor) was used to detect increase air flow when throttle was opened more
Its also used to save gas, when you are driving and you take your foot off the gas pedal to coast, PCM sees throttle is closed so it turns OFF the fuel injectors until RPMs are under 1,300 or so then turns them back on at idle levels
If throttle is Wide Open(WOT) PCM ignores O2 sensors and pumps in extra fuel for extra power because MPG is not the priority at WOT
TPS gets 5volts and sends back 1 to 4volts to PCM
Spec is .6 to .9volts when throttle is closed
4.5-4.9v when throttle is WOT
The anti-diesel screw(idle screw) can be used to fine tune TPS closed throttle voltage so its below 1volt
Center wire is Return voltage back to PCM, you can use a sewing needle to pierce that wire and test with KOEO, move TPS slowly open and closed looking for dead spots or short, 0v or 5v
Check that WOT shows 4.5v

If ECT sensor is working and you still have slow startup
Try cycling key on and off 3 times, this builds up any lost fuel pressure
Then try to start
If it starts faster then you are losing fuel pressure when vehicle sits
Not all that unusual, but shouldn't happen
Running fuel pressure in a 1993 is 30psi
And you need about 15psi for start up
When you shut off the key the fuel pressure should hold above 20psi for MONTHS and MONTHS
But it can bleed down from leaking FPR(fuel pressure regulator) or check valve inside fuel pump
The fuel not leak out so no smell its just leaking back into gas tank and no pressure for start up

Each time key is turned from off to on the fuel pump runs for 2 seconds, that adds about 10psi of fuel pressure
So if you think it might be a fuel pressure issue on start up, cycle key off and on 3 times to see if that helps