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4.0 OHV miss at idle


Bobby'sFFRanger

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3.5"
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33"
I’ve had quite a go at this truck, which my family had owned since new (‘94 Ranger 4.0).

Recently replaced motor w/reman long block (S&J). Can’t get it to idie right.
First, we verified vac leaks, cuz idie was high. Then, we got new IAC, MAF, EGR, EGR pos sensor, ECU, TPS, reset the IAC and diesel screw, finally have idle at about 800. New plugs and wires too.

But there’s a miss. A slight one at idie, small shake in the truck. When started cold, IAC does it’s job, it chokes and idles high, then settled into a shaky 800. You can hear the burble of a miss at exhaust tip every other second. Higher Rpm’s are smooth and dont seem out of sorts.
Occasionally when warm started, will start with a heavier miss that revving won’t smooth out, then after about 10 seconds something adjusts internally and it settles into the regular tolerable shaky idie.

When driving, it pulls strong, no codes. But with hood up, when you review from idle, there’s a popping-like sound coming from driver side bank exhaust manifold. Happens only ar lower rpm’s, at throttle tip-in. Can smell a hint of exhaust from that dude too (EGR side).

Shop that installed motor is expensive and reluctant to work on an OBD1 motor. They keep trying to convince me it’s fine “because it’s old”.
Any help would be appreciated. I have too much time and money in this truck.
 


franklin2

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I would do a compression check. If one or more cylinders are low, I would check the valve lash, a couple of the valves may be too tight. I am assuming the 4.0 is the same as the 2.9 with the valve adjustment on the tip of the rocker arm?
 

Bobby'sFFRanger

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I would do a compression check. If one or more cylinders are low, I would check the valve lash, a couple of the valves may be too tight. I am assuming the 4.0 is the same as the 2.9 with the valve adjustment on the tip of the rocker arm?
Thanks for this. Honestly, I’m a bit wary of a comp test, afraid this debacle will continue with a bad motor.
Not sure on the valve lash; are the valves adjustable on the OHV?
 

RonD

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No, 4.0l OHV doesn't have a valve adjustment

1994 4.0l with Manual trans should idle at 625-650rpms
Mine does
It also has all its original sensors, except O2s, which are only good for 12 years or 100k miles
I have 400k on my 1994

You have a Calif Ranger if it has an EGR valve in 1994, should also have a Cam sensor and synchro, does it?

After engine is warmed up and idling
Unplug the 2 wire connector on IAC Valve, RPMs should drop, AND CEL(check engine light) should come on
Adjust the anti-diesel screw counter-clockwise until engine is barely running, 500rpm or so
If RPMs won't drop that low then shut off the engine
Remove Throttle sensor, it may be holding throttle open, leave it plugged in
Then restart engine and see if you can get RPMs lower
If not there could still be a vacuum leak, you should be able to STALL any engine by closing the throttle plate enough

If not, or if so, plug back in IAC Valve and RPMs should go up to 650 , maybe 700, not sure what the emission specs were for Calif Rangers in 1994, but I wouldn't think a manual would ever idle at 800RPMs on purpose, automatics yes, but "in gear" only, 750 in Park or Neutral

Did the new Computer(ECU) numbers match the old one
F47F-12A650-XXX<<< these last numbers need to match
Especially for Calif Ranger
 

Bobby'sFFRanger

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3.5"
Tire Size
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Idle is at 800 because that’s what I wanted when we finally fixed the vacuum leaks that had it at 1100. I find it better for city manual driving, especially on tip-in up hills.

The RPMs aren’t really the issue, except that they wave up and down. I’m sitting in it at idle as I type, they flux about 50-75rpm every other second. There is also a minor to moderate shimmy in the block that can be felt at idie through the truck, the quiet burble of a miss if you listen to the exhaust pipe.

Additionally, when under the hood, if I rev the motor by hand at the TB, a sound from the drivers side bank, or exhaust manifold, kinda like marbles rattling or even a stretching sound(?) can be heard at initial throttle tip-in. Maybe “after fire”?

And occasionally, on warm starts, it’ll start-up with a heavier miss, heavier stutter. After about 10-15 seconds, it’ll smooth out to its typical slight shake and slightly wavy idle. No codes, and computer is OEM and code matched (used).

Drives great, pulls strong, and while on the road you’d think it was perfect. But these idle issues tell me there’s a problem. Especially since everything is new, I’m just being nickeled and dimed by repair shops.
 

franklin2

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You really can't decide what idle rpm you want, the computer is programmed for a certain window for the idle rpm. If you turn the idle up, you run the danger of putting the TPS out of range. I am not sure of the specs on your truck, but most of them the voltage from the TPS at idle should be 1 volt or less. If it gets above 1 volt, it puts the computer into a different fueling strategy. This may be what the hunting is all about.

I have been around this block a time or two also trying to make it do something it's not programmed to do. Very frustrating.
 
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RonD

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You want the lowest idle to trouble shoot, once the issue is found you can do what you think is best, but as franklin2 said, the computer decides the idle, and will set codes if idle is higher than it "wants"

At lower RPM a misfire or partial fire is more noticeable, that's why "it runs great driving" but rough at idle

You can put a can of Seafoam or similar injector cleaner in the gas tank, I do this once a year on my 4.0l and it smooths out the idle for 10 or 11 more months, lol
At idle dirty injector tips DRIP fuel instead of spraying it, and with less air flow you can get a partial air/fuel mix and partial ignition

4.0l needs a good spark so gap is 0.054", quite wide

The "back fire" means not all the fuel was burned in a cylinder so next cylinder, on that side, to fire's HOT exhaust ignites that unburned fuel in the exhaust manifold on that side, so "pop"

If it does this at idle then put a stick or metal bar on each exhaust manifold and put other end by your ear, and listen for the "pop", to see if its only on the one bank/side of the engine
Maybe narrow down possible cylinders with the issue
 

Bobby'sFFRanger

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Tire Size
33"
You really can't decide what idle rpm you want, the computer is programmed for a certain window for the idle rpm. If you turn the idle up, you run the danger of putting the TPS out of range. I am not sure of the specs on your truck, but most of them the voltage from the TPS at idle should be 1 volt or less. If it gets above 1 volt, it puts the computer into a different fueling strategy. This may be what the hunting is all about.

I have been around this block a time or two also trying to make it do something it's not programmed to do. Very frustrating.
Thanks for this info. Please know, much of what I “think” is what I’m being told by the shop I’m having to use for anything other than the most basic.
My idie really isn’t much higher than it has been over the last 30 years - the needle wants to sit perfectly horizontal on the tach, which I thought is between 700-800. It just waves up and down about 50rpm. Also, A/C unit kicking on drops RPMs down a good 200+ lower.


My TB is a BBK - not because I cared to have a fancy one (I know it’s not any better) - but because finding an OEM one was difficult. The shaft in the BBK leaks air, but manufacturer says they’re not air tight at the shaft, ie: it’s supposed to leak. Seems wrong, but they wouldn’t warranty it.

Stretching/snapping/rattling sound at driver’s side engine bank isnt loud, but noticeable at initial throttle/rev. I suppose a faulty injector would justify the shaky idie, sound from block, misfire etc.

I have run a simple STP injector cleaner in my last two tanks of fuel, no improvement. Either those additives are worthless or my issues are more than cleanliness I suppose.
No codes from any of this. I’ll order a set of injectors and take a look at plug gaps. Thanks for your input guys.
 

RonD

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The injector cleaners work but just use the one can
If it doesn't make any difference is because dirty injector tips wasn't the issue
 

Bobby'sFFRanger

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2WD
Total Lift
3.5"
Tire Size
33"
The injector cleaners work but just use the one can
If it doesn't make any difference is because dirty injector tips wasn't the issue
Can an injector have simply failed? Or do you mean if cleaner doesn’t work, it’s compression?
 

RonD

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Injector can fail electrically so cleaning wouldn't help

Failed injector would usually cause a full time misfire at any RPM and speed
And for sure cause Lean code on that side of the engine
 

Bobby'sFFRanger

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Seattle, WA
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1994
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Transmission
Manual
2WD / 4WD
2WD
Total Lift
3.5"
Tire Size
33"
No codes at all, though my OBD1 computer seems to only care when my EGR and related friends are unhappy.

What idie speed should I expect with A/C turned on? My compressor turns on anytime my HVAC goes right of OFF, but clicks on and off every 20 seconds or so.
When it clicks on, my RPM drops to near 500,then pops back up when the comp clicks back off (back and forth). Is this normal, or a weak idie symptom of my supposed injection problem? And would any of this also occur if my fuel pressure was low or regulated incorrectly?
 

RonD

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1994 Heater/AC controller seen here: https://www.2040-parts.com/_content/items/images/11/1134211/001.jpg
Last year for the slider type setup, 1995 and up used all knobs

In a 1994 Ranger the AC compressor should only come on when AC button is pressed OR if Vent slider is moved over to Defrost, either should cause the AC light on dash to come on

This sends 12volts to the Computer(pin 43) and to/thru Pressure switches to activate the AC Clutch
The pressure switches are what cycle the AC clutch on and off

The Computer should bump up the idle by 50-75rpms when it gets 12v on pin 43, using the IAC Valve
Computer will also raise and lower idle based on Pin 10, 12volts after pressure switches when AC compressor is actually on
But you shouldn't notice idle going up or down much since it all happens very fast, but would be a slightly higher base idle with AC on
 

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