4.0 OHV runs like S#!* when clutch is released

ive been have a persistent and worsening problem with my 93 ranger ext cab 4x4 manual with a 4.0 OHV motor that's my daily driver. runs fine then all of a sudden and for no reason it will lose power start to miss (like its running on 3cylinders) and the engine will completely cut out over half throttle. it is a completely random occurrence but is happening more and more frequently. the CEL comes on but ONLY when it is messing up and it immediately goes out when it clears up, the problem comes and goes like flipping a light switch. I tried putting a scanner on it to see what codes come up but since it doesn't act up when I have the scanner around and it doesn't save codes I cannot pull anything up. also what is weird is sometimes I can lightly press the clutch pedal when it acts up and itll run fine. yesterday when I got home it was acting up bad, I let it idle in the driveway trying to see if I could figure something out and it was cutting in and out about 5 sec of running bad then 5sec of running good and it just went back and forth doing that, the exhaust smelled like and old carbed motor (like it was dumping fuel). anybody have any ideas? im leaning towards O2 sensors but im tired of throwing parts at it and so is my bank account.


ive done the usual 4.0 problem fixes in hopes of fixing the problem with no success. cleaned MAF, new TPS, new coil pack, new idle air control, new coolant temp sensor, plugs/wires are about a year old, fuel pressure good, fuel pump replaced about 2yr ago, throttle body cleaned

Computer ignores O2 sensors at idle and at WOT(wide open throttle)

So to see if it might be O2 sensors, take it out for a drive and push gas pedal to the floor(WOT) and see if engine comes back on line.
Thats how I tracked down a mechanic reversing the wires on my '94 4.0l O2 sensors, new clutch put in.
O2s are also not used for the first few minutes after cold start, 3 to 5 minutes depending on outside temp, so if cold operation still has the cutting out then it can't be O2s

Unplug MAF sensor, engine may seem to run rough but see if it does that 5 second thing you describe.
Also use a volt meter to test MAF with engine running
Under TESTING in this article: http://www.therangerstation.com/tech_library/CleanMAF.shtml

It will show the wires to hook up to, I used sewing pins to pierce the wires to test live voltage.
Raise RPMs and voltage should go up, range is 0 to 5 volts but 3.5v is WOT, keep RPMs at say 2,000 and see if MAF voltage stays stable, if it jumps up you would get sudden rich mix.
Voltage changes with air flow, higher flow=higher voltage, so if engine stumbles it will pull in less air, BUT..........you are watching for a voltage change just BEFORE engine stumbles, it will, of course, change WHEN engine stumbles.

On the upper intake is the IAT(intake air temp) sensor, drivers side in the middle, 2 wires.
It is similar to ECT sensor, in voltage readings, 5 volts in and Temp voltage out
test which wire has 5volts, key on
Then test the other wire with engine running, voltage goes down as temp goes up, but what you are watching for is a sudden change in voltage, if voltage jumps up then computer would add more fuel to richen the mix for the COLD air.
Much like O2 sensors can do

And it could be Computer is starting to go
I would put it in self test and see if it gives you the 111 OK response.
And if possible pull it out and open the top, look at circuit board

Should look like this: http://www.auto-diagnostics.info/ford_eec_iv

If you see any discolored areas then Capacitors have leaked which can cause all sorts of issues

Outside of the clutch pedal arm hitting wires under the dash and causing a short or disconnection, I can't think of a reason that would effect engine operation.
The clutch pedal switch can send computer 5 volts when pedal is up all the way, IN GEAR, and 0 volts when pedal is to the floor, NEUTRAL, but the switch is not activated until pedal is almost touching the floor.
On manuals the 5volts IN GEAR was not always used, on automatics it was always used to bump up idle RPMs when IN GEAR.
Taking the LOAD off the engine by pushing down on the clutch pedal a bit would make it feel like engine was running smoother

thank you for the very informative response

I have had the problem with reversed O2 sensors before on another ranger and know that problem well. now that you mention that I don't think O2s are my problem, engine dies at anything over half throttle, even WOT.

I have unplugged the MAF while it was running bad and it made no change to the rough running.

Im going to try and test the IAT sensor tomorrow to see what I can find out about that. ill also do the MAF test. hell a buddy of mine has a 98 with a 4.0OHV I might see if we can swap MAF and maybe IAT to see if I can get a change.... my problem is definitely getting worse, its now doing it at any time, cold, hot, driving, stopped, etc. and the clutch pedal trick is no longer effective. shutting the truck off and restarting seems to be the temporary fix until I can get this figured out.

Could be computer issue if rebooting solves the problem even temporarily

well I worked on it today. I didn't like the idea of piercing wires on the wiring harness to test the sensors so I bit the bullet and bought a new IAT sensor and a new clutch switch.

i took the IAT sensor out and it was dirty. it also had the element that was inside hanging outside the 3-prong shroud thing so i thought for sure that was the problem so i cleaned it and pushed the element back to the center and drove to advance to get a new one, truck ran great so i thought problem fixed........that was until the ride home. put the new sensor on in hopes it had messed up again, NOPE. as a last ditch effort i then took the clutch switch off and looked at it, it was also kinda dirty and one of the springs for the slide was stuck at the bottom. i cleaned it and put it back on, still didn't fix it. just for giggles i took the clutch switch back off and vice gripped the switch at the bottom (clutch depressed) and put it out of the way and went for a drive. 20min drive, ran awesome, not a single sputter. thought i had it whipped so went and got a new switch, put it on and went for a drive, 5min in and CEL flickering and running like s#!*.

im thinking something in the ECM is messed up on the clutch circuit inside. i am still able to remedy the issue temporarily by putting pressure on the clutch pedal (even after the new sensor). only thing i can figure it would be, something cross-feeding in the ECM to the ignition/fuel circuit making it miss and run bad.

pulled the PCM today. think ive found the problem. I took the cover off the PCM and saw that 2 of the little blue capacitors have char marks around the bases of them. one of them looks to have fluid that has leaked out of it at one point. I have a rebuilt one ordered. im hoping for the best

While not common it is one reason older PCMs fail

Picture here if failing capacitors: http://broncozone.com/topic/20788-fuel-delivery-problem/

got my new ECM in today and installed. no more skipping, truck runs great.

Nice

Thanks for the update and fix

Any update is she still running good? Mine is having similar issues but it's an automatic


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yes. still running great and its my daily driver

RoidedRanger said:

got my new ECM in today and installed. no more skipping, truck runs great.

Click to expand...

My 4.0L automatic has the same problem.

Your last two posts say 1) have a rebuilt PCM on order and 2) ECM replaced and truck runs great. <Yay that's an awesome feeling - I hope I have it soon.>

Question, was it the PCM or the ECM that fixed it. Sorry I don't even know what those stand for.

See my new thread today, "running rough, bad miss".

TIA

Peg

peglomaniac said:

My 4.0L automatic has the same problem.

Your last two posts say 1) have a rebuilt PCM on order and 2) ECM replaced and truck runs great. <Yay that's an awesome feeling - I hope I have it soon.>

Question, was it the PCM or the ECM that fixed it. Sorry I don't even know what those stand for.

See my new thread today, "running rough, bad miss".

TIA

Peg

Click to expand...

Power Control Module and Engine Control Module.

Some times they are synonymous, but I think on some vehicles PCM is for the transmission. No idea on a ranger, I have a stick. I've always called it an ECU, engine control unit...



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Had the same troubles a couple months ago.. step on it and would lug down... had another set of plug wires laying around and decided to put them in..... fixed the problem... the wires i took out looked good but there was something not right in them....