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Rough low Idle


ratdude747

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My 1995 4.0 has developed a low rough idle. Warm or cold, she idles at 250-500rpm and very roughly, often times failing to start. Give it some gas, she will start/run but with reduced power. No CEL either.

I originally suspected fuel pressure trouble, which was found, and after replacing the fuel filter, fuel pump/sock, and pressure regulator I was able to get to the specified 30-40psi (at least from what I have measured during dead head and attempted idle tests). However, the issue still persists.

I cleaned out the throttle body, which was FULL of carbon. Slightly better, but still no good.

Disconnecting the EGR valve's vacuum line did nothing.

I did some scanning with my bluetooth scanner, and found that the upstream O2 sensors are reading lean and the short term fuel trim being around +35-37%. Since it's both sensors, I'm thinking that I am running very lean and not that both sensors went bad at once.

No obvious vaccuum leaks; I was getting pulsed vacuum at the EGR valve control hose (so it is pulling vaccuum).

At this point I'm stumped... suggestions?
 


RonD

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EGR valve should have no vacuum present unless you are driving and engine is under a load, just idling the EGR valve needs to be closed all the way.

Put a longer vacuum hose on the EGR valve so you can suck on it, or use a vacuum pump on it
Start engine and apply some vacuum, suck on the hose, engine should drop even more RPMs even with a little vacuum and stall out right with even more vacuum.
If a little vacuum does seem to change engine RPM then EGR valve could be stuck a bit open, causing your poor idle.

Your IAC Valve is suspect in any case, computer should be reacting and opening up IAC Valve more in response to idle RPMs lower than 650, manual trans, 750 automatic
Cold engine idle with either should be 900-1,100RPM

Remove IAC Valve from intake, plug in its wires
Turn on the key
IAC Valve should open all the way
Unplug it's wires and it should close all the way
Plug them back in and it should open all the way again
Repeat as much as you want to make sure it is not sticking.

IAC Valve gets 12volt with key on and computer gives it full Grounding, so it is open all the way for starting.
After startup the computer starts to pulse the IAC Ground, which in essences lowers its voltage so valve starts to close reducing the air by-passing the throttle plate, computer actively "watches" RPM via Crank sensor, and sets IAC Valve to maintain Target idle, Target idle is based on Coolant/Engine temp.

My '94 4.0l has warm engine target of 625RPM, it stays between 623 and 628 last time I tested it, computer and IAC Valve is a very precise setup
 

ratdude747

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(In Canada right now, not around to work on truck)

Follow-up: the O2 sensors are reporting that its running very lean. All of your suggestions would imply that it is running rich.

Unless the O2 sensors are wrong, I'm personally thinking that I have some sort of massive vacuum leak or injector control issue.

That said, I earlier suspected the IAC... Not opening at idle but then again with it removed (ports open to air) the issue persisted, which seemed to indicate that a lack of air isn't the issue.

Also when trying clear flood mode it tried to redline... Perhaps a TPS issue? Scanner shows 19.2% throttle at idle and 92% (or so) at WOT. My 1995 3.0 had the same behavior so at first I didn't suspect the tps.
 
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ratdude747

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Came back to the states today. Did some more testing:

  • Rechecked fuel pressure, 30PSI when barely idling.
  • Checked supply voltage to the injectors via the EGR solenoid connector (my schematics show it being on the same node of the circuit), 11.98 volts when barely running (12.0 volts at the battery in such condition, not enough power for the alternator to work?)
  • Unplugged a large vacuum line, when starting with an artificial vacuum leak it went to 1000RPM fo a split second then died.
  • Pulled the fuel pump relay while running, engine died immediately.
  • Still had pulsed vacuum at the EGR valve control line with the solenoid unplugged. Leaky solenoid?

Based on those, it seems that the injectors are getting power and the fuel pressure is good. Since it runs OK at high throttle, I don't think fuel flow is the issue either.

Right now I'm thinking I'm having a vacuum leak somewhere as the O2 sensors and STFT indicate I'm running very lean, which would explain the IAC not doing anything (if you're running lean, adding air isn't going to help) and I highly doubt both sensors went bad at once (if it were a common O2 sensor power/ground issue I wouldn't be seeing "good" singnals on my scanner, right?).

So unless somebody here has another suggestion, my next move is to see about getting some sort of fogging device and rigging it to my intake to look for leaks. No, I'm not doing cigars and I'm too impatient for the propane/carb cleaner routine (although if I must, I will).
 

ratdude747

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UPDATE: After a week+ of tinkering and work, I've gotten her mostly driveable but still not running right.

I did find a medium-large vacuum leak at the EGR valve, mainly due to work I was doing. Fixing this required a new EGR tube and manifold fitting (As my flared joints were rusted solid- I had to cut out the tube with a hacksaw) and a recycled EGR valve off my wrecked 3.0.

Fixing that leak brought my cold (ECU reset) STFT from the 40's (all numbers are rough averages between banks) to around 25%. Once the ECU compensated the STFT was normal and the LTFT was 15%, which dropped to 9%ish after an hour's drive home.

This said, everything runs fine (good idle) when the O2 sensors are warmed up and the ECU is in closed loop. Open loop, especially stone cold, she will not idle well and if stone cold stalls w/o manipulating the throttle.

I did spray around with carb cleaner, the only leak I found was at the EGR valve again (due to poorly tightened bolts and an unsettled valve gasket), which AFAIK is fixed. Might still be a brake booster though?

Barring that, the only other issue I can think of is a bad MAF- the MAF is responsive (not dirty?- I've cleaned mine twice), but can they also fail reading low? If it wasn't for the epoxied screws I would have swapped the MAF sensor from the 3.0 over- the Aluminum shells are different but the raw sensors are the same.

Ideas?
 

RonD

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What temp does the ECT sensor show at cold start?

and just after starting?
 

ratdude747

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ECT at cold: 62.6F (IAT says 57.2F)

At cold after start (feathering throttle to maintain idle) Same thing.

At cold idle dying: Same thing as well.

By the time I got to work, I was up to 182F, so the IAT seems to be right.

---

Something else I noticed... at the 600ish RPM it idles at closed loop, there is not much torque. As in if you put in gear, it stumbles/stalls if you don't give it gas soon enough (I have an Automatic). Also the Alternator voltages drop to just above battery voltage and the A/C cycles but doesn't cool. AFAIK it has the stock pulley too (not underdrive). Almost seems like normal idle is more in the 800's, which is where I have to push it to for my torque/accessories to come back.

:dunno:
 

RonD

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Yes, automatic trans idle, warm engine, would be 750 in Park, 800 "in gear".
Manual trans is 625-650

Cold start at 60degF should have idle of approx. 1,100, computer in Choke Mode/Open Loop, there is no O2 sensor feedback for 3 to 5 minutes after cold start, they need to heat up above 600degF before they can work, so computer ignores them

If you switch Scanner data to Grams per second(g/sc) on MAF data you should get approx. 1 gram per cubic liter of engine size, at warm idle, 600-700RPM
so 4.0L engine should be approx. 4 g/sc, 2.3l engine 2.3 g/sc, 5.0l engine 5 g/sc

You could also test the IAC valve to see if it is getting correct voltage from computer
When key is on 1 wire will have 12volts, direct from ignition switch, the other wire goes to computer and is the Ground wire for the IAC Valve, computer Pulses the Ground which, in essence, lowers the voltage at the IAC Valve closing it, faster pulses raises voltage and opens it.
At key on, IAC Valve should show close to battery voltage, so valve is opened all the way for starting, fully grounded via computer
After startup engine should surge above 1,500rpm because IAC valve is opened all the way, and then as computer starts pulsing the ground, voltage goes down, the IAC Valve will start to close and RPMs drop to "target" value in computers memory:
1,100 cold
750 warm
This takes 1 or maybe 2 seconds, it happens very fast, the surge and then drop to target RPM, assuming you don't touch the gas pedal, TPS(throttle position sensor) will override computer control.

If you use pins to pierce the IAC Valve wires you can hook up volt meter to see if computer is doing what it should in IAC Valve control

TPS can be tested with pins in the wires as well
Top wire should show 5volts with key on, volt meter grounded
Center wire, throttle closed, should show under 1volt, .69 to .99v is Ford spec
Throttle Wide Open(WOT) should show 4.5 to 4.99v on center wire


Under 600 RPMs an alternator can not produce enough volts, even at 650RPMs all 3 fields need to be working and Rotor needs full voltage to get enough AMPs out.
Alternators don't produce full rated AMPs until engine RPMs are above 1,500
So your voltage readings at battery can be normal, if RPMs were low.
But could also be you have 1 failed Field, this shouldn't cause any other issues except dimming head lights at idle.
Not uncommon for 1 field circuit to fail as alternator gets older, and usually not a problem since engine spends most of it's time above 1,500RPMs, so battery stays charged
 
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ratdude747

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Will check those tonight. Alternator is a rebuilt one from my wrecked truck that is a year and a half old. I swapped it thinking I had a field out, nope.
 

ratdude747

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At cold start I'm only getting 3.2 g/s max. Maybe it is the iac?
 

RonD

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3.2 at MAF at what RPM?

At 600-700RPM it should be just at or slightly above 4.0g/s, on a 4.0l Engine
3.2g/s would be correct for 3.0l engine

And would be even higher at cold start if RPMs are at 1,000+

MAF is reading ALL the air(well 10% of all the air) that is coming in to the intake, that includes IAC air and throttle plate air.
IAC Valve doesn't use "outside air", it is on a passage in throttle body that by-passes throttle plate

Vacuum leak or bad PCV valve can lower MAF g/s because ALL the air isn't passing through MAF.

3.2g/s is 25% off of expected 4.0g/s so a big difference for air/fuel mix
 
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ratdude747

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Oh sh*t.

Iac is getting 12v to open when open circuit. Nothing when the iac is connected. Issue is on the control side. Probed at the ECU connector (pin 83) and the same thing.

So it's either a fried ECU or a bad ECU connection?

I'll take a look and if necessary open the ECU to look for a fried switching transistor/ cracked solder joint.

ECU wise, remind me, what years and vehicle styles will swap? Will any 1995-1996 auto 4.0 work? 4x2 only?
 

ratdude747

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3.2 at MAF at what RPM?

At 600-700RPM it should be just at or slightly above 4.0g/s, on a 4.0l Engine
3.2g/s would be correct for 3.0l engine

And would be even higher at cold start if RPMs are at 1,000+

MAF is reading ALL the air(well 10% of all the air) that is coming in to the intake, that includes IAC air and throttle plate air.
IAC Valve doesn't use "outside air", it is on a passage in throttle body that by-passes throttle plate

Vacuum leak or bad PCV valve can lower MAF g/s because ALL the air isn't passing through MAF.

3.2g/s is 25% off of expected 4.0g/s so a big difference for air/fuel mix
that was at 500rpm, stumbling idle.

AFAIK I've gotten all of the vacuum leaks fixed. spraying carb cleanernear all of the potential leak points did nothing.

but given that my IAC is not opening I think the reason for the low flow is found.
 

ratdude747

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Cracked open the ECU, nothing obviously blown. Connector looked fine, so unless 0v loaded is normal, I'm condemning the ECU.

Ouch...
 

ratdude747

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Forgot to mention: iac valve read 10 ohms, so not shorted.
 

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