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2.5L ('98-'01) Yet another P1506 high idle thread


Conflict

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I've read everything related. Successful solutions have been throttle plate adjustment bad, dpfe bad, iac bad, dirty idle passageway, maf bad, egr bad, and high voltage. I haven't seen anyone admit to a vacuum leak, bad pcv, or dirty throttle plate being the issue. Coolant temperature is also a usual suspect, I've heard. I'll be trying a new dpfe tomorrow. Mine doesn't respond to vacuum being applied. But it seems like too much egr would lower idle, not increase it, which is my problem. So I'm not hopeful. I've shut off all vacuum inputs to intake (other than egr) while running with no effect. I completely blocked off the IAC passageway. Still high idle. TPS output is 1 to 4.2V with 600-850 ohms smooth operation. 5.9 Vref. EGR audibly responds to vacuum applied. I haven't actually watched the valve function. It's a nightmare disconnecting the EGR vlv beause it doesn't align with the throttle body and I have to battle that hard tube to get the bolts in. If anyone knows a trick to that, please let me know.
When I first start up, my idle will come down to 1200 after a few minutes but after I start driving, it's 1500 to 2000. My old scan tool says my TPS is 20.7 at idle and 41 giving a little gas. I don't know the units. MAF goes from 7.7 to 31, tracking with TPS changes. Coolant and intake air temp are accurate. Advance is 12 to 26. Short term is -3.4 to 8.5. Long term is .7 to 4. O2 sensors are .08 to 8 and .08 to .9 for 11 and 12 respectively. Those numbers don't mean anything to me but the fact that they change implies they're good, right? If any or all of this way off base, please let me know. I don't want to go to a mechanic, they have expensive lifestyles to support.
 


RonD

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P1506 Idle Air Control Overspeed Error

This means the computer is closing the IAC valve and idle RPMs are not going down

RPMs are RPMs, just the crank sensor involved, computer sees say 1,100rpms and closes the IAC Valve to get 700rpms, and nothing happens, so P1506

Which on the face of it means there is probably an Air Leak, other than the "planned" air leaks
IAC Valve is a "controlled" air leak, just like the throttle plate
PCV Valve is a "controlled" air leak

Air controls RPMs, not fuel
If you add more fuel you get a flooded engine
If you add more air you get higher RPMs

All the sensors are monitored and will set codes if they are out of spec
If a sensor is out of spec, and sets a code THEN you test it
Nothing wrong with testing and looking at sensor voltages or parameters, but none are really involved with high idle and P1506

Best test for vacuum leak is warmed up engine
Unplug 2 wire connector on IAC Valve, RPMs should drop to 500-600 if there is no leak and if IAC Valve is not stuck open, which can happen but way down on the list

With high idle unplug larger vacuum lines on the intake first, one at a time and plug the intakes port with your finger
Looking for idle to drop, engine may even stall, either is good
PCV hose, brake booster

Then start doing the smaller hoses
I think the 2.5l may have some UNDER the intake

You can spray soapy water around the intake, if any is sucked in the engine will stumble
And worst case the engine bay is cleaner and NOT on fire using combustible sprays, lol

Every one has had a vacuum leak, may not admit it, but its very very common on all gasoline engines
 

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This kinda reads as a vacuum leak but I know from experience that they don't run well with them. Wouldn't the fuel trims be adding fuel instead of taking it out if there was unmetered air in theory? The system volt to the TPS should be 5V not close to 6V and it's measured TPS seems high to me for being in a reasonable voltage range... It's almost sounding like the idle screw is just screwed in too far or something like that.

Like Ron said, with everything plugged and IAC unplugged it should be a low slow idle, if not maybe back the throttle stop screw out, for good measure maybe count turns.
 

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thanks Ron and Scott. I backed out the throttle screw 6 flats and it's totally drivable now. resting idle is 900 rolling is 1400. Measured TPS down to 19.2. Voltage out is .9V. CreepinJeepster said "Closed Throttle can be anywhere between 0.6-1.0 volts". I've had this truck for 13 years and never touched that screw. Why was it all of a sudden bad now? I know I have other issues. The PCV blows through easily in both directions. New one from autozone I can only get a wisp of air through both directions blowing really hard. Don't know which is worse. Should I keep buying them until I get one that acts like a check valve?
Before and after screw adjustment, unplugging IAC makes no difference. So maybe I need to keep backing the screw out until the idle controller needs to use the IAC?
Changed out the DPFE. The new one reacts to small pressure changes. The old one just sits at .8 volts out and won't budge. That changeout made no noticeable difference on a test drive. Vref for DPFE and TPS was 5V not 6.
 

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I've had this truck for 13 years and never touched that screw. Why was it all of a sudden bad now?
I would pull off the air tube at intake end and check if throttle plate is loose, 2 screws
Could also be the throttle rod bushings are worn, allowing a bit more air in
 

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I recently changed my spark plugs and cleaned the throttle plate while I had the intake off. I'm embarrassed to admit it didn't occur to me I might have opened things up a bit. It was pretty bad. I will go back in and check for looseness.
 

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Good thread if old .

I replaced my IAC valve and the 3,000 RPM idle stopped but now it seems to take a long time to return to the correct idle speed, this makes me wonder of the throttle plate might need cleaning ? .

This truck is way modern for me so I don't know bupkis about what to do .

Also, what is "DPFE" ? .

TIA,
 

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DPFE is basically a EGR flow sensor

If it takes a while to return to idle on these when coming to a stop it can be the speed sensor on the back of the trans (I don't think yours has this) or the one on the rear axle in the middle on top.

There's not much to adjust or do on these, if you haven't cleaned the mass airflow sensor I would recommend doing that too as it can help (use MAF cleaner or electric parts cleaner only)

And to add to one of your notes a couple posts ago you have two oxygen sensors, one just after or in the exhaust manifold and one after the catalytic converter, the front one or sometimes called upstream sensor is the one that does a great majority of the work.
 

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Is there a detailed thread on cleaning the M.A.F. ? .

I understand they're delicate .
 

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+1 ^^^

Delta Pressure Feedback
or
Differential Pressure Feedback
 

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Okay, I thought that was the rear ABS sensor, I cleaned it back when I first got the truck, it was filthy .
 

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That's the RABS sensor which also drives your speedometer.

For the MAF, just get a can of MAF cleaner or electric parts cleaner (both spendy and the same thing I believe) and spray off the sensor with the engine off. How delicate they are depends on the style, Ron will probably correct me if I'm wrong but I'm pretty sure it's based on a mild current applied to the sensor testing the temperature of the element based on airflow and ambient temp? Whatever, there's a little yellow deal in a plastic spoon thing that likely has some dust on it, usually spraying them off is enough...

This style fuel injection is more complicated than a basic carb but not too bad compared to some of the janky things that've been done over the years like air pumps, vacuum feedback carburetors with miles of vacuum lines and don't even look at a modern fully emissioned diesel if you think '90's era EFI is complicated :). I'm just poking fun...
 

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" there's a little yellow deal in a plastic spoon thing that likely has some dust on it, usually spraying them off is enough... " - how I love technical jargon .

I *think* the MAF sensor is at the far end of the air intake hose, right on top of the air filter box, yes ? .
 

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I'm trying to figure out why there would be any pressure in the differential......

Did I ever mention how much I love this bare bones base model work truck ? I had to go a distance today and fill the bed with good crap I bought at auction, this 20 year old truck just ambles right along and gives me little troubles .
 

scotts90ranger

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Yes, the MAF is the sensor over by the air cleaner in the intake tube.
 

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