96 F150 5.8 High Idle... and I've replaced everything! Please help!
 

Hello,
I have been searching everywhere for the answer to my high idle problem.

Here is the current condition:

The truck was working fine.
Next, during cold weather, it would idle very low at start up.. I tapped on the EGR valve and the rpm's would come up to normal.

Now, the engine idles very high (maybe 2K?) ALL the time.

So I searched the forum and the internet for possible solutions.

Here is what I have replaced:
EGR valve
EGR vacuum solenoid
PCV and hoses
TPS (Throttle Position Sensor)... (When I installed it, I set the minimum at 1.0 volt)
IAC (Idle Air Control) solenoid
Plenum Gasket
Throttle Body Gasket
Thermostat
Charcoal vacuum hoses

I have sprayed around the engine with water, but the idle never changes.

There are no trouble codes.

I have 15hg of vacuum.



If I manually pull some vacuum on the EGR valve, the idle will go down.



I would welcome any suggestions....




Here are some photos....

https://www.f150-forums.com/attachme...ine=1322336982



https://www.f150-forums.com/attachme...ine=1322336982



https://www.f150-forums.com/attachme...ine=1322336982




https://www.f150-forums.com/attachme...ine=1322336982





https://www.f150-forums.com/attachme...ine=1322336982





https://www.f150-forums.com/attachme...ine=1322336982

Here are some more photos....


https://www.f150-forums.com/attachme...ine=1322339597



https://www.f150-forums.com/attachme...ine=1322339597



https://www.f150-forums.com/attachme...ine=1322339597




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https://www.f150-forums.com/attachme...ine=1322339597





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https://www.f150-forums.com/attachme...ine=1322339597





https://www.f150-forums.com/attachme...ine=1322339597




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https://www.f150-forums.com/attachme...ine=1322339597

Sounds like you might have a vacuum leak still. Start your engine up. Take some carb and choke cleaner and spray around the vacuum hoses and the EGR area. If the RPM's rev higher when you spray in a particular spot, replace the vacuum hoses. I had a similiar issue on a car of mine. Turned out it was a crack in the EGR tube.

If you unplug the IAC, does the idle decrease?

If it is a vacuum leak, what sensor(s) would tell the injectors to add more gas?

Is it in the exhaust? Is there some other intake sensor that is telling it to add gas? Or.... is extra gas being added?

The mass air flow sensor is upstream of the throttle body. The throttle body is not opening, so something is telling the engine it needs more gas.


And why would opening the the EGR valve cause the idle to go down?

From what I understand, the EGR valve start to open during cruising speeds.

I will try this and report back.
I understand the IAC operates when the AC (or some other power-robbing device) comes on.

Do you have an O2 sensor on this vehicle? Vacuum leaks, cracked EGR tube, etc. screw up the fuel mapping and the O2 sensor can see it. If it senses its too lean, more fuel is added to the system.

If the O2 sensors are operating in "closed loop", then the signal from the O2 sensors will tell the PCM to richen the A-F mixture to compensate for the additional air.

The EGR adding exhaust gases to the intake charge does not cause a lean condition, due to the lack of oxygen. Adding exhaust gases to the intake charge
lowers the combustion temp, which lowers NOX emissions.

The PCM uses the IAC to increase idle speed for various reasons, AC compressor being on is one of them as you mentioned. Another condition that will raise idle speed is the ECT sensor indicating a HOT condition. If the ECT sensor is defective, causing a false HOT condition, the idle speed will be increased.

The ECT (Engine Coolant Temperature) sensor... is it the same one that drives the instrument panel gauge? Or are there two different sensors?

The instrument panel gauge never gets above medium (normal) temp.



Update....

In digging around on the internet, it looks like there are three sensors. An air tem sensor mounted on the air box, a gauge temperature sensor mounted on the manifold, and the ECT mounted near the thermostat.

I'll have to verify that tomorrow. Hopefully, there is a test to determine if the ECT is good...

Separate sensors. The ECT sensor has a 2-wire connector, the temp gauge sensor has a single wire connector.

10-4....

I appreciate all the input, folks!

Yes....
I unplugged the wiring connector and the idle is normal.

This is a new IAC.


So.... something is telling the IAC to energize?

Here is the ECT (Engine Cooling Temp) Sensor


https://www.f150-forums.com/attachme...ine=1322418839



https://www.f150-forums.com/attachme...ine=1322418839



https://www.f150-forums.com/attachme...ine=1322418839



The block is about 60 deg F.

The resistance is 21K

The 22k reading is normal.
I bought another sensor and it reads the same.

As you heat the sensor, the resistance goes DOWN.

I think I'll check the connector for voltage. NO sensor connected (if the wire was broken) would mean excessive heat... unless the computer sees it as a failure.


CORRECTION.... No sensor would be infinte resistance which would be the COLDEST indication.

With key on, but not running, the ECT connector has 4.66 VDC.

This would mean the wires are not broken going to the sensor.



So what other items would tell the IAC to engage?
The AC is not on.
The ECT is good.

?????

I don't think so. When the IAC is unplugged the engine should idle extremely slow, to the point of almost stalling. You may want to check the voltage on the throttle position sensor again, to make sure the throttle is going to the rest position.

If the sensor was not connected, this would cause an infinite resistance, which would be in the COLD direction, not HOT.

Did you measure the 4.66V across the 2 connector pins with the connector off the sensor?

Measured the 4.66 with the sensor disconnected.


Thanks for pointing out the "infinite resistance" mistake....

I double checked the TPS sensor and voltage (1.01 VDC) before I bolted the throttle body back on. The throttle cables didn't move the throttle arm. I opened and closed the throttle and watched the voltage change.

It always went back to 1.01 VDC.


Is the IAC "variable" or is it "two position"?


Thanks again for all the help....

The IAC is "variable", or analog, not digital (open or closed) :D:D

When you set the TPS voltage, did you rotate the sensor, or adjust the throttle plate stop screw?

When you measured the 1.01V, were you using the GND terminal on the TPS sensor? Do you measure 5V (or close to it) on the 3rd TPS terminal?

TPS:
I rotated the TPS to get the 1v, then I tightened the screws (I also added purple Loctite on the screws).

Everything I have read says NOT to adjust the stop screw.

I followed the instructions in the Haynes manual. It showed the two wires to measure across to find the voltage. I did not look at the other wire as it was not mentioned in the Haynes manual.

By the way... I'm in Raleigh.

Nice to meet you neighbor, I'm in Northern Durham County!

The 15 inches of Hg is on the low side. Is the vacuum gauge steady at this reading, or is it fluctuating? One other source of a vacuum leak is the vacuum line going to the brake booster. Have you tried pulling this line from the brake booster, and plugging it?

The 15" stays steady.
I have not tried the booster yet.

Should I keep looking for vacuum leaks... or is there another sensor or device that might be causing it?

I would keep looking for a vaccum leak.

I have sprayed all the hoses with carb cleaner and there is no change... other than the paint is gone!

if you cannot get the TPS to read .6 to .8 volts you have to adjust that screw.basically you don't want to touch it unless there is no other way to set the TPS to spec.at 1.0 volts this makes the PCM think it is at part throttle and it caused all sorts of bizarre behavior,even unintended acceleration when you put it in gear.

what i do when setting it is i put the TPS at mid travel then adjust the idle screw to get .8 at the TPS.try it and report back with your findings Deet.:)

Update...

I just borrowed a scan tool from my neighbor. I hooked it up with the engine OFF and it said that the TPS was at 20%.

This has GOT to be the problem.

I am going out to start wrenching and I'll let you know if setting the TPS down to 0.6 or 0.8 VDC works. The scan tool will verify that the TPS is reading lower...

sounds like you are going to conquer it.did you see the video in your PM box at Y.E.com.this is more or less how it should behave when idling down,even with a cold engine.

With the throttle body just about wide open, the TPS reads 88%.

I tried adjusting it and it will only go to 18% at rest.

So I completely removed the TPS to see what it would read at rest.

With the TPS completely disconnected from the throttle body, it is reading... 10% open.


I don't know if the TPS is "bad" (although it is new).

I can't make it read any lower.

adjust your idle screw,hook up a digital VOM to the signal return and V Ref wires (i think,i am very rusty) then adjust the screw on the throttle body.the aftermarket parts these days come out of China and the quality often leaves very much to be desired so the chance of a defect is very high.i would use a genuine Motorcraft part on that one for sure.

I'm going to measure the voltage next...

Adjusting the screw won't change anything. When the TPS is completely disconnected from the throttle body it still reads 10%.

speaking of idle screw,how to set base idle.
unplug the IAC Valve then adjust the idle screw with the engine warm to about 600 rpm,check the TPS voltage and make sure it never goes past .99 volts at closed throttle, .6 to .8 is what you want.

either have a wiring problem or bad PCM.:(

one other thing,remove the TPS from the throttle body,plug it into the harness and then check the voltage.it is possible something went wrong when installing it.did you check the voltage at the TPS with a DVOM before?

here is a video i stumbled across,instead of using body ground for ground use the black wire for the ground as it is the ground from the PCM.the 3 wires on this is the Vref wire,the signal return wire and the ground.you would want to hook up to the green and black wires on a 1990 Mustang.the 351 in your truck even though it is OBD2 is similar to the OBD1 is on the Mustang.OBD is short for On Board Diagnostic.the later ones this capability is enhanced and monitors more functions that OBD1 does.this one is Supercharged and this is why he sets the voltage that high,a naturally aspirated near stock or stock engine does not require this to be set that high.this will cause drivabillity problems done this way though.

If I unplug the TPS, the scan tool reads 0.0%

yeah,there is something wrong with the TPS or something went wrong in the installation.with the TPS removed from the throttle body and plugged in into the harness what does it read?

i go by process of elimination and is one of the best ways to diagnose problems i found.parts changing will bite you where it hurts.

good move the way you did there BTW.