My son's 1984 F150 with 4.9L is still very original and we hope to keep it that way for as long as possible. [My 1986 with the same engine has been converted to Duraspark, so I am familiar with that.]

The hot engine idle speed of his truck varies from 900-1200 RPMs. When the A/C compressor comes on, the idle speed drops to a more normal speed, but still a little high. The truck starts and runs well, but fuel mileage has lowered.

Chris TuButis provided me with the instructions for reading the blinking light. There were two codes: 23 and 42.

There is no binding of the accelerator cable and no apparant vacuum leaks. Pinching the PCV valve hose does lower the idle, but replacing the PCV valve resulted in the same high idle condition. Pinching the power brake booster hose had no effect. The 4 screws holding the carburetor halves together were loose, but tightening them did not help. Turning the idle air mixture screw in did help lower the speed, but still not normal.

The truck does still have the hard plastic vacuum lines from the solenoids to the EGR, etc., so I will replace them.

The 12v stepper motor is working and the timing [with spout disconnected] is right on.

What do you all think?

Thanks!

 

Make sure the choke is all the way straight up, and the screw that hits the cam for the fast idle on the choke is not touching the cam. This thing is a weird mixture of mechanical carb, and electrical controls all working together(or fighting each other).

 

23 = Throttle Position Sensor values are out of range, I would pay attention to that, first.

 

Yes, that's a excellent place to start. You now have to figure out is it a electrical problem with the position sensor, or a mechanical problem with the linkage causing the sensor to be off. Some of these problems are head-scratchers sometimes.

 

Thank you, Chris and Dave. My Ford Technician friend is trying to help me. He did try to check the TPS adjustment. I will check the choke adjustment and the screw / cam relationship.

 

KOEO = no codes stored

KOER = the above posted codes

The choke is completely open when warm and the fast idle screw is not contacting the cam.

It seems that something [?] tells the engine to idle up, since mechanically there is no binding or contact from the idle screws. I suppose this is the 12V stepper motor. The question is, what tells the engine to idle higher? And richer?

Again, it starts easily and runs well, but is on the rich side [the exhaust pipe is dark, but it does not puff out black smoke].

The only sensors I am aware of are the MAP sensor, the O2 sensor and the temperature sensor located by the thermostat housing. I believe the MAP sensor would only affect timing and the O2 sensor "reads" the exhaust and tells the computer to adjust the lean/rich mixture, but I would not think it would contribute to the increased idle speed. The temperature sensor "tells" the computer that the engine is cold/hot and that might contribute to both the richness and the increased idle speed. With the engine at operating temperature, I disconnected the temperature sensor and the engine reved even higher [from the ~1000-1200 RPM starting point]. So, I would think the sensor at least recognized the "perceived" lower temperature and increased the speed???

Any ideas? Thank you!

 

The 23 is the TPS, already said that.

The 42 is an oxygen sensor problem, although I don't remember which one.

I would first deal with the TPS problem and see if the O2 sensor problem goes away at the same time.

BTW the KOEO test contains two sets of codes....

KOEO - The computer checks for connectivity to all connected sensors and if they're reporting values within range

CM - Continuous Memory - These codes come after KOEO and are what the computer has stored from the previous N driving cycles.

A misbehaving TPS can easily have the computer set the idle speed higher, a misbehaving oxygen sensor can easily make the computer enrichen the mixture.

 

I know most of us frown on "throwing money at parts", but I am wondering if it would be prudent at this point, to purchase a new TPS and see if that would correct the problem. Then, if still rich, but at normal idle speed, replace the O2 sensor?

The O2 sensor is a Bosch unit. Not having ever replaced a TPS, would Motorcraft be available?

 

If the computer is telling you the part is misbehaving, that is an excellent reason to replace said part (assuming the wiring & connections are good).



Yes, replace the TPS, clear the codes, see if the O2 sensor (or any other) comes back, proceed from there.

 

I found a Motorcraft replacement. Scheduled delivery time is 3-5 days.

 

Late to the party, but...
When you did the KOEO test, did you open the throttle fully so that the computer could tell if the TPS is working? If you didn't, that would explain the code 23. I found out the hard way. Hope you didn't.
Stop at AutoZone and ask to see their sensor testing book. It has hundreds of tests to check different sensors, including the O2 sensor.
Best of luck.

 

Are you sure about that? You shouldn't move the throttle during the engine off test. You should move the throttle when it tells you to during the engine running test.

 

A little update... The TPS arrived and this past weekend, I installed it. While everything [air cleaner, etc.] was off, I looked down the carburetor to make certain the throttle plate was fully closing and it is. The throttle linkage is not binding. The engine was cool and the choke closed most of the way.

Once the TPS was installed, the battery was reconnected and the engine was started. The fast idle worked [~1500 RPMs] and the choke quickly came off and the choke plate was properly positioned [straight up]. The warm idle was still too high [my heart was heavy] at ~1100 RPMs [same as before]. I looked at the adjustment screw for base idle and it does not even touch the carburetor body [my technician friend had backed it out]. The high idle adjustment screw was not touching the cam. Taking a screwdriver, I pressed against the base idle adjustment screw and it cannot be moved to lower the idle speed.

I then took carburetor cleaner and sprayed where the manifolds mate with the block and noticed a lowering of the speed when spraying by the #1 cylinder. It was abrupt as the red straw is missing from the can and the spray pattern is diffuse and evaporated almost immediately with the fan wash. My son, who was in the truck, said the drop was about 100 RPMs. So, it seems the truck does have a vacuum leak.

We checked and tightened the upper manifold bolts, but with the hot engine, were unable to tackle the lower ones. I am not sure I have the appropriate length of extensions and deep well sockets needed, but the technician friend will have them. After tightening what we could, the idle speed was ~100 RPMs lower than before.

After driving the truck yesterday for a more extended time period, he reported that the idle speed dropped another 100 RPMs. I wonder if the computer is relearning or the vacuum leak sealed somewhat due to thermal expansion.

I do appreciate the input provided by the members who have taken the time. I'll get back with you after tightening the lower manifold bolts.

 

Did you disconnect the battery before replacing the TPS? I've learned to do it that way as there is no other way I'm aware of to tell the computer you changed that component; I changed one once without first disconnecting the battery, it idled WAY fast! So, I out of habit disconnect the battery and let it re-learn its idle speed.

 

Yes, the negative strap was removed prior to doing any of the work.