Greets all. 1993 f150 4x4 xlt 5.8L

I'm chasing a fast idle problem. I've read a boatload of fast idle posts, have made some progress, but am stuck. After replacing the TPS for a related problem, I was left with an improved throttle response, but with a fast idle of about 1000 rpm.

The only TDC generated is 311: Thermactor air system fault. Looking into this, the code generally seems related to catalytic feedback sensors, and since most of my exhaust system is rusted off aft of the manifold, I discarded that as the problem.

After coming up to speed on the IAC valve, and noting that when it was unplugged the idle droped to 650-750, I replaced it...with no effect. It still idles at 1000 and drops when unplugged. Grrrrr.

Suggestions?

 

Based upon other posts, I am proceeding as though the problem is a vacum leak.

I'm not real experienced, and based on the multitude of noises, I've not been able to detect it by listening while it runs. I'll proceed by the 'listen with a hose' method, and post back.

Thanks

 

Unfortunately, the IAC can check good, and still be defective, according to www.fordfuelinjection.com.

1. I would set the TPS voltage between .91 and .96 v, also check that voltage sweeps up and down. The voltage should go up and down as TB blades open and close.
2. Check the O2 sensor, ACT and ECT sensors and for vacuum leaks.
3. Throttle cable for binding.

 

Terribly sorry for the multiple posts, but I thought I'd add that I disconnected the battery (pos side, since I couldn't get the neg off) after installing the IAC with no success, just in case the computer needed to forget the TPS idle setting...with no success.

 

Check my first post. If nothing can be found, and all checks good, disconnect battery and ECM firewall plug. Then, pull the ECM from under the drivers side kick panel. Ensure no aftermarket chip is installed, and remove cover and check for burnt circuitry.

 

I did confirm that the throttle cable seemed free and clear, and when tweaked manually it returns to the idle position against the stop screw. Fornuately, I saw the warning on adjusting the throttle stop screw, since that was my first inclination.

I don't believe my TPS is adjustable, since the holes are round and not slotted. Is there a setting to adjust it somehow? I read one very helpful post wherein the individual posted a voltage table for various motors, but the resolution was too low to read. My Ford book said to confirm the test voltages were as depicted 'in the chart', but I could find no chart in the book or any reference in the table of contents.

My inexperience leaves me at a disadvantage with all of the acronyms used here. Might I trouble you to spell out what ACT and ECT mean?

Thanks.

 

It may be best if you spent the money on a Haynes Repair Manual. The Air Chage Temperature (ACT) sensor is in the upper intake, as well as the Engine Coolant Temperature (ECT) sensor, near the thermostat housing, do not confuse this with the engine temperature (temp light or gauge) sender. The ACT will most likely be oil fouled, and need to be replaced. The ECT can be wire brushed if not to terribly discolored.

Check the vac lines for cracks, check for vac leaks around the upper and lower intakes, check around the throttle body for vac leaks, and check the PCV valve grommet for cracking and or shrinking.

The TPS is adjustable. You may need to take a rat tail file and elongate the screw holes in the TPS. Some brands have them, and some do not. TPS voltage should sweep from minimal voltage to 5 volts and back to a minimal reading.

James

 

For what it's worth, I do have the Chiltons manual.

Thanks for the ACT/ECT info. I'll check both, measure the TPS, survey for vacum leaks, and report back.

Thanks.

 

The idle speed in park is high at about 1000 rpm, while in gear it drops to around 700 rpm. Dunno if that is significant. I've read here that they should be roughly the same.

I didn't quickly realize that the 1000 rpm was high, since it was an improvement over the problems resolved by the replacement of the TPS.

Slowly...ever so slowly....I realized that she was idleing too fast after I determined to have the transmission serviced due to the clunk occuring when shifting into gear at the higher RPM.

 

Could this fast idle issue be related to the bypass air idle speed control?

Since the only error code is thermactor related, and that BAIS control is part of the managed air thermactor system, I thought I'd ask.

My book says service is by replacement, but gives no diagnostic.

 

Check for vacuum leaks. Intake gasket, throttle body gasket, air inlet hoses, all vacuum hoses and connections. Search YouTube or Google. Best way to do it is with propane, but carb cleaner works if you do it with the engine cold and are extremely careful. Do not do the carb cleaner method unless you have a fire extinguisher or at least a garden hose on and ready just in case.

Clean the throttle body with throttle body cleaner and a clean rag. Make sure the throttle is not binding.

You can even remove the IAC valve see if it's caked with carbon. Carb or throttle body cleaner may help with that.

These are just some pointers and by no means is this everything. Could also be a fuel pressure regulator, or something else.

Timing set correctly? As stated before, check to ensure the TPS is correctly set. You may have a few things going on.

 

I did check the low voltage on the TPS, and is was about .91v. I saw a chart from Ford for the target voltage which said it should be .65v, but even after elongating one of the holes I could only get it to about .86v...then I checked the post above and saw that about .90v was suggested, so I left it.

I've already replaced the IAC, with no change.

I'll keep searching for a vacum leak, but I don't have the experience to be able to hear the leak...OR, I don't have one. What vacum lines I can recognize seem in pretty good shape. I'll give the torch test a run tomorrow.

A little more accurate description of the problem:
When the truck is started cold, it idles at about 1250 in park. If I goose it, it will settle to about 900. When put in gear, it idles properly at 750...and when stopped while driving it settles at 750.

Based on the helpful suggestions, it could be any sensor on the vehicle, so I'll keep plugging along...or accept it as is.

Any comment on the possibility of it being related to the thermactor error code? (since bypass air idle speed control is part of the managed air thermactor system)

Thanks to all.

 

TPS information from the service manual

Observe voltmeter while moving throttle slowly to Wide Open Throttle (WOT) position. Release throttle slowly to closed position. Voltage should change smoothly from about .6 volt at idle position, to about 4.5 volts with throttle wide open.
If voltage is not as specified, ensure throttle stop screw and linkage are adjusted correctly. Remove TPS and check for damaged, corroded, or misadjusted pins. If pins are okay, install sensor. Ensure sensor is seated correctly.

Code 311
Secondary Air Injection (AIR) system inoperative during KOER (Bank #1 w/dual HO2S)

 

EsJayEs: a very interesting post...veerrrry interesting.

First: you confirm what I'd read elsewhere, that the TPS should be bottoming out at about .6 to .65 volt. I wonder if that might indicate that my TB is not closing completly, or that my TPS is faulty. I did remove the TB and shine a light from the back to see if light was visible through a gap. I observed none, however I did notice that the lower 'hole' had some obvious deposits on the cylinder walls. I've not cleaned it yet because I read here about the contamination tolerant design, and the warning not to clean it. I think I'll clean it.

Second: you have a different definition of Code 311 than I. Can you confirm that yours would be applicable to a '93 351? My code list comes from the Innova Ford Code Reader specifically for 81 to 95 cars and trucks, which is the device I used to read it. I have confirmed that in my device documentation the 311 code is the Thermactor air system fault previously mentioned.
EDIT: After a little research, I found a code listing that shows the definition you reference, however it is titled: EEC-IV/OBD-II which may be in error. My code reader book says that EEC-IV is OBD-I and EEC-V is OBD-II. The OBD-II list shows your definition, and my OBD-I list shows my definition. If you indicate that this is somehow incorrect, I'll contact the reader manufacturer for clarification.

Note: I'm not exactly sure what your notation: Bank #1 w/dual HO2S means.

Finally: is the throttle stop screw that you suggest adjusting the DO NOT TOUCH screw on the side of the TB that I've read so much about, or is it the screw on top of the throttle body where the throttle cable linkage is?

Thanks for taking an interest

 

That's exactly what it said in the service manual when I pulled it up for your truck. Just passing it along.

Bank 1 refers to the cylinders on the passenger side of the truck. H02S means "heated oxygen sensor"

Now what I'm about to post is something that really shouldn't have to be monkeyed with, however there is a section in the service manual regarding how to set the idle speed. However considering your TPS voltage at closed throttle is high, I'm going to say the simple answer is to try and adjust the throttle stop screw. but keep track of exactly how you rotate it so that if you need to, it can be returned to where it was.

Idle speed adjustment
Idle speed on all engines is controlled by the Powertrain Control Module (PCM) and Idle Air Control (IAC) valve. If control system is operating properly, engine idle RPM is fixed and cannot be changed by standard adjustments. Some engines do not have an idle RPM specification. A scan tester must be used on these systems because control strategy and idle speed are controlled by PCM. Before performing idle speed adjustment procedure, complete a thorough basic inspection and self-test (KOEO, KOER, Continuous Memory self-tests) to verify proper operation of systems which may contribute to idle speed control problems. See appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.
Before performing idle speed check and adjustment procedures, ensure throttle bore and both sides of throttle plate(s) are clean. Ensure throttle plate does not stick in bore and/or linkage is not limiting throttle plate operation. Throttle lever MUST rest against throttle stop screw. When idle speed control device is disconnected to set minimum air rate, engine may stall. This is acceptable only if throttle plate and linkage are not stuck.
Some throttle bodies use sludge tolerant throttle plate designs; they are identified with a Yellow and Black label. These units must not be cleaned, because a protective coating is applied to the lip of throttle plate(s).
Turn off all accessories before checking and adjusting idle speed. Ensure cooling system is filled to correct level. Ensure there are no vacuum leaks or unmetered air entering the throttle chamber on mass airflow design systems. If vehicle is equipped with electric cooling fan, check and adjust idle speed when fan is off.

3.0L, 4.9L, 5.0L (With E4OD/AOD) & 5.8L
Connect scan tester to diagnostic connector. Enter KOER self-test. A single-pulse tone indicates entry mode has been successfully entered. Observe scan tester readout.

If a constant tone, solid light, or STO LO is displayed, base idle speed is within range and adjustment is not necessary. Exit self-test.

If a beeping tone, flashing light, or STO LO display flashes at 8 Hz, TPS is out of range and adjustment is required. Perform adjustment and repeat test.

If a beeping tone, flashing light, or STO LO display flashes at 4 Hz, base idle speed is too high. Go to step 7).

If a beeping tone, flashing light, or STO LO display flashes at one Hz, base idle speed is too low. Go to next step.

Check throttle plate for presence of throttle plate orifice plug. See Fig. 4. If there is no plug, rotate throttle return screw CLOCKWISE only to adjust idle speed until conditions in step 2) are obtained. If there is a plug from previous service, remove plug and rotate screw in either direction to obtain conditions in step 2). After adjustment, screw MUST be in contact with throttle lever pad.

If idle speed is too high, turn engine off. Using tape, block off orifice in throttle plate. If orifice is already plugged, go to next step. On mass airflow engines, reconnect air cleaner hose. On all models, restart engine and check idle speed. If engine stalls, crack throttle plate open with throttle plate stop screw. If idle speed is still too high, go to next step. If idle speed drops to specification or drops lower than specification, go to step 9).

Perform KOEO self-test. If fault codes are present, repair as necessary. See appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section. If TPS fault code exists, check for proper TPS output. See THROTTLE POSITION SENSOR (TPS). Ensure adjustment screw is in contact with lever pad. If fault codes do not exist, see TESTS W/O CODES article in the ENGINE PERFORMANCE Section.

Turn ignition off. Remove air cleaner hose. Remove tape from throttle plate orifice. Select correct color plug for orifice. Plugs are available in Throttle Plate Plug Kit (FOPE-9F652-AA).

Reconnect air cleaner hose. Start engine. Recheck idle speed. If idle speed is not within specification, turn adjuster screw CLOCKWISE to obtain correct idle speed. After adjustment, screw MUST be in contact with throttle lever pad.

On all models, perform a KOEO self-test to verify TPS fault code does not exist. Move throttle plate to verify it is not stuck in bore and linkage is not keeping throttle from closing. On models with automatic overdrive transmissions, check throttle valve pressure adjustment.