91 Ford Bronco High idle and Bucks at low RPM

i have a 91 ford Bronco EB with a 5.8. Upon initial start up its seems normal, high idle for a few seconds and slowly decreases to an idle of 700-800..    after driving it, it idles to 1400 rpm. Also when driving around town, when the truck shifts to 3rd and 4th gear the truck bucks (chugs) almost as if its not getting fuel. when starting from a stop, the truck has no immediate throttle response. step on the gas and it falls on its face. once in a while it will run OK. When i am on the freeway and i step on the gas to pass... the truck will downshift and bog and slowly gains speed. 

I pulled all the  codes checking the KOEO and KOER. i only have a temp sensor that i replaced with a new one but the code remains. 

need help to get the throttle response and power as well as rid the truck of the bucking issue. 

I've put the following into the emissions of the truck...  

MAP sensor

idle control valve.

Throttle positioning  sensor

full tuneup

cleaned the throttle body

new fuel injectors

o AC,

WELCOME

What is the Code:

Some possible high idle causes;

suspect ECT sensor or it's connector/wiring.

Testing; "..."But due to its simplicity of design, the ECT is rarely at fault when problems occur. Before testing the ECT or any other EFI component perform a self-test, trouble codes received during test can be used as a diagnostic tool along with other indicators. To test an ECT sensor you will need a volt meter. You can test the ECT by back probing the harness while reading the voltage returning to the EEC. Or you can removing the connector completely and test the resistance between the 2 pins on the ECT..." READ MORE Source: by Ryan M (Fireguy50) at fordfuelinjection.com

"Before you start blaming the engine coolant temperature sensor and replacing it make sure the rest of the coolant system is in good condition. All of the following items will affect the ECT:

*Coolant level

*Radiator Fan

*Water Pump

*Water Pump and Fan Belts

*Thermostat

*Base Timing

*Engines general condition

*Harness and wire general condition

See site for diagrams and test chart & connector pin-out info

The Engine Coolant Temperature (ECT) sensor is a thermal transistor, which means it allows less electricity to pass through the sensor the warmer it gets. The ECT receives the �Signal Return� voltage from the EEC, then allows a certain amount to return back to the EEC. Because the ECT is in direct contact with the engine coolant flow it changes resistance in response to the temperature of that coolant.

The ECT is third in command in the hierarchy of EFI sensors, this means this sensor is very important when calculating fuel ratios and timing curves. This is because of simply chemistry; fuel and spark are constants in the equation. Air is the biggest variable in combustion; it changes density greatly over a range of temperature. So keeping track of the changing temperatures of the incoming air and the engine in which it is burnt becomes very important. But due to its simplicity of design, the ECT is rarely at fault when problems occur.

Before testing the ECT or any other EFI component perform a self-test, trouble codes received during test can be used as a diagnostic tool along with other indicators. To test an ECT sensor you will need a volt meter. You can test the ECT by back probing the harness while reading the voltage returning to the EEC. Or you can removing the connector completely and test the resistance between the 2 pins on the ECT. The engine temperature must be greater than 50�F (10�C) to pass the KOEO Self-Test and greater than 180�F (82�C) to pass the KOER Self-Test. To accomplish this, the engine should be at normal operating temperature.

Values were calculated for VREF = 5.0 volts. These values may vary 15 percent due to sensor and VREF variations."

MIESK5 NOTE; if VREF is Not 5.0 volts or close. then there is a wiring/connector terminal issue from EEC to ECT or possibly EEC damage. But other Sesnors' (such as TPS, EGR, etc.) codes would come up if EEC is damaged or VREF is off. VREF wires share the same reference voltage circuit w/ many sensors & share the same ground circuit.

miesk5 NOTE; batty should be 12.6 volts at rest after a charge

DTC 21, 51, 61 or 116, 117, 118 "...Failure in either the circuit or temperature sensor will show code 21, 51, 61 or 116, 117, 118.Unplug the harness connector. First, check the signal voltage at the connector with the key on, engine off. Should be approximately 5.0v. Then, check the resistance of the sensor cold. Should be between 58,750 to 40,500 ohms. Plug in the harness connector and warm up the engine to normal operating temperature. Then, unplug and check the resistance of the sensor hot. Should be 3,600 to 1,840 ohms...'

Source: by Seattle FSB

IAC valve gasket mod by Nelbur; http://broncozone.com/topic/22199-possible-computer-issue/

See my a Vacuum leak test in post #20 @ http://broncozone.com/topic/23994-90-58l-getting-continuous-code-33-and-running-code-44/

Not sure if it's the same sort of thing with my 94 5.8L... Upon start up seems normal, after driving for 10 mins or so....idle goes up to 2000-2400 any time I touch the gas it sputters and acts like its missing or starving for fuel. I don't have a CEL on so not sure if there would be any codes to pull koeo

Sounds like a bad fuel pressure regulator.....it's pretty easy to replace yourself and the part doesn't cost to much, I think about $30

I have a similar issue with the occasional engine skips a beat. It has been tuned up recently, new plugs & wires, new fuel filter, new fuel pressure regulator, ran a can of seafoam, everything else is perfect except for the engine skips a beat and causes the truck to buck, it may do it at idle, it may not, it may do it at high speed or it may not, it may doing it going up hill or down hill or it may not, it may drive 5 miles and not do it or it may do it 3 or 4 times in one mile. When you first start it cold, the rpms swing  between 500 to 1100 then settles down and idles at 750 to 850. I have cleaned the IAC and gave it a shot of penetrating oil. About ready to take it to a shop unless anybody else has a DIY suggestion. Thanks!

Yo 87,

Is check engine light on while eng. Is running?

Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19

http://broncozone.com/topic/14269-code-reader/?pid=74587&mode=threaded

A possible issue is:

Curb Idle and Fast Idle are controlled by the PCM and IAC and are not adjustable.

Engine Load, TPS Sensor and the IAC are very closely linked, they must all work together to maintain a good idle.

Idle Air Control (IAC); also called, Idle Air Bypass (IAB), Idle Speed Control (ISC), Throttle Bypass Air, Idle Bypass, Inlet Air Controller, Inlet Air Bypass, Intake Air Bypass, Intake Air Control

Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; "...Hard cold starts, hesitation and stalls on initial start-up or during idle or decel may be caused by sludge in the throttle body and/or idle by-pass valve. Sludge deposits or oil film on the throttle body bore and plate or the idle air by-pass valve may cause one or more of the following conditions. Hard Cold Start, Stall On Initial Start-Up, Stall During Idle, Stall During Decel, Rough Idle, Rolling Idle, Hesitation During Acceleration. A new idle air by-pass service kit (F2PZ-9F939-A) is now available for service use to correct sludge contamination concerns of the throttle bore and plate only. It eliminates the need to clean the majority of past model throttle body applications. Cleaning is not required on sludge tolerant throttle body designs released for 1991 and newer model years..." Source: by Ford

try unplugging the IAC motor. If the idle drops, its usually due to an input signal to the computer which results in the computer raising the idle speed. If this is the case, look at all of the computer inputs to make sure everything is up to par. If the engine RPM does not drop when the IAC motor is disconnected, then either the IAC motor is not functioning properly, or, there is a vacuum leak in the engine. Stop the engine, remove the IAC motor and block off the passages. Restart the engine to see if there is any change in engine RPM. An idle speed higher than base idle specs indicates a vacuum leak. If the engine RPM is at base idle specs, then the IAC motor is bad. Resistance specs are 7-13 ohms. A good resistance reading does not necessarily mean that the IAC motor is good. Occasionally, an IAC motor will become weak, and will actually bypass too much air, causing a high idle condition even though everything else is operating properly.

Nelbur posted a diy here;

Idle Air Control (IAC) Modification in an 86; "...I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally. For most of my trucks life it has been dying when it was started hot. It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. It would never die when cold as the IAC would keep the idle speed up, and it never died at stop lights. Now it's worst situation seems to be when it has been shut down for a half hour or so, and restarted. I wonder if the heat soak is fooling the ETC into thinking the engine is warmer than it really is. I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow..."; miesk5 Note, Tomco makes an idle plate P/N 8491 = to Ford, P/N F2PZ9F939A; Like Ford’s service kit, Tomco’s spacer plat e (arrow) is installed between the air bypass valve and its mounting boss using two gaskets. The two holes in the plate are for bleeder screws, which provide a minimum air rate adjustment independent of the throttle opening.

Source: by Nelbur at Ford Bronco Zone Forums http://broncozone.com/topic/22150-fast-idle-problem/?pid=115710&do=findComment&comment=115710

I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally.

For most of my trucks life it has been dying when it was started hot. It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. It would never die when cold as the IAC would keep the idle speed up, and it never died at stop lights. Now it's worst situation seems to be when it has been shut down for a half hour or so, and restarted. I wonder if the heat soak is fooling the ETC into thinking the engine is warmer than it really is.

I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow.

Have since replaced the ECM, TPS, ECT, F INJ RELAY, FIXED CRACKED VAC LINES, FUEL FILTER, FUEL PRES REG, and the occasional miss still persist. There are no codes. About ready to yank the distributor and start all over.

add ignition control module & stator to the list. What is the little condenser thing next to the coil? It looks extremeky corroded. Maybe that be an issue?

Yo,

Ok.

Condenser is Capacitor, Radio Noise Suppression for sound control... should not be the source of the problem, replace it anyway because some old sources claim that when it fails it causes ignition problems. Disconnect it and take Bronco for a test ride.

How did spark plugs look?

And, did engine skip before spark plug wire change?

Spark Plug Wire Routing & Firing Order TSB 94-04-10 for 87-93 5.0

The firing order for 1987-1993 5.0L is 1-5-4-2-6-3-7-8. If #7 and #8, or #2 and #4 spark plug wires are routed next to each other at the separation bracket, an induction crossfire condition can occur.

by Ford via Tank92 (Tank) at

http://www.supermotors.net/clubs/superford/registry/media/299011

Misfiring

Owners should always follow the manufacturer’s suggested service interval unless there are driveability problems due

to misfiring. Obviously the ideal service interval for a spark plug is to replace it before it starts to misfire. Some of the

possible causes of misfiring are listed below.

Spark Failure: The electrical current is not strong enough to arc across the electrodes of the spark plug.

Possible Causes:

• Ignition coil voltage is too low due to battery condition

• Ignition coil voltage is too low due to ignition coil or igniter condition

• Voltage leak between the coil and the spark plugs

• Dirty or fouled spark plug

Spark Failure: The required voltage is too high to ignite a flame kernel.

Possible Causes:

• Spark plug electrode wear

• Spark plug gap is too wide

• Defective connection between the high-voltage spark plug wires and the spark plug terminal

Failure To Ignite: The spark plug fires normally, but the air/fuel mixture does not ignite.

Possible Causes:

• The spark plug gap is too narrow

• The air/fuel ratio is too lean or too rich

For more info, see [PDF]Technical Training Manual - Autolite - Autolite Spark Plugs

AT

https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.autolite.com/media/14882/technical_training_manual.pdf&ved=0ahUKEwjCwbvL_ZrLAhWFdj4KHTq7DlEQFggrMAc&usg=AFQjCNHGIx6V1LUid6T8WkbSspkcpLtCwg&sig2=sRTDa80ns0obWNMoWEJRMw

Will bbl...

Thank you for the input!. On the advice of my wife I went and unplugged & replugged # 3 spark plug wire. ( I had an exhaust leak that cooked it pretty good so I went for it first) Just drove about 100 miles without a single hiccup!!

This is a classic case of hijacking someone else's post.  Please do nto tag your problems on someone else's post, start your own thread.  I am going to start enforcing this forum rule.  I will delete any that I find. IT DIVERTS ATTENTION FROM THE ORIGINAL POSTER'S PROBLEM. 

Gotcha Supreme Commander. My apologies. No harm intended. It ended up being a cracked vac line to the little vac switch just to the right of the ign coil. The lower line. I think this effects ****** on the distributor.