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I have a 1990 Bronco 5.0L that decides to cut in and out and sometimes stalls... always fires right back up cant find the issue I've done alot of work to it and the issue still persists. Will also do a quick dramatic drop in idle then go back to 2500 rpms when trying to keep it steady at 2500 rpms.
1. Rebuilt engine and replaced every internal component with new components "stock build"
2. Rebuilt heads
3. All new gaskets FEL-PRO
4. NEW SENSORS " ECT, ACT, 02S, MAP, IAC, TPS, EGR, EGR SOLENOID
5. NEW COOLING SYSTEM "All components"
6. New Catalytic converter with everything connected as the stock was
7. New ECM ... double check new ECM "Good"
8. New injectors from rock auto Pink MotorCraft.. it said OEM replacement but they aren't orange like the originals.
9. 1 year old Fuel pump and filter
10. New fuel pressure regulator
11. NEW plugs, wires, distributor, and coil. Triple checked firing order and spaced wires properly
12. Battery and cables
13. Ignition switch at the bottom of the column
ONLY COMPONENTS NOT REPLACED
1. Smog pump and components
2. Knock sensor
3. Wiring
4. Alternator is only 2 years old and its motor craft
5. Multifunction switch in column.
I have a feeling it may be a wiring issue or a bad ground but I'm not entirely sure ... I've check all the vaccum lines several times and they are flawless "used starting fluid also" ... the truck runs great it's just this issue and I've read everywhere and cant find a solution
.. yes its timed properly and has proper fuel pressure..
If anyone can help point me in the right direction that would be awesome
 

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Yo BRONCOADDIKT,
Welcome!
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
See if you can get it to run long enough to reach normal operating temperature.
If not do the key onm engine off portion.
.
Try unplugging the Idle Air Control (IAC)

Location pic by Steve
Testing..
"First let me say this little thing has many names. But they all talk about the same item under the hood. Here all the names I've had the torture of learning throughout the years:
•Idle Air Bypass •Idle Air Control •Idle Speed Control •Throttle Bypass Air •Idle Bypass •Inlet Air Controller
•Inlet Air Bypass •Intake Air Bypass •Intake Air Control.
This is really easy to test. First as with all problems you should gather the trouble-codes from the computer. Follow the codes for testing and repair. If you get a code that points to a problem with the IAB start the vehicle and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the IAB. If the engine begins to stubble or stalls the IAB is functional and does not need to be repaired. If the engine idle does not change you should remove the IAB for inspection.
The IAB can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAB to function properly. Remove the IAB and clean it. There are 2 halves to the IAB, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAB needs to be replaced." By Ryan M.
...
This is by Nelbur; "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."

Nelbur mentioned this Ford kit in..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..."
Buy a Motorcraft IAC in event it needs to be replaced.

See my vacuum leak test in post #11 @
"When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, such as the power brake, the unit will not function correctly. Or Air Conditioning when in MAX mode may switch to Defrost."
●□●

Here are the 1990 Bronco Pre-Delivery Shop & Electrical & Vacuum Troubleshooting Manuals (EVTM), Partial by member Kingfish999 in Google Drive @ https://drive.google.com/drive/mobile/folders/0BzRTW4yeT2u4UkhzR0ZydDd3MzA?usp=sharing&tid=0BzRTW4yeT2u4NXB6NW5neFRoZGc
Find page # of item desired in contents, then flip screen down like spinning a thumbwheel, then stopping periodically to see what page you are on.
Suggest you download for faster scrolling and to use the table of contents index.

1990 Ford Bronco Brochure, partial by El Kabong @ 1990 Ford Bronco Brochure pictures, videos, and sounds | SuperMotors.net

For any Bronco issues or to chat about it's planned modifications or build, it's better to post each seperately in Noobie Bronco Tech Questions. Flame free zone. This will get more attention and you can build up your post count to get into other sections such as Bronco and Ford Parts/Accessories (75 posts required to view). I'll move this thread to the Noobie section for better visibility.

To save you time and for better responses, please fill out your Signature with location, year, engine size, transmission type, transfer case type (manual or electric shift), locking hub type (automatic or manual) info & major mods such as a Lift, etc. .
From the navigation, near the upper right-hand corner, click on your avatar and then select “Account Settings” from the drop down menu.


From your “Account Settings” page you will then see more navigation option of the left side. Select “Signature”.


Now you can simply enter your signature information in the text editor and click save.

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Discussion Starter #5
I checked all vaccuum lines thoroughly... this issue seems like a short of some sort ... could if be the multifunction switch?
 

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Electrical controlled engines can give you trouble with only one wire partially f#€£Ed up either in the middle from friction rubs or something as simple as a rusty connection, especially when your talking about something over 10 years old. Wiring problems are a nightmare tracking down but as easy to fix as a cheese sandwich. If you got wiring issues I wish you the best of luck and seriously keep a log book of each wire or connection you check so in 3-4 weeks you remember what you have checked. Might be worth checking your transmission filter as it’s easy enough to drain and service while your there, as someone mentioned if the filter drops that could be an issue.
 

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I've replaced the IAC twice so I know that's not the culprit... it has an AOD Trans
Yo BRONCOADDIKT,
You can inspect the multifunction switch
Multi-Function Switch (MFS) Failure Causes; "...Mechanical components wear out or electrical connections are faulty; If the switch assembly is not operating properly, turn signals, hazard warning lamps, windshield wipers/washers and the headlamp beam selection may not operate properly. Related Components to Check; The switch itself, the electrical connections to the switch and the connections to each of the affected components (items such as turn signals, wipers, etc.)..."
Source: by motorcraft.com
Multi-Function Switch Replacement by Kevin L at kevin-long.tripod.com via web.archive @ http://web.archive.org/web/20070905175145/kevin-long.tripod.com/mfswtch.html

Try unplugging the MAP Senso and start the Bronco.

Did you have the opportunity to try the self test for codes?

How was the IAC passage in the throttle body?
 

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Discussion Starter #9
Miesk5 the IAC was fine...
I'm gonna replace the multifunction switch for good measure...
How do I properly test the map ? When I unplug and start it, it will fire up and run with an engine light and it run extremely rich
 

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Discussion Starter #10
Miesk5 the IAC was fine...
I'm gonna replace the multifunction switch for good measure...
How do I properly test the map ? When I unplug and start it, it will fire up and run with an engine light and it run extremely rich
 

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Miesk5 the IAC was fine...
I'm gonna replace the multifunction switch for good measure...
How do I properly test the map ? When I unplug and start it, it will fire up and run with an engine light and it run extremely rich
Yo BRONCOADDIKT,
I undestand the IAC is ok, but how about the throttle body passage?
□⊙□
MAP SENSOR Testing, Symptoms & Overview; A multimeter that can read frequency is normally required to check the sensor's output.
150654

150651


150652
 

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I have a 1990 Bronco 5.0L that decides to cut in and out and sometimes stalls... always fires right back up cant find the issue I've done alot of work to it and the issue still persists. Will also do a quick dramatic drop in idle then go back to 2500 rpms when trying to keep it steady at 2500 rpms.
1. Rebuilt engine and replaced every internal component with new components "stock build"
2. Rebuilt heads
3. All new gaskets FEL-PRO
4. NEW SENSORS " ECT, ACT, 02S, MAP, IAC, TPS, EGR, EGR SOLENOID
5. NEW COOLING SYSTEM "All components"
6. New Catalytic converter with everything connected as the stock was
7. New ECM ... double check new ECM "Good"
8. New injectors from rock auto Pink MotorCraft.. it said OEM replacement but they aren't orange like the originals.
9. 1 year old Fuel pump and filter
10. New fuel pressure regulator
11. NEW plugs, wires, distributor, and coil. Triple checked firing order and spaced wires properly
12. Battery and cables
13. Ignition switch at the bottom of the column
ONLY COMPONENTS NOT REPLACED
1. Smog pump and components
2. Knock sensor
3. Wiring
4. Alternator is only 2 years old and its motor craft
5. Multifunction switch in column.
I have a feeling it may be a wiring issue or a bad ground but I'm not entirely sure ... I've check all the vaccum lines several times and they are flawless "used starting fluid also" ... the truck runs great it's just this issue and I've read everywhere and cant find a solution
.. yes its timed properly and has proper fuel pressure..
If anyone can help point me in the right direction that would be awesome
Have you checked function/connection.. of the Engine Coolant temperature sensor ??? as mine acted this way and this was the FIX
 

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Discussion Starter #13
I will check misek thank you!
Bsd4083 I did check the temp sensor, both the act and ect and they are working as they should
 

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Discussion Starter #14
So I haven't had a chance to test the map but I did test the ect and act connector unplugged and they only show 4 volts going from the green and purple wire ... shouldn't it be 5 volts?
 

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So I haven't had a chance to test the map but I did test the ect and act connector unplugged and they only show 4 volts going from the green and purple wire ... shouldn't it be 5 volts?
yo,
VREF=5.0 volts.These values may vary 15 percent due to sensor and VREF variations.
150716

150717

150718
 

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Discussion Starter #16
Miesk5
What about the tab and tad solenoids and check valves? How do I check those and what are the functions??
 

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