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1988 FSB, 351W, Towing Package
102 Posts
Discussion Starter #1
This is a topic I’ve seen posted around the internet, but suggestions vary wildly. Looking at the truck 2 weeks ago before I bought it, it started and ran fine (and supposedly has for years as a daily) but while looking at it it quit wanting to start at all. The guy replaced the fuel pump relay and it went back to starting and running fine, so I bought it. 2 weeks later, I’m in my driveway with it after driving around and it dies on me when I get off the throttle. It would start but would die if it wasn’t throttled. I was floored, but remembered the fella replaced that relay so I made sure the connection was tight and Jiggled it around and low and behold she cranked right up and ran well for another week. Well, yesterday it began the same thing, it’ll start up but die after a couple seconds. Before the guy replaced the fuel pump relay, it wouldn’t try and start, I’m not sure if these are related. I’ve read about bad ford grounds and such, but I’m not sure where to start. I can hear the fuel pump come on when turning the key on. All else I’ve done is disconnect the negative terminal for a couple minutes to see if the computer would sort itself out. Can anybody help?

1988 FSB, 351W, Towing Package
102 Posts
Discussion Starter #2
Also worth mentioning, I can smell gasoline after getting out and giving it some gas.

Super Moderator
25,018 Posts
Yo Caldwell88,
Try a Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Something wrong

Most likely perp is the IAC. I."t would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator." by Nelbur
Try unplugging the Idle Air Control (IAC)

Location pic by Steve
"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 incl some jowens126 HVAC Control Panel pics/info @ Help with dtc codes and idle

If no codes, find the source of that rich smell. Any exhaust system leaks or rusted pipes, etc from exhaust manifold to tailpipe? Beware of carbon monoxide!

Black Exhaust Smoke is an indication of rich fuel condition.

Fuel Injectors: A leaking or dripping fuel injector will cause a rich fuel condition.

Fuel Pressure Regulator: A stuck closed fuel pressure regulator will cause a rich fuel condition.

Fuel Return: A restricted fuel return line will cause a rich fuel condition.

Plugged fuel filter, air filter, spark plug condition/wrong reach or vacuum leaks.

Any leaks in fuel lines or tank, evap valve atop tank, evaporative system, fuel filter, throttle body/ gasket loose, fuel rails/injectors,?
Gas Cap loose

Excessively rich idle caused by an incorrect MAP Sensor calibration
High exhaust back pressure due to plugged cat/muffler,
EGR pipe rusted.
Exhaust manifold to cylinder head leaks can cause an oxygen sensor (in closed loop) to read lean which in turn causes the EECIV to add more fuel
Here is the 1988 Bronco Dealer Brochure via member sneal @ 1988 Bronco Dealer Brochure part 1... site upgrade rules limit maximum of 10 images per post

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Ford Hoarder
78 & 92
6,805 Posts
Easy check, pull vacuum line going to the fuel pressure regulator, if fuel there it is bad.
Other then that, sounds like maybe ignition related to me, especially if gas smell is present. I know the pickup in the dizzy can go bad and cause these issue, but I would trouble shoot that and not just replace it.

91 Bronco, 351 lightning motor, 6 inch lift 35inch tires
88 Posts
If you smell gas the diaphragm in the fuel pressure regulator has let go. Like dude said. Itll shoot right out of it
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