86 Bronco II Quitting on Me!

[Mhollis1]

Get a volt meter on the high pressure pump leads and see if the voltage goes down with pressure.

I Like this!…I mean fuel pump/pumps are working on start up (can hear the spinning at least and will run for 20 minutes until everything gets warm…pressure gradually falls from 30psi to less than 5 psi. In the last 4 minutes of run time till it quits…will not start again till it cools off (10 minutes) and repeats the process unless you get up to a good constant speed and drive on without stopping. Once you stop and accelerate over and over at lights/ stop signs it will quit again. Constant Cruising speed motor maintains a cooler temp than at idle. Question is what would cause voltage to drop when getting Warm and what exactly is getting warm?

 

[JerryC]

If the voltage drops my guess is bad connections adding resistance.

 

[Uncle Gump]

While it could be a voltage issue... it could also be just another junk fuel pump. They sell a bunch of those these days.

I also doubt the fuel pressure issue has anything to do with the TFI module getting warm.

 

[Mhollis1]

While it could be a voltage issue... it could also be just another junk fuel pump. They sell a bunch of those these days.

I also doubt the fuel pressure issue has anything to do with the TFI module getting warm.

 

[Mhollis1]

Could be a bad pump, however the issue I am having was present before I replaced both pumps…is still possible but thinking something else is happening somewhere that is causing loss of pressure or voltage when it gets warm.

 

[Uncle Gump]

@Mhollis1 please don't take offence to me posting your private message question here in the open forum... The only reason I'm doing it is simply to allow anyone else having your same question aware of my answer... I'm mean... helping is what we try to do here.

Your message is as follows...

"I posted an issue on here back in July with my Bronco and you offered up some advice which I greatly appreciate. Still have not figured it out however I was looking on another forum and the individuals having the same problem I was. He Described it as being a faulty rear fuel pump booster… is it just me or is this some sort of booster that I’m not aware of for the rear pump"


First off... I'm not aware of any "fuel pump booster" in your fuel delivery system. Your fuel system is a modified version of a low pressure fuel delivery system that was used to supply low pressure fuel to a carburetor. The "modification" was to install an inline high pressure fuel pump and inline filter after that pump to meet the delivery demands for fuel injection. Side note... Not all vehicles received the additional inline filter... my 86 was one of them... it does seem that most did, but it is not a "for sure" thing.

I have actually heard both of your fuel pumps referred to as "booster pumps". The high pressure pump because it "boosts" pressure to useable numbers for fuel injection... also the low pressure pump because it "boosts" fuel delivery to the high pressure pump. I've also heard the in tank low pressure pump referred to as a "lift pump". Regardless of what they are called they're called in the field... this design of two separate pumps to feed a fuel injected engine was poor and was soon changed to a better way of doing it... not to mention the cost savings the changes made. They changed to a single in tank high pressure pump and an inline filter. Simple and effective and really is the standard way of doing it across the industry. I recently modified my 86's fuel system to reflect these changes and it simplified the mess they created and with the limited use I have with it so far... it works well. I want to do an article of what I did and have saved all the part numbers and took photos along the way to document my entire process. I'm willing to share all of this with you if interested in going this route. The big question is still the fuel pump I chose. I don't have enough time on it to say that choice was a good one... it's also why I modified the rear floor of my truck to allow a fuel pump change in a matter of minutes not hours.

Now for what I know about your early design fuel system... the frame mounted high pressure pump is great for "pushing" fuel at a higher pressure. What it's not good at is "pulling" fuel from the fuel tank. If you're having issues with the low pressure pump... be it cutting out while driving or maybe restriction from trash in the tank etc... it could be causing problems with supply to the high pressure pump and starving it. I've heard similar complaints to yours and it was stated that the problem was more repeatable with lower fuel levels in the tank. This is due to the high pressure pump being lower then the fuel level in the tank and the pump was being gravity fed fuel. Once the fuel level was lower then the pump... gravity flow stops and the high pressure pump doesn't have the "pulling" power to overcome it.

Not sure this answers your question or will fix your issue but it's what I know, and I hope it helps

 

[rstanovich]

change your Key switch if you haven't already. im betting that you or someone in the past has had a s-load of keys hanging from the switch and its worn out. cheap to replace, before you do when the engine stops I bet the switch is Hot.

 

[FIngerstyler]

I have posted to this thread earlier because i am having probably the same issue as Mhollis1. I also am still searching for the fix. As a recap what a mechanic replaced for me: Distributor, wires, plugs, condensor, 2 TPS's, MAP sensor, o2 sensor, temp intake sensor, coolant temp sensor, battery, battery cables, alternator, 3 ICM's, 9 ECM's, 2 HP pumps, 5 lift pumps, ignition switch, electric connectors at inertia switch & both fuel pumps, fuel filter, relays and their connectors. I still have the problem, i can idle it in the driveway for 20-30 minutes and we can watch the fuel pressure gauge drop from about 40 to <10 when it runs bad and conchs out won't restart until 10-20 minutes. Usually when i get stuck i'm not far from home and restart it and just get it home in less than 20 minutes. I don't get any codes really on the odb1. Last week i did the short pin test to turn on both fuel pumps, thing is i can not hear the tank pump, can not feel any vibrations with my hand on the tank, can not hear anything when i put a funnel in the filler neck and my ear next to it. I can hear the HP pump and it sounds a little whinny. I'm getting someone in a couple of weeks to test my theory -- the wire to rear tank though already checked for V drop at cold maybe is corroded and gets heated causing lower voltage only at that time to lift pump so pump somewhat quickly burns out from low voltage leaving the HP pump to miracously keep it running until HP pump gets heated up shuts down. And somehow the HP pump is now the only one that actually runs. Just a non-mechanic researcher hypothesis. But someone said car won't run without both of them working. Yet one shop says "yea we checked voltage and it is good at the lift pump, but 12 volts straight to the pump and it won't run (that was at the 4th pump)" Not giving up $@#!%^^@#$&

 

[Uncle Gump]

I have posted to this thread earlier because i am having probably the same issue as Mhollis1. I also am still searching for the fix. As a recap what a mechanic replaced for me: Distributor, wires, plugs, condensor, 2 TPS's, MAP sensor, o2 sensor, temp intake sensor, coolant temp sensor, battery, battery cables, alternator, 3 ICM's, 9 ECM's, 2 HP pumps, 5 lift pumps, ignition switch, electric connectors at inertia switch & both fuel pumps, fuel filter, relays and their connectors. I still have the problem, i can idle it in the driveway for 20-30 minutes and we can watch the fuel pressure gauge drop from about 40 to <10 when it runs bad and conchs out won't restart until 10-20 minutes. Usually when i get stuck i'm not far from home and restart it and just get it home in less than 20 minutes. I don't get any codes really on the odb1. Last week i did the short pin test to turn on both fuel pumps, thing is i can not hear the tank pump, can not feel any vibrations with my hand on the tank, can not hear anything when i put a funnel in the filler neck and my ear next to it. I can hear the HP pump and it sounds a little whinny. I'm getting someone in a couple of weeks to test my theory -- the wire to rear tank though already checked for V drop at cold maybe is corroded and gets heated causing lower voltage only at that time to lift pump so pump somewhat quickly burns out from low voltage leaving the HP pump to miracously keep it running until HP pump gets heated up shuts down. And somehow the HP pump is now the only one that actually runs. Just a non-mechanic researcher hypothesis. But someone said car won't run without both of them working. Yet one shop says "yea we checked voltage and it is good at the lift pump, but 12 volts straight to the pump and it won't run (that was at the 4th pump)" Not giving up $@#!%^^@#$&

Does your 86 have two fuel filters? The 86 could have a replaceable cartridge (But not always) in the canister and an in line... just throwing that out there.

I won't say your theory is wrong... but these 12 volt system parts have a requirement to operate continuously... unharmed/unaffected between 9 and 16 volts. Not that all parts are properly built to spec. LOTSA junk out there these days.

You know you have a fuel issue... I know what I would do... especially considering all the pumps and $$$ that your truck was blessed with. Run new wiring from the pump relay to the tank... and update to the in tank high pressure pump. Toss all that factory shit out and do it the way it should have been done to begin with (I even built new lines for a clean install). Once done... re-test the system. I just know I wouldn't spend another dime chasing the problem until that factory stuff was removed and properly discarded.

 

[FIngerstyler]

Thanks for your reply Uncle Gump! My 86 always just had 1 filter in that reservoir. Another mechanic said buy the carb with intake mani kit! But next month i hope to run a hot wire from relay to lift pump, i'll run it out the side of hood and tape it to the outside if i have to just for temporary test purposes and see what that does, also make a new ground for it.

 

[Mhollis1]

Ok fellas I am about to quit on this Bronco…but before I do could anyone have the time or patience to send pics of where their ground wires are located? I have either a rear fuel pump problem ( being fuel pressure is slowly dropping after the motor warms up ) or a ground issue. TFI is getting ridiculously hot ( like burn your fingers hot and it has been relocated on a heat-sink away from the motor) and I do not believe that is normal. Warm sure but can’t really touch it hot seems like something is a miss.

 

[JerryC]

The manual lists many ground points.

Here are what I consider the big three.

Core Support by the battery. The larger of the wires is coming from the negative battery terminal.


Firewall, above the drivers side cylinder head. It's the braided flat cable.



Engine passenger side front just above the oil pan.

 

[FIngerstyler]

I'm still thinking of this everyday, another mechanic has mine for 3 weeks now. I'm supposed to hear from him Monday. I know all my grounds especially those 3 have been inspected and cleaned and i still have this same exact problem as Mhollis1 on my 86 B2. In talking with a friend that is an electrical engineer and explaining all the parts and issue especially when we put a pressure guage after the HP pump and watched it fall from 40 to 5 - he said would be 1 or more of several things: either 1 or both fuel pumps bad, 1 or more wires that supply both fuel pumps, the ECM that controls fuel pumps, the fuel pump relay or wire(s) involved with fp relay.

 

[Jeggles]

Sounds like a fuel pump overheating and boiling the gas inside of it. I had that problem before. It can be caused by a couple different scenarios. Low voltage to the pump being the primary reason. Voltage drops, amps (and heat) increase. Try icing the fuel pump (rail mounted high pressure pump) when the truck dies, and see if you can restart it without the 20 min wait. If that allows you to restart and run vehicle then you know what to figure out at least. To truly see the real working voltage the pump would have to plugged in and running. Testing the voltage with pump unplugged(no load) can give deceptive readings(you can read "soft" voltage. Some other causes of a high pressure pump overheating can be a clogged or restricted fuel return line or a clogged gas tank vent. Just a thought.

 

[DanGleebitz]

Hello;

I've resolved all my starting issues, and have had a good amount of time to confirm their success. Things I believe fixed the issue.

1) Crash sensor kill switch (passengers footwell) -- I just bypassed it entirely
it would get hot and fail.

2) Other helpful things (optional)
-new plugs/wires/coil/airfilter; all increased performance
-IDK who this'll help. But my BII was running rough, and the fix was bypassing a junction that was just floating around in the engine bay; if youd likemore info I can try and get photos, but to this day, idk what it is/does.

 

[Uncle Gump]

Hello;

I've resolved all my starting issues, and have had a good amount of time to confirm their success. Things I believe fixed the issue.

1) Crash sensor kill switch (passengers footwell) -- I just bypassed it entirely
it would get hot and fail.

2) Other helpful things (optional)
-new plugs/wires/coil/airfilter; all increased performance
-IDK who this'll help. But my BII was running rough, and the fix was bypassing a junction that was just floating around in the engine bay; if youd likemore info I can try and get photos, but to this day, idk what it is/does.

I would like to see pictures. We can probably help determine what is.

 

[DanGleebitz]

@Uncle Gump

 

[Uncle Gump]

That's a head scratcher... I have no idea what I'm looking at there.

Are those wires soldered to something? What color wires are they?