Turning to you guys for help, I'm a bit baffled here.

First off, specs of the bronco are:

1990 Mustang 5.0 w/ factory harness
Sherman fuel tank
Single inline fuel pump mounted on frame rail below drivers seat
2 inline filters (the large metal ford ones)
Rubber hose making the connections up to the factory fuel rail/return line
Newer gas cap


The problem began last week when I was driving home, got stuck in some stop and go traffic for at least 20 mins on Thursday, I believe it was up in the 90's that day. As I slowed to a stop at a light, the bronco surged for a few seconds and died. Got it pushed out of the way into the shade to diagnose, and noticed when the key was on I couldn't hear the fuel pump unless I got almost under the truck. This is not normal as my fuel pump is normally very loud I can hear it over everything else at idle. Popped the hood and depressed the schrader valve to see if there was any fuel, a little ran out but a TON of air (what I presume is fuel vapor) released from the line. Went back to the fuel cap and opened it, again lots of vapor pushed its way out. Left the cap off and the truck alone for 20 mins or so (vapor continued to pour out of the filler neck, I could see it) turned the ignition on again and heard the fuel pump refill with fuel, started right up and was able to drive home no issues.

Figuring it was some sort of anomaly, I blew it off and started to prepare for last weekends camping trip to the dunes. Sat morning drove down to the breakfast without a hitch, continued on after that to the royal gorge bridge again without issue. After we headed towards Medano Pass, but as soon as we were driving through Westcliffe it started to shutter again and stalled out. Same result as before, air in the fuel lines, very high tank pressure. We were in the sun this time so about a half hour passed before I was able to get it going again. Made it literally a few mins down the road and stalled again. Did this twice before we made it to the pass road. Once we had made it on to that road, we drove for approx 45 mins without issue except for one hill climb that it stumbled at the top for a second but didn't stall.

Next day we went to the dunes, to Zepata Falls (way cool btw) and started to head home via I-25 to be closer to civilization if something were to go very wrong. Died once on the way to 25, filled the tank with 87 because I read somewhere once that the boiling point was higher and I was willing to try it all. Left the gas cap off for the next part of the trip as well for the same reasons, maybe the pressure was causing my issues. Made it to Pueblo before it gave up again, pulled off the hwy and waited for it to cool again. At this point I've decided to head for the nearest gas station for some bags of ice to lay on the fuel lines on top of the motor, stalled again on the way there.

Grabbed my ice and was feeling hopeful, made it halfway to the springs before.. (put my gas cap back on here because it clearly wasn't making any difference) then to just north of monument.. by this point the sun is down and I've noticed my trips between stops are getting longer, the heat from the sun had a clear effect on it. Went through 2 bags of ice but made it home 4 hours later than I would have liked.


I know its a lot to read but I wanted to give a thorough explanation of what's going on. Fuel pump does not get hot, I don't believe the filters to be clogged because it would always run bad, and the metal fuel lines were hot to the touch.

I'm attaching a thread I found on a Corvette forum of an almost word for word explanation of an identical problem. https://www.corvetteforum.com/forums/c4 ... k-huh.html

I know the other users mention several times to check spark, I'm sure this is not my problem, because of the clear change in sound by the fuel pump and the vapor in the lines.


I appreciate any help guys.

you might want to relocate your fuel pump as close to the tank as possible and consider a fuel accumulator.

I had the same issue with an inline pump. The pump itself would get hot and vapor lock. Ice on the pump cured it for a while. My pump had the foam insulation around it. I cut most of it off. It helped but I just went intank to prevent it again.

Sounds like you will need a carb spacer(unless it is fi) and coated headers to get the heat out of the engine compartment. Read up on coated
headers and how they take the heat out of the engine compartment and transfer the heat under the car. (Burn your butt instead)

I have had that problem with my 64 vet, 71 vet and my 57 truck (454) in TX. Steel headers and it was hot under the hood. I put a 1/2" carb spacer and coated headers in two of them. I am putting a 351w with about 400hp in the 66 bronco, and I expect the same problem .. So I am starting with electric fans and ceramic thermal headers. I have heed assured that a spacer is not necessary in Colorado.
I am rid of the vets and trying to get rid of the truck. I want this bronco engine to run without that problem.

Thanks for the really good right up about the details, that helps a bunch.

I agree with the other guys that the best solution is to change your fuel supply / pump arrangement.

Another consideration is your exhaust system. Do you happen to have dual exhaust ? If so, most people have had luck going to single exhaust down the passenger side to keep the heat away from the fuel lines (I assume they are mounted along driver's side frame rail).

Having suffered from fuel delivery issues for months, just recently, I feel your pain. Before my engine rebuild I had a Holley Red LP pump at the tank, and the stock HP pump in the engine bay. Never a problem, so when I rebuilt the engine, I replaced both pumps with new units (carter 4070 and E2000). From then on, I would have issues once the engine had been warm a while. I never did get any vapor, but I installed fuel pressure gauges on the fuel rail, and one somewhere in the fuel line. After what I thought was a faulty E2000 pump, I replaced it with a Bosch (part # in another thread somewhere). Still had issues; so as a last ditch effort, I removed the LP pump, and put the HP pump right next to the tank. I haven't had any trouble since. Seems the LP pump was not keeping up with the HP pump and causing the HP pump to get hot and fail.

In your case, I would move the HP pump to as close to the tank as you can, after a filter of course. Then you could see where you might be able to insulate any fuel lines from external heat.

Really appreciate all the reply's on this guys. First off I only have a single exhaust and its on the opposite frame rail, the heat coming off of the pipes was the first thing I considered as well. Secondly I'm going to give the accumulator a shot, I have one in my parts bin from BCbroncos so I'll be replacing the filter before the pump with that unit. I don't know how much closer I could move the pump to the tank, its probably less than 2 feet from it now with some hose slack. The pump is wrapped twice with foam to attempt to keep the noise down as well, the thing is just so loud already I'll try and keep that as is.

Any thoughts on if this will help with the tank pressure? It started I believe when I got the new fuel cap, the old one didn't have the rubber seal and would pour gas out of the filler neck when I was off camber.


Lastly, and related to fuel but unrelated to this problem, when I turn the key to 'prime' the fuel pump before starting, it will never ever stop running. Doesn't do the typical 3-4 second prime and stop. I am getting computer codes stating that it can't read the fuel pump etc. No idea what controls this.


Accumulator should arrive next week so I'll post up how it goes. Also need to get a welder going and patch up a few things, it kind of destroyed itself for no reason this trip..

These pumps are designed to push fuel, not suck it. So you really need to get it close to the tank. The only exception I can think of is if your tank is much higher than the pump allowing gravity to feed the pump. I have a ~3" piece of hose, my filter, another 3" hose and then my pump.

I decided against the accumulator only because I like to keep things simple if I can get away with it. Didn't need more fittings in the fuel system to worry about.

Some caps are designed to vent, but I don't recall for what reason. So maybe you need a venting cap, or put a vent into a charcoal canister?

I run a Carter low pressure pump, then accumulator, then airtex e2000 high pressure pump. Seems that is the 'old' way of doing it and many are just running the HP pump close to the tank.

If your tank isn't vented, you need to get it vented, not just a vented cap. A vented cap isn't enough.

Also, I'd check with EFI Guy about the pump running on. I think I've read that is a sign of ECU failure. He can fix ECUs and has a good DIY on YouTube if you wanted to try it yourself. But I'd just get it to Garry and let him deal with it if it needs fixed.

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Another possible culprit is the fuel pressure regulator. You didn't say what the fuel pressure is running now/while cool vs. at issue.

In my case I used to deploy six months at a time, where the Bronco would sit and I'd usually forget to put some Stabil or similar product in the tank. Then the first real heat wave of July it'd start acting up and I'd have to replace the FPR.

I would also watch the fuel plumbing with an accumulator. Many of the EFI versions have the fuel return running into the accumulator. If you're heating the fuel up, you want to send it back to the tank, not shorten it's return loop to the engine...

Also helps to have a bigger/fuller (more full?) tank (general edification).
HTH, Joe

Another idea for the fuel return line is to plumb a cooler (like the early Bronco PS cooler) into it, cool the fuel before it goes back in the tank.

thanks for all the replys guys.

I've done a ton of research on this and all the info still comes out 50/50 with regard to the dual pump/accumulator vs single HP. Obviously the best option would be a drop in but I have an old tank w/o the provisions for it so that option is out.

At this point I'm just going to give something a shot because internet research is splitting the conversation. Going to try and move the HP to the crossmember in front of the tank and see what happens.



As far as the venting of the tank, I've read about drilling the cap, adding a marine style vent to the tank, charcoal canister and purge. Those emission pieces have been removed by the PO and all the factory bronco emission stuff is long gone as well. I plan on garage parking it so the emission components sound like the best solution however I'm afraid they won't be enough due to how much pressure is building. I could be overthinking this but its a serious burst of air that releases. I haven't driven the truck in over a week and opened the cap today and lots of pressure came out.



With the pump staying on constantly I haven't really thought much more about that part as of yet, but if anyone has ideas I'm all ears. I have a spare ECU and no change when I swapped it so I'm doubting that, it was my first thought as well.



I can't help but think that the pressure and vapor lock are connected somehow, but I can't find out why.

So quick update, one problem solved!

Since I did so much reading on the pressure building and the need for a charcoal canister/purge as a vent system, I ran over to the yard this morning and grabbed a canister from an f-150, hooked it up to my purge valve line that was just plugged with a screw, ran some hose down to one of the small 5/16" ports on the tank and tada! pressure build up alleviated I think. Drove for about 10 mins which is long enough to get it built up usually, removed the cap and the smallest little poof came out so I'm calling it a win.

Should update this again, 2 new filters, moved the pump back as far as I could from its original position. It’s now right in front of the rear crossmember. Time will tell if that’s going to fix it, since the summer heat has passed.


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Well sure enough, its warming back up and here we are again. Re-reading this thread is giving me a few other ideas but here's an update on where I am now with this. Drove around quite a bit today and ended up vapor locking again. I'm not 'quite' sure this would be the proper term so here's exactly what happened..


First, update on the fuel system and more specs. I'm now in addition to the same HP pump, running an LP pump right before it. No accumulator right now, just one pump into the other, right in front of the rear crossmember mounted on the frame rail. There are 2 new filters, one before the pumps and one after. The fuel lines in this truck all the way to the rail are 5/16" rubber fuel hose, both delivery and return.


Driving down 25 today and I feel the truck start to stumble, when I apply more pedal it bogs down more and when off it actually seems to run better (I'm thinking this is a supply/demand thing with adding more air to the engine). Pull off to the side of the highway and it shuts down. As many of you know, the HP pumps these days have a distinctively loud whine to them, and when this "vapor lock" happens, you can barely hear it whir. Its for whatever reason dry, only air (vapor) is spinning around in the pump. After 5ish mins, I turn the key to prime and hear the pumps whine come back so I know its once again full of fuel. I start the truck and head to the nearest gas station, (had about a 1/4 tank so I'm thinking the volume may cool it down). When we arrive at the station, maybe 5 mins away from the break down, I crawl under the truck immediately and start to feel every part of the fuel system for heat. Neither pump was even warm, fuel lines feel the same even right past the drivers side header. If the fuel is getting hot, I surely couldn't tell it felt completely fine to me. (I'm expecting some sort of excessive heat here, correct me if I'm wrong but this was barely luke warm) I jump back on the road and just a few miles down I can feel it buck again so I pull over and let it rest for maybe 2 mins, drive a little more it bucks again so I let it coast as much as possible because we were almost to our destination. It seemed to figure it out and was just fine as we drove through the park.

The drive home after was totally uneventful as well, like nothing happened.


Plan going forward is to double check the LP for function, and replace the fuel lines with new rubber, adding a fiberglass heat wrap as well up around the engine. (I doubt this will change anything but what the hell why not)

Open to new ideas, I have Moab in a month and really need this solved before that drive. Thanks again guys.


Oh! also, I found out why my pump would never stop priming, the relay was grounded to the body instead of the tan/green wire from the ECU like its supposed to be.

I'd remove the LP pump, put the HP pump in it's place and hook up the supply line again and see if that stops it. I took another trail ride Sat. and since I did just that, I have not had any fuel issues.

I had tried that before adding the LP pump, unfortunately no change. I verified the LP was working as well, I'm going to hook up a fuel pressure gauge and take it for a drive tomorrow since it will be pretty warm out. Maybe that will lead to some answers.

Read up a lot last night about heat soak and vapor lock, etc., one of the reasons for fuel pump cavitation is inlet restriction which lead me to think maybe my supply line at 5/16" isn't enough and I should bump it to 3/8". Not sure that would help however, seeing as the pick up tube coming from the tank is 5/16" as well as the stock metal fuel rail which would just cause it to bottleneck. Maybe that small increase in liquid space would help but I'm not overly confident with that being the solution.

Another would be that my HP pump is old and slightly cavitated causing the fuel that runs through it to slowly increase in heat and over time it causes it to turn to vapor. Which makes sense, but again my pump(s) and lines were barely warm.

I'd like to find out what's causing this before I throw a new HP pump and FPR at it just because. Fingers crossed the fuel pressure gauge will enlighten me.

I installed a gauge on the fuel rail, and I also installed one inside the cab. But the one inside the cab never helped me because I had already fixed my issue. I can get you details on the gauge I installed inside if you'd like it. It was one of the cheapest I could find, but it's been working great.

Fuel pressure at prime is 38 psi, at regular idle 33 psi. 41 psi when vac line from FPR is removed and plugged with my thumb.


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Drove it for about an hour up and down 285 toward conifer for the elevation, and no issues. Tried to make it hot and run it hard but it was just fine, FPR seems good to me as well, it runs around 30 at coast to 40 at WOT.

Stopped a few times to check heat on the pumps and lines and they were perfectly cool.

I don’t know if I’ve mentioned this or not but it’s now a working theory of mine, but it seems like this only happens when the sun is beating down on a warmer day. It’s a bit overcast here so I think that may be why it ran just fine. That being the case, I think it has to do with the fuel rail itself on the manifold being the source of the overheating, pushing that hot return fuel back into the tank.
Working off that, I could wrap the rail in something, replace it with one of those aluminum rails, or add a heat sink cooler to the return line.

Does this seem feasible to anybody or am I way out in left field?


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Which low pressure pump are you using? It takes a good one to keep up with the flow of a high pressure pump especially when its hot. Carter 4070 will work. I've tried others that didnt. If your doesn't it creates vapor lock between the pumps.

I’m using a Mr. Gasket 12s, 7psi and 35gph. Looks like that carter is 72gph.

I’m still not finding my HP pump to be hot though, everything I’ve read said it will be “uncomfortable to touch” if the HP is cavitating.


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I'm sure that's your problem. It will only get worse as it gets warmer.

I’ll give it a go, ordered the pump and a 13” heat sink cooler. I’ll try the pump out first and add the cooler to the return line if that doesn’t work. If it solves the issue, the cooler is one less part I’ll need to buy for my hydro assist.

Will keep you updated, thanks for the help.


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Per viper’s suggestion, I installed a new fuel pump tonight. First impression is that now the HP pump is completely silent. Fingers crossed that the issue is solved.

Will update later on.


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Still don’t think that’s the final answer to this saga. Ran it around today a bunch to see what would happen, and it’s stumbling again. Didn’t fully die this time, but it may have just been because I reached my destination first. Fuel pumps are luke warm at best, hoses about the same, but the metal rail on the engine (the stock one) is hot. I can leave my fingers on it for 3-5 seconds before it’s uncomfortable.

Pulled codes to see if that would lead me anywhere and got a pass with 29, 95, 96 continuous.

29 is vss stuff
95 and 96 have to do with the fuel pump circuit, I don’t believe it’s the culprit because of the way its all wired up with aftermarket pumps.

So, return line cooler? Try to keep heat away from the stock fuel rail?


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I assume you have replaced all fuel filters?


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All new yes


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