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Discussion Starter · #1 ·
78 bronco 400 motor auto transmission
2 issues
It runs hot
When hot won’t hardly run uphill.
I’ve replaced plugs, wires, 4 core aluminum radiator, air cleaner, 160 thermostat and fuel pump.
Usually runs 190, after sitting idle climbs to 210 then doesn’t want to run at all.
Any suggestions
 

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Discussion Starter · #3 ·
It will rev up as normal then when you put it in gear under load it sputters and has no power and dies
 

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Had same issue going up hill, was a bad fuel pump. As for running "hot" are you sure the thermostat is the right side in?


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My 460 with a 165* T-stat will idle up to 200 at a drive thru on a hot day if the cars in front order half the menu, but no issues. Maybe the choke flap isn't opening enough after it warms up?
 

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Discussion Starter · #6 ·
I put a new fuel pump in, I am pretty sure I installed the thermostat correct
I’m running out of ideas
 

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Have you checked the timing?
Vacuum advance?
After that I would look at the carburetor

I had these type of problems before
It’s usually the timing. Runs great in the driveway but won’t hardly get to the street

one other time on a new engine same symptoms. Turned out the carb jets were too small
 

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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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First thought is that it might be a partially clogged fuel filter. It's not affected by heat, but can be almost clogged, have crud fall away while it's not running so it seems ok when it first fires up, then pick up more crud & get more clogged again while it's running. So it might seem like it's getting worse because the engine warms up, but that might be just because it warms as it's running longer. Symptoms of a clogged filter typically show up under higher fuel demands, like driving uphill or at higher revs. And I think a lean engine runs hotter, so it could be that restricted fuel is causing the higher temps, not the other way around.

Because you talk about it acting up when hot, another thought is vapor lock. When the fuel is heated it can boil & turn to vapor in the fuel line. Mechanical fuel pumps can't pull vapor, so it doesn't run. Typically it's a complete shut down, not a "hardly runs" situation, so this seems less likely. Cures include re-routing fuel lines away from hot exhaust or installing an electric fuel pump back at the fuel tank. Back at the tank fuel is cool & liquid. If fuel in the line turns to vapor between the pump & carb, the rear pump continues to push cool fuel thru, which clears the vapor & still delivers fuel to the carb.

Another vapor lock issue is fuel boiling in the carb itself. Cures are reducing under hood temps, fixing cooling issues so the engine runs cooler, & adding a phenolic spacer under the carb to separate it from manifold heat.
 

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I agree on the timing.
I have had issues with the stock dist. vacuum advance not doing what you would think. I have also seen an engine over retarded cause this same issue before.

Along with the fueling comments above. I have seen a weather'ed rubber line created a vapor issue as it will actually pull in air in to the line threw the rubber. Worth checking out. The spot I had this problem with, was right where it went into the tank.
 

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Discussion Starter · #10 ·
I just changed the fuel filter, I don’t think that’s it
I’m thinking a vapor lock issue
 

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I just changed the fuel filter, I don’t think that’s it
I’m thinking a vapor lock issue
How old is the gas in that tank? Mine was essentially molasses and would essentially burn up the fuel pump/filter since the tank and gas were so cruded up. You give the tank a look?


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Driving Stuff Henry Built
-90 xlt, 351w, e4od, man 1356, 3.55, sag, warn hubs, 35s. -73, 400, np435, d20j twin, 35s
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Another possibility is a clogged sock in yhe tank. Same thing as a filter, it can be mostly clogged, have crud fall away when the pump's not flowing, then clog more when it's running.

Another thing that can cause issues after running for awhile is a fuel tank vent that's not working. As fuel is pulled from the tank air needs to be let in to replace it. If the air can't get in vacuum builds in the tank & makes it difficult for the pump to pull fuel from the tank. The test is easy. Run it until it runs poorly, then pop the gas cap. If you hear a rush of air past the cap, something is wrong with the tank venting.
 

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Could be ign module on inner fender. Sometimes they don't initially die outright, they just don't work when hot.
 

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I had vapor lock issues in BOTH my broncos during the summer of 2018
All I can figure is a change in gasoline as the cause.
I had been driving the one bronco since 1995 and never once had vapor lock. Suddenly both trucks???
I put a heat shield blanket on the fuel pump and heat tape on the fuel line
Solved both problems.
when the issue acts up you can try pouring water on the fuel pump to cool it down
That’s what worked for me
 

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What thermostat are you running? You are aware that the M-blocks take a special thermostat? See the Thermostat tab here: 351M AND 400
 
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