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Strange goings on with Ford Ranger

I have a '77 Ranger which will not respond to the idle control when it is started in a half-warm condition. About a mile or so down the road everything is back to normal but letting the hot engine sit for 30 minutes will cause the idle control not to respond (the engine dies when you take your foot off the gas to shift gears).

If the warm idle speed is too low adjust it higher. There is no idle speed control on the engine other than the cold start fast idle cam which may also need adjusting. It would be helpful to know whether the engine is a 4 cylinder or 6 cylinder.

What you describe might be vapor lock.

Vapor lock occurs when the fuel in the fuel line begins to boil.

And if you use gasoline that contains ethanol, vapor lock is more likey to occur on an engine that has a carburetor when it’s hot.

To find out if this is the problem, carry a bottle of water in the vehicle.

And the next time it acts up, open the hood, and pour the water over the fuel line into the carb.

If the engine starts and idles fine, that’s vapor lock.


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Real possibility on a '77… that never would have occurred to me, since it’s been decades (since carburetors) since I’ve seen it. A tip of the hat to you.

I own a carb’d 8 cylinder (302) Ford truck of just few years earlier vintage. Probably the first thing I’d check if my own truck developed this symptom is the idle solenoid. It’s supposed to poke out a little against the throttle positioner when the ignition system is on to increase the warm idle speed a little. Beyond that there’s a whole slew of stuff that could cause this. Most of it is pretty simple to discover, for example is the choke plate fully open when the engine is warm, where on the idle speed cam are you operating at the various stages of engine warm up, is the timing correct, any signs of ignition system problems, spark plug condition etc. Those are the areas I’d look first. On a vehicle of that vintage lots of possibilities of course, including major engine problems. If all else fails, make sure all the routine maintenance is up to date, and check the compression.

I’ll add one more idea. Warm idle is where vacuum leak problems show up the most. Vacuum leaks are probably the most common problem I’d had w/my truck. A thorough investigation of the vacuum system would prove/deny that idea.

When checking for vacuum leaks don’t forget to check that the holes where the shaft of the throttle plates have not worn too large. They can be bushed.

Driveability problems like this were common in cars of that era and before until they warmed up. There was usually and adjustment screw on the fast idle cam and either a choke adjustment or an electric assist to warm the choke as well as idle mixture and idle speed screws in the carb that all had to be balanced against each other. All these things had to be free of dirt, rust and stickiness and corrosion to work.

Ah yes, they don’t make cars like they used to, Thank God.

It is a 6 cylinder and this only (I thought) occurred during the winter,
until yesterday. I like the idea of a vapor lock since the fuel does
contain ethanol (see Tester’s comment). Also, that would be an easy thing
to check. The shop had checked for sticking throttle, etc. but found
nothing. I am a good customer of theirs and they did not charge for any of
their checking but couldn’t find anything.

I remember, as a kid, my dad driving through the mountains of southern New
Mexico and using half a grapefruit for the same reason. It was a 41
Studebaker, flat head, with four doors that opened to the center. There
was a clear view of the pavement around the clutch and break peddles.

Do you know if the gas recirculates to keep the line cool? An old Buick
did that and had the same problem (but worse) caused by some metal filings
in the fuel injector restricting the return flow allowing it to overheat.

Thanks for your suggestions as well as a thanks to Tester.

The gasoline does recirculate a bit in my carb’d Ford truck. Whatever doesn’t go into the carb from the fuel pump is routed back to the gas tank. There’s an important-to-prevent-vapor-lock vapor separator gadget at the point where in incoming gas line splits and some gas goes back to the gas tank. On my truck it is a metal split tube arrangement. Located about 8 inches from the carb. Might be a good idea to remove and clean it out.

However, I’ve never had a vapor lock problem on my truck, and I expect there’s a little ethanol in the gas here. Unless your pump gas there has ethanol content greater than 10%, I guessing that’s not your problem. I expect over the years there has been some work done on the truck’s exhaust system; so good idea to inspect all that, to make sure none of the exhaust pipes, muffler, etc have been re-routed too close to the fuel lines. If the exhaust system is too close to the fuel lines, that can cause vapor lock independent of ethanol in the gas or not.

Thanks, GeorgeSanJose, I’ll take a look early in the morning when it is
not 100 degrees (really!) out there.

Your 77 Ranger does use a carburetor, right? The late 70’s is where the Ford engine, ignition, and fuel system configurations started to change in the direction of electronic ignition & fuel injection.

It starts up ok when warm right? It just won’t hold a steady idle after start-up when warm. If it is cranks a long time before it will start, that could point to a flooding problem, overly rich in other words. I had a problem like that a while ago where the power valve was leaking gas into the intake manifold, preventing the engine from starting after sitting when warm. But once it started it ran and idled mostly. The idle speed did tend to wander a bit though. Is your engine idle speed going up and down seemingly at random?

It has a carburetor.
Starting cold: 1-2 cranks and it starts and runs pretty fast. After
about 20 seconds it slows down to a reasonably high speed. No further

Drive to Home Depot and buy some screws (3 miles). Come out and Temp
gauge shows about half-hot (1/4 way up from cold).
Starting at 1/2 hot: 1-2 cranks and it starts. Engine quickly dies.
Restarting (if I allow it to die) is a little harder 3-5 cranks. (Could
the ‘harder to restart’ indicate vapor lock?? - just a thought). Continues
to try to die after each gear change requiring a foot on the gas and break
at the same time.

Drive about 1 mile: Everything back to normal.

Normally this only happened in the winter and I thought it had something to
do with sticky oil or something affected by cold weather. That was until a
couple days ago and it was 100 outside (probably 110 in the parking lot).

The sequence of events is always the same and the solution always comes at
about the same time (1 mile of city driving). There is something very
repetative about the problem but very hard to recreate on demand.

Make sure that idle solenoid is popping out like it should when the key is “on” and it holds the throttle valve slightly open with your foot off the gas pedal. If that appears to be working, and the routine engine maintenance is up to date, vacuum system verified ok, then there’s probably a partial clog in the carb’s idle and/or off/idle circuits. A carb rebuild is required to fix that.