1985 Ford F150 4.9L 6cyl plow truck not idling/won’t stay running

[DarkSaga]

Hello everybody,  I own a 1985 Ford F150 Lariat XLT pickup truck with a straight 6 cylinder 4.9L automatic 300 cid engine (long bed, dual gas tanks). This truck is a plow truck and is only used to plow my ½ mile long dirt driveway. It sits a lot and doesn’t get driven as much as I know it should (bad on me).

Anyway I've had the truck 10+ years (it’s never run superb, but it always started and ran) and of late I've had a terrible time keeping it idling/running.

OK, working in reverse chronological order:

Dec 2021: A local truck mechanic attempted to get the truck running and has not had any success so I am getting it back from him this week.

***Before the mechanic got my truck I removed the bed to install a brand new 19 gallon (front side mount) Poly gas tank with new sending unit... during the install I discovered that a rodent had chewed through the wires to the old front tank sending unit. This is what most likely created my initial STARTING/IDLING/RUNNING problems...lack of fuel! (The fuel gauge doesn't work on the rear tank and it leaked so I rarely used that tank.  

Because this truck is only needed for plowing I plan to only use the new front gas tank and eliminate the rear metal gas tank.


 Fall 2020: Basically the only way to keep it running now for just a few seconds/minutes it to spray starting fluid into the carb and have someone start the truck with the accelerator to the floor and hold it there as it struggles and chugs. I have to hold the choke plate wide open to keep it idling/running for a minute or two.

Some important notes:
<>The exhaust is black and sooty (can be seen on the ground).
<>The spark plugs are about 2 weeks old. They are black and sooty when I inspect them after trying to start the vehicle.
<>The truck has a single barrel Carter YFA carburetor… I rebuilt this, but was concerned that I might have muffed it up so I bought a cheap Kipa brand carb off of Amazon that is temporarily on the truck now. I mainly wanted to see if the Kipa produced any better results. It did not.
<>At times the fuel mixture is misting up out of the carb when I’m holding the choke plate open. Also on the Carter lately when it was trying to run the choke plate was flopping (opening and closing) violently.
<>I connected fresh gas from a gas can directly to the fuel pump and checked the pressure at the filter right before the carb connection and got a steady 6psi. It wouldn’t even idle or run any better with gas pumping straight from the gas can to the carb.
<>When I disconnected the fuel line from the fuel pump there was just a dribble of gas. I lightly blew compressed air back to the tanks and they gurgled a bit and then gas flowed up to the line. The fuel sending units may be failing (definitely need to investigate this further – probably will need new tanks and sending units).
<>When it sits overnight and I start it up it starts right up and idles for about 45 seconds and then will just start struggling and die. (This happens with either carburetor installed).
<>This truck has the TFI unit mounted on the side of the distributor (have not removed it yet but cleaned up the connections).
<>All the vacuum lines are old/original and have not been replaced yet (and need more thorough inspecting).

Brand new parts in 2020:
Fuel filter, air cleaner, mechanical fuel pump, spark plugs + plug wires, distributor cap & rotor, coil, battery, starter solenoid, starter motor, oil catch can (the air filter was getting oily), Kipa carburetor


 2020 Recent things I’ve checked:
<>The rotor rotates when the engine is turned over by hand.
<>The rotor is pointing at #1 spark plug location when the timing marks line up on the engine. Rotated it to the ZERO (0) mark and the (---) straight scratch line on the engine.
<>The spark plug wires are wired to 1-5-3-6-2-4 firing order.
<>Used a spark tester and each spark plug had spark that appeared fairly strong and consistent.
<>(COLD ENGINE) Did a dry compression test and the cylinders (#1-6) were respectively: 110, 118,116,110,110,112 (I'm inexperienced with compression testing and may have done things very wrong)
<>Attempted to do a vacuum test but could not get a reading (the truck won’t stay running or idling very long).
<>Tried to use my OBD2 scanner, but the plug didn’t match up so I guess I need the Ford adapter for OBD1... if that will even work.


***My apologies for the very long post, but I wanted to include as much info as possible...I am a novice mechanic at best and just try to listen and learn and fix things when I am able to. Anyway thanks in advance for your help I greatly appreciate it!

[Note: the mechanic put the original CARTER YFA carb on the truck and had no success either.]


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[Gary Lewis]

It sounds like the problem isn't with the carb as you get the same results with either carb.  Which is strange because it also sounds like the fuel/air mix is very rich since you have to hold the choke open rather than closed.  And that rules out broken vacuum lines.

But you said "When it sits overnight and I start it up it starts right up and idles for about 45 seconds and then will just start struggling and die. (This happens with either carburetor installed)."  From that I can think of two things that might do it:

Clogged Exhaust: If the exhaust system is plugged, as the pressure comes up in the system the engine will struggle to run.  But, the pressure would then go away after the engine dies and it should start and run ok for another 45 seconds.

Ignition: You said it is a TFI system.  The distributer-mounted TFI systems cause all sorts of headaches, and especially when they get warm.  If you have the money you can test this by replacing both the TFI module and the pickup in the distributor.  But if you have the time you can test it by using a heat gun to warm up the TFI module before starting the truck.  If instead of starting and running well for 45 seconds it struggles to start or run then you probably have found the problem.

[85lebaront2]

The other thing in his description, choke flapping open and closed coupled with the truck sits for a long time, valves sticking maybe?

Are we having a special this month on 1985 six cylinder trucks?

[Gary Lewis]

I forgot the choke flapping. Can the unloader do that much?  Maybe only if there’s almost no tension on the choke stove?

Yes, valves could be an issue. If the thing didn’t start and run well for 45 seconds I’d be wondering about cam timing.  It the six has gears and I’d think they’d strip rather than skip.

[mat in tn]

well now that is a list of symptoms! im looking at the carb first. you claim to have seen some misting and that is not normal. i have no experience with whatever import carb you bought. the yfa is remarkably reliable when right. unfortunately, carb mechanics are a dying breed.  this can be as simple as a stuck float valve allowing the carb to overfill. it certainly sounds like it is getting fuel. electrically, start by just doing a good spark test the old-fashioned way. even if it runs poorly, it will prove that it can. place a clothes pin on the choke flap to keep it open a little so that it cannot seal and that will take one variable out.
btw. you have the carb 4.9 which has a mechanical fuel pump. the fuel senders in the tanks are nothing more than a pickup tube with a hard plastic screen on the bottom. the level sender is the only electrical part and it only Grounds the gauge through a resistor operated by a float. the engine doesn't even know it's there and it will not affect how the engine runs. a clogged pickup screen can.
one thought on the exhaust. does it have a cat/converter? they are known for getting clogged on old trucks and i have seen hundreds of trucks come in without them. not my call i just recommend my favorite muffler shop (legality). go ahead and laugh but I've also restored a few that the mufflers were FULL of acorns after sitting for a season. that gets a lot of looks when i stand the muffler on end and shake them out.

[DarkSaga]

Thanks for the comments fellas. Keep your ideas and suggestions coming.

I'll have the truck back soon and will be attempt to do some more work on it before it becomes brutally cold. Unfortunately I have to work outside on the vehicles, so I'll do what I can before the weather becomes a major obstacle.

Hopefully with your help I'll be able to figure things out.

***The truck does still have it's catalytic converter. Anyway at one time last year I needed to find and repair some exhaust leaks so I ran a shop vac (in the blower mode) and forced air thru the tailpipe. I was able to detect the exhaust leaks this way up to the front near the engine, but I did not hear any nuts or debris rattling around in the exhaust system. That doesn't mean that there aren't any or there isn't a clog of some sort...I suppose the interior of the cat could be collapsed... but I wanted to mention that I pushed fairly high pressured air thru the exhaust system and there didn't seem to be any major obstructions.

[mat in tn]

well, that sounds fair enough for it to be able to start and run. in which case you would have mostly a quieter running engine with a loss of power if it were merely restricted but dirty. by quieter i mean after the converter but would make any leaks between it and the engine louder. therefore, easier to find.
one additional tip. when you get it back and get to work on it, narrowing the scope is going to be helpful. the idea is to reduce the variables as much as possible. one thing often overlooked and assumed is the vacuum "system". which can make things worse and very hard to define. below the carb on the intake is a vacuum "tree". you may want to undo all hoses from it and cap them with vacuum caps. Take note of how many ports are there, which hoses go where, and if any are capped already (or should be)

[RenoHuskerDu]

Did you look down the carb throat while the motor is running rough? If the bowl is overflowing, that would take a few seconds to develop. After it fills, raw fuel will dribble down the throat of the carb and make the motor chug and smoke.

The carb choke flapping is a puzzler. That butterfly should be connected to a choke pull-off device, wired to hot 12v when the ignition is on. There will be a certain amount of movement allowed in the linkage, but I've never seen one flap. Ever. They are weighted to stay open.

As for the converter, aren't you allowed to delete all that stuff after the vehicle hits a certain age in your state? We are so allowed in Texas. It certainly simplifies things. Delete/block air injection and its pump, delete or gut catalyst, delete EGR and block off its ports, replace funky emissions carb (looks like you did that already). Just run with a PCV valve.  We had a 4.6 melt a converter and run very weak. The internals of those converters are now in a bag in the shop. Runs great and sounds better now.

[DarkSaga]

Thanks mat_in_tn and RenoHuskerDu for the comments and ideas of things to check on. I'll have the truck back this week, so if the weather is cooperative I'll start working on the truck again.

Just so you guys know, this truck will not be inspected/registered to be on the road in NY state. I only use this truck for plowing my road (when it's running) so I can bypass and eliminate any emissions parts on the vehicle. I think the original owner removed or blocked off most, if not all of the emissions stuff anyway. You guys just point me in the right direction and I'll try your suggestions!

[DarkSaga]

Here are some old pictures of when I took apart the original Carter YFA carb... just wanted to post them so you all could see the condition it was in at the time and if you notice anything out of the ordinary or incorrect. Obviously it's quite dirty, but I cleaned it up as best I could before reassembling it with some newer parts from the rebuild kit.














*If too many images bogs down the page or is frowned upon just let me know and I won't post so many. Thanks!

[DarkSaga]

On a side note I just wanted to mention that I had seen this video in the past where these guys turn the inline 6 into a beast with huge power. I don't need a supercharged plow truck but I was wondering if I should just get rid of the Carter YFA and the current distributor system and just add some parts similar to what they did in the video. I don't mind putting money into the truck as long as it will start up every time and work great for my plowing needs. -->

https://www.youtube.com/watch?v=N5bnl8knexU

[Gary Lewis]

I don't think you want to go doing that with your 300.  They are a very reliable engine and are/have been used in all sorts of industrial things due to that.  So unless you need the power I'd stay with the standard setup.

Note that I'm not saying you have to keep all the emissions stuff.  But that the YFA carb and DS-II or TFI ignition are quite capable of giving you both power and reliability.

On the carb pics, they don't seem to be of such a large size that they'll cause a problem.  But I wouldn't put that many pics that are each close to the 1Mb limit on as it could cause problems for people using cellular data or low-speed internet feeds.

[DarkSaga]

Sorry about the amount of large pics that I posted Gary. Next time I'll just post small ones (and if you guys need to see more detail in any I can always enlarge them later on).

As for modifying the truck, I'm just getting fed up with it not running and am looking for a solution. I took it to a reputable local truck mechanic and he couldn't even get it running (though I know he didn't spend that much time on it because he has a fleet of other trucks he's always working on that bring in the buck$ for his business). Because it's older and has a carburetor maybe this mechanic wasn't the right guy to take it to, but usually this type of set up should be easy to fix for a seasoned mechanic.

Anyway at this point my options seem to be: 1) Get expert advice from the forum and fix it myself, 2) Be lucky enough to have a forum member come look at it (and pay them $ to fix it or help me fix it), 3) Modify the engine [$$ more costly] or 4) put a new fuel injected engine in it [VERY $$$ costly].

Certainly #1 above will be the cheapest and bring the most satisfaction if I get the truck working right... but I could end up going in circles and just get frustrated with it. I'll start with trying to fix it myself, but I'm really going to need some help from you guys in the forum.

[Les Kerf]

Your compression readings are pretty low (did you hold the throttle wide open while taking the readings?) but it should run well enough to plow snow.

These engines are known to have the harmonic balancer slip, causing the timing marks to be way off.

I would unhook the exhaust at whatever is the easiest place (maybe right at the manifold) and see if it will run longer than 45 seconds. Yes, it will be loud!

[85lebaront2]

Go junkyard prowling and find a nice 1978 or earlier F150/250 with a 300 six, get the distributor, DS-II box and as much of the wiring as you can and the nice simple YFA carb with minimal emission stuff on it. You can get the DS-II wiring from the bigger engine F series trucks up through 1987, up through 1986 the left side front wiring is set up for EFI. feedback carb or none depending on the engine as long as it is a gas engine.

Once you have these in hand, you can get rid of all the computer crap (hint, sell it to someone who needs it). On your carb, you can see where the throttle body to float bowl gasket had started to come apart.

[mat in tn]

i think i missed where the compression was measured. when you get the truck back and get into it we can see what tests you do and maybe help from there

[DarkSaga]

Hey guys,

I'm certain now that I did the compression test incorrectly back then (it was the 1st time I ever tried a compression test and my methods were wrong), so don't go by the compression numbers I posted. I did a number of things incorrectly with that test, so just disregard that info.  I'm still learning...

[mat in tn]

not to mention a lot of "hobbiest" quality compression gauges are very inaccurate. they rarely read high. they are an excellent example of you get what you pay for

[DarkSaga]

I finally got my wounded F-150 back from the mechanic. I didn't get a chance to look over it super closely yet, but I can see they disconnected and failed to reattach some stuff and broke a thing or two. That's annoying, but at least it's back and I can start to work on it again soon (weather permitting).

Hopefully you all can help me troubleshoot things so the source of the issues can be determined and corrected. Thanks in advance!

[Gary Lewis]

Glad you got it back.  We stand ready to help.