Okay, facts first, I've got an 81 F100 custom, w/ inline 6.

Lately I've been having troubles with her, She'll go for days running fine, then all of a sudden start running rough, die when I come to a stop, and then refuse to start. Hour or so later, cranks right up and rumbles perfectly. No observable pattern to when or why. Except never happens at speed, only when going slow or coming to stop.

So, I've gone through the basics, checked and replaced sparkplugs and plug wires, replaced some old vacuum caps on the carb (Had two that were dry rotted), replaced the coil (plugs were sooty, so I decided to replace it), checked the timing (adjusted slightly, but it was pretty darn close to begin with) and replaced the ignition module; I even put in seafoam in case I had some moisture in my gas. All in all a lil over $100 of "can't hurt anything" but didn't help either.

My last guesses are either egr valve or carb. I really want to avoid taking her to the shop, I just don't have the money (and I need new tires, soon).

 

The ignition module is often the #1 culprit for a vehicle shutting down when it gets hot. They're sensitive to heat and can cause owners of Duraspark II equipped vehicles to chase around a ghost that will leave them scratching their heads. It's good that you've replaced it, as it may have given you trouble at some point. That being said, when you mentioned your timing was off a hair, did you make sure the harmonic balancer hadn't slipped? If it did, your timing is off if you set it by the balancer mark. You'll have to find TDC of the compression stroke on the #1 cylinder and remark the balancer and re-time the motor to get it right. Also, did you have the distributor tested to make sure it was good? The pickup modules are known for going bad as well.

If you think that you've got water in your gas or trash or anything like that, you'll want to go out and get an inline fuel filter that you can t- into the fuel line that leads up to the mechanical fuel pump. A lot of times, bits of rust and such can make their way into the carburetor and clog up the needle and seat, causing a condition that leads to flooding and a rich-running motor. The inline fuel filters that thread into the carburetor aren't really known for their quality, as they have just a small mesh screen that is used to filter the fuel and remove particulates.

Thinking about this further, I wonder if you have an overly rich condition (like a gummed up needle and seat, fuel pressure gone too high, etc.) that would cause the motor to stall. When you left off of the gas and apply the brakes, typically vacuum goes up in a quick fashion when the throttle valve is closed, and when this occurs, you're basically cutting off the supply of air to the motor. I'd want to look at the choke setting and adjustments on your carburetor to make sure the choke isn't getting closed for whatever reason and causing your motor to flood out.

Also, is your carburetor and gas tank both venting properly? Are the fuel lines in good shape and have adequate are flow around them? We need to make sure that vapor lock and such are not concerns that you may have.

You say you've checked for vacuum leaks, did you check to make sure the carburetor was tight to the manifold? The inline-6 is known for causing carburetors to vibrate loose. It is held on with two 1/2'' nuts and studs.

If you think the EGR valve is suspect, test your EGR valve. Take the line off leading to the valve and plug it where it connects to the water-heated vacuum switch at the front of the motor. Then, take a spare piece of vacuum line and plug onto the EGR. Crank the motor and let it run and warm up. While it is running, slowly suck on the vacuum line going to the EGR valve and note any changes in how the motor runs. A bad diaphragm in the EGR valve causes a vacuum leak, and if the motor starts running worse, the EGR valve is good. If the motor doesn't change, then the valve is probably bad.

I'd also want to look at the brake booster. Is your brake booster making a hissing noise when you cut the truck off? It could be a bad booster causing a vacuum leak when you let off the gas and apply the brakes. With the vehicle running, can you pump the brake pedal at idle and the motor start acting up? And, if it hisses when you cut the vehicle off, you might have a bad booster.

If the booster is bad, the master cylinder might be as well, so you'll want to unbolt that from the booster and check to see if brake fluid is leaking from the master cylinder into the brake booster.

Is your transmission an automatic or a manual? If automatic, check the vacuum lines running to and from the transmission. If manual, I wonder if the clutch is fully disengaging every time you press the clutch pedal.

Do you have air conditioning? How are the vacuum lines running to and from that?

This is just a general guide to give you things to look at. If you find anything that looks suspect, let us here at FTE know and we'll do our best to help you.

 

In case you might have missed it, I edited my post because another member here brought to my attention that the way I have formatted it might have caused a little bit of confusion. We here at FTE do not recommend just randomly throwing parts at a vehicle and we believe in firmly diagnosing the problems before we go to tackle a solution and get a vehicle fixed. If you have any questions or concerns about any advice given here, just let us know and we'll address the issue.

Thank you, and welcome to Ford Trucks Enthusiasts.

 

Thanks for the advice.
We have a lead! maybe...


Okay, today I fired her up, difficult to start and idled real rough, since it was sounding just like it has everytime she's died on me, I walked around to watch the engine. While it was idling and about to die, (died about 30-40 sec after I started her) I saw fuel spilling out under the carb. When I saw that I remembered that everyother time she'd died on me and not wanted to start, I'd smelled gas (can't believe I forgot to add that earlier).

Now, literally right before I fired her up and this happened, I'd replaced the fuel filter, but there was no leakage around any areas I messed with, so while the rough idling might have been caused by air in the line, the spillage shouldn't have been.

Thoughts?

If I can get her to run smooth a little later I'll go ahead and test the EGR valve how you suggested, but I'm really thinkin it's the carb now.


Thanks, Stephen.

 

Your float needle in the carb is not seating. It works just like a toilet, it has a float and a valve and it has a storage bowl for the fuel. If the needle doesn't seal off, then it lets the bowl fill up too full of fuel and it spills over into the engine.

99% of the time, dirt is the problem. Since you have just changed the fuel filter, it might do it once or twice more and then clear up. If it does it and won't clear up, take the plastic handle of a good sized screwdriver and tap on the top of the carb near where the fuel line hooks up. This usually will dislodge any dirt under the needle and let it close off.

If this keeps happening, check the fuel filter again. If it's dirty again, I would put a new fuel tank on it.

 

Well, I've had it running several times, and she idled rough, but no more fuel spillage. Maybe no dice?

While I had her runnning once and she wasn't idling rough, I tested the egr valve, definetly didn't like me sucking on it, I'd say it's good.

I checked out the brake booster, didn't seem bad, no response from pumping the brake, didn't hear any hissing, so I'm still thinking somethings up with the carb, but I can't tell what. Should I just start looking at rebuilding it and seeing where that leaves me?

 

Check to see if the carb didn't vibrate itself loose, along with Franklin's and 1983F1503004x4's suggestions.

There are four screws at the bottom of the YF and YFA 1 barrel carb that likes to vibrate loose. These screws are accessible once the carb is removed from the intake manifold and turned over. The four screws hold the mid section and throttle body together. If loose it will cause a vacuum leak through the sides and destroy the gaskets.

Oh and what state does your truck come from? California or is it 49 state emissions? You can tell which by reading the bottom of the emissions lable located on the radiator support. Compliant to California emissions, or compliant to EPA etc...


I would also go and test the ignition module just for good practice and either eliminate or confirm this component as a problem.

 

Your EGR and brake booster sound like they're in good condition.

I would have the new module checked like suggested. Many parts are known to be bad out of the box. Starter relays for our trucks are notorious for this. If it checks out good, then on to other things.

Of which, it could be a few things. I'm betting it's a gummed up needle and seat like already stated. With a YFA, you could have a bad accelerator pump diaphragm, but these usually only go bad once they've dried out, and I doubt it's that since you've been driving it before and it just randomly happens every so often. Your fuel pressure is more than likely fine, and the carburetor is more than likely venting correctly like it should.

If you feel comfortable with doing so, and you're positive that the carburetor isn't loose or anything like that (don't forget to check the 4 screws holding the throttle body to the main body of the carburetor), I'd either put a rebuild kit in it yourself or have a mechanic rebuild the carburetor for you.

Once that's done, get an inline fuel filter from the auto parts store that has a paper element in it, and T- that inline into the rubber hose that comes from the hard line off the gas tank to the input side of the fuel pump. This will filter out the trash that might be getting pulled into your carburetor. The carb mounted filters aren't worth much, and it's just good insurance to make sure you aren't pulling trash into your carburetor in the future, as something like this can happen again if the opportunity arises.

 

I've actually got the carb apart being cleaned right now, I'll be putting in the kiit as soon as I have a chance. I had to take it off to check it, so I decided to do a rebuild also, since nothing was obviously loose, and little things keep leading me to towards the carb.

How do you suggest I check the module? I've seen it suggested before elsewhere, but am not really sure how to do so.

 

I think when you caught it spilling fuel, you found your main problem.

 

You take the ignition module to a auto parts store to have it tested on their machine in the same way they can test alternators and starters.

They will usually do this for free.

 

Well....

Not to get my hopes up, but I've finished rebuilding the carb, reinstalled it, and ran her up and down the road a few times, purposefully coming to a stop several times (on hills, flat areas, etc.) to give her a chance to stall, and nothing.

I'll give her a few days of driving before I call it, but I'm cautiously optomistic. Keep your fingers crossed.

Thanks for all the help along the way guys,
Stephen.

 

Did you find many dirt and rust particles in the bottom of the bowl inside? That's where they go when they finally get around the needle valve after sticking it open.

Like I said if you start having trouble again, and find particles in the fuel filter, it might be time for a new fuel tank. It's a common problem and they are not that expensive.

 

I didn't see a whole lot of junk, just a lil here and there. So, nothing obviously wrong, but there must have been a stuck jet, loose screw or something. It still wasn't working before I did the rebuild, but now that it's done, so far she's running fine.