Calling all mechanics!

[aimn]

I have a 1978 Ford F-150 SuperCab pickup truck. It has a 300 straight 6 in it. I have driven it a few times in the last few months and it ran great. My bro inlaw came over to take it back to his house (70 miles) and he said it back fired most of the way. He said "at times I couldnt go faster than 40mph". I know when I started the truck before he left that it was bone dry on gas, it had a half a tank two weeks ago and hadnt been driven. Either someone thieved it or there is a leak in the tank.
My question is, what causes backfires? Where do I start troubleshooting? I am thinking line dryer first. The check the distributer cap. I have replaced the plugs recently. Can someone throw me a bone here?

 

[jemcam]

Line drier? Care to elaborate on that one?

Anyway, when you replaced the plugs, did you drive it afterwards, and did it run okay?

Could be timing, but if you drove it after you put spark plugs in it and it ran fine, it's hard to say.

If it's out of gas, it wouldn't run, and certainly not for 70 miles.

Start by verifying your firing order, which would be stamped on the intake manifold, then start at the distributor on the #1 plug marked on the cap. As the rotor spins, the rotor should hit their respective marks on the distributor cap to each cylinder. Did you pull all the plug wires off, then throw in new plugs and then hoped you got the wires on right? The best way to do it is one at a time. Harder to screw it up that way.

Sounds like it's a timing problem of some kind. Start by verifying your plug wires are in the proper order, then check the timing.

A clogged fuel filter would cause a loss of power, but usually not backfiring.

 

[aimn]

Of course I put gas in it, I thought that was a given. I have driven the truck several times since I have done any work to it, it ran great. I had a half tank of gas in it, never drove it for a month or so, went to start it, doh! the tank was bone dry! I DID put gas in it, thats when the back firing started.

PS.....line dryer is alcohol that evaporates any water in the lines or tank.

 

[Colt45]

sounds to me like your timing is off.. and i hate messin' with timing..GRRRR

 

[jemcam]

How many miles are on this truck?

This is just a guess, but it's possible it could have jumped timing. The Ford 300 6 uses timing gears, so it's not quite as common to jump timing as it is with a timing chain & gears, but it's possible.

I'd still start with the timing, but if it didn't jump time for some reason, it's possible that you have a fuel problem of some sort. It doesn't really sound like a fuel problem (water in the line, or clogged filter)since you're getting backfiring. That usually indicates timing.

Also, since it's carbuerated, water or trash is less likely to be a culprit than it would be if it were fuel injected. It still has the stock 1 bbl, right?

 

[thedarkwolf]

If it was completely outa gas you could have sucked some loose rust or dirt into the fuel line and its blocking it or the carb.

 

[aimn]

Ok......I will replace the fuel filters and have a buddy of mine check the timing. Water in the line wont cause backfires?

 

[chiwawa626]

i think u need to clean ur car.

 

[thedarkwolf]

I had a whole lotta water in my tank last year and I only remember it sputtering a lot no back fires.

 

[87horsepower]

Fuel pump may be on its way out.

 

[TimberWolf]

The "First Diagnostic Rule" is: Start with the simplest things first, and never assume anything . . .

The above is even more important if you are relaying information to others in a quest for diagnostic assistance. You are the eyes, ears, and sole source of (hopefully) accurate information.

Try the following:

Start with "cheap and easy" - Verify the firing order and correct ignition timing, as jemcam suggested. Then run the engine in a shaded area (not in bright light), and visually examine each wire from the plug boot to the distributor cap for any stray sparking, especially near the plug well and any place the plug wires contact or cross over each other.

Stay clear of the fan and hot manifolds!

If you can reproduce the backfiring at a particular RPM, have someone assist while you use plug boot pliers (or insulated gloves) to pull one wire off at a time (in order). If the backfire stops on a particular cylinder, replace the wire and/or plug.

Replace the fuel filter. Add some fuel system drier. Water and/or sediment in fuel can cause a backfire symptom because it can foul or ground the plug momentarily - allowing an excessively rich mixture in the affected cylinder - and then fire intermittantly.

Not all engines respond the same way to a particular problem. Carb and/or intake manifold air leaks also produce backfire symptoms.

A leaky diaphragm in the fuel pump can also cause backfire symptoms, but considering expense, it's a bit further down the list of things to check.

Tell us what you find. Details are important . . .

 

[purplehayes]

<< My question is, what causes backfires? >>

Backfires are caused by the mis-timing of the valves. Instead of the fuel combusting in the cylinder, part of it is combusting in the exhaust. It can also fire out of the carbuerator [sp?] which is kind of scary if you've never seen it before.

PH

 

[aimn]

Ok.....I will check the firing order and plug wire placement. I cant imagine anyone openning my hood and moving plug wires around though! I put plugs in it months ago, it ran GREAT afterwards. It would be easy to believe that the timing is off. Would the fuel pump going out cause a fuel leak? It will be a week before I get out to bro inlaws to work on truck, I will post an update.

It sure is nice to have more minds than one, to try and figure this out! Thanks!

 

[PG]

Hey, good luck.
I love those old 70's Ford trucks. I had a '75 F250 4X4 myself. My dad still has a '79 F250 2WD with a 460. It only has about 48,000 miles on it. I'd love to get that one some day.

 

[jorken]

While we're at it... what would be your guess to a cost of replacing a Ring Gear and Flywheel for an '87 Jeep Wrangler?

 

[PG]

While we're at it... what would be your guess to a cost of replacing a Ring Gear and Flywheel for an '87 Jeep Wrangler?

What do you mean?

The ring gear mates with the pinion gear in a drive axle. You would need to change not just the ring gear, but also the pinion. That can be expensive! The ring and pinion change takes special tools and experience. The gears need to mate properly so that they don't wear out prematurely.
Also, since a Jeep is 4WD and has 2 drive axles, you need to change BOTH ring and pinions so that the gear ratios match. And the front and rear axles are different, so you need a different set for the front compared to the rear.

Why would you want to change the flywheel? I guess a bigger flywheel would store more energy for low speed off roading, but I wouldn't waste my money changing a flywheel unless it really had problems and needed replaced. If it has cracks or the gear teeth were worn down, then it would need to be replaced.

 

[351Cleveland]

Also check to be sure the distributor cap isn't cracked. A crack in the cap will cause a crossfire that pops off under heavy load.

 

99% of all backfiring is caused by a lean mixture
Running your tank dry and then filling it stirred up the sediments on the bottom of the tank.
Clean your carb (Complete dissassembly) and replace your fuel filter(s).

 

[87horsepower]

<< Would the fuel pump going out cause a fuel leak? >>



Sometimes mechanical fuel pumps will leak when they fail. Mechanical pumps are relatively inexpensive and easy repair. Intank electric pumps are the one that can be a pain in the arse.

 

[Soybomb]

Yeah you replace the ring and pinion as a set. What did you do to yours? Did you run it without lube and break it?

 

[TimberWolf]

<< Ring Gear and Flywheel for an '87 Jeep Wrangler >>


He's referring to the starter ring gear - and broken teeth, cracks, or heat spots can require replacement of the flywheel / gear assembly.



<< Backfires are caused by the mis-timing of the valves >>


Not quite accurate - Backfiring may be a symptom of incorrect valve timing; but can also, and more commonly, be caused by worn valves and seats, excessively rich or lean mixtures (due to jetting, float settings, injector problems, plug fouling, or air leaks), and ignition system componant or ignition timing problems.



<< I cant imagine anyone openning my hood and moving plug wires around though! I put plugs in it months ago, it ran GREAT afterwards >>


Can you imagine spark plug boots not fully seated? Eventually they can retract the clip off of the plug,or pull the wire out of the clip, and cause misfiring. Carbon-core wires also deteriorate after awhile (3 to 5 years) causing misfiring. Make sure they are fully seated and in good condition.

 

[JohnnyKnoxville]

Hmmm sounds to me like a muffler bearing going bad

 

[jorken]

Yes the starter ring gear, sorry for not being more specific I wasn't aware there were more then one ring gear and flywheel in these things.