May someone here can tell me why my truck backfires when I let off quickly on the gas pedal?I'm on my second muffler,it splits them wide open.Around town it doesn't do it.It seem to only be when I'm going around 55-60mph and let off the gas fast.I replaced the anti backfire valve & the smog pump.It doesn't do it with the air pump belt off.Maybe somebody can help me figure this out.Thanks! Bobby
The anti-backfire valve operates off of engine vacuum, have you checked the vacuum lines and solenoids for leaks, correctly hooked up, proper operation?
If this truck has a juice can vacuum reservoir, that's the first place I'd look.
It does have a square plastic vacuum canister next to the battery on the passenger side,it has 2 lines that 1 runs to the carb.and the other one runs along the frame rail,I assume this one crosses over and goes to the fuel tank mounted on the drivers side.This 300 6 cyl does have the electronic feedback carb.and that is new also.The engine does have a slight shake when it idles.I haven't found any vac. leaks yet.I've spent a lot of time trying to figure out why it keeps backfiring.I was thinking about taking the whole exhaust off,rodding the converter.And putting a straight pipe or glass pack muffler on it.Any other suggestions are welcome.I would like to get it fixed.Thanks! Bobby
That's the charcoal canister, has nothing to do with your problem. Try looking at the underhood vacuum diagram, if there is a vacuum reservoir, it should be shown there.
It's backfiring because either; It's running extremely rich The air pump isn't diverting on deceleration as it should There is an air leak in the exhaust allowing air to be drawn into the exhaust stream which gives the same effect as the air pump not diverting. Combinations of the above.
The air pump is controlled by two solenoids which are controlled by the ECM. The solenoids control how vacuum is applied to vacuum diaphrams on the air pump switching valves. The solenoids are; TAB (thermactor air bypass) TAD (thermactor air divert) The factory service manual has the proper procedure for diagnosing the operation of these two devices.
I don't see any of this curing the backfire problem. I -do- see the backfire getting you a ticket if that truck backfires in front of a cop who's a bit on the jittery side...
First thing is to assess whether the related systems are intact and functioning properly, Ford used reject tomato juice cans for vacuum reservoirs, 20 years is plenty of time for one to rust out, they also liked to use hard plastic vacuum lines, 20 years is plenty of time for them to get hard and brittle and crack.
You wouldn't believe how many supposedly unsolvable driveability problems are caused by things as simple as this.
The truck has a different front clip on it because it has the eng. as being a 302.And it doesn't have a juice can anywhere on it.I believe the truck must have struck something in the front,so there is no telling what parts are missing.I've had the truck about 6 yrs.I thought I had the problem solved by installing the new air pump,until I got on the interstate with it.You gave me a lot of good info.I guess I need a factory manual.I may take the manifold off and see if ther's a leak somewhere.I took the vent hose off the carb. and it's wet with gas.it has a check valve on it and another valve 1 line going to canister and the other one to the antibackfire valve.Thanks for the info.! Bobby
There should be a diverter valve in the air pump system that "diverts" the air pumps out put to atmosphere when you release the throttle so that you don't get the back fire. The valve is usually at the "Y" where the line from the air pump splits and goes to the manifolds, but may also be mounted at the pump. The canister you found earlier is the secondary charcoal canister for the fuel tank vent, it stores gas fumes and then the fumes are feed via the line to the air cleaner housing.. Then secondary only has two lines on it, the primary three.
Whitelightning
Another possibility ... although I don't know if this applies to carbureted engines. A friend had a similar backfire situation and the cause turned out to be the TPS (Throttle Position Sensor) harness. When the engine vibrated "just so" the harness would wiggle "just so" and the engine would backfire. When I wiggled the harness while the engine was idling it died.
HTH,
-D
I put a new diverter valve (antibackfire valve)on it,and it still backfires.I don't believe this truck has a TPS on the carb.it's electronic feedback.I broke a bolt off on the exhaust manifold,I cut the exploded muffler off with a ripsaw,I got the tail pipe off.Now I've got to jack the trans.up and take the crossmember off to get the exhaust pipe that has 2 cat converters on it off.Then I can take the intake-exhaust manifold off to get the broken bolt out.And check to see if there are any vacuum leaks or cracks in the manifolds.Probably won't get much done tommorrow,suppose to get in the teens,and the ground is pretty cold.I forgot to mention it was getting 13MPG.it has a C-6 trans.an overdrive would be nice.I'll keep yall informed on my progress.Thanks! Bobby
Instead of dropping the trans.I just cut the back cat converter off,and dropped the exhaust pipe down with the front cat converter.There's a pump reactor tube that runs fom the smog pump down between the two converters.Both converters don't have anything in them,so I guess someone already cleaned them out.I'm just going to put the one converter back and put a 2 ft. section of pipe in place of the back one.And a cherry bomb muffler in place of the split one.So now I'll be taking the manifod off to get the broken exhaust stud out.If it still backfires I'll just take the belt back off the pump. Bobby
Do you think the gutted cats might have anything to do with it?
Whitelightning
I would think the gutted cats. would probably have some effect on making it backfire.I also found a small pinhole in the check valve that connects on the carb.spacer plate,coming from the smog pump.Also exhaust gasket was shot.I replaced the trans servo cover gasket,while the cat was out of the way.I finally got the broken stud out,that was a real job!Easyout wouldn't take it out.Drill bit and rethreading.I got two studs to put in the head to hold the heavy manifold in place,while I get it bolted back in place,it doesn't have dial pins,and it's to heavy to hold with one hand while putting the bolts back in.Hope I can get it back together today. Bobby
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