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Old 02-07-2006, 14:01   #1 (permalink)
bobby swift
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86 f-150 4.3 Backfires?

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

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Old 02-07-2006, 20:01   #2 (permalink)
aarcuda69062
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Re: 86 f-150 4.3 Backfires?

In article <28707-43E90A36-923@storefull-3118.bay.webtv.net>,
bobbyswift@webtv.net (bobby swift) wrote:

> 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.
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Old 02-08-2006, 10:01   #3 (permalink)
bobby swift
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Re: 86 f-150 4.9 Backfires?

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

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Old 02-08-2006, 21:01   #4 (permalink)
aarcuda69062
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Re: 86 f-150 4.9 Backfires?

In article <19933-43EA236F-26@storefull-3115.bay.webtv.net>,
bobbyswift@webtv.net (bobby swift) wrote:

> 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.


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.

> 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.


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 was
> thinking about taking the whole exhaust off,rodding the converter.And
> putting a straight pipe or glass pack muffler on it.


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...

> Any other
> suggestions are welcome.I would like to get it fixed.Thanks! Bobby


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.
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Old 02-09-2006, 12:01   #5 (permalink)
bobby swift
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Re: 86 f-150 4.9 Backfires?

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

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Old 02-09-2006, 14:01   #6 (permalink)
Whitelightning
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Re: 86 f-150 4.9 Backfires?


"bobby swift" <bobbyswift@webtv.net> wrote in message
news:28011-43EB9510-280@storefull-3117.bay.webtv.net...
> 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


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Old 02-09-2006, 15:01   #7 (permalink)
Derrick Hudson
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Re: 86 f-150 4.9 Backfires?

On Thu, 09 Feb 2006 04:12:19 GMT, aarcuda69062 wrote:
> bobbyswift@webtv.net (bobby swift) wrote:


>> 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.

>
> 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.


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

--
How great is the love the Father has lavished on us,
that we should be called children of God!
1 John 3:1

www: http://dman13.dyndns.org/~dman/ jabber: dman@dman13.dyndns.org
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Old 02-09-2006, 19:01   #8 (permalink)
bobby swift
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Re: 86 f-150 4.9 Backfires?

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

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Old 02-13-2006, 09:02   #9 (permalink)
bobby swift
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Re: 86 f-150 4.9 Backfires?

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

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Old 02-15-2006, 09:01   #10 (permalink)
Whitelightning
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Re: 86 f-150 4.9 Backfires?


"bobby swift" <bobbyswift@webtv.net> wrote in message
news:1187-43F0B6E1-269@storefull-3113.bay.webtv.net...
> 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


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