[cschieck2002]

Just as the title says. Loud "thump" (like someone kicking the firewall)after I start the truck. Bought the truck two years ago and it has always made this sound. Bought with 49k on it and now it has 64k- seems to run just fine- always has. Serviced the tranny at 63k- NOTHING in the pan, shop said it looked awesome. Done some mods (see signature) since I bought it- hasn't affected it one way or another. I'm just really, really curious to what it is cause I'm about to drive it up to Happy Valley-Goose Bay, Labrador, Canada- about 3000 miles ONE WAY.

Thanks in advance- Chad

 

[gregrob]

[ QUOTE ]
Just as the title says. Loud "thump" (like someone kicking the firewall)after I start the truck. Bought the truck two years ago and it has always made this sound. Bought with 49k on it and now it has 64k- seems to run just fine- always has. Serviced the tranny at 63k- NOTHING in the pan, shop said it looked awesome. Done some mods (see signature) since I bought it- hasn't affected it one way or another. I'm just really, really curious to what it is cause I'm about to drive it up to Happy Valley-Goose Bay, Labrador, Canada- about 3000 miles ONE WAY.

Thanks in advance- Chad

[/ QUOTE ]

Mine did the same thing a while back. Blew the Big air filter slam off. /ubbthreads/images/graemlins/eek.gif

 

[Oneof6]

Chad, most likely the sound that you are hearing is your blend door thumping open or shut. When you shut the truck down with the controls in any position other than OFF the door, vacuum controlled, moves or leaks off. Then you come out the next day, sleepy, carrying a hot cup of coffee and fire it off. Just about the time, you want to back up the vacuum pump has charged up and pulls the door back into it control set point position. The result is a heavy thump that sounds like you described it. Try turning off the HVAC control before you start up and see it it quits. I is a habit with me that I turn it to off when I shut down for the day. Thought I had run over the dog the first time it happened. I have had more than one PSD both would do it and many posters report the same thing.
Another note; I like that area you are living in, Casper, Wyoming. I go out there to service equipment at a power plant in Glenrock just east of Casper and have some friends out there. Great area, just a little to cold for me in the winter. Heck, a cold winter for me is having to wear socks.
Good luck, 106

 

[KRP]

Turn the Heat/AC switch to off and it probably won't do it. It is the vent door closing when the engine starts and the vaccuum pump starts sucking I believe.

 

[cschieck2002]

Thanks alot man! Never bothered me until I decided to move and drive it 3k- then I figured I better ask for peace of mind.

appreciate the help.

 

[jimmyendicott]

Oneof6 nailed it. Its the mixer door. Mine does it too.

 

[gregrob]

[ QUOTE ]


Mine did the same thing a while back. Blew the Big air filter slam off. /ubbthreads/images/graemlins/eek.gif

[/ QUOTE ]


How does the mixer explain it blowing off my air filter?

 

[KRP]

Gregbob, Were you using ether to start it the time it blew the filter off?

 

[Oneof6]

[ QUOTE ]
[ QUOTE ]


Mine did the same thing a while back. Blew the Big air filter slam off. /ubbthreads/images/graemlins/eek.gif

[/ QUOTE ]


How does the mixer explain it blowing off my air filter?

[/ QUOTE ]

I does not, I don't believe that the two are connected or that the blender door position on one of these trucks can do that. If you had a thump that did that, and I am sure you did if you say so, then it was caused by something else. Starting fluid maybe?

 

[rlarkin]

[ QUOTE ]

How does the mixer explain it blowing off my air filter?

[/ QUOTE ]

It doesn't. You obviously have a different problem.

 

[SteveBKFLD]

The thump most people hear is not a blend door (how can a plastic blend door back that kind of sound anyway). This is a myth that has spread faster the the myth of Sasquatch.

The thump most people hear is a slug of refrigerant entering the accumulator (IIRC). This is a different phenomenon than compressor slugging, which is a thump when the compressor first starts turning due to liquid refrigerant collecting in the compressor. There is a TSB about accumulator noise, and a revised accumlator, but its only for earlier modes. I don't think Ford (or its supplier) ever got the system redesigned to eliminate the slugging, since its still a trait of these trucks (on a hot restart). You hear the thump on the firewall because the evaporator and accumulator are mounted to the firewall, and when they vibrate with the slug of refrigerant, it is transmitted throughout the firewall. The intensity varies with the previous off time of the engine and heat soak. I'v even heard it between compressor cycles, when its cool out (50's) but sunny and I have the AC on).

BTW, In order to hear it, the HVAC controls must be in a position to run the AC compressor, which is all settings but floor only, panel vents only, and off.

 

[Oneof6]

[ QUOTE ]
The thump most people hear is not a blend door (how can a plastic blend door back that kind of sound anyway). This is a myth that has spread faster the the myth of Sasquatch.

The thump most people hear is a slug of refrigerant entering the accumulator (IIRC). This is a different phenomenon than compressor slugging, which is a thump when the compressor first starts turning due to liquid refrigerant collecting in the compressor. There is a TSB about accumulator noise, and a revised accumlator, but its only for earlier modes. I don't think Ford (or its supplier) ever got the system redesigned to eliminate the slugging, since its still a trait of these trucks (on a hot restart). You hear the thump on the firewall because the evaporator and accumulator are mounted to the firewall, and when they vibrate with the slug of refrigerant, it is transmitted throughout the firewall. The intensity varies with the previous off time of the engine and heat soak. I'v even heard it between compressor cycles, when its cool out (50's) but sunny and I have the AC on).

BTW, In order to hear it, the HVAC controls must be in a position to run the AC compressor, which is all settings but floor only, panel vents only, and off.

[/ QUOTE ]

So I assume that you highlighted "most" because there are alternative theories. Doesn't matter, the solution is the same, turn it to off if the thump startles or bothers someone. I have been told by Ford Mechanics that the sound is the blender door, never had a reason to question it when someone gives a diagnosis and a working solution. Now I am curious, I think that I will unplug my pump one morning and start up in AC and see if i get that slug knock you are referencing. 106

 

[ACD2]

It is the blend door. Just for kicks and grins, I got my head down where the door is and had my wife start the truck. Sure enough, hiss, thump. Switch positions at the controls and hear the same thing, just not as loud.

 

[gregrob]

[ QUOTE ]
Gregbob, Were you using ether to start it the time it blew the filter off?

[/ QUOTE ]

NO! /ubbthreads/images/graemlins/eek.gif don't use that crap on my truck. Filter was fine.
Started truck, THUMP!, turned it off and popped hood, filter blown off boot.

 

[corapeake]

Mine "THUMPED" once........it was a cat!! /ubbthreads/images/graemlins/eek.gif didn't kill him but the belt took a big hunk of hair !!!


Sorry for the side track, I couldn't resist. /ubbthreads/images/graemlins/biggrin.gif

 

[psd1234]

[ QUOTE ]
It is the blend door. Just for kicks and grins, I got my head down where the door is and had my wife start the truck. Sure enough, hiss, thump. Switch positions at the controls and hear the same thing, just not as loud.


[/ QUOTE ]


It is not the blend door.

If you want to know what your air mix, blend and recirc doors sound like then just turn your key on without starting the engine and cycle the HVAC switch through the different positions.

You hear vacuum apply and release and if you listen closely you can hear the doors move. You will have to try really hard though. After a couple cycles you will deplete vacuum and you will hear the vacuum pump under the hood turn on.

The noise is compressor slugging. The refrigerant migrates into the compressor overnight while the truck sits. Their is a delay when you start the truck before the compressor comes on to allow the idle speed to stabilize before applying a load (all cars have this).

The liquid refrigerant gets pumped out of the compressor on engagement and sends a pressure wave through the system making the knocking noise. This is similar to water hammering in your house.


Anyone who says it's the blend doors is taking someone elses word for it. Anyone whos says they have confirmed it themselves in some way like disconnecting the vacuum or whatever is just plain lying. I have no idea how such an inaccurate rumor got started.

To prevent the noise wait a few minutes to turn the a/c on so underhood temps come up and evaporate the refrigerant out of the compressor.

FYI, the a/c compressor is on in every position except "feet only" and "face only".

 

[JimmyDee]

[ QUOTE ]
The thump most people hear is a slug of refrigerant entering the accumulator (IIRC). This is a different phenomenon than compressor slugging, which is a thump when the compressor first starts turning due to liquid refrigerant collecting in the compressor. There is a TSB about accumulator noise, and a revised accumlator, but its only for earlier modes. I don't think Ford (or its supplier) ever got the system redesigned to eliminate the slugging, since its still a trait of these trucks (on a hot restart). You hear the thump on the firewall because the evaporator and accumulator are mounted to the firewall, and when they vibrate with the slug of refrigerant, it is transmitted throughout the firewall. The intensity varies with the previous off time of the engine and heat soak. I'v even heard it between compressor cycles, when its cool out (50's) but sunny and I have the AC on).

[/ QUOTE ]
This is the correct answer.
Jim

 

[Oneof6]

"I have no idea how such an inaccurate rumor got started." Well maybe you didn't read the entire postings. Granted I took the diagnosis as accurate when I was told that it was the blend door about three or more years ago. And that diagnosis was confirmed at my dealership but we all know that their accuracy can vary greatly. I really don't know that I have a need to PROVE it one way or the other since the solution to both is just to turn the HVAC to off. I guess a simple way would be to un plug the compressor prior to the first start up a few times with the HVAC on AC and see if you get the thunk. If you do then that proves the blend door theory. But a blend door can partially open or close with one level of vacuum and close suddenly when the vacuum builds up. Curiosity will only cause me to do so much effort so I will just continue to turn it to off at when I shut down for the day. 106

 

[ACD2]

[ QUOTE ]
[ QUOTE ]
It is the blend door. Just for kicks and grins, I got my head down where the door is and had my wife start the truck. Sure enough, hiss, thump. Switch positions at the controls and hear the same thing, just not as loud.


[/ QUOTE ]


It is not the blend door.

If you want to know what your air mix, blend and recirc doors sound like then just turn your key on without starting the engine and cycle the HVAC switch through the different positions.

You hear vacuum apply and release and if you listen closely you can hear the doors move. You will have to try really hard though. After a couple cycles you will deplete vacuum and you will hear the vacuum pump under the hood turn on.

The noise is compressor slugging. The refrigerant migrates into the compressor overnight while the truck sits. Their is a delay when you start the truck before the compressor comes on to allow the idle speed to stabilize before applying a load (all cars have this).

The liquid refrigerant gets pumped out of the compressor on engagement and sends a pressure wave through the system making the knocking noise. This is similar to water hammering in your house.


Anyone who says it's the blend doors is taking someone elses word for it. Anyone whos says they have confirmed it themselves in some way like disconnecting the vacuum or whatever is just plain lying. I have no idea how such an inaccurate rumor got started.

To prevent the noise wait a few minutes to turn the a/c on so underhood temps come up and evaporate the refrigerant out of the compressor.

FYI, the a/c compressor is on in every position except "feet only" and "face only".

[/ QUOTE ]


So then, esplain why it still does it in the winter time, when I have my AC compressor disconnected? I unplug the comprressor in the winter time, and open the heater valve, and leave it that way until spring time. I found that either with or without the AC running, my windows still fog up, so why bother wasting fuel for something that isnt even doing the job? So, no compressor turning=no "slug", yet I still get the thump. /ubbthreads/images/graemlins/shrug.gif

 

[pilot4life]

I dont think that it is the blend door as my truck never makes that noise when I have it on Max A/C. I think it is the door that switches between interior air and outside air, as when I leave the truck in the normal A/C position I always get this noise. Just my thought.