96 F250, 4x4, 5.8L, Auto OD, 8800GVW, 340 gears, 3" exhaust, Rad fan delete(2X Flex-a-lite Fan kit #295). Bigger tires, slightly under geared.

I got a strange hissing noise when i'm towing my trailer and it comes and go's.
Its definately coming from the engine compartment.

This only happens when the truck is under load and I lose ALOT of power, usually need to downshift to 2nd to keep up to 80KPH on hills nevermind 100KPH speed limit.
Really feels like the timing is being retarded, or the computer is going into safe mode.
The truck shows no codes in the computer. Strange thing is the truck does have alot of power, just not when I tow my 7000lbs travel trailer.

Anyone have any ideas?
Should I chip the truck, if this will fix it?

 

with 3.55 gears and 33's I was effectively running a 3.01:1 ration in my axle. Towing a 3 quads and 2 dirtbikes and all my camping gear and gas put me in first gear towing up hills....UGHHH!!! I could barely make 40mph.... with a bad trans. So I think thats gonna be most of your problem, and A chip will help but the first thing I would do is fix that axle. Thats the most highly neglected part of the truck when people put bigger tires on they never change the gears.

 

Gears is one thing I need to do(4.10's probably), but whats with the hissing noise followed by a lose in power?

 

Its probably pinging. I would try running mid grade fuel instead of regular. If you are already running mid grade then try premium. Pinging is really bad for the engine. I would go with 4.10 gears.

 

I have heard it ping under load with reg fuel, only with my trailer on. I have used premium and the hissing noise followed by the power loss still remains.

 

Check the vaccume hoses on the manifold there might be a disconnected hose. None the less your power loss isnt due to hissing its due to your gears flat out. The hissing may annoy you but its not causing your loss in towing power. I had a line on the top of my manifold that was in the wrong spot and it made a nice whine.....until I plugged it where it was supposed to go.

Let me stress that the loss in power has nothing to do with the hiss...I have the same loss when I was running the 3.55's it was like no matter how hard I floored it I didnt have enough power and it just redlined in first gear. It seemed like a total loss of power but it was the mechanical disadvantage, think about it 3.00 gears thats highway....you cant pull crap with that.

 

So gears it is.

 

Wait a minute! Does the power loss coincide with the hissing? In other words, when you don't hear the hissing is the power normal?

 

A hissing noise develops under increased load stress accompanied by a drastic power loss? I would seriously seek out where your vacume leak is. And yes, a serious vacume leak can cause you to lose power. I would also watch your temp when the hissing develops. Just out of curisosity, what weight is that trailer your tugging around with 340's?

 

True but I bet ya that when the vaccume leak is fixed it will still tow the same with those gears....How big are the tires your running? Are you sure there 3.40's ive never seen them.

 

This is exactly what is happening. It is only heard for short periods of time then go's away. Almost like a valve that opens then closes and once it closes I get my power back. If this were stopped I would be able to keep a better constant speed.

If this was a vacuum leak it would always remain. No?


The trailer weighs in at 7000Lbs, its a twin axle 2005 27 foot travel trailer.(not a light model)

 

I am going to go back to the stock tire size next pull of the trialer and run Premium fuel, and see if that helps.

Also just to be sure, I'm gonna get the axle code off the truck just to make sure I have the 340's.

 

When does it go away? when the grade flattens out or it just goes away? Im pretty sure mine did the same thing up hills and it was just a dog. Pulled fine on straits but up hill it was like I was dragging a battle ship.

 

I had a hissing sound under my hood, with a loss of power and worse gas mileage. I had a rotten fuel pressure regulator. I'd look into it. It's a pretty cheap fix.

Ryan

 

It comes and go's off and on once and a while. The grade does not matter. Its like someone opens a valve for 30 seconds or so then closes it. During the hissing I lose alot of power.
I will look into that fuel pressure regulator, if that might be a cause of the hissing sound.

My truck is lousy on gas too, might have something to do with it.