on cold mornings, my 1996 f350 7.3 automatic, crew cab long bed 4x4 has a low rpm "hiss" on cold days, where the hiss comes from I don't know, but it seems to be coming from the engine bay, not too loud, not very noticeable, you can hear it in the cab pretty good but only when you let off the throttle. the hiss comes on warm days too for just about 2 seconds. on fire up you will hear: start, 1-2 second hiss with a slight rpm drop of 100, then normal idle normal rpm. on a cold day the hiss stays, and stays hissing until about 1200 rpm and feels like it is kind of hindering the trucks performance or holding it back to where only responds well at a 1/4 throttle and better. if the then everything works fine past that rpm bracket. it sucks fuel when it hisses, turbo guage is showing slim to none when it hisses, need some help thanks!

 

Do you have a modified or aftermarket air intake?

 

Does it go away when it's completely warmed up?

Sounds like your EBPV - exhaust back pressure valve. Closes when ambient temperature (intake air temp) is below... oh... something like 38F. Opens when engine oil temp reaches 160F or when throttle demand (pedal position) is high. It's probably doing exactly what it's supposed to. It closes to increase exhaust back pressure, to get the engine to heat up faster. A lot of fellas have found that the supposed faster warm-up is not worth the impact on performance, so they permanently wire the valve open, and disconnect the electrical connector to the actuator. Another option is to remove the valve plate. There's even an aftermarket pedestal available without the valve.

 

madpogue I think you nailed it! I looked that up and it does sound like that hiss is coming from the turbo area, even thought my turbo was sticking, even though it looks like a new one. but the guy that dropped my motor in I don't think hooked up the engine heater up and this year i'm due for a new set of glow plugs. this was a 55,000 mile junk yard motor, lost a injector then a cylinder in Virginia, so had to do a engine swap at 255,000. but with that moved up to the big turbo pipe, a straight through no cat, no restriction, 3 1/2 exhaust with just a straight through muffler, the guy that did the drop in I think changed the thermostat to 160 degree (good or bad?) so my gage barely gets off of cold. if I go 6 miles, shut down, sit, start back up, the hiss is gone. sucks diesel when it hisses in this 6 miles, 1/4 tank! and hissing stops just past 1/8 pedal, so it must be this valve. also looks like this thing could be taken off, valve cut out, holes stick welded up with a nickel rod, and put back on, better flow. or the one with no valve put on, who makes it? but how do you trick the computer into thinking its still working without getting a code? love the info you gave me and thanks! couldn't have got this far with out it!

 

That 160F thermostat is waaaaay too cold.

 

I suspected the EBPV also. It is louder when the intake is not stock.