1997 7.3 "hissing" & "stumbling"
 

I have a 6.0 and spend right much time over on that forum but am less familiar with the 7.3 and was hoping you guys could help me. My Dad's 1997 F-250 with the 7.3 has a problem while cruising down the road. About every 20 seconds or so the truck will make a "hissing" sound and start slowing down on it's own. This hissing sound and stumbling associated with it will only last about 2 or 3 seconds and then it goes away. The best I can describe the sound is that it sounds almost like you are driving on a wet road. I thought at first the sound was the A/C compressor kicking in & out but that is not it. It is not very loud,if the radio was on you probably wouldn't even hear it. As far as the "stumble" it feels a little like a truck with a Jake Brake set on low. The truck will only do this while cruising at a steady speed on flat road. While idling or accelerating it will not do it. When the truck does it,the instant you apply any throttle at all the stumble and the noise disappear immediately. To give some background on the truck it is bone stock and my dad is the original owner. It has about 105,000 miles and the only repairs have been a water pump early in it's life and a couple of oil leaks along the way other than maint. of course. Also,I thought about the possibility of it being a tranny issue but I checked the fluid and it is full and looks good and I tried driving in & out of O/D and it made no difference. Any help is GREATLY APPRECIATED and Thanks in advance!! BTW,I had a friend check for codes and there are none.

Its most likely the EBPV closing when it shouldn't . You can just unplug it . Some one may have a pic of the location ...

Thanks for the help! I am assuming an EBPV is an exhaust back pressure valve? Unplugging it won't hurt anything?

If it's cold out < 30 degrees it is doing what is is supposed to do. The PCM is partially closing the EBPV attempting to warm up the engine faster. This causes the EGT to increase 100* - 200* above normal. Problem is it is not very efficient and somewhat annoying, especially if you don't know what is happening. Once the EOT gets above 175* it will stop doing it, unfortunate in the winter it's hard to get the EOT up that high in a short drive.

As Rick indicated most just unplug it and it does not hurt anything. When you first start the truck the PCM will check to see if it's operational and if not just ignore it. It will throw a code if you have a scanner but it won't light the CEL.

Thanks for the help! I never noticed this before but I don't drive the truck very often. I drove the truck about 35 miles last night and it did it the entire time. I would think the oil temp. should have been up to 175 by then? Unless the thermostat is stuck or something. He doesn't have any gauges on the truck but it was normal by the dash gauge for what that's worth.

Under ther driver side of the turbo you'll see a two wire plug. That's the one you want to unplug

I went out a while ago and tracked down the tube,sensor,and the 2 wire pigtail at the turbo. I will unplug the pigtail at the turbo and test drive this evening to make sure that is my prob. If it is (and I'm sure it is) I will verify that the truck is heating up to full temp. and go from there. Thanks again for the help!!

Was having a similar problem with our 97 this month. It was "whistling" when I would increase the RPM, at around 1500-1700. I did a search on the forum and found out where the two wire plug was. This thread has a better description of it's location - near the turbo; the other thread said near the drivers valve cover - that's the 6 wire plug...(the retainer clips like to break off when they're 12 years old!))

After I unplugged it I let it warm up for about 10 minutes and it still made the pitchy noise. I decided to drive it aound the neighborhood - about 10 miles worth, and it got a little quieter but I could still hear it when accelerating. Went for a longer drive the next day and it stopped making the noise altogether. :-jammin

It seemed to work just fine for many years, why do they start to act up? Is there a linkage binding?

The tube from the manifold on the Rt manifold gets clogged up with soot .. It the sensor itself goes bad ...