Every 5-10 seconds, I hear this hissing sound. It’s hard to pinpoint exactly where it’s coming from, but it sounds a bit louder when I’m under the truck. Any ideas?
You should grab one of those needle stethoscopes to track down the noise. They’re super helpful for stuff like this.
Could it be an issue with the exhaust back pressure valve?
Ozzy said:
Could it be an issue with the exhaust back pressure valve?
I’m pretty sure the 7.3IDI doesn’t have a back pressure valve.
Ozzy said:
Could it be an issue with the exhaust back pressure valve?
I’m pretty sure the 7.3IDI doesn’t have a back pressure valve.
Gotcha, my bad!
Maybe it’s a leak in the vacuum system? These trucks have a belt-driven vacuum pump for the power brakes since there’s no natural manifold vacuum. There’s also a low-pressure vacuum switch (near the passenger battery) that might trip a warning light, and a vacuum storage canister near the firewall on the driver’s side. Check the hoses and connections for leaks. Just a thought, but it’s worth checking. By the way, your engine sounds great!
For reference, it’s a 1990 F250 with the ZF5 5-speed and 4x4.
I’d try taking off the belt(s) and see if the noise goes away. That’ll give you a starting point.
Do you think it could be a super high-pitched vibration? That seems more likely than an actual air leak.
Asa said:
Do you think it could be a super high-pitched vibration? That seems more likely than an actual air leak.
Another thought—could it be the A/C compressor cycling? Try turning the air on and off, or spray some WD-40 on the pulley to see if the noise changes.
@Asa
The A/C compressor isn’t hooked up right now (no belt), but I’ll mess with the HVAC controls and see if that affects anything. It might be some kind of vibration, but it’s weird how it comes and goes. Thanks for the tips!
@Tan
Got it, good luck! Maybe give all the pulleys and belts a quick spray of WD-40 and see what happens.