Hi all I bought a 94 f250 idi factory turbo a few months ago.
just finished working all the kinks out and now diving into the smaller stuff.

since I bought it it makes a loud hissing sound from the HVAC control/radio area of the dash. Happens only when accelerating and seems to correlate with my pedal/rpms. I was guessing some sort of boost leak so checked out the turbo area and it's not leaking any oil and theres nothing obviously wrong with any of the hoses etc.

was wondering if I'm on the right track and where to go from here on troubleshooting any help would be greatly appreciated.

I also get horrible gas mileage with this truck, I've had 3 of these trucks and this one is a fuel hog. I'm getting between 8 and 11 mpge. Not sure if that would be part of having a boost leak it's my first turboed idi.

 

Also relevant the truck seems to have a ton of power at least compared to my old NA trucks that I've had just working off memory.

 

I say boost or exhaust leak, or maybe just the normal turbo sound. It's been a billion years since I drove one of those and don't remember what they sounded like.

 

Could be a vacuum leak

 

i agree. do the heater controls work properly, or does the air divert to the defrosters when you accelerate?

if you have a boost leak you usually get black smoke on acceleration.

 

Only a puff of black smoke on shifting gears, heat seems to work I'll check if it switches to defrost on accelaration.

 

My guess on fuel economy is timing. I'll bet it needs some advance, because they all do.
If you really want to fix it, plan on upgrading your IP and Injectors - they are about a 100K mile wear item(for best performance), but tend to 'run' for a long long time after that... just getting worse and worse performance.

Hissing noise - Probably vacuum leak from one of the HVAC hoses or solenoids. Higher RPMs = higher speeds of the vacuum pump = more vacuum. Especially because I'll bet that vacuum pump isn't putting out nearly as many CFM as it originally did.

 

Thanks for the replys guys, I've been busy with work but I'll have to check out my vacuum situation. Makes a lot of sense since my brakes lose vacuum every once in a while and my my vacuum pump and brake booster are brand new.

timing makes a lot of sense, injectors are 30k miles old have the receipt from po. Pump was rebuilt around the same time so timing has to be it just filled up today at 11 mpg so would like to figure that one out. Most expensive fuel wise idi i've ever driven.

 

You can check the function of your vacuum booster by depressing the brake pedal with the engine off. Start the engine and the vacuum booster will pull the brake pedal to the floor.

If you suspect a vacuum leak you can use a cigar/vaper and blow smoke into one of the vacuum hoses in the engine bay (with the engine OFF). You will see the smoke leak out of the vacuum system wherever the vacuum leak is. There's a professional tool technicians use to generate smoke to do this, bu they're of course quite expensive.

 

On hissing, maybe someone had a nylon tubed mechanical boost gauge, and the tubing is bad / broken / etc?

 

When I first bought my '88 IDI, there was a hissing noise from the climate control when I shut down the engine. I took the thing apart and the vacuum selector was busted. Crack allowed some of the remaining vac in the system to hiss. Found a replacement selector in a junk yard and it's fine ever since.

 

There should be a hose on the passenger side firewall that feeds vacuum into the cab, disconnect that and see if it goes away

 

Thanks for all the replys guys, disconnected vacuum line to cab and it was just pulling vacuum there didnt have anything to plug it off at the time so I plugged it back in. Sounds like my issue is in the HVAC unit though. Going to troubleshoot it more when it gets warmer then 20 during the day here.

 

As Tom mentioned above, check the function of your HVAC air doors. Can you get air from your vents, defroster and floor? If not then this indicates a vacuum leak in your HVAC unit. It's designed to default to defroster if the unit fails or looses vacuum.