Hi all, I have a 1981 F-150 300-I6, California Emissions with Feedback Carb.

I'll try and describe what the problem is.

I have a intermittant whistle that comes and goes, when the truck is at idle or closed throttle. It does not whistle when I have my foot on the excelerator.

It does not whistle in the mornings, when the engine is cold, but as soon as the engine is warm it will whistle, but only after the engine is put under a load, and then the excelerator is let off completely. Like climing a long hill then coasting down the other side.

It will last anywhere between 20 seconds to a minute and a half, then be gone.

If the ambient temp is above 80, after about 20 mins, it wont whistle again that day, untill the engine is cooled again.

If it's below 80, it will whistle as soon as it's put under a load, then the throttle released for coasting down the hill.

Some observations,

The idle is normal. However when it whistles it does have a rough idle, that suggests intake leak. As soon as the whistle stops it idles great and as smooth as it did when new. (it never did idle completely smooth).

I checked the torque on the manifolds and it's to specks.

I've tried pulling over, and open the hood to listen for where the whistle is coming from, but as soon as I pop the hood, and get my head under the hood, the whistling stops.

It's frustrating as heck, and it's been doing this for about a month now.

Fuel mileage does not seem to be affected too much. Last averages was 19.6, 20.2, 23.1, 25.4 and 18.3, which is a average of 21 MPG.

Any advice on this? Opinions? Anyone have a problem simular to it?

It's got me scratching my head...

 

Anything connected / related to the air conditioning, maybe? Only other thing I can think of is something vacuum or EGR related.

 

Doesn't have Factory or dealer air.

I will check the EGR valve. Defective ones have been known to cause intake leaks. Good call there.

I also just had a thought about the thermactor system. It dumps the air pump air to atmosphere at closed throttle. I will also check the bypass and diverter valves and the check valves.

I'll report back later this evening if I have any joy.

Thanks again.

 

tighten the screws on the carb, Mine did that. you have toremove the carb, the ones makeing the wistle are from the bottom. make sure you get a new carb to intake gasket

 

Thanks Dustybumpers. I will check those 4 screws tomorrow (I know exactly which ones you are talking about...) and put in a new carb to intake gasket.

The Egr and the Thermactor system checked fine.

Also did a computer diagnostic, and passed with flying colors.

Will try tightening the carb. Will report back if any change.

 

Check the throttle shaft in the carb. I had an 82 that whistled constantly at idle, hunted for it for about a month and then pulled up on the shaft and it quit. They wear over time causing a vacuum leak and in my case a very annoying whistle.
-Johnboy

 

Took the carb off, had to rethread one of the 4 screw holes, as it was stripped.

Put a new carb to spacer gasket on the truck, tightened the carb down.

Same whistle, no change.

Checked for a vacuum leak in the carb throttle body, where the throttle linkage goes through. No leak that I can detect. I even wiggled it back and forth. No change.

Any other thoughts? It's weird that it comes and goes like it does.

 

check the bolts on the back of the engine that hold the intake exhaust on. spray some carb cleaner all over the intake to head while engine running, listen for idle to rise, to find a leak, another thought, I had the vaccume line to the trans suck itself flat 1 time, made kina buzzing/wistle. replaced it, it solved the noise

 

Already checked those, they are torqued to specs.

I also checked the three that hold the exaust and intake together below the carb. (Log manifold), one of the studs backed out and was loose. Used locktight and retorqued to specs. Did not replace the gasket as it requires removing the manifolds from the engine. However I will pay close attention to this area while doing tests.

Tried the carb cleaner trick. No luck with that either. Will try making it leak and running out open hood and spray area above before it stops whistleing.

I also didn't replace the spacer to manifold gasket, and now I wish that I did. I may just buy a new carb to spacer, and spacer to manifold gasket and see what that does. They are cheap enough and it wouldn't hurt.

As for vacuum hoses I did notice that the brake booster hose is starting to crack. Will replace that do some more testing and check back tomorrow.

Thanks for the Ideas.