‘97 460 idle too high ? Rough.
 

My 97F250HD has a 7.5 with 58k miles. It’s been a warm weather 5th wheel hauler its entire life (before I bought it several months ago). I can’t get the idle below 900 rpm. I’ve adjusted the idle screw until it’s off the hard-stop and the idle still won’t drop. Is there a cold temp vac solenoid (or something else) keeping the idle high ? I’d appreciate any suggestions on how to proceed. Also, inside the truck, the idle feels a little rough. With the hood up, looking at the engine while running, I can’t see any shake or vibration. Not sure where to start on the rough idle either. Haven’t pulled the spark plugs yet. Thanks

Wrong 7.3. This is for the gas motor 7.3.

OP mentioned spark plugs. Is this a 7.5L gas engine?

I'll move the thread once we clear that up.

Sorry, it’s the 460 engine (7.5?).

Vacuum leak? Or carb sucking extra air through bad gasket? My John Deere tractor had a leaky carb and one of the symptoms was an elevated idle that I couldn’t get rid of. Rough idle can be a vacuum leak.

Thread moved to this forum for the '97 HD trucks.

Vacuum leak sounds likely, thanks. I’d forgot about that possibility. There are lots of vac lines running around the engine. I’ll pull them off one by one to see if rpm is impacted (or isn’t). Some carb cleaner sprayed on accessible gaskets may also reveal a leak.

Everyone here seems to have much better results using an unlit propane torch to check for leaks. I've always found it to be a waste of carb cleaner.

Interesting, never heard of propane before. Certainly cleaner. I’ll give it a try, thanks

A attach a hose to the torch and bring it close to the spot you suspect a leak, vapor is easier for the engine to pull in than liquid, but also easier for the fan to blow away.

Will do, thanks.

Update: no vac leaks detected with propane. Checked every visible vac line under hood (including booster) - no visible damage. Measured vac at the intake tree at just over 20 in Hg at idle (950 rpm). Does that vac level seem typical ?

Don't forget to check the intake gasket and throttle body gasket. Honestly I'd replace the throttle body gasket if you haven't, it's $2 on Amazon and super easy

I did propane both of those gaskets but couldn’t get full coverage. Will keep them in mind if I can’t find another vac leak.

I don’t think 900 is abnormally high for these trucks. Mine has always idled around 700-900. I wish it was closer to 500-600