I bought a 1991 F150 XLT Lariat 4x4 today. It has either 124k miles or 224k miles (darn 5 digit odometers). It has a very high idle (2700-2800 RPM). Owner said they had local Ford dealership look at it. They replaced the throttle position sensor, and one other part (I can't remember ). It didn't help much. The dealership told them that it was likely a vacuum leak. Are all the vacuum lines near eachother? How easy will this be to check/fix?

Thank you for your help.

 

Does it idle that high all the time? Or just when first started?

 

The brake booster is hooked to a vacuum port on the back of the intake manifold near the passenger side firewall. The rest originate on the vacuum tree on the top of the intake manifold. From there they branch out all over the engine compartment.

Do not forget the gasket between the upper and lower intake as well as the lower intake to head gasket. Plenty of areas to methodically check.

 

Double post.....

 

Thank you.

Yes, it idles high all the time. Driving down the highway it runs just fine. It idles high enough that the brakes almost wont hold it on a gravel road. If it's in drive or reverse, the RPMs go down a little, but it lurches forward/rearward when you put it in gear.

How would you reccomend testing these lines for leaks? Would you spray carb cleaner around the lines, or would you inspect each line?

 

I test each vacuum circuit with a handheld vacuum pump/gauge. Keep in mind several of those lines feed other components away from the engine. You can spray them down with gallons of carb. cleaner and never find the true cause of the leak.

 

my '89 F150 had a totally shot gasket between the upper and lower intake. replaced that and it helped but it didn't cure it so there must have been leaks elsewhere. sold the truck not long after. I'd start by hitting those intake gaskets with carb cleaner and replacing as necessary then checking hoses.

 

Ok, thanks!

 

If you can't find a gasket leak, or a vacuum leak. It could be the idle air control.

 

Ok, thanks!

I decided to change the air filter today (something I always do right after I buy a vehicle). When I get the airbox apart, the smell of oil and gas was very noticeable. I took the filter out and what used to be a white paper filter was now a yellow and black (and damp) paper filter. It stunk like oil/gas mixture. I happened to look down in the airbox and there was an oily sluge and some standing oil down in the bottom. Is it sucking oil out, and could this be causing the 2800-2900 RPM idle?

While I was taking Diesel Tech. classes, I remember the instructor saying that the old Detroit 2 stroke engines used to suck oil and then run away. I wonder if this is a simillar problem.

Any suggestions or advise would be greatly appreciated.

 

that little filter is for the PCV valve. sounds like you need a new one, though if it smells like gas there may be bigger problems.

the PCV valve is on the valve cover. a hose runs from the airbox where the small(breather) filter is to the PCV valve.

 

Ok. Thanks!

 

Ok,

I have noticed an intermitent grinding noise coming from the engine compartment. I can't locate where it's coming from. It sounds like metal on metal, but goes away after a while, then comes back.


I just remembered something, I have also noticed a hissing noise coming from both sides of the engine. I assume that might be the vaccum leak (a DUH! moment on my part ). Would this be the intake gaskets? How easy is it to replace the gaskets? Should I have a shop do this?

 

That is NOT the PCV valve. That is the intake for the PCV system. oil on that is caused by overpressure the the crankcase (normal blow by) caused by a nonfunctional PCV exhaust side. This could mean the valve or any of the hoses connecting to it. They are known to become carbon fouled. Check your Haynes for the valve location, it should be on the back of the intake on the passengers side with a hose to the valve cover, if its the same as my 5.8.
-Mike

 

Yes you are correct. The PVC check valve is on the passenger side valve cover, near the firewall, there is a hose goin from it to the plenum.

If your taking the intake and plenum off, this would be a great time to replace the PVC valve