Hi Guys,

I've recently gone through a bout with my '86 F250 351W. I had sparking at the ignition coil, which after changing wires was found to be a bad ignition coil. I have since eliminated the arcing with a new coil, but have a new problem.

The truck will run OK on fast idle. Once the choke opens up and the truck is at temperature, it will fall flat on its face. Bumping up the curb idle screw doesn't do much of anything, which immediately clued me in to a vacuum leak. I can turn the curb idle 1/4 turn and still can't get a proper idle which in my limited experience shouldn't be...

I hear a loud sucking sound, so all things points to vacuum leak but have been yet unable to find it. I will try the starter fluid probe tomorrow morning.

One question - if there is a leak so bad as to hear it.... where might be the usual suspects? Unfortunately I am a pretty poor mechanic and am learning on the old girl as I go. Could someone please explain how to diagnose an EGR issue?

FYI - the carb is a Holley 4160 that replaced the 4180 2 years ago. The mechanic that did this left all the emissions stuff in place, which may add to my problems and confusion.

Also - I have reset the idle adjustment screws back to factory setting - 1 1/2 turns out. The truck will stay running with my foot on the gas, but once idling, forget about it.

Thanks.

 

The EGR valve is not supposed to be open at idle but they stick after a while as they build up carbon inside.

If you have a hand-held vacuum pump, you can attach it to the EGR and watch the diaphragm as you generate vacuum.

Another way would be to watch it as you kick up the throttle - it should open up at cruising RPMs with a warmed-up engine.

An EGR valve stuck open at idle can make for a really crappily-idling engine.

 

Chris-This is unrelated to the thread, but what is the little black and white square in your signature?

Jw...

 

It's a QR code:

QR code - Wikipedia, the free encyclopedia

 

Ahhhh....I see I see....

 

The old starter fluid test seemed to point out the EGR valve as the culprit. It "looked" like it was closing fully so I am going to go ahead and block it off.

With the 4160 in place of the 4180 the EGR valve was never getting vacuum in my setup. I do not see a benefit to run through hoops to get it back in the loop - the truck sees <2000 miles a year so I'm not saving the world by any means.

Side note, I'm hoping this diaphragm (more than likely now cracked or whatever) has been leaky for a while and will help my startup problems which no adjustment of the choke could ever overcome. Stay tuned.

 

I have the same truck and I had a leaky large hose going under the front of the carb...and once I forgot to put it back on after the rebuild, or sometime when I was working on it and it runs badly...hand-inspect all of your vacuum lines.