87 f250 vacuum leak at manifold 351w 4v

[sjcrft]

Ok this one has me stumped ,I tried to help my friend and opened a bee's nest with one ,truck was purchased new and always had an idle problem from day one,dealer rebuilt carb about 4 times & adjusted idle to 1300 rpm and said this was the way it should be.lol ok now 15 years later truck started having more idle problems I tried to find with carb cleaner (propane wasn't working) with all the vacuum lines off(disconnected & capped) I'm down to the intake manifold gasket where i found a leak on the back passenger side ,Changed Gasket ,same Result. then we changed the intake manifold(edelbrock) & new gaskets again.now i'm at a loss,how can i check if is the head is cracked?it also has a small spot on the drivers side as well .I'm Just about at the point where I tell my friend new motor.because cost wise we are just about at that point but this motor only has 80000 on it .any idea's would be helpfull at this point just to get me out this one

Thanks

[RahX]

did you replace the upper AND lower gaskets or just the upper plenum+gasket?

[Crasen]

have you tried cleaning out the EGR valve? If the EGR valve is stuck slightly open it can cause the engine to idle like it is a vacuum leak with a lean misfire when it is in fact a density misfire from the EGR valve allowing exhaust gas to enter the intake at idle.

[sjcrft]

Originally Posted by Crasen

have you tried cleaning out the EGR valve? If the EGR valve is stuck slightly open it can cause the engine to idle like it is a vacuum leak with a lean misfire when it is in fact a density misfire from the EGR valve allowing exhaust gas to enter the intake at idle.

egr gone with the old intake and it it carb not efi

[unclebubbles]

Originally Posted by sjcrft

egr gone with the old intake and it it carb not efi

You didnt mention what kind of carb it is, but this will apply to any brand 4bbl. Since its always given trouble, and your pretty sure you have the vacum leak fixed , i would check the carb. Take it off, and check the screws on the bottom that hold the throttle plate on. Make sure they and the ones that hold the top on are tight. Gaskets sometimes can dry out and shrink from heat, and create vacum leaks. Check the secondary butterflies and shaft , and make sure theyre free, and seating properly. Make sure the linkages arent binding some where, and holding the secondaries open slightly. This can cause the high idle. Make sure the butterflies are properly adjusted. If it has always had this problem, it could have been improperly adjusted when new. The dealer could have used a wrong gasket when they rebuilt it. Carbs for different applications can have slight differences, even though they look identical externally, and the kits usually come with all the gaskets for all apps. Its easy to get the wrong throttle plate gasket on some carbs, and cause problems like yours. Good luck
Later
Ed
.

[Crasen]

So can you be more specific about the problem? Is it a misfire, surge, high, or low idle? Does the problem only show up on idle or does it appear at higher rpm's as well? Does the problem only appear at certain temperatures? If it is a misfire have you done the normal tuneup with plugs, wires, cap, rotor, ignition timing, and fuel filter. If the carb hasn't been rebuilt yet there could be some dirt clogging up a port. The only way I know to check for a cracked head is by taking it off and having a machine shop magnaflux it, if it has aluminum heads to go with the edelbrook intake I believe they have to use a die penetrate to check for a crack