Hi all, thanks for all the help these past year on my f150, this forum saved me money I didn't have.
I have my 97 f150 4.2 that im having a difficult tracking down a vac problems once the truck warms up. I was originally getting the code for imrc bank one opened 1518, and I just fixed the imrc actuator bushing in the back and that solved the problem for a few days but now once the truck warms up the original problem pops up, and code 174 pops up.

I have replaced the pcv, the elbows, and most of the vac hoses. I have an aluminum intake manifold plenum which I have taken off and cleaned up the butterfly in the ports and I just replaced the gasket too, I also want to say I found a broken vac line at the vacuum reservoir behind the battery and just capped in for time being, not sure that would matter though.

It runs beautifully when cold for 5 min, I have taken the plenum off several times to check all the vac lines for cracks, leaks , and replaced a few, I know its running lean and it must be a vac leak somewhere but I am lost now.
My question is, when warm I have zero vacuum to either imrc's, how do I check to see if my imrc's are working properly? These things are expensive so i want to be sure before ordering them Thanks,

 

A 1518 is more often a symptom of a vacuum leak than it is of an actual IMRC problem. The 0174 just corroborates this.

The 97 and early 98 4.2's had bad lower intake gaskets.

IMRC's are supposed to stay closed till you hit 2500 to 3000 rpm under load.

 

Thanks glc, I just finished replacing the lower intake manifold gasket, and I am still with the same problem.
I popped off the intake plenum and checked the imrc's to see if they are closing the secondary intake ports and they both are.
My understanding of the imrc's is that the imrc is designed to open the secondary intake ports at around 3200 rpm, allowing more air into the system for greater power at higher rpms. At lower rpms (everyday driving), the imrc is supposed to keep the secondaries closed for greater torque and economy.
In my case when the engine is cold the secondary valves are closed and the truck runs great under load and all speeds, but when it warms up the secondary valves open and stay open instead of fluctuating between my driving conditions. It only takes till it gets to operating temp that it throws the 1518 codes for both banks and eventually the lean codes.
I have 20psi vac on the lines to the imrc's with the truck running, I am perflexed on what else to check.
I did find the broken port on the vacuum resorvoir box behind the battery but that was found before the repair of the lower intake manifold gaskets.
Both the imrc hold a vacuum, what else shoud I try, thanks.