4.0 idle

[American1990]

Ok guys, I got a 1989 ranger that i did a 4.0 swap on 1 year and half ago, been running perfectly fine, so i put headers on it and broke some manifold bolts and ran it like that for a awhile with no Y pipe or exhaust. Well I cant take the loud noise and exhaust leak anymore, so I got another used 4.0, with manifolds and everything, now I'm running full exhaust it it quiet...yes. Ok, so here my problem, the truck wants to idle at 1,000 RPM no matter what i do. I used my old IAC and still no difference, the MAF fine, it never did this with the old engine. There no visual vacuum leak that I can hear. The engine does look like it had work done to the upper intake, but I'm not losing any anti freeze from what i can tell so i don't think it a intake leak. When I let off the gas and put it in neutral while driving the RPM rev up and down, two or three times before going back to 1,000 RPM. When I'm in gear and left off the gas the trucks keeps going like it has gas then BAMMM it starts to engine brake and slow down. I unplugged the MAF and the truck idles a little higher but not much then goes right back to 1k after I replug it in and unplugging the IAC or TPS does nothing while it running.

how can I check for vacuum leaks?

 

[adsm08]

You can spray brake cleaner or carb cleaner around the engine and listen for changes to the idle. The better way is to introduce some sort of visible vapor (smoke) into the intake and look for it to come out somewhere.