So today I had to put the 914 on hold to replace the upper manifold gasket on my '91 F-150. V8 5.8L by the way. The vaccum leak caused by the faulty gasket was really sucking a lot of power out of my engine. So I get it replaced, check, double-check, and triple-check to make sure I've got every wiring harness and vaccum line that I disconnected hooked back up, and that every bolt is torqued to spec, and I do. So I fire it up and... same sh*t- there's a vaccum leak somewhere. Granted the engine is idling smoother now than it was, but it's still rough.

Now, knowing my luck, the upper manifold gasket needing replaced was only hiding the fact that the lower manifold gasket has a break in it somewhere, but it's been my experience that these gaskets rarely break or leak unless they were either installed improperly or the mounting bolts have come loose. Of course this is something I can't really check without once again removing the upper intake, and at this point in my day it's a moot point since I have to be to work in less than 2 hours.

So can anyone give me an idea of what else I might check? Like I said, I made sure all vaccum lines were hooked back up, so the only thing I can figure is that maybe I was getting a little too rough unhooking/hooking the lines and cracked and/or broke one of the plastic lines. I suppose I'll double check that while I'm awaiting a response.

Here's the rundown:

-Before replacing the gasket, the engine ran rough at idle (about 400-500 RPM) but smoothed out once speeding up and the RPM's rose to 700-900 with no problems during acceleration.

-After replacing the gasket, the engine still runs rough at idle, but now I'm noticing the leak is worse when accelerating, but still smooths out once the engine reaches 700-900 RPM.

Thanks!

If I remember, the old way to check this out would be to start it up, let it idle, and spray throttle body cleaner or starting fluid(sparingly-its very flammable!) at suspected leak points and you will know when you hit one as the idle speed will change when the motor sucks your flammable substance into the combustion chamber(s).

My 94 with the 5.8 idles at about 900 rpm in park. In gear it drops to about 800 at the lowest. 4 to 500 rpm sounds way to low at idle. Other than that I really don't have any more info.

I've already tried that- at least w/ the places I can access. Half the injectors and the other half of the lower intake are covered by the upper intake, so it's not really possible to do any spraying around that half of the engine.

So maybe there's something else wrong? I just recently gave it a tune up- plugs, wires, cap, rotor- so I guess I can double check that and make sure all the wires are on tight. If one is loose and it's not firing on all cylinders, that can create a similar sound as a vaccum leak.

As for the idle speed, the book I've got is telling me that since it's feul injected, idle speed is not adjustable as it's controlled by the TFI-IV/EEC-IV system (whatever the hell that is.)

Go to the parts store and get a few of the variety packs of vacuum plug.
They are basically hoses with end...little stubs.
Label and remove all the vacuum lines from the intake manifold.
You need none of them to run the car at idle.

Start the car and get it to idle.
If it will not idle with all the vacuum lines plugged AT THE MANIFOLD, then you need a new air flow sensor, or intake gasket.

For the airflow sensor, there are normal voltages that you should be able to read with a volt meter at idle. You need to create a few jumpers to get the the correct terminals.

If it does idle during this test, its a slow and methodical replacement of one vacuum line at a time, each time you test the motor.

Rich

one possibility idle air control valve aka throttle air bypass common thing to replace on the Ford v8s of that era

Which is hooked up to the intake via a vacuum line.

Rich

Actually it is not hooked up to a vacuum line it is bolted to the upper plenum just behind the throttle body. Has an external electrical connection only
Click to view attachment

Well guys I appreciate the advice, but as it turns out, it was the MAP sensor. Only $80 new...

By the time I got done fidgeting with it the other day I was running late to work, so I had to just let it go. I was having probs connecting to the 'net yesterday so I couldn't check this thread, but what I ended up doing is basically what you said towle. Instead of using vac plugs tho I just dug around for various size bolts I have laying around and used those to plug the lines. Unhooked 'em all, plugged 'em, then fired it up and it idled smooth. Then one by one I hooked them back up and found it was the MAP sensor. I just kept it plugged off until I got a new one.

Thanks again for the advice. I probably wouldn't have even posted about it, but when you go from driving a '75 stepside chevy for 11 years to a '91 F-150 the sudden change can be overwhelming. It's not that I've never encountered newer engines before, but there's just soooo many components on these newer engines that I'm not familiar with them all. So to try and diagnose the problem is basically shooting in the dark for me.

351's in the early 90's had problems running cylenders lean with injector problems. (i'd check fuel filter for flow/restriction too). what i've seen cause a simular problem to vac. leak was a partially plugged cat. converter. does it pop back thru intake on acceleration (similiar to intake leak). check vacuum at idle then when on a test drive. acceleration vac. will basically be non existant depending on severity. disconect the converer and test if you suspect it.

it sounds more like an egr issue if your vacuum leaks are fixed and external leaks confirmed good. check the egr for being blocked open (carbon crap). that will cause low idle by dumping in inert gasses, then engine smooth out as r.p.m.'s help overcome lean mix

p.s. change that metal flex e.g.r. tube if it's original or oldish, time to fix while your in there

opps didnt read to the end...

glad you got it.

mike