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390 FE erratic idle

8K views 12 replies 6 participants last post by  65Galaxie_390 
#1 ·
I have built a lot of engines in my day including an 8 second street car, but all Chevrolet. I have had several Mustangs both fox body 5.0 and 4.6 which never needed any major work on them. Other than my 7.3 in my F250, this is my first Ford I am digging into. The FE is a strange creature and I love the challenge!

Anyway, to the point. This car has a lot of unknowns and neglect from all the previous owners so I am slowly fixing all the issues. I thought I had a vacuum leak on the intake so I pulled the intake (thank God for the internet). The car had an erratic idle, would be very slow to rev, and no power. So I cleaned everything , polished the intake, and reassembled following the instructions. I also found that the vacuum advance had no diaphragm in it so I replaced it and I really thought I had this solved. I fired it up and it is a lot better, but I still have the erratic idle and it wants to die sitting at a stop light if I don't have my foot on the gas.

The car has a Lunati .554 lift bracket master cam (installed by the PO) and stock torque converter so I expect a little issue at idle. I now think I either have a stuck lifter or worn cam lobe because I hear some clacking when I have the engine above 2000 rpm. Very hard to hear at idle, but I think it is doing it there as well. I am going to do more inspections today and find which lifters are causing the issue. The pushrods are all straight which I inspected when I pulled them out. I feel stupid for not checking the lifters when I had it pulled apart. Anyone replaced a lifter without removing the intake on one of these bad boys?
 
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#3 ·
That is a good thought, but the old girl has manual brakes and brake cleaner confirmed I don't have any vacuum leaks. I also plugged all the vacuum lines and she still ran like dog poo so I know it has to be internal. This thing should be responsive on the throttle and it is just a dog even with total advance at 36 degrees. I may try some Seafoam of Marvel Mystery oil before I tear back into it to see if it will free up, but I am going to make sure my suspicion is correct and check the lifter bleed off on all the valves. I will report back what I find.
 
#4 · (Edited)
What kind of carb and intake? My 1964 started having those symptoms all of a sudden, and I rebuilt the carb and that fixed it. I suspect the problem was a blown power valve.

I've also read people having a problem with the spacer between the carb and manifold and associated gaskets. Not sure of the exact details but apparently if you use the wrong gaskets and/or spacer, there can be a gap. Worth checking with brake cleaner if you didn't already cover that area. You mentioned plugging all lines - to include the PCV one, right?

I've also heard of intake gaskets leaking "internally", to where oil/air could get sucked in from between the intake and heads. Do any of the plugs look oil fouled? I guess you could check that with some type of spray, too, with the engine running and the valve covers off. But not too much since whatever spray will end up in the oil.

Pat
 
#5 ·
Definitely an issue in the valve train with as much clacking as there is. Got some Seafoam running through its veins, but hard to tell if the cam is damaged without pulling it apart. I may just give up and tear it down again. The trans is leaking all the time now at the front seal so I really want to ditch the FMX and look for a C6 if I have to pull it out anyway.
 
#8 ·
The timing is right on and I have now discovered something very weird. I have an Offy 360 intake on it and just replaced the intake gaskets. The car had a vacuum leak which was why I replaced the gaskets in the first place. Anyway, I sprayed brake cleaner around the carb base and it idled up which indicates vacuum leak. Put new gasket on, then another, and another...no change. For giggles, I used a towel and blocked the carb while I sprayed the intake and it idled up. Now you know that the intake is part of the head and with the valve covers on, how can this happen? This is pretty weird. I siliconed the crap out of the intake ports so how in the world can this happen? I am stumped.
 
#9 ·
Some intakes like the Edelbrock do not have a flange wide enough to block off the 4100 spacer. The leak will show up where the PCV plugs into the spacer, underneath. The gasket is not strong enough by itself to hold the vacuum.
 
#10 ·
I am not sure what spacer you are referring to since this is a square bore intake? It has a Holley 700 double pumper on it right now. I had an Edlebrock and the Holley on it and they both do the same thing. I will post some pics when I get a chance or even a video. Never seen an intake do this before, but it is a Vintage 1975 Offy intake so who knows? I got to say that even with the vacuum leak and more than one bad lifter, she still sounds pretty good. If I have to pull it back apart then I have no worries. My biggest concern is the vacuum leak. Is it a bad intake or am I just missing something?
 
#12 ·
Here is a little update.i was able to do a little more investigation and found that the lifters are hydraulic, but there is no bleed off at all. I was able to pull one lifter squeak one by the opening to investigate. I had to use a vice to compress enough to pull it apart and clean. I soaked in fresh oil and replaced it. Fired it up and let it run for a while and then tested. It had very slow bleed off like normal. The others are solid as a rock. I know that new lifers are in order now, but I still have a vacuum leak I can't find.
 
#13 ·
Found it!! So after an Edeljunk carb (cracked baseplate) and my favorite Holley 4778-2 700 double pumper. BOTH had vacuum leaks!! My 4778 is leaking through the throttle shafts. I turned the gas on a small propane torch and was able to pinpoint the leaks which was huge. I confirmed the fix by putting my son's 850 on and...viola, runs like a champ. I did also find that the Offy 360 has another small screw in vacuum port just below the carb on the passenger side that is really hard to seal up. Lots of tape and it finally sealed. I am very glad I found this and don't have to rip it apart again. The lifters are going to bathe in Seafoam in my oil, but the new carb took care of the crappy idle and runs good. Hope this can help someone in the future and realize you can have more than one bad part.
 
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