1980, 351M, C6 auto.

Owned truck for 10 days, so little history on it, so bear with me.

It'll start and idle what I consider pretty well. Every time (except once) I drop it into R from P, or D from N, it will cut out. Not stall, sputter, etc. - stop like you turned the key off. You can crank it right back up.

Recently, I was able to actually get it to stay running while shifting from P to R. Dumb luck - didn't do anything but move the lever. It loaded up as I heard the idle drop pretty good, and it idled back up when I moved from R to N. When I went from N to D, it's like I pulled the key out.

Idle speed during my most recent cut-outs was anywhere from 1000-1200 RPM. It has little vacuum system equipment left - I've been over most of it and no obvious significant vacuum leaks. I will have a manifold vacuum gauge setup in the next couple days to get that data point.

The ignition lock cylinder is not real smooth and I don't get a good spring feedback going from Run to Start, so I've had the habit of manually pulling it back to disengage the starter. I believe I've shut down the engine a time or two accidentally by going too far. So I know I have this issue.

I jacked up the back end and spun the tires over by hand - nothing seemed to be in a bind or serious drag. I have not attempted to run it yet with the rear off the ground - will do in another day or two.

Let's say the engine tuning is sufficient and the driveline isn't loading it down when engaged. Is it probable that I am disturbing the ignition switch enough when shifting that I am causing it to shut down? I didn't see any crosstalk between the shifter and switch rod, but, especially with the potential "tenuous" position of the key, it got me thinking they could be related. Considering the key switch lacks any real feel for the detents at various positions and I have no spring feedback, should I be looking to replace both lock cylinder and switch?

The truck is coated in just about every fluid known to man underneath. I want to get it driving enough to get it into the yard, wash it down, and then assess what I have. Trying to avoid performing a bunch of maintenance in the engine bay if it'll just have to come out for other things anyway.

Thanks for sticking with me.

 

Try capping all the vacuum connections and see if that helps, if so you can reconnect vacuum lines one-by-one to isolate any leaks.

 

being that old it would help to spray the ignition tumbler and switch with wd-40 to loosen it up. the grease in there could be dragging. my 80 f350 needed this too a while back.

 

Thanks guys. I think I was premature on chasing an ignition switch problem. I think I just have a poorly tuned engine right now. Will pull plugs, check compression, and if all looks ok, replace plugs, wires, cap, and rotor. Confirm timing, probably throw in a carb overhaul as well.
I did check vacuum and it is near 20" Hg at 1500 RPM in park - does fall off to 16" at 1000 RPM idle. Gauge appears to be pretty steady. I have got it to run in reverse with the wheels off the ground, but going to drive results in instant shutoff. So I'll do the things I should do regardless and check back in when I reach some sort of resolution.

 

I have a similar problem with my 86 Bronco that often cuts out when steering wheel full lock, change gear selector to Reverse and apply the slightest acceleration to start backing up (also most times on a gradient).

All ideas welcome as I've never worked on a vacuum system other than to clean up spills.

 

Quick recap - truck would instantly die in R or D but idle fine in P and N.

Replaced plugs, wires, cap, rotor, and checked compression. All very reasonable for a 40 year old engine.

During some troubleshooting, realized if I put slight key cylinder pressure toward the Start position, I could keep truck running in R. No such luck in D.

Buddy came over and we hotwired it. Had no problems maintaining any gear. Further inspection revealed that the ignition switch on the steering wheel side was tilted up off the metal base because the tangs on the metal base were not properly grabbing the switch. Peened the tangs down and was able to drive the truck through the neighborhood!

 

Great, thanks for the info.!