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Before I go full moron & replace this engine & deal with it later, I should ask opinions about the knock it has.
Its a 400 in a 79 2wd f250 with a C6. Currently the engine is essentially bone stock. It doesn't knock or make any concerning racket at idle to 35mph ish. Above 40 - 45mph & up on the speedometer and under a light load on the accelerator it makes a racket that sound like a rod knock. At that same speed under a heavy load on the accelerator it does not make the rod knock sound. If you let off of the accelerator & just coast at the those speeds, it also does not make the rod knock sound.
Let the speed fall back down to 35mph ish & lower, you can do whatever you want. It makes no concerning noises.
I define light load on the accelerator as giving it enough gas to maintain speed. Heavy load is picking up speed, rapidly. Sometimes.
I've changed the oil & torn apart the oil filter and there is no metal in either the oil or the filter material that I can find. The plugs are newish & look good. Not carbon build up, not running hot, not soaked in fuel or oil. Old plug wires though. The truck has great oil pressure as well & doesn't run hot. Heck, aside from this, it runs amazingly well.
That is off of an old recycled gauge. I've a new one coming, but the mail is exceeding slow in my neck of the woods. Very slowly, and they loose stuff a lot.
What you're describing does sound like a rod bearing wiped out. under load it tends to keep it held down but when drifting along it lets is slop around. and since your engine is well known for that exactly it's pointing towards it.
Spark knock is also possible but it's a much different sound.
Detonation would occur worse under WOT, but you can get a pinging and surge at cruise speed if you have too much vacuum advance. if you want to rule spark out disconnect your vacuum advance and retard your initial a bit and see if anything changes.
I honestly haven't got a clue if the EGR is present, let alone working. I'd be surprised if it is working if present. I've planned on replacing the intake & carb from the get go. So I've not given that particular area as much attention as I probably should have.
How many miles do you estimate on the timing chain?
Being a '79, good chance it still has a thermactor air pump. I recall one making noises on a Plymouth police car, but on a bet I took the belt off and it stopped, I lost the bet but learned a little.
The odometer shows 75k & change. I wouldn't be surprised if it's rolled over at least once, if not more. I do have a new double roller timing set to put in it, just haven't gotten to it yet. Most of my attention with this one has been on the brakes and front suspension.
This is the only picture I have of the engine. I won't be home until Friday to take more, unless I can get the wife to take some.
The oddball wires have since been removed. I did replace the duraspark box since it had puked it's guts out on the inner fender. Used the one below.
About the chain, I was thinking maybe a little slap.
I think I hear a little in mine at times, but mine will do it at idle. I never hear it while running down the road.
Rod knocks I've heard are like dull thuds and pretty rhythmic. Even when mine lost oil pressure in 1992, it never knocked, bearings were just really worn thinner than new. My main bearings were worn really bad too. I never took the crank out as it looked fine enough. My oil pressure drop was likely due to excess clearance suddenly overwhelming the oil pump's capabilities ... though I did replace the oil pump too.
I might would try an oil change, new filter, and something like 20w50 to see if anything changed.
I don’t see any of the smog equipment that would be associated with a 79. Either CA specification, or FED spec.
The air pump was removed, along with an along with a lot of vacuum lines, and the diverter valve crimped shut there on top of the manifold.
Can’t see an EGR valve, but that’s directly behind the carburetor anyway, so would not be very visible in the picture.
But you can almost guarantee that even if it’s there, it’s not connected. There were not only vacuum lines to it but control valves and amplifiers and other rigmarole and such.
I also see something that could be very bad in the long term. Depending on how long it’s been this way, it could have caused some mischief already.
You do appear to have a functioning PCV valve in the driver side valve cover/oil filler cap. but the clean air inlet on the passenger side rear is not connected to an air filter.
It would’ve originally been connected to the factory, air cleaner housing, but that appears to be long gone. If you can’t plumb it to the existing air cleaner base, I would stick some kind of an air filter unit into the tube.
Same type people use as breather filter elements directly in valve covers.
Just plug it in into the end of the hose for a quick, even if temporary fix.
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