Hi guys, new to this forum

I recently picked up a '79 F350 with a 460 in it with a C6 auto trans. Someone before me had already installed an Edelbrock 1405 on the intake, stock intake still.

My main question is, when these trucks were new back in the day, did they idle super smooth or did they still have some shake? I thought perhaps just by having a big block it would move while idling in gear. Even when looking at the truck for a test drive, it had this cyclic intermittent idle shake.

My basic problem from day 1 is that it's never had a nice smooth idle. It always shakes some, almost like it misses a cylinder once in a while or something. It's mostly noticeable when you put it in drive and keep the brake on. I've checked the RPM to make sure its 850rpm and lower , should be at 650rpm in drive (auto transmission)

I've already put all new plugs, wires, dist cap, and rotor. It has electronic ignition with the magnetic sensor.

I also double checked the timing - the label on the engine says it should be 8 degrees at 650rpm, so I loosened the dizzy and checked the timing with the advance unplugged and yep got it to 8 degrees and locked it down.

The problem is it still runs rough at idle - the main issue is when I go to put it into gear it shakes quite a bit, I hate it. Everything else seems to work good when you accelerate, etc. There isn't any surging or hesitation while driving. It's only the idle that stinks.

I took the idle adjustment screws out and blew them out with cleaner and with compressed air and this actually seemed to fix the problem Monday when I was test driving it around it actually seemed to idle pretty smooth, but today I started it up and it was shaky again and not as smooth as it was Monday. And the other thing is, even if it does smooth out, it doesn't do that until it gets nice and warmed up. At cold unless you rev it up, it will shake a lot. BAH

I'm not sure what to do next . Maybe I need to just do a carb rebuild. I was thinking the only other thing I could check is the fuel pressure, but at idle an engine requires so little fuel I don't think the fuel pressure would be a problem. I guess maybe if the pressure was too high that could cause issues on an Edlebrock.

I have checked for vacuum leaks and haven't found any (spraying carb cleaner around), at least nothing where I suddenly hear the idle change.

Any thoughts guys? I know there are still so many things it could be I think. I thought maybe I should get a new coil and new ignition module - that would make the entire ignition system pretty much new.

I hate just throwing parts at stuff, I read a lot of people that do that and never get anywhere except to shrink their wallet.

 

So, I'm thinking maybe it's an EGR valve that is leaking. It swear I smell what seems like exhaust fumes coming out of the carb. So that seems to be the final thing to check, everything else seems ok - I plugged every single vacuum port I could find and it still idles rough

When the person put the Edelbrock on the intake manifold they left the EGR spacer in place and just unplugged the EGR from vacuum. But this means if it sticks open or leaks at all, it will get sucked right into the intake.

 

Ok guys, I think it's the EGR valve. Beyond being able to smell exhaust when idling and having the hood open, I took the EGR valve off and it looks like this

I took some compressed can air and blew into the valve side and I can feel air coming from the other side, so I think it's "open" and not sealing the exhaust from coming in at idle like it should.

 

Definitely interested if you find it's the EGR valve. Mine used a phenolic spacer, but one of the tabs broke so I had to source a steel spacer with the EGR valve built into it. I just put a cap on the EGR. Tonight mine started running rough and I noticed when I got it home and took the air cleaner off, it was sucking a lot of air, a lot more than usual. I couldn't pin-point it, but I'm wondering if something was going on with that EGR valve. Let us know what you find if you figure out a solution. My dad had an aftermarket spacer on this at one point, I may get it out and install it in place of this plate I am currently using.

 

Yep getting the new valve tomorrow morning, I had to order it , nobody had it in immediate stock.

I did try to make a temporary blocking plate but that didn't work, most likely becuause I just used some thin sheet metal and it still wouldn't seal well enough - I could tell because spraying carb cleaner at it would make the engine start to behave better.

I'm hoping this is the problem! I'd like to see my vacuum gauge go back up to 20 lbs instead of hovering around 15-17 and running terribly at idle

I just know only a few days ago I had this thing purring like a kitten at 600 RPM idle even when driving, and then it started to misbehave, I think it may have been because my curiosity got the better of me and I started to push on the EGR valve to see if it would move, that was a bad idea , this valve is probably the original valve from '79! So..it's probably my fault it opened, broke and got stuck. When I took the valve off there was a bunch of carbon chunks sitting the exhaust side of the pipe. I sucked em out with a shop vac. I think they were what was left of the EGR metal valve :P

 

Maybe I missed it but did you do a compression test?

 

No, I haven't done a compression test yet. I did have great vacuum readings for the first week I owned it, and the truck runs great while on the road, it just doesn't idle very good right now.

I did replace the plugs and they all looked good. I have thought I may rent the tool to test compression just to check it, but I haven't done so yet

So far the only things I found broken on the engine are the Vacuum advance was shot (leaked air) and now looks like the EGR is shot. On an old truck I would expect these types of things since they are rubber and/or have hot gases through them all the time. Water pump leaks, I will replace that soon. But the rest of the engine seems sound

 

Ok guys,

Replacing the EGR with a new one - even though I'm not using it. Put on new EGR and new gasket, put some high temp RTV around the gasket holes. It seems to have fixed my insanely rough idle. I'd still wish for a higher vacuum and even smoother idle though.

Once I got my idle RPM down to 700ish, it seems to be smooth now when in gear and doesn't shake like crazy anymore, Vacuum reads about 20ish , it's not "solid", probaby some wear to the valve guides

I should do a compression test one of these days just to double check the condition of the cylinders just to know

I also kind of want to pull the carb off the intake just to check the gaskets there and make sure nothing is leaking there too. But for now it seems to be running decent again. It seems to idle even smoother in gear after driving it for a bit, probaby warms up the transmission fluid and makes the torque converter not such a resistance to idling in gear.

So, for now, it seems to have solved the really bad stumbling I was getting. It still seems a bit moody but that's from having a manual choke you got to play with the choke to keep the right ratio as it warms up.

 

Not very many people put a factory cam back in those when they rebuilt 'em, and most RV bumpsticks will give you a lumpier idle than stock - sounds like you got the EGR leak fixed, and you might be able to smooth it out a little more with some tuning and minor leaks, but you might not get to glassy smooth.

 

Niko, do you know what carburetor you're running, by chance?

 

It's never been rebuilt or cammed from what I can see, everything looks bone stock except for the new aftermarket carb. It's an edelbrock 1405 with manual choke.

That's kind of what my question was, how smooth were the idle in the original "back in the day" truck. Right now at 650rpm or so when I sit in gear after warming up it seems to be pretty smooth again (thankfully). It's not glass smooth though like my Lexus car is But that may be expected with a carb vs modern EFI

 

Its a Edelbrock 1405 with manual choke

 

Well guys I don't think this solved the whole problem. As usual.

It really acts like it is "missing" when I'm trying to idle cold . Maybe it is something electrical.

This is basically going to take forever to track down god damnit. I wish there was a scope I could put on the plugs to see if they are all firing correctly in order. I dont want to become one of those guys that just throws parts at it, but it looks like I'm not giong to have a choice.

I guess I should test the alternator too maybe the voltage is dropping or some crap.

Who knows.

I do know that it really seemed to start having problems after I sprayed some degreaser/water to clean the engine off some. I probably screwed something up with water. I'm an idiot and I'm sure this running problem is entirely my fault somehow

 

You didn't say anything about the water earlier.
Pull your dizzy cap and check for moisture and use an ohm meter on your spark plug wires.
I went thru a low water crossing and you can imagine what happened next.

 

460's should idle silky smooth if all is as it should be. If it were me I would replace the HEAVY cast iron intake with an aftermarket aluminum dual-plane NON-EGR. I know there is an argument to be made for a properly functioning EGR system on a gas engine. But IMO that theory falls apart when saddled with 70's era non computer controlled emissions equipment that is difficult if not almost impossible to source when replacement is needed. Get rid of all the TVS (thermo vacuum switches) and related maze of connections. Only things needing vacuum should be PVC, brake booster, interior heat/air controls, distributor vac advance (ported), modulator on trans if automatic, and cruise control system + reservoir. Connections on carb and one vacuum tree on intake will cover those needs.
I have also had carburetors do exactly what you are describing, probably dirty/plugged idle circuit.