I have this sound coming from my truck. 1981 F250 with the 6.6l 400 cid. I don’t know what it is and it sounds like it is coming from the top of the engine. It is really random and doesn’t really get fastet with rpm. Spark plugs we just replaced. Any ideas help. Thanks.

https://drive.google.com/file/d/1y_b...w?usp=drivesdk

 

Welcome to FTE.

Did this start after the plug change?
Truck lack power and runs ruffer now with the new plugs?
If so I bet you pulled all the plug wires off the 1 side, changed the plugs and then put the wires back on right?
Go back over all the plug wires to make sure the firing order is right, I think you got 2 wires on the wrong plugs.

It also sound like you have an exh. leak from the sounds of it.
Dave ----

 

+1 on that! A little history would help. Which came first, the noise or the spark plug change?

Beautiful scenery, BTW.

 

I definitely do have an exhaust leak, pipes are rusted through just haven’t got around to fixing it yet. It started before the plugs. I pulled the boot and changed the plugs one by one because I assumed the order was correct. I haven’t actually checked the wire order yet, it is how I got it from the last guy. I think I am going to change the wires and hopefully get them back in order.

Things I have done,
fuel pump/fuel lines
alternator, and subsequent wiring/voltage regulator
battery, terminals, wiring
starter relay, wiring,
belts,
tried to tune/set idle the carb (key word tried + tach doesn’t work)
oil/other fluid changes
spark plugs

last guy said he did pushrods

Thanks for the advice,
-Dagny

 

If it's an A/C truck it sounds like an issue with one of the vac motors in the heater plenum as the frequency seems to change with engine vacuum.

Also sort out your exhaust leak before trying to do any engine tuning as you will never be able to properly dial it in.

 

It was an A/C truck but the last guy took out the compressor because it wasn’t working. I also took out the condenser. Is there anything else I should unhook/unplug concerning the A/C or any vacuum lines? Anything else that might contribute to the sound? The heat does work buy we haven’t done anything concerning it.
Thanks

 

All of the damper flaps in the heater plenum under the dash are vacuum controlled. It does sound like one that is continually cycling. And the frequency does seem to change when engine vacuum is high (deacceleration etc) It tough to diagnose a sound like that remotely so it is just a guess.

One thing you could do to rule that out is to disconnect the vacuum supply to the interior of the truck at the vacuum manifold/tree on the cowl/ firewall under hood. If you disconnect the vacuum supply and the noise stops you know it is in the interior and in all probability somewhere in the heater plenum/controls.. If not back to the drawing board..

 

That could be an issue?
Why did he change the push rods as I have not really heard of a need?
You would not have the old ones by chance do you?

I don't know and I don't think the valves / rockers are adjustable so if the wrong push rods were installed, to short or long, could cause issues.
Now I also wonder if he installed longer rods to over come a flat cam?
To me that is what it sounds like, a flat came. When gases cant escape after being fired like in a bad exh. valve lobe, the gases come back up the intake side with a little pop.
At first it may not happen all the time and it mostly happens at a little higher RPM than idle and under power because the RPM is not at idle.

First I would go back over the firing order as the PO may have crossed the plug wires, an easy fix if so.
1 way to see what hole it may be from is to pull 1 wire off at a time and see if the noise goes away. Then you can try a compression test on that hole to see what you get.
You can also pull the valve cover run the motor at idle and see if the rockers are moving for that hole.
If they are moving and can be adjusted might try and back them off a turn and see what happens.
If not moving that the motor needs a cam, push rods will not fix that.

That is a good test if it is the HVAC but I did not hear that happening. Then again the wife talls me I don't hear anything she tells me
Dave ----

 

Can you duplicate the sound while parked, with the transmission in neutral? Any fluctuation in idle speed? Or do you have to be driving so there's a load on the engine? I agree a compression test is a good idea.

Kinda wondering why the previous owner replaced the pushrods. Makes me wonder if he was trying to fix the noise, or perhaps accidentally caused it after changing the pushrods for some other reason.

For the possibility of a bad HVAC actuator, try selecting Defrost or Off. IIRC, that shuts off vacuum to all actuators and they will all relax to their spring-loaded default positions.

 

Thank you guys for all this information. I’ll try to go through these things and diagnose. The spark plugs wires are all in order.
Last guy said he did pushrods because the last ones were “bent.” I don’t know it that is even an issue with these engines as this is my first Ford and he had a lot of other fords so I guess I took his word for it.
The truck will make the sound at idle/in neutral and is really pretty the same when I put it in gear and start moving.
I might have found another issue as when I got it the distributer hold down was loose and so I have no idea what the current ignition timing is. I read in my manual it should be around 6 BTDC as a base but also haven’t gotten to that yet.
Thanks,
-Dagny

 

Just thinking out loud about some basics. Are you positive about the engine model? Is that per the VIN tag? There's always the possibility somebody had swapped in a different engine with a different firing order. Anything could have happened over the last 39 years.

I'm not well versed on the different models, but it wouldn't hurt to look online for how to identify Ford V8s. Once you're positive what is installed, then you can confirm the firing order. There's no guarantee the previous owner had it correct.

 

Was the distributor loose, too, or just the clamp?

The ignition timing is very important, so definitely investigate further. Also, make sure the timing marks are accurate. On the crankshaft pulley, the outer sleeve (with the marks) can slip relative to the center. To verify, remove the #1 spark p!ug. Insert a soda straw or something similar against the piston. Do NOT use a screwdriver, dowel, or anything rigid that could break. (I do not care to discuss how I know this...) Use a socket and breaker bar to bring #1 piston to TDC and compare to the marks.

 

I am pretty positive it is the 400 per the fuel pump location and bolt pattern and a few other parts; I know it’s not a 351c/w. It could be a 351m, as I know it is just a bored down 400, but at this point I seriously doubt it. I did get the engine specs off the vin but I do think it is the original engine.
A good question to ask though (who knows what’s happened to it in the almost 40 years its been around)
Thank you

 

A 351m is a destroked 400, bore is the same. 351M 4.00" bore x 3.50" stroke. 400, 4.00" bore x 4.00" stroke.

Externally there is no way to tell a 351M and 400 apart.

 

Found some time and started taking stuff apart. Took off the valve covers to find a mess on cylinder 4. Intake rocker arm fell off (bolt came loose) and exhaust push rod was bent big time. All the other pushrods were fine. Put a new pushrod in, torqued the rocker arms down, hand cranked it to make sure nothing was super stuck. Looked good, started and drove it, no sound. When I started it also smoked like hell, so I disconnected spark plug 4 for a moment, stopped smoking. I reconnected it and after a while it stopped smoking, so I assume that cylinder 4 had not fired properly the whole time I had it. Im gonna re check and see if the pushrod got bent again. But what really causes a pushrod to get bent in the first place?