It's been driving me crazy thinking what is wrong with the truck.
After installing the new cam (Cam specs;
* Advertised duration: 270 intake/280 exhaust
* Duration at .050: 204 intake/214 exhaust
* Gross valve lift: 448 in. intake/472 in. exhaust
* Lobe separation: 112 degrees )it runs good when cold. Once it warms up good, it doesn't idle as good and makes a rattle/knock sound. This sound is only when the rpm's are up. not at idle.
Acceleration and steady cruise above 40 is great. Low speed (0-30) cruise isn't very smooth after it warms up, but when cold it runs great.

This rattle/knock sounds as if it is coming from the lower intake area, if that makes sense and have been told it was a rod knock but it isn't a deep metal sound at all.

What has been done;
1)Every sensor has been replaced
2)Rechecked pushrods for proper tightness and to make sure they weren't bent.
3)Intake manifold and TB cleaned and new gaskets - checked for vacuum leaks.
4)New - distributor and module (thought the noise was coming from this at first), distributor cap/rotor, ignition wires and plugs.
5)Ignition coil tested good.
6)Water temp normal
7)Oil pressure normal
8)Fresh gas (91 oct. )
9)After running with new plugs they look very good

What this is not;
Not an exhaust leak
Not valvetrain noise (lifters/rocker clatter etc.)

The strangest things I have found;
1)When attaching a vacuum guage to the vacuum "tree" on top of intake the motor instantly dies. Remove guage before it actually dies (attached for only 1 second or less) it stays running. Whith the vacuum line disconnected for these attempted test's the idle smooths out but dies when you plug the vacuum leak?????

2)The compression check;
120 across the board (hot or cold). Unfortunately I do not know what it was before the cam swap.

Where I left it before my surgery I had tried to pull the oil pan to inspect the lower end but found I need to move the AOD back at least 1/2". I have the pan resting on the K-member with the oil pump and pickup removed, well the pump is out and the pickup tube is at the bottom of the pan.

What bothers me is the compression - The motor only has 40K miles since a rebuild. When I did the cam swap I didn't think about pushrod length. Could too long of a pushrod cause the low compression readings from leaving the valves open a hair and possibly cause some kind of ignition knock or rattle (not a ping) and that is what I am hearing after it warms up and causing it to run a bit rough at low rpm?

I was hoping someone else has experienced a knock like this and found it to be a severe spark knock, because I'm almost positive those pushrods are too long from what the compression test revealed.

What does a burnt valve sound like, I've never had one and this could be the problem as well?

SORRY FOR THE LONG POST! But I have been down rocovering and thinking alot about it. Hopefully I'll be able to start wrenching on it again soon

Thanks for any thoughts on this crazy truck. At least you know a Ford man when you see him - He'll be the determined one!

A rod rap only sounds on deceleration. You rev the engine up and as it slows you hear the rap-rap-rap... as the connecting rod catches up with the rod journals on the crank. Also, a rod rap isn't a lingering problem. It get quickly worse as you run the engine and the rod bearing quickly turns in the rod's bore and you lose oil pressure and BANG! So we can rule out rod rap.

A burnt valve is very noticeable. A burnt intake will backfire through the intake, an exhaust valve will backfire through the exhaust. This problem will also quickly get worse and progress to a no-run condition. So we can rule this out.

At first I thought of piston wrist pin noise or piston slap, but you said it is quiet when cold and noisy as it warms up, which are the opposite symptoms of those two problems. So I'd rule that out.

Since this problem appeared with a cam change, this might give us a clue. Here are what pops in my head:

*Did you break in the cam properly by running it at 2000-2500 rpm for a 1/2 hour immediately after first start up?

*Did you check to make sure you had proper rocker arm geometry with the different cam? You could have the rockers hitting the valve covers or rocker arm studs or tops of the retainers if the geometry is wrong. I've seen all three conditions before. You don't have much lift, but stranger things have happened.

*Are you running about 10 degrees of initial timing? Can you make the sound disappear by altering the timing?

*How did you adjust the valves?

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'70 Mustang Mach 1 - '72 Mustang Sprint - '94 F-150

I think this is where it's at as I don't have a lifter bleed down tool to check the lash as the book suggest's.

Looking at the compression results of 120psi, I have a feeling the pushrods are too long causing the valves to stay open a hair.

I was very reluctant to drop the pan to inspect the lower end because of the great oil pressures hot or cold but I was desperate.

I'm am running 10* initial timing.

What are your thoughts on the strange behavior with the vacuum? I should be able to hook up a guage to it. I'm thinking this has something to do with the valvetrain geometry as well (valves staying open).

As soon as I'm able to get back to it I'll take a harder look at the valvetrain again.

THANK YOU!

120 psi isn't very far off for a low compression engine such as yours.

The engine dying when you plug the vacuum tree is very strange indeed. Do you currently run it with the port open?

How does the engine run with the distributor advance unplugged and that vacuum line plugged?

Can you get a vacuum reading from some other port other than the tree at the back?
I hope you feel better soon, recuperating from your surgery.

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'70 Mustang Mach 1 - '72 Mustang Sprint - '94 F-150

There isn't a vacuum advance, that's controlled by the computer.

The vacuum tree is on top of the upper intake with a cluster of 3 hard vacuum lines and one single vacuum line. That is the one I disconnected.

I'll have to remove the cluster and use one of those ports next.

Why don't you follow that line and see what it leads to. If it leads to a MAP sensor that would explain it.

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'70 Mustang Mach 1 - '72 Mustang Sprint - '94 F-150

Yeah, I thought about injectors too but not sure how to isolate one to rule that out.

maybe that is 1/2 of your problem the other half is why will it run with that extra air coming in and die with it pluged off.
This is a total guess. I dont know if that port should have any "real flow".