Howdy fellas...The 'new' 400 I threw in the Bronco is starting to develop a miss. Plugs are new, wires were swapped over (but less than a year old), and there is a minor timing issue. All of those things are going to get fixed/replaced shortly. However, since it just started coming on in the last month or so, I'm thinking a valve going bad. So my question is, how do I test for that (so I can buy parts accordingly)? How do I know if it's an intake or exhaust valve? Is this what a leak down tester does? Will a compression test yield any applicable answers (when tested 5 months ago, all cylinders tested within 10psi of eachother)? Thanks for any input.

Just thinking out loud here, i could be way off track with this one.

I would think if it was an intake valve you would have some black in your intake runners... Like you do on the inside of your exhaust manifolds...
Pullin the intake would let you know. as for testing i believe the leak down test would work... But i dont know if that is the correct test or not...
Vince would be the man to ask...

UTfball68 said:

Plugs are new, wires were swapped over (but less than a year old), and there is a minor timing issue. All of those things are going to get fixed/replaced shortly.

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I think it's a little premature to orry about a valve being the cause of your miss...you need to get the "timing issue" squared away first.

Bricks said:

I think it's a little premature to orry about a valve being the cause of your miss...you need to get the "timing issue" squared away first.

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Very much so agree. I was just jumping the gun a little because I was trying to figure out what kinda problems there could be and realized I don't have a clue how or what to test for a bad valve. So I was more looking to enlighten myself and prep just in case the other fixes aren't the right ones.

A vacuum gauge will tell a lot about the condition of an engine.
http://www.google.com/url?sa=t&sour...0ID_DA&usg=AFQjCNGioYW0giOPJBXh528lQec0enC6vQ

That and a leakdown tester

Alright thanks fellas. Doesn't Jurg have a thread around here about using a leak down tester?

A leak tester will tell alot. I would get the timing right before worrying about a valve as well.

Also change the rotor and cap. Or at the very least check em for wear and damage.

A drivers license will tell you a little really quick. Take it and hold it near the tail pipe...If it sucks it up against the tail pipe, there is a good chance of a burnt exhaust valve. Explaination: If you have a burnt exhaust valve and the piston on that cylinder drops for the intake stroke, it will pull air back through the tail pipe.

73F100Shortbed said:

A leak tester will tell alot. I would get the timing right before worrying about a valve as well.

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Will do...was just getting ahead of myself

1985 Ford F-150 said:

Also change the rotor and cap. Or at the very least check em for wear and damage.

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Planned on that as well.

chuck101st said:

A drivers license will tell you a little really quick. Take it and hold it near the tail pipe...If it sucks it up against the tail pipe, there is a good chance of a burnt exhaust valve. Explaination: If you have a burnt exhaust valve and the piston on that cylinder drops for the intake stroke, it will pull air back through the tail pipe.

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Good call Chuck...I had forgotten about that one, but I had always heard a dollar bill. Either way, it'll get done.

Get your piston to TDC on the compression stroke, then, remember the air hose fittings for changing valve seals? Use those to
"pressurize" the cylinder. If you hear air from the oil fill cap, bad rings/piston. If theres air in the tailpipe, bad exhaust valve. If you hear it through the intake, bad intake valve.

Never thought about that, vince your a smart one!

Truckin4life said:

Never thought about that, vince your a smart one!

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That's why he's the man. Thanks Vince.

Yup, that does explain why he is a certified ford tech!


Makes you think if you faced with these problems every day, what kind of things could you come up with to solve problems quickly and efficently.

compression test anyone.....
And the intake gaskets on the 351M/400 have a bad rep...

393clevor said:

compression test anyone.....
And the intake gaskets on the 351M/400 have a bad rep...

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Gonna do it this weekend...and I used a valley pan, still bad news?

Ben, you have been at work what? 20 min and already on FTF?

Truckin4life said:

Ben, you have been at work what? 20 min and already on FTF?

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Hey hey hey...I got here at quarter til 8...and I'm waiting on another department to send me some info before I can continue.

Meh, this was the first place i was at too...
Its all good, i guess its apart of the "cubicle crowd"