Exhaust Leak/Burned Valve

Can someone please refresh my memory. Status - pretty sure I've got a cracked exhast manifold - haven't pulled the heat shield yet but it definitely sounds like one under the hood. Engine runs like one cylinder is not firing or compression is weak/gone. Engine getting harder to start, is idling rough and consistently missing on one cylinder. Pulled MAF sensor and wires are clean and intact. Check Engine light is on and won't extinguish.
The Question: Can an exhaust leak at the Head/Manifold joint cause an exhaust valve to burn over time? I thought the leak I was hearing was at the welds of my muffler (until it fell completely out of the car last week without any real warning). Now that it's welded back in, I can really hear what I think is a pretty severe leak under the hood. :banghead:

Forgot the add previously - it's a '98 with a 2.7 liter 4 banger.

I don't think so, but a cracked manifold can cause the O2 sensors to give errant readings to the ECM, which can cause the engine to run poorly...typically rich. Also, run a compression test and check the spark plugs for soot and oil fouling. This can indicate a rich condition or oil leaking past the valve seals, respectively. A fouled plug will act like a dead cylinder until the oil is burned off (unless the leak is severe).

To check for a burned valve, run a leak-down test. Hook up compressed air through the spark plug and turn the engine until the valves should be closed on the compression stroke. If you hear hissing from the exhaust pipe, the valve is burned or leaking.

Good luck!

how about back to basics....cracked spark plug on install...crossed wires firing out of order...if it backfired in the past then how about blown exsaust gasket...cracked distributor cap...or...cap is cross firing because of moisture....the valve will burn but not that fast...it would take about 1500 miles but would have to be firing right...dead wire...take care of the hard start before you worry about exhaust leak...the engine would run fine without an exhaust manifold...check all the basics first...then find the leak...you can get the engine to run fine without worring about exhaust leak...lasty but not sure...you might have a timing belt that slipped a couple of links....uncle darryl...bremerton

Regretably, the leakdown test is beyond capability but the rest of the checks are within my scope of ability. I'm hoping for a bad plug or possibly a wire bad. The truck has 105,000 miles with 55,000 on the plugs and wires. Nothing has been touched (yet). Once out of idle range, the engine runs smoothly at speed, both at high idle and down the road. I'll be pulling things apart this afternoon to take a look. I"ll have to check the wires the old fashioned way - pulling each off the coil and listening/looking for the spark at the coils and the same for the plug end by grounding the plug on the valve cover. I know it's not the best way by far but it is currently all I have at my disposal. I'll post the results after I do the checks. Thanks for all the input so far. ..........

Latest update - burn pattern on all plugs looks good, cleaned and regapped to get through next paycheck or two. HOWEVER, found the exhaust manifold cracked nearly in half - I think the fact that it is bolted to the head is probably the only thing keeping it together - same situation as the plugs - next paycheck or two. This has only graduated from what I think may have existed for some time to this extreme in the last two weeks or so. It's chugging like an old tractor engine. At road speed do the other 3 cylinders' exhaust create a scavenging effect on the #3 cyl exhaust thereby rendering the leak unnoticeable above say, 1500 rpm?:banghead: