Chevrolet Repair: rough running lumina, vacuum leak, timing chain

Question
I have a 1991 lumina with 3.1 V6.It runs rough at idle but runs better as you increase engine to about 1800rpm then it starts surging like it has a bad miss.The ignition module was changed first with no effect.Compression is good,fuel pressure is good.With a vac.gauge hooked up the reading is 10 and goes up to 20 when engine speed is high.When you rev engine to wide open quickly vac. reading goes up.The mileage is 180,500 and a friend told me it could be a bad timing chain.I would appreciate any advice you can give.Thank you,Larry  

Answer
Hi Larry, The first thing I would do is locate the cylinder that is weak or not putting out at idle. Now I would check for a vacuum leak on that cylinder. Remember that these can also leak vacuum inside the engine. Remove the oil fill cap and check for a large amount of vacuum there. Also this could be a bad fuel injector, check spark plug to see if it is covered with gas. If not then the injector might not be spraying fuel. You need a special tool to fire the injector to see if it is working.  Hope this info helps.

              Mark