Before 1 day its rpm dont go above 5 and just after hour it started to misfire and now the car is not getting start. I have got the codes 61-1 and 86-1 And no other codes.

Sounds like a wire connection may have gotten pulled when the alternator was replaced. Some vehicles have coil packs that could have got wires disturbed. Dont know what your vehicle is since you didn't put year make or model.

I have had similar problems on a VW Polo 1.2. It could be one of two things that I know of ( it could be another fault too).

The first, less likely, is a coil pack failure. Modern day engines have a coil per cylinder that provides a spark to each cylinder at the correct time. When one fails you will be running on 2 / 3 cylinders ( on a 3 / 4 cylinder engine respectively). This is unlikely, however, as usually coil packs progressively die and you would notice it getting worse over a number of days or more likely hours before it failed.

The second problem I have had, which was almost identical to your problem; my car would start fine and sometimes run without issue. After driving it for a while I may start to get slightly jerky drive, then get a severe loss of power. Turning off the engine for 5 minutes usually fixed it but there were some occasions I had to leave it for up to half an hour. There was even one point this happened queing up to a toll booth on a motorway, and I was stranded in the centre lane for 5 minutes until I could start it again.

Both errors, though, should present an error code, most likely indicating a misfire and / or post-cat emission issues.

Get your car checked at a dealer with a diagnostic machine or, alternatively, if you have an android phone you can do it yourself with a minimum of outlay. You can purchase a cheap OBD2 bluetooth adapter from ebay or similar, then download this application https://play.google.com/store/apps/details?id=org.prowl.torquefree. You can plug the adapter in to your car, connect your phone wirelessly to the adapter and check for fault codes.

If it's a coil pack failing, you will likely see a misfire registered on only one cylinder. If it's the cam sensor, you could show a misfire for several cylinders. Either way a diagnostic check will point you in the right direction.

how to stop a Chevy Tahoe from misfiring and smelling gas

I think you have an engine wiring harness issue going on here.

What did the spark plug look like?Did it have oil or gas on it?

I would assume the check engine light is on now because the EGR is not working ?
Depending on which hose you are talking about, it may be causing a vac leak near cylinder 5. There is a solenoid valve that cuts off vac to the EGR valve at idle which may not be working.
The EGR valve should not be open unless the engine is above 1500 RPM or so.

Sounds like fuel problem to me?? Have you changed the fuel filter ?

Try taking it to get a dignostic test.

po 300 is random/ multible cylinder misfire detected po507 idle system rpm high-- go to this site for the po507 meaning--www.obd-codes.com/p507 === please go to this site for the po300 code---- www.obd-codes.com/p0300

Random/Multiple Cylinder Misfire Detected have a good day !!

Replace the wires and coil boots.........