Not all grilled chicken, mind you. But chicken that has been grilled on a grill that hasn’t been cleaned in a while tastes disturbingly similar to poop (if you’ve ever had a bad septic tank with a crack in it, you probably know what taste I’m talking about). You get hints of the taste on other meats cooked on long-uncleaned grills as well, like burgers.
What causes this poop-tasting phenomenon?
I’ve never had this occur to me, ever.
But just a WAG: if the grill isn’t clean, then it’s probably got some half-burnt rotting food on it. Food rotting is just getting digested by something other than people (bacteria and other bugs). So chemically it probably is very similar to poop. Clean your grill, people.
(Also, if your septic tank cracked, wouldn’t it leak out into the yard? Why are you out there tasting it??)
The scent is so strong you can actually taste it. It’s not pleasant.
Never experienced or heard of this either.
I suggest you do a good burn on your grill, get that sucker screaming hot and keep it that way for an hour.
Old chicken tastes nasty no matter how you cook it, septic and vomit inducing and… Grills don’t frequently hold on to rotten meat. Perhaps you are taking the former for the latter.
Old chicken is about the only way I see this making sense. Never encountered the taste before, but I’ve smelled old chicken, and it does have a certain poopiness to it.
Moderator Action
Since this concerns food, it’s better suited to CS. Hopefully the cooks over there will have some ideas about what causes the bad taste and also may have some advice on how to avoid it.
Moving thread from General Questions to Cafe Society.
When chicken starts to go around the bend, it gets a smell often described like rotten eggs. Or ammonia. I imaging cooking bad chicken doesn’t help with the smell/taste.
A grill can retain certain flavors from prior cooking. I don’t do much cleaning of my grill between cooking, but I definitely scrub the grill bars down after cooking fish because I can pick up that flavor on mild meats like chicken (sometimes, anyway, and when I do, it’s not a rotten or sewage-like flavor, just a fishy flavor).
If there are fats or drippings going rancid in the grill, I could see that being transferred to meat, but I have never experienced exactly what the OP is describing.
Also, what fuels are being used? Lighter fluid can lend a gasoline-like flavor to any meat if you don’t let it burn off first. Charcoal briquettes should burn until there is white ask on the outside, or you’ll often pick up a chemical odor from them. If you burn wood in a grill, use hardwoods because softwoods like pine can have a flavor like turpentine. If you’re burning anything that has gotten wet, there could be mold/mildew growing on it.
I’ve never actually had the situation happen to me at home, but it’s happened at a few eateries I’ve been to. I always just assumed it was uncleaned grills. But now you got me thinking they were trying to feed me near-rancid meat 
[SARCASM]
Thanks ever so much!
We are having grilled chicken for dinner tonight.
How can I ever repay you?
[/SARCASM]
wrong barbecue sauce.
Never experienced the poopy taste. My only complaint about barbecued chicken is that most people apparently have never heard of a meat thermometer. So to be on the “safe” side, they cook it until you could resole your boots with it. Same with burgers and sausage. I’m really tempted to bring my Thermapen to these affairs.
Doesn’t work. The minute people like that cut into a piece and see anything remotely juicy or (gasp of horror) the redness that sometimes leaks out of the bones, they throw it back on the grill, Thermapen and common sense be damned.
Doomed to a life of tough, tasteless burgers and sausages like sawdust. 
Depends on what the chicken ate as well. Too often chicken that ate soy tastes like soy.