I know it sounds odd but try an SOS pad like the one used for doing the dishes and a little warm water, works for me. It will be a bit messy so make sure to put a towel between the pipes and the engine, etc. to catch the mess.
After washing the area carefully with soap and water, use a 0000 steel wool pad. You can do this wet or dry. Your call. It may take a bit of work, but repetition rather than force will do the trick without harming the chrome finish.
06RoadKing has the procedure that works for me, oven cleaner on warm pipes, but protect everything in the area from overspray. Leave on for about 30 minutes, then use a clean white cotton tee shirt. :s
Do not rub with anything abrasive, unless you want "satin" finish...micro-scratches will show. I have done the other using nylon scrubber...still leaves scuff marks that show on my pipes...the first time...
oven cleaner on warm pipes as well but spray onto towel then put on pipe. Leave towel on for 30-1 hour and rub. Repeat if needed.
Tried "Mr. Clean" eraser before but didn't work well.
Similar problem on more than 1 occasion i got the stuff off by heating it up using a gas blowlamp and using a bit of soft wood as a scraper all came off and a final polish with autosol