Overheating Exhaust

[verdemar]

Just joined the forum, hope someone can help!

1998 VW Polo 1.4 CL Petrol

My daughter went on a short trip to the supermarket and back (5 mile round trip). On the way there she noticed the engine was running a bit rough and lacked power but she made it to the shop and rang me for advice on whether to leave it there or attempt to drive it home. I asked if it was overheating and if it would start and tick over OK. The temp was normal and it did start etc, so I said drive it home!

She limped the 2.5 miles home and when she got out of the car she noticed smoke pouring out from underneath, she looked under the car and flames started to appear, in panic she dialled 999. She saw something burning fall from the car and then it started to die down. The Fire brigade arrived and checked over the car but couldn't find the cause and the fire had now disappeared although the car was full of smoke. They started the car which would still run although still lumpy. They left saying they had no idea what caused it and to get the car checked by reputable garage (which they would say).

I arrived home and checked under the bonnet and around the car generally but couldn't see anything wrong. I then jacked it up and got underneath and found that the exhaust centre box rubber hanger had completely burnt away leaving just a charred residue behind, nothing else in the area was abnormal or gave a clue as to why this had happened. I then investigated the uneven running which turned out to be a dodgy ignition lead on No. 1 and so I replace all the leads and the engine is now running fine.

Obviously my daughter is concerned about driving the car now as the reason for the fire is a bit mysterious. All I can put it down to is that the unburnt fuel from the non-firing No. 1 cylinder was being pushed out through the exhaust and caused secondary explosions in the exhaust causing it to overheat and ignite the rubber hanger. However she said the car did not misfire and the burnt hanger is near the centre box a long way from the engine (at least half the length of the exhaust away from the engine under the rear seat of the car).

I've searched the web for exhaust fires but can't find anything similar to this. I'm now wondering if a blocked cat or centre box in the exhaust might have caused the exhaust to overheat and the hanger to burn, maybe this was caused by the misfiring. Surely these are designed to withstand high temperatures so it must have got very very hot.

Has anybody any ideas to explain this, do you agree with my theories or think there is some other reason not related to the mis-firing.

Comments invited.

Mike

[R32North]

Unspent fuel cvan be ignited anywhere along the exhaust.... its can make its way to the very end and ignite on the way out when ut hits air..... BlueFlame back-fire!!

Is the exhaust the orginal of replacment, ie Kwik-Fit/ It maybe that its either old or a cheap exhaust and if made of thin metal wold heat up quite quickly if unburnt fuel vapour was being ignited by a hot piece of exhaust pipe, ie think section of old piece of pipe!

What condition is the exhaust it alongs its length/

ALso, I'd consider replaceing that part of the exhaust with new, as the excessive heat and combustion of fuel vapour in the pipe maybe have damaged it..... poke it with a screw driver to see if its likely to hold out or give out!

[verdemar]

Thanks for your comments, I later found the carpet above the hanger melted in about a 6 inch circle over the area. Don't know what make the exhaust is but is probably not the original as it's in quite good condition for an 8 year old car.

Since fixing the misfire with new leads and fitting a new hanger all seems fine so I think it was just the ignition of unburnt fuel in the exhaust which caused it to overheat. The design of the hanger is that it sits along side the pipe about 1/2 inch away instead of being above the exhaust. I think this design means the hanger is going to suffer easily if the exhaust temp goes up.

Seems OK now anyway, thanks for your feedback.