Keaton85 Posted August 28, 2010 Report Share Posted August 28, 2010 come on guys? what's the reasoning behind low octane fuel causing a burnt valve? I can't see one as of yet, since low octane fuel mostly causes extra carbon build-up and pre-detonation. I could see carbon getting in the valve seat and causing the issue, yet this would be a very widespread issue since a lot of people are running low octane in there beaters! I can't see the pre-detonation causing an issue though since it's also compensated by knock sensors to some degree. extra heat? idono but I still don't see it. Any info here? Link to comment Share on other sites More sharing options...
Keaton85 Posted August 28, 2010 Report Share Posted August 28, 2010 I retract my last post, damn those suckers are in the hot seat Here is a good read. Ive never really payed much attention to exhaust valves till now. vacuum leaks with a lean mixture, sounds like 75% of Volvos out there! http://www.aa1car.com/library/ar1192.htm Link to comment Share on other sites More sharing options...
Scootdiggity Posted August 28, 2010 Report Share Posted August 28, 2010 come on guys? what's the reasoning behind low octane fuel causing a burnt valve? I can't see one as of yet, since low octane fuel mostly causes extra carbon build-up and pre-detonation. I could see carbon getting in the valve seat and causing the issue, yet this would be a very widespread issue since a lot of people are running low octane in there beaters! I can't see the pre-detonation causing an issue though since it's also compensated by knock sensors to some degree. extra heat? idono but I still don't see it. Any info here? Dont know if this is your problem but about the same symptoms my friend had the same problem a few years back 250k on engine one mechanic diagnosed as burnt valve the other said clogged fuel injector, guess who was right? The mechanic who said the clogged fuel injector, ran industrial injector cleaner through fuel system and all is well to this day. Moral of story check the little things first. I have had these engines to 300k and have never had a burnt valve, but I run stock settings on turbo, which were proven at factory, most people dont. When you do performance mods its the price you pay for wanting to be faster, as many have seen on here, volvos failing because builds were not correctly done. Link to comment Share on other sites More sharing options...
Gizmo Posted August 28, 2010 Report Share Posted August 28, 2010 I had a cylinder burn both exhaust valves. Upon pulling the head, every other cylinder was utterly fine... could have reused those valves in the rebuild. This makes me thing the valve failures are related to something less system wide than fuel grade or valve seals... Link to comment Share on other sites More sharing options...
Gilhuly Posted August 29, 2010 Report Share Posted August 29, 2010 Personally, I believe that there is a correlation between burnt valves and regular gas as I have had conversations with mechanics who have done tons of rebuilds and have seen it myself. However, that correlation may not be causal as they say, but spurious. People who cheap out on gas probably also hesitate to spend money on regular maintenance. I think the article Bing references makes a good case for seafoaming and cleaning some crud off the valve seats. regular gas does burn hotter, though and this has to contribute some. Link to comment Share on other sites More sharing options...
Gilhuly Posted August 30, 2010 Report Share Posted August 30, 2010 Everything you ever wanted to know about gasoline and octane OK, knowledge is power and I pull back from octane as a cause of significant temperature differences and go with poster's assertions that crap leaking down and baking onto the valves and creating thermal inefficiency is a much greater possible cause for a burnt valve. Vale stem seals are important. Link to comment Share on other sites More sharing options...
Gizmo Posted August 30, 2010 Report Share Posted August 30, 2010 The guy who rebuilt my head, Dave at D&F Performance in South Jersey, who has worked on a number of these engines, seemed to think that oil leaking down a seal wouldn't be enough of an issue to cause failure. He was also not in the octane camp either though. He thought I might have been running too lean. I wonder if the fact that I had only one cylinder fail, and it was #5, might speak to cooling inefficiencies in the earlier 5-banger heads? This exhaust valve burn is a pretty consistent problem with these heads it seems... People who've had failures before, (1) Where was it?, (2) what were the mods on your car at the time, and (3) what was the condition of the non-burnt valves upon rebuild? To restate, (1) I burnt Exhaust valves 9 and 10 (both exhaust valves in Cyl 5) (2) Car was running 13psi on a 15G with a stock map (MBC) (3) all my other valves were within spec Also of note, the springs were worn and the seals were crap, but the valve guides were within spec. Carbon buildup was not extreme.EDIT: also, '96 Turbo head. Link to comment Share on other sites More sharing options...
Keaton85 Posted August 30, 2010 Report Share Posted August 30, 2010 Ive only come across one burnt valve and I'm doing the head job this week. It was on a extremely well taken care of 98 S70 GLT with almost 300K miles! I can't stress it enough that the head I pull looked like it had less then 100K on it. Ive seen heads with half the miles that looked like crap! Also the cylinder walls still have there original cross hatch markings! So idono, maybe this is just a fluke thing that comes up due to some valves having imperfections during manufacturing. Give me E100 and call it a day haha! Link to comment Share on other sites More sharing options...
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