Hey guys I have a 1983 F150 with 4.9L. Sometimes when I shut off my engine it will make this hissing noise. I have no a/c hooked up so it can't be that. The weird thing is though, when I shut off the engine, the engine turns over one more time and then a hissing noise will follow... Any thoughts on this?
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I have always heard the term "dieseling" when a gas engine runs just a tad after ignition is turned off. Something with the timing is off a bit. Probably retarded a hair. As your motor rotates, it builds compression and still creates a little vacuum. Your probably hearing all the leak down with the key off. Because its not firing and sending it through the exhaust as normal.
Check for vacuum leaks. If there are none when was your last tune up? It could be timing. I just resolved this issue on my 83 f150 300 with a tune up. I knew the timing was right and there weren't any vacuum leaks though.
ya its dieseling, usually caused by overheating and or too advanced timing. check ur timing and check ur coolant/thermostat. did u put special plugs in it like the tri tipped ones? if so, go back to basic, check ur fuel, have you changed grade/brand or started using an additive?