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Tractor Talk Discussion Forum

Octane requirement

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RandyR

05-08-2005 17:46:59




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Hi folks. What causes an engine to need higher octane gas as it ages. My vehicles all tend to need more octane as they get higher mileage on them.
Also, how do I fix it, get back to regular grade like it used to use? Thanks!




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Leland

05-09-2005 17:53:12




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
Just pour a slow steady stream of water thru intake while running you will have to speed motor up about a quart or 2 will help knock carbon off piston tops.



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gerrymn

05-09-2005 11:12:21




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
Don"t believe that the timing can"t be adjusted just because the manual says so. My 92 F-150 pinged badly just going up hills. The service manual stated that the timing could not be adjusted. I looked at the distributor, it was held down w/a clamp in the normal fashion. Someone had goofed with it and advanced it to try to compensate for a tune-up they had botched. I loosened the clamp and reset the timing to spec. and it no longer pings.

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Old Pokey

05-08-2005 20:06:57




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
What vehicles are they, and what are the symtoms that cause you to use higher octane fuel?



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old

05-08-2005 19:54:12




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
Easy way to cleamn up carbon build up it the spray a light mist of water in the carb with the engine running.



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Gerald J.

05-08-2005 19:45:35




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
There are several "motor honeys" in the automotive departments and parts stores that promise to remove such carbon build ups. Didn't help my '98 F-150. Turned out to be a contaminated mass air flow sensor that was running the mixture lean leading to pinging. The Ford shop cleaned the sensor wires with carburetor cleaner and its run fine since.

Gerald J.



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MarkB_MI

05-08-2005 19:29:12




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
As Jerry said, knocking on old engines is usually caused by carbon buildup in the combustion chamber. Another sign of carbon buildup is "dieseling" during shutdown, although fuel injection has made that a thing of the past. Some fuel additives claim to remove carbon, it's worth a try.

I have heard of mechanics removing combustion chamber carbon by squirting a little water down the intake while the engine is at a fast idle. I have not tried it myself. There is a danger of damaging the engine if you overdo it.

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Jerry/MT

05-08-2005 18:57:32




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 Re: Octane requirement in reply to RandyR, 05-08-2005 17:46:59  
Assuming the ignition system (distributor, coil, electronics, and associated wiring) are OK, usually the combustion chamber volume decreases due to carbon build up. This causes an increase in the compression ratio. You can reduce the advance to accomodate that or use a higher octane fuel. If it's a modern vehicle, it's handled automatically if it has a knock sensor.



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RandyR

05-08-2005 20:33:05




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 Re: Octane requirement in reply to Jerry/MT, 05-08-2005 18:57:32  
It's a 4.6 litre V8 in a 94 T-Bird. It pings in hot weather with the A/C on, accelerating up a hill or flat road. Have tried a new mass air sensor, professional decarbonizing, water in the intake, engine cleaners in the gas and straight into the intake. Mechanic says nothing is wrong. I asked about the EGR system being plugged, and he says it's not. Have tried different brands of gas. The timing can't be adjusted, nor can the computer be reprogrammed. I'm told the computers of that year were not reprogrammable. Any ideas?

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Jerry/MT

05-09-2005 19:51:59




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 Re: Octane requirement in reply to RandyR, 05-08-2005 20:33:05  
Don't know whether your '94 has a knock sensor but I suspect it does and you might have that checked to see if it's operating properly.. Another alternative is premium fuel (ouch!) but that's pricey right now.



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dhermesc

05-09-2005 10:01:16




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 Re: Octane requirement in reply to RandyR, 05-08-2005 20:33:05  
Antiknock sensor is out. My old Gran Marquis was doing the same thing until I had it replaced.



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Jon H

05-08-2005 21:14:03




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 Re: Octane requirement in reply to RandyR, 05-08-2005 20:33:05  
Not familiar with a 94 model 4.6, but I would bet it has some sort of knock sensor system to control detonation. I would look for a bad sensor,incorrect sensor mounting or wiring problems.



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