Join Date: Sep 2006
Location: Burlington, Ontario
Re: 1989 Grand Marquis 5L efi stumble
Mixing different vicousities of oil is not recommended. Now I've never seen the phenomena, but apparently two different grades of oil sometimes react with each other much like oil & water reacts - they don't mix. I am a little skeptical - because how are you to know what oil to use if, for instance, you're adding oil to a car that does not belong to you? I'm sure that different grades of oil are mixed all the time without ever knowing.
However, to purposely mix two different grades of oil together - what purpose is there in doing that? Pick an oil, and use it - don't complicate matters by mixing your own oil.
What does your manual state is the correct oil to use? Personally, I wouldn't deviate from that. In our climate, I wouldn't run anything thicker than 10W40 in your engine during the winter. Again, I stress following the manual's recommendations, and not my "best guess." I use 10W40 - only because I don't have a manual, and I know by experience that 20W50 is too thick for cold weather. As for harm - by using an incorrect oil on a car as old as yours, you're not going to do anything worse that time & everything else has already done to the engine.
Another reason for using the correct oil is also for fuel economy. The thicker the oil, the harder the engine has to work. How much fuel will you really save in the long run? Well, this is something that has been debated...
I will say this much - the climate in Southern Ontario over the last 20 years hasn't been cold enough to warrant the use of a block heater on a Crown Vic. If you need a block heater, you are using too thick of an oil (or you travel a lot in the northern part of the province). My '85 Crown Vic never came equipped with a block heater, nor has anyone added one. It has never failed to start during the coldest of winter days. The story might be quite different if I lived in a northern part of the province. Most cars in this part of the province don't need a block heater. Diesel-powered cars are a completely different story.
One more thing about the fuel - have you checked or replaced the fuel filter recently? Sometimes one that is just starting to get a bit plugged will react that way - fine until you reach a certain speed, and then it starts to misbehave. A fuel filter that is really bad might allow the car to run at slower speeds, but once you reach a certain speed it just won't go faster, and the engine just lacks power. However, this doesn't explain why you smell fuel. You can also check vacuum hoses & the PCV valve. Vacuum hoses are often overlooked. On a car as old as yours, many hoses will be cracked, hardened, or pourous. Hope some of this helps..