Hey Guy's! Folk's call me Bill, I have another Question for the experts,

My 94 dakota 4x4 came with the Magnum V8 It never passes a gas station it dosen't like! I been running 87 octane in it since i bought it from my Mom (almost a year ago), question is would i benifit from 91 octane? It does ping every now and then, Okay! every day.

gas mileage is around 12-14 tops!

Should it do better than that?

 

The epa estimation says it should do that and maybe a little more. I would say your doing about as good as its gonna get. I dont know that running 91 would increase your mileage noticably, but it might stop the pinging. Take a flashlight and look down your intake and see if theres a bunch of pooled oil. If there is you may benefit heavily from changing your plenum gasket.

 

Yes it will help with pinging. The best thing you can do for pinging is change spark plugs. Use Autolites 3923. Get them from any part store and ask for them by part the number. Ya just like said above check the plenum too.

 

I had never gotten better than 12 town and 14 highway since I've owned my 95 v8. I run 89 octane (usually with 10% ethanol too) and never have a problem with pinging. atleast I dont think I do...

I was forced to use my truck for a long work trip (roughly 500 miles out and 500 miles back). the week before I left I removed the Cat and put in a glass pack and a brand new air filter. When I know im going to put an entire tank into it on the highway I will use premium. you will notice better mileage.

Moral of the story, intake and exhaust gave me 17 mpg highway on the plains and 15 mpgs highway in the mountains around Billings MT. I have never seen numbers like that, even with my v6 I had before. Also, ran the tires at 44psi for the trip, usually 35 for normal driving.
Start with simple/cheap things. new plugs, air filter, tire pressure, driving style (go easy on the skinny pedal).

I dont recommend the exhaust mods only for fuel mileage because its a little loud on the highway now, but we dont have emissions here and that cat was super plugged and restrictive. I noticed a power addition when I did the exhaust. Seems more responsive to the loud pedal now.

 

Sounds like pretty normal mileage for an auto, especailly around town.

Premium will not help anything (mileage or power wise) on an engine tuned to run on 87.

Definitely check out the plenum gasket, it's sure to be blown.

 

I will give a look down the plenum and let you guy's know what i find, How far does it have to be taken apart to see? I'm thinking it will look good knowing the old owners. the truck only has 87000 just turned this week... My Grandfather bought it new, my dad owned it for a while, I bought it for my son because he can not drive a stick shift, hey i might as well keep it in the Family Right?. My mom said they always filled it using premium 91. I have not changed the plugs yet so i will do that this weekend, also check/change all the filters. something i still need to do is the Bypass hose, tstat and have the rad cleaned out, that should keep it on the road for a little longer.
Thank's for all the good advice.
Bill

 

In the case of the plenum gasket, maintenance habits have zero effect on it's longevity. The manifold is aluminum, the plate is steel, the different expanding rates will blow the gasket no matter what.

 

All you have to do to check the plenum is take off the air filter housing, open the throttle body butterflies and look down into the intake with a flashlight. If you see significant amounts of oil, theres a good chance the plenum blew! Also, it doesn't really matter how well the truck was taken care of, this is a common issue with these engines cause of a flawed design. But the blown plenum will cause the engine to suck in and burn oil which causes knocking/pinging.

Also, if the truck has been run on 91 octane for a long period of time and now you want to run it on 87, you should reset the computer. It's hard to say if the computer knows how to compensate for the octane change, but it could have tuned itself for 91 over the years.

 

Has the engine been rebuilt or the heads decked? I ask because a buddy of mine had a 3.9 Dak that had a rebuilt engine that pinged on 87 but not on 92. The only thing we could figure is that it had a slightly higher compression than stock which cause the 87 to ping. My 96 V8 auto 2wd Dak gets about 17-19 on my normal driving. K&N filter, shorty headers, electric fan, and underdrive pulleys all seemed to help with mileage.

 

On an engine designed and tuned to run lower 87 octane fuel, if you run a higher octane, it can, and will, cause excellerated carbon build-up in the combustion chamber, which will cause pinging, hot running, etc....

You can de-carbon your engine with the seafoam method, then replace the O2 sensor, and run exclusively on lower grade. This should help.

The octane rating rates how flamible the gas is - the higher the number, the less volitile it is, so it takes more compression and more heat to properly burn the higher octane fuels.