I've got a 94 slt 318 2wd. When I bought my truck, the previous owner said that the engine had been rebuilt and replaced. He didn't say how long ago, but everything was pretty clean, no oil or dirt on it. It still looks really good, no leaks except for the radiator. Right after I got it, I was running regular(87-89) in it and it was running great. Smooth idle and plenty of power and throttle response. Tire smoke after the 2-3 shift with my foot in it was quite common. About October, the idle got rough and power dropped off. I did the usual plugs and wires, air filter, oil change....blah blah blah. And it was still acting up. Then I filled it up with premuim(91-93) and the problem went away. She was back to her old self.

My question is, why didn't run like crap the day I got it, and is there any way of checking to see if the block and heads were machined during the rebuild? (Increased compression....Without taking the engine apart)

P.S. I've been running premium ever since the tune up.

 

I believe you have another problem that high octane gas is covering up if it ran fine on regular gas for a while. You can't really shave the heads so far that it increases the compression so much that you need to run 93 octane. That high of compression comes from high compression heads and pistons.

First, check to see what computer is in the truck. If the computer has the factory sticker on it, there is no reason to run 93 octane. If it has a mopar performance sticker on it, you need to be running 93 octane.

The only other thing you can do is try to contact the previous owner and ask, or tear down the engine to see what parts are on/in it.

 

An aftermarket computer or reflash makes that kind of a difference? I know that there is a lot to be done with most efi engines, but I was always under the impression that unless the compression went up, hugely, regular or midgrade would do.

 

Yes, a computer can make that big of a difference. Compression is one of the two factors for determining the nessessary octane rating. The other is ignition timing... which is what the performance computers change.

 

i know this is probably out there but grease, oil, etc in your tank will drop your octane down from 87 in the tank. you live in Michigan so guys grease and oil undercoat there trucks for the winter. i usually put some grease around the filler cap because it was a bad spot for rusting on the salt roads of Nova Scotia.
it only happened once but i got some gas and there was a small amount of grease on the end of the nozzle. i cleaned it off but i imagine it would screw things for a while. now i always look at the nozzle before i put it in the filler neck. it's rare but if all else fails keep running the high octane through and switch back down the road.
i switched over to platinum plugs for my tune ups and found them excellent for smoothing out the idle.

 

The only thing octane does is change the bang in the cylander. 93 softer bang and 87 harder bang. Depends on the engine build. You can use 93 and be ok. But why pay more when your engine is built to run 87.

Now if you look at cetane thats diesel and you obviously need to compress the air to get diesel to fire but the heads and valves are designed to take that much of a bang.

 

Um, it doesn't sound like you have a grasp of what octane is.

Octane is an additive put in gasoline to resist detonation. It changes how the gasoline reacts to heat and pressure. If you have an engine designed to run on 87 octane, it is a low compression motor with late ignition. The 87 burns easier because of the lower octane rating, so it burns faster. If you have an engine designed to run on high octane, it is a high compression motor and/or an advanced timed motor where the spark occurs further from TDC. If you have low octane gas in either situation, it will burn too fast and spark knock, so you need the higher octane to slow down the detonation. The added power does not come from the gasoline itself, but from the increased heat and pressure by igniting the gas earlier (in an advanced timed motor) or compressing it more (in a high compression motor).

If you run 93 octane in an engine designed to run on 87, the gas will burn colder and slower, and you will have increased emissions, increased carbon buildup, and decreased power. If you continue to do so for a long period of time, it will eventually lead to bad build-up problems in the heads and upper cylinders. So you are absolutely not correct in saying you are ok to use 93 in an engine designed to run 87.

 

That's what I was trying to say without using Google

 

Lol, I didn't use google, I just know how gasoline works.

 

it's not " softer or harder bang " it's the ability of the gas to ignite. 93 is cleaner, ignites quicker and travels through the cylinder faster ( more bang for the buck ) 87 is slower to ignite and slower to travel through the cylinder, less power.

the problem with high octane is that if you have carbon in the cylinders it pre-ignites the gas before the plug fires and you get a rattle in the engine. (friend of mine put it best in motor vehicle repair, sounds like putting a chain in a bucket and swinging it back and forth). the lower you take the octane rating down the rougher it runs (contamination will do that).

sorry slt but octane is not an additive to decrease pre-ignition. it's to increase power by increasing the octane. this also increases the chance of pre-ignition. but i'm telling you something you already know slt.