ok... lets set this up.
My 98 Lumina and 99 Monte Carlo both had the 3100 SFI V6 under the hood and would only run well on 93 octane. The 96 Beretta I have now has the same engine but gets better mileage on the 87 octane swill. What the fuck?

It would be interesting to see what the PCM is doing while driving. It could be adjusting differently. I wonder if you can swap the PCMs on those even...

Hmmm....My '98 Lumina did well regardless of which gas I put in it. Interesting.

If it don't spark knock on 87 your just wastin octane.

What about the weight, cam profiles, tire width, inflation, options, and as Skelly said, PCM. All affect MPG.



Edit, if your 'retta runs fine on 87, then running a higher octane could hamper MPG if the compression is less and isn't burning the higher octane fuel properly before jumpin on the exahust stroke..........

maybe it was the due to the transaxle gearing, PCM parameters, the weight difference of the car... who knows. 87 just seems to work better in the Burrito.

Posted by Samurai maybe it was the due to the transaxle gearing, PCM parameters, the weight difference of the car... who knows. 87 just seems to work better in the Burrito.

Another question, is it a 3 speed or 4 speed auto. Or hell, a 5 speed manuel. I can't remember Sam, but I know you have probably factored those in already. I remember being amazed when I bought my 00 Cavi that it was a 3 speed auto, didn't know they still made em outside of Wranglers...............

Posted by jeepdave Another question, is it a 3 speed or 4 speed auto. Or hell, a 5 speed manuel. I can't remember Sam, but I know you have probably factored those in already. I remember being amazed when I bought my 00 Cavi that it was a 3 speed auto, didn't know they still made em outside of Wranglers...............

they were all 3100 SFI V6's with the 4speed automatic.

Posted by Samurai they were all 3100 SFI V6's with the 4speed automatic.

I'm gonna go with weight and like you said, gearing.

Posted by Samurai ok... lets set this up. My 98 Lumina and 99 Monte Carlo both had the 3100 SFI V6 under the hood and would only run well on 93 octane. The 96 Beretta I have now has the same engine but gets better mileage on the 87 octane swill. What the fuck?

Not unreasonable to expect a 9:1 compression engine to run on higher octane. I wonder how retarded the timing is on your Beretta.

Posted by big dave Not unreasonable to expect a 9:1 compression engine to run on higher octane. I wonder how retarded the timing is on your Beretta.

that is one of the bitches that I have with GM/OBD II PCM's... you're locked out of timing your engine. The computer assumes full control. That's why forced induction is such a bitch on the V6's. There are guys that can program them, but they charge a premium for the knowledge.

One thing that I am noticing is that the Beretta seems to be geared higher (numerically lower) than either the Monte or Lumina... i am assuming it's because of the weight? Also, the tranmission in the Beretta is not the OD-D-2-1... it's the more annoying OD-D-L, under a certain speed the transmission goes from 2nd to 1st... Scares the shit out of you.

Any of the Jeeps with the AW4 automatic transmission, the shift points are higher up than most automatics out there. I have no idea why those crazy guys at AisnWerner did that, but it was weird for me at first because I thought something was wrong. But a little reading fixed that up.

I know with my Cherokee though, with the misfire at idle, I get 15 mpg on my normal driving week. I have not done a highway road trip yet to see what it goes up to. I know most other Cherokee owners get 17-19 or so on simular driving so its interesting that one misfire is costing me that much. I've been looking at finding someone with a DRBII code reader from Mopar so I run it and see what it actually is doing to adjust the problem. I know from my own troubleshooting, the PCM seems like its adjusting at anything above 1100 RPM.

Swap out injectors Skelly and see if the mis fire moves. If so ya gotta clogged or weak injector. If you pay the shippin I got a couple extra layin around ya can have.

Posted by jeepdave Swap out injectors Skelly and see if the mis fire moves. If so ya gotta clogged or weak injector. If you pay the shippin I got a couple extra layin around ya can have.

I already swapped in some Neon injectors and the misfire still is there on #3 regardless. I've taken it to a retired Mopar Mastertech locally who is 100% its the piston ring.

Posted by Agent Skelly I already swapped in some Neon injectors and the misfire still is there on #3 regardless. I've taken it to a retired Mopar Mastertech locally who is 100% its the piston ring.

Marvels Mystery Oil? Pull the plug, pour some in, let it sit overnite and see how it runs in the mornin.

i was thinking more bent or stuck valve, collapsed lifter or bent pushrod.

Posted by jeepdave Marvels Mystery Oil? Pull the plug, pour some in, let it sit overnite and see how it runs in the mornin.

I might try that.

When I first got the XJ, even with after the tuneup, I was getting 12-13 MPG. A local redneck (Yes, thats what he is exactly! Nice guy) suggested putting in my crankcase some stuff called "Engine Restore" On the next fillup, I jumped up to 15 MPG.

Posted by Agent Skelly I might try that. When I first got the XJ, even with after the tuneup, I was getting 12-13 MPG. A local redneck (Yes, thats what he is exactly! Nice guy) suggested putting in my crankcase some stuff called "Engine Restore" On the next fillup, I jumped up to 15 MPG.

Wanna buy some land in florida?

Posted by jeepdave Marvels Mystery Oil? Pull the plug, pour some in, let it sit overnite and see how it runs in the mornin.

Are you serious? Will you buy a bottle of my pee if i said its a trans rebuild in a bottle?

Posted by Agent Skelly I might try that. When I first got the XJ, even with after the tuneup, I was getting 12-13 MPG. A local redneck (Yes, thats what he is exactly! Nice guy) suggested putting in my crankcase some stuff called "Engine Restore" On the next fillup, I jumped up to 15 MPG.

That stuff is nothing but really thick motor oil. Do the math.

Say what you will, but I have soaked a cylinder with a stuck ring and the next day it wasn't. Its not a cure all, but the shit does what it needs to do now and again. Plus, we are not talking high performance high reving car engines here. Think more tractor. Doesn't always work, but not all of us have a garage or even pavement to work on, so if a mechanic in a bottle gets me 40,000 more miles out of an engine that really needs replaced/rebuilt then I'll chance it. Got nothing to lose and everything to gain.

Posted by jeepdave Say what you will, but I have soaked a cylinder with a stuck ring and the next day it wasn't.

Ive poured a cup of plain 30 weight oil in a cylinder and it freed it up over a lunch break. Im talking about a Studebaker with over 300,000 miles that hasnt run in 30 years because the head gasket failed and leaked water into said cylinder. Theres nothing special about Marvel Mystery Oil.