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Discussion Starter #1
This is my first post.I'm having fuel mileage problems with my white rm.I have a lot of fuel tank pressure.and I think it must be linked with bad fuel mileage.My gold Rm got 23-26 mpg.I recked it.So i bought this white one,it gets15-17 not good performance.I changed the plugs and wires and new fuel filter.It already had a new cap&rotor.It has 256 posi with 235 75 15 mechlins tires new when i bought it.I've been reading the posts to see if maybe the opti might be weak.what are the signs that it is weak.The gold RM had 308 non posi.with 225 70 15 tires.I was reading about the motovac treatment and I'm going to try that next.The white Rm has 100k and the gold on has 150k.no codes on the white RM.Any suggestions.Thanks tornado.
 

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A couple of things that I noticed about your message...

- If the fuel vapor pressure is not relieving into the charcoal cannister, then it could affect economy and power. [Some pressure is normal; too much or too little is a problem.] Check the lines going into the charcoal cannister. (It's in the right front fender well underneath the battery.)

- You went from a 3.08 rear end with undersize tires (27.4" tall versus factory 28: tires) to a 2.56 rear end with oversize tires (28.9" tall). [You are actually getting about 3% better mpg than you think because of the tire being the wrong size.] The latest info on that other forum suggests that a 3.23 gear set in wagons may get the best economy. If you still have the gold car, you might consider swapping the rear axle, tires, and PCM.

- A Motovac treatment by a competent mechanic will not hurt.

- Some folks have had good luck replacing O2 sensors (while a few have reported no improvement).


Several years ago when my friend was looking for an SS as a second car to his Camaro, I test drove over two dozen SS's over several months. I noticed a HUGE variability in the cars' performance. There is a lot of possiblities for the differences but one seems to be the transmissions. This brings me to two other recommendations for improving your economy.

- Change the trans filter fluid and filter. [There is a huge debate as to whether to flush a trans and completely change over the all fluid including the torque converter versus the factory method of just changing fluid in the pan and the filter. At least do the factory recommendation.]

- Get a custom tune. PCM Performance (www.pcmperformance.com) did mine and it was worth every penny. [They were also one of the least expensive places.] This added about 1-1/2 to 2 mpg for me and a huge performance increase. They advance timing and adjust fuel maps to make full use of whatever fuel octance you specify [87 octane for me] and raise the trans line pressure to extend trans life.
[If you want to tinker with it in the future, consider an OBD-I conversion. Basically they modify a '94-5 PCM to plug in and work in a '96. This was done a lot a few years ago before the tuners had really perfected OBD-II programming. It is still a viable option today because you can get a adapter to laptop for about $50-$80 and software is free to about $100 versus several times that for an OBD-II setup. You also have the benefit of a second PCM if you screw up something in the other PCM.]
 

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Discussion Starter #3
Thanks for the infomation.I still have the gold RM,and planned on changing just the pcm next.Do you think I will have to change the rearend and pcm as a pair to adjust for the difference in gear ratio.I havn't found any information on how the canister and fuel tank work together,but it's been a problem before i bought it as they had an aftermarket fuel cap on it and I changed tthat out with a factory cap with no difference.The trans has had regular service and the fluid is clean.(1 owner dealer serviced full life)No slipping.But the performance is sad more like 180hp instead of 260.
 

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Thanks for the infomation.I still have the gold RM,and planned on changing just the pcm next.Do you think I will have to change the rearend and pcm as a pair to adjust for the difference in gear ratio.I havn't found any information on how the canister and fuel tank work together,but it's been a problem before i bought it as they had an aftermarket fuel cap on it and I changed tthat out with a factory cap with no difference.The trans has had regular service and the fluid is clean.(1 owner dealer serviced full life)No slipping.But the performance is sad more like 180hp instead of 260.

I don't think that changing the PCM will make any difference. I was only suggesting changing the PCM to get the speedo correct after the axle change. It won't blow up without changing the PCM, just the speedo and shifts will be off. Changing the PCM is simple compared to the axle change, so why not do it.

You aren't getting crap for performance because you are running the most non-performance axle and oversized tires.

The evap cannister takes the expansion vapors from the tank and "holds" them. Periodically [start up, other?], the engine sucks the vapors out to burn them. (I know it does more, but thats all I can up with on the spot.)
 
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