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Old 04-08-2018, 05:10 PM   #1
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Join Date: Jul 2005
Location: troy, Ohio
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95 Geo Prizm poor gas mileage fixed!

Put a used tranny in this one- drove it 200 miles- but was getting low idle and absolutely pitiful gas mileage- about 7 mpg - 71 miles on a tank of gas!! I even went so far as to set up the idle stop to get it to idle faster. It was running really rich- would blow out black smoke and blacken the spark plugs bad.

Finally found the issue where the vacuum hose was pulled off the MAP sensor on the firewall near passenger side. Plugged in the hose while running, idle increased about 400 RPM, and now I had to set it back to where it was before the tranny change. What was amazing throughout this entire ordeal was that there was never a service engine light!! It was not so much that there was a vacuum leak, as that the ECM never was able to properly read engine vacuum and adjust injector pulses accordingly....Live and learn and pass it on...
Automotive A/C Engineer with:
'99 IH 4700 Toy Hauler
(2) '95 GEO Prizms both maroon
'99 GMC Yukon
'95 Chev 3500, 454 Dually Crew Cab- 145k miles-
Wife's Camel trailer puller.
'94 Astro- 370k miles
'94 Firebird Formula- 5.7L 180k miles- gone-
'92 Chevy Lumina Van 3.8L 264k
'86 GMC S-15 - 2.8L 154k
'87 Buick Park Ave . 187k
'86 Buick Park Ave 3.8L 199k miles- gone
'77 Chevy Vega- 2.5L 175k miles gone but not forgotten
'68 Camaro 396 4 spd RS/SS -72k miles-

Last edited by brcidd; 04-08-2018 at 08:02 PM.
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