I have an 87' T/A, 305 TPI, auto, 2.73.

Just recently my mileage has gone down to 14 mpg from 16 mpg. It also seems to die after 4000rpm. Could my cat be clogging? The mileage wasn't that great to begin with.

What mileage do your TPI's get?

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1986 CAMARO Z28 ->
151,000 original miles, 43,000 on 89' vette 350 block, Holley 650 DP, Weiand intake, 1.6 rockers, 1 5/8" headers, no cat, 2 1/2" Flowmaster catback, Mallory coil, March pulleys, B&M Ripper shifter, T5 with 3.35 1st gear and 0 miles, 90% Centerforce clutch, 3.73 posi, Eibach 1" drop springs, Spohn LCA's, Spohn adjustable panhard rod, Subframe connectors, all bushings are urethane, 2 12" Sound Streams, 2 Infinity 6X9's, 2 Pioneer 6X9's, 800 watt Lanzar Vibe amp. ET 13.22@ 106.4MPH 3300lbs less driver and speakers Next mods-> Extreme Energy 268 cam, Holley street dom intake.

1987 TRANS AM ->
73,000 miles, 305TPI, auto, gutted air box.

 

City or highway? Mine gets about 24MPG highway, 15 or so around town since it idles alot at red lights. If that's the best it can do on the highway, especially with that rear you're running, replace the plugs, wires, cap, rotor and oxygen sensor, clean out the intake and check the TPS and CTS output, and see how it does then.

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, Accel coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Ruger P95DC, hot wife, new oil filter,
thick rubber floormats, towels in the seats.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

How's the T-stat? If it is sticking open, then you will be running too cool and this affects gas mileage. A weak CTS can to do this too.

Whens the last time a tuneup was done?

 

The engine is getting hot enough, it actually got better mileage when it was cold out than it does now. I am going to check the cap and rotor. The only reason I'm asking is because it was a sudden thing. Also, about a week ago I cut the belt on the smog pump not for power but because the belt was squealing, I have heard from people that it will not affect anything, but heard from others that it does.

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1986 CAMARO Z28 ->
151,000 original miles, 43,000 on 89' vette 350 block, Holley 650 DP, Weiand intake, 1.6 rockers, 1 5/8" headers, no cat, 2 1/2" Flowmaster catback, Mallory coil, March pulleys, B&M Ripper shifter, T5 with 3.35 1st gear and 0 miles, 90% Centerforce clutch, 3.73 posi, Eibach 1" drop springs, Spohn LCA's, Spohn adjustable panhard rod, Subframe connectors, all bushings are urethane, 2 12" Sound Streams, 2 Infinity 6X9's, 2 Pioneer 6X9's, 800 watt Lanzar Vibe amp. ET 13.22@ 106.4MPH 3300lbs less driver and speakers Next mods-> Extreme Energy 268 cam, Holley street dom intake.

1987 TRANS AM ->
73,000 miles, 305TPI, auto, gutted air box.

 

Definitely not the smog pump so you can rule that out. It only diverts air by the O2 sensor in open loop operation if it's working properly so it shouldn't really affect fuel mileage. Mine's been gone for almost a year with no adverse side effects. I would check the O2 sensor if it hasn't been replaced or you don't remember when it was replaced last.

 

Ditto on the O2 sensor.

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'88 IROC TPI 355, '69 truck 4-bolt, fully balanced, 9.75:1 compression, fully ported '70 truck heads(for now), Harland Sharp 1.6 Roller Rockers, Comp Cams CS 252AH-12, ported '86 TPI converted to speed density, Crane AFPR, Flowmaster 3in. exhaust, Edelbrock TES headers, all emissions equipment is still intact, plan on burning my own chips, many more mods and parts on the way as time and money permits =(
Best E.T. with TBI 305: 16.11 @ 82mph
Haven't been to track with new motor
If you have any questions on converting from TBI to TPI email me at jdkchevy70@yahoo.com
In memory of Dale Earnhardt, I'll miss cheering for that black #3 on raceday!!!