1991 GTA (305 5.0 TPI) - Poor Mileage, Running Rich, & Backfires too! - Third Generation F-Body Message Boards

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1991 GTA (305 5.0 TPI) - Poor Mileage, Running Rich, & Backfires too!

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Old 08-13-2008, 03:00 AM   #1
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1991 GTA (305 5.0 TPI) - Poor Mileage, Running Rich, & Backfires too!

Hi everyone,

First of all, I'm posting this after reading about 30 pages of threads, so I really hope I'm not repeating what's been discussed (and sorry for the length) but I think this is unique.

When I bought my car a few years ago, it ran perfect and got nearly 30MPG hwy. I took off the stock manifold & cat, and disconnected the smog air pump, then installed shorty headers with 2.5" pipes and kept no pump and no cat. Last year, the mileage plummeted and it was running extremely rich so here's what I've done:

- replaced o2 sensor 1 year ago
- replaced PCV valve ~1.5 years ago
- replaced fuel filter this spring
- replaced EGR 2 years ago
- checked for vacuum leaks (none)
- stock thermostat
- ran tests with with Snap-On scan tool ... everything appears good
- replaced stock distributor cap with MSD distributor cap this spring
- installed MSD blaster coil this summer
- checked/gapped plugs last summer
- installed 8m blue streak wires 2 years ago
- tested fuel pressure regulator this summer (good)
- replaced injectors 1 year ago
- replaced ECM with working ECM from another car this summer
- replaced PROM with new stock PROM last week

Now, the mileage is a little better (~23 MPG) but it still smells rich and proof of the richness from two other weird happenings:
1.) when I maintain 130km/hr (I think that's 75MPH) for a few minutes, the SES light will come on and fire an EGR code (but it's done that ever since I replaced the EGR valve so I know it's a lie)
2.) when I step on it (past 75MPH) then immediately let off the gas, it backfires at the exhaust, which makes me think there's so much excess gas in the pipe that it's igniting.

I read some other threads that suggest a thorough analysis of the sensors, and also that without a cat the o2 sensor isn't getting enough built-up heat. I do have a DynoMax cat from my previous '86 TA that I could install. But before I go ripping up my pipes or dumping more money into sensors, I wanted to check with you.

I've talked to 4 different mechanics and they can't see anything wrong up front because it passes the (limited) OBD-II scan test and runs at perfect idle ... but I know you guys are smarter ;-)

Can anyone help?

Thanks in advance!
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Old 09-02-2008, 03:05 PM   #2
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Re: 1991 GTA (305 5.0 TPI) - Poor Mileage, Running Rich, & Backfires too!

UPDATE: I have installed a resonator in place of the cat to create backpressure. The backfire/pop is now gone but mileage is still poor and the car still smells. I spoke with a mechanic and he said that a heated O2 sensor will do nothing as it only gets hotter faster and nothing else.
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