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. I also posted to the general tech board but got no reply.

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. I don't have the stock pipe anymore. Last year, the mileage plummeted, it was running extremely rich, and even backfired at high speeds 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 3 weeks ago
- installed resonator in place of exhaust last week

Now, the resonator got rid of the backfiring/popping, but it still smells rich and the mileage isn't where it should be (~23MPG instead of 30)

I spoke to a mechanic about a heated O2 sensor and he said it'd be useless as all it does is heat up quicker.

Also, 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!
-Scott.

 

my car runs really rich to but mine misses and stuff i just changed my plugs before and it was fine

 

There's a chance it could be the plugs and/or O2 sensor cos my numbers may be wrong as to the time when I changed them (i.e. 2 years instead of 1). I do store the car every winter for roughly 6 months and so the car does sit ... maybe the plugs could be gunked up.

My mechanic suggested that the headers may be the cause of decreased mileage because the power/sound increase is the trade-off for mileage. I'm not so sure cos I know that a more efficient car will also get better mileage.