two problems, need help please
 

i have a 93 rx-7 with 68k miles and the only mods i have added to it has been a downpipe and an apexi exhaust. after the installation and a couple of months, the engine has been backfiring alot. have no idea what it could be? any suggestions?

theres also another problem: when the engine gets to about 3500 rpms, the engine start to lose power and studders. what could be the cause of these two problems?

thanks everyone.

When's the last time you changed your plugs? Worn or foul plugs can cause alot of backfiring and a lost of power.

As you open up the exhaust flow on these cars, some backfiring will occur. However, an excessive amount is not normal.

Perform the normal car maintenance if you haven't yet:

* Change oil/filter
* Change air filter
* Change spark plugs and check to see if plug wires need to be replaced
* Change fuel filter
* Flush coolant
* Check and clean grounding locations (as well as battery connections)

If you get real ambitous, also change the tranny and differential fluids.

See how the car behaves after having the normal maintenance performed. If these things have recently been done, let us know and we'll see if we can offer some more suggestions.

Make sure your O2 sensor is still plugged in at the harness.
After putting my UIM back on last time, I forgot to connect the O2 harness and the thing was backfiring like never before.
Your 3500 problem sounds like insufficient grounding. Add some and upgrade all the factory points.

Raj

mahjik ive changed all that and i just put in a new o2 sensor in last week.

the only thing i havent done is to check the grounding points. but why would that affect it?

that 3500 studder sounds like the common 3000 fd hesitation. I know that on my first fd it did it and we changed the ground that looks like shit going to the firewall on the passenger side with a better thicker gauge wire and i think my hesitation stopped or least got a lot better.

how much boost are you running? could be ignition breakup, could be fuel cut could be many things...

You dont have a loose vacuum hose do you? Mine had some of the same problems one day. I looked under the hood and I had a vacuum hose not attached. Put it back on and works fine now.

i will check the vacuum hoses and look into a new ground wire.

thanks to all for the help, ill post any successes