black smoke and back fires under boost
 

my friend has a 3 gen, black smoke under boost, back fires, and will no want to rev (wants to die out) if you rev it with out getting into boost then it's fine , only at 4 or 5 psi it will do this.

The first two symptoms are normal.

A few questions....

1 - is the MAP sensor hooked up with no tears in the rubber hose going to it? (Do a search on MAP sensor if you don't know where/what this is)

2 - Is the car getting choked with unburned gas? This can be caused by several things such as a faulty injector dumping fuel in or a blown motor where you aren't getting proper combustion so it runs crappy and blows raw fuel out the back.

What modifications does he have? Does the car idle smoothly? Does it start up fine? We need more information to even try to make a good guess.

Brian

are you running a pfs pms? Because when I had mine on there it used to backfire a bit.

Matt

the car starts fine, idles fine revs fine, but only under boost will the back fire and black smoke come out and the motor want to die like to much fuel had been injected (hints the black smoke)and we checked the map and the vac line to it and it is fine, tried a map off my dads 3 gen and made no diff. mods full turbo back exhaust, intake, intercooler, chip, dont know were the chip is from. thanx

you said you have a full turbo-back exhaust... that is a LOT of airflow for the stock injectors... could this be fuel cut due to 100% duty cycles??? I don't think this could be happening at 4-5 psi though....

Brian

He may be hitting fuel cut...Full exhaust and intake

its an RX-7 we backfire.

I hate to say this, but how old are your turbos? I have the same problem have you're having. While, I obviously can't be certian that your situation is the same, i'll tell you what's happening to mine.
My primary turbo, which I just bought used mind you, is shot. The seals are gone. I'm spitting oil into my intercooler like you read about. :rant:
I'm getting the same black smoke that you're talking about. I'm back firing like an old man after dinner as well. The black smoke is from running rich. I'm running rich because the ECU is expecting a certian amount of boost, thus sending the appropriate amount of fuel, but the turbo is gone, so it's not getting the boost to effectively use the fuel.
I'm not saying that this is your exact situation, but it is a possible one. May you not have the near unfathomable bad luck that i've had.
:banghead:

but doenst the ecu supply fuel according to what the map sensor tells it? If he is only boosting to 4-5 psi the computer should only supply gas for 4-5 psi because that's what the map sensor is getting at the back of the uim.

Good question.....
I honestly don't know the chain of commands that the cpu/ turbos follow. I can only tell you what was told to me when my issue was diagnosed.
It would appear in my case that no, the map sensor isn't making the adjustment. I can tell you that both times my turbos went, the same situation occured. I got a rich, backfiring, slow or low boost situation.
I'm sorry I can't be more specific. If I was actually thinking of keeping the twins, i'd be more on top of the situation, but since I got the bad news, I lost all patience with the car, and it's sitting in front of my house waiting for a single turbo.
I have some people I can ask to try to get the answer to your question though. I'll see what I come up with.

when it starts to back fire, the motor dose not want to rev any higher, thats when the black smoke comes out. i dont think its the turbos because they come on fast and strong, and the map is fine. my friend has ran on this mods for the last year. he has the fuel we live at 5500 feet above sea level. could the chip messed up?

When you say that the motor doesn't want to rev any higher, what do you mean? Does the car stall? Is it like fuel cut?
I suppose the ecu could be bad, but it's really tough to tell from what we're hearing here.
Could a faulty map sensor have anything to do with this?

we used my dads map sensor and it still did it so the map is fine, it sounds like it stops reving because there is to much fuel, thats why it smokes black and pops

Hey. I had a simular problem. My Rex would smoke a little on start up and at high RPM. This problem just got worse over a few months. So I decided time for its first rebuild. (78K) Come to find the outter oil control rings o-ring on the rear rotor was deteriorated and broken. It was held in place by the ring but not functional.

So for some insight. If it does not smoke at full boost at low RPMs but it does at High RPMs and a little at start up then I would say its the oil control o-rings

Oh yeah also the plugs will get fouled really fast (cause drivability issues)

Have you pulled the trouble codes? Is the check engine light on?

secondary injectors ok?

this sounds a lot like limp home mode, try resetting ecu

I initially thought the same thing, but if he is boosting to 5 psi, then it's not limp mode. Also, if it's limp mode, the computer should be throwing some sort of trouble code.

there are no trouble codes at all. if you just rev the motor with out boost will the secondarys come on? because then it could be a bad secondary injector, letting to much fuel into the motor making it pop and have black smoke come out.

exactly
I think once you go past 3500 rpms or something the secondaries come on even without boost

i dont think the motor need the secondaries just to rev (when not under load) that is to much fuel i think, that would be why is revs fine with out a load, so i think the second the map see the boost the secondairs come on and let to much fuel in and that is why is pops and smokes, so i think the secondaries are messed up some how.

I've seen a lot of people report similar symptoms when they're secondaries are screwed up.

According to the FSM, they come on ~3000 rpm depending on engine load. So freely revving the engine should not turn them on.