ok i got a real head scratcher here guys. 87- 6port- manual trans. starts up cold and runs fine. after it runs itself thru the warm up cycle and the rpm go to its normal 750-800. it just DIES. no fuses are bad. i got a lil bit of fuel in it.(i mean its almost on E but ive seen it lower. i run midgrade by the way). it seems like its a fuel issue to me or wiring. i no there is no limp mode for an s4. and wen i start it up again after the first initial start up, ill give it a tiny bit of gas. . (less than 1/10 throttle so i dont accidentally flood it) itll just bog down and die. i am a mechanic but my weak side is diagnosing **** of this nature.

-any ideas at what i should look for would be helpful.

-its really wierd tho that itll start up fine cold until its done warmig up. i mean if it didnt start up at all i probably wouldnt be asking for help.

--oh and i forgot to mention, the day before i was poking around with the console switches(highbeam) trying to put a separate switch in to keep the flash to pass lights on just as daytime driving lights like if it was cloudy out. i did accidentally pop a turn signal fuse cause i couldnt find my mazda book with wiring schematics in it. i replaced the fuse.

 

Will it stay running if you give it enough gas? How do the spark plugs look? Have you adjusted the TPS? Did it just start doing this, and if it did, how did it run before? Have you messed with timing at all? What does it do if you run it with the fuel pump check connector jumpered? (that forces the fuel pump to stay running) Get reading in the FSM and also check the variable resistor (located behind the passenger headlight). Is the initial set coupler still jumpered?

 

Check your accelerated warmup system components. Check that the thermowax pellet is not jamming the throttle cams shut. Check for vacuum leaks around the intake manifold.

-Bad or missing tps wires or any wiring related to idle control would be evident on a cold start as well as warmed engines.

-Another thing to check is the BAC valve. Jumper your check plug to the negative battery
and see if it clicks on and off.

-Just remember to always disconnect the battery when doing electrical work. The only thing I could think of that's linked to the fog or headlight wiring would be the electrical load sensor in the ECU. Hopefully that's not damaged.

 

^ what they said. More specifically, I'm thinking TPS adjustment or vacuum leak.

Wouldn't be load sensor or anything fancy. GL!

edit: press in the plunger on the TPS in a little next time you know it's gonna stall as it's warming up. I have a feeling the thermo/fast idle/start up system starts closing the throttle (TB - air), and the computer is still giving more fuel than the engine needs.

 

1) no it wont stay running wen i give it gas.
2) didnt think to pull the plugs only cause it seems to fine on it warm up cycle.
3)didnt adjust the tps. i dont think it woulda moved anywhere. the car was running mint the day before.
4))yes it just started doing it. right after i was trying to put that flash to pass switch in. i musta hit a wrong pin on the headlight plug but it only popped a fuse wen it happened tho.. as far as i no.
5)i havnt touched the timing at all.
6) location of the fuel pump check connector?
7) ive never looked in the "FSM" before, wheres it at on the website.

 

just went out real quick to poke around. i used a pencil to push the tps the rest of the way in from were its usually possitioned and started it up. it reved up to like 4 grand then shut off i think it may be the BAC valve. i used my handy dandy steth0scope and put it on the bac valve to the left were there is the sticker with part#.. with key on engine off(obviously)lol. and its making a constant chatter-clicking sound.. is it possible from me tryin to wire in that flash to pass switch i some how fudged it up?

 

FSM= factory service manual
It can be viewed here: http://www.rotaryheads.com/PDF/2nd_gen/

The fuel pump check connector is near the airflow meter. It's a 2-wire yellow connector, sometimes it has a rubber boot over it.

When it just straight up dies, does the tachometer go to zero right away as the engine spins down, or does it chuga chuga chuga, approach zero, and then die because the rpms go so low?

It's looking more like you may have a Kentucky Fried Computer

 

The BAC is a pulse width modulated solenoid. That means the ECU opens/shuts it several times per second. The buzzing noise is good.

 

no cha chuga chuga chuga over here. just straight off. and as bad as a fried computer sounds i liked that kfc remark =) i might have one from my vert

 

wait nvm i just remembered my vert was an S5. aw poo

 

It may be a stretch, but see if the tach drops to zero before the engine completely dies. If you're losing the tach signal to the dash, that could be indicative of ignition dropping out (i.e. dead/dying coil). Most likely though, you let the smoke out of your ECU. Not sure how, but stranger things have happened.

 

2 guesses here.
check the Afm and also the water temp sender connection at the back of the waterpump.

 

i still think its a ecu. it makes sense cause i was stickin my nose in somthin i should just left well enough alone with the wiring lol. but ill still try it out tommrow morning. and if i need coils its somthing i have in stock lol. thanks alot tho for ur help. i was scratchin my ***** over it

 

Sudden issues like that which happen right after an arc is drawn on the car somewhere usually ends up to be something solid state that has given up. None of the sensors on the car that come to mind right now are solid state. At any rate, he says the car runs just fine until it gets to closed loop, then dies. You still need the AFM to start the car, and even with a thermosensor completely unplugged or shorted, the car will still run. As I recall, the ECU will see 0 or 5v on that sensor and disregard it, reverting back to a standard temperature (around 70F I think).

 

Pull the cover off the ECU top and bottom and search for burns and popped resisters.

 

update bump. finally got the right toole for checking ignition spark were u can put the set screw to a certain gap. leadings arnt firing.. did some researching and was wondering if the S4 alternator back connector is an 12v ignition source for primary coils? found a picture for converting s4 and s5 alternators and it says on the s4 the one pin is an ign switch. any ideas? and i did actually swap my alternator to an s5 w.o the help of that diagram