help alltrac running bad!

my buddy bought an alltrac shell and it was completely gutted at the time, but came with all interior except no motor. so we swapped in a jdm motor from a full clip and he used the jdm wire harness, but extended the harness in order to fit. we have the motor running and all but, its running kinda weird. apparently the car idles fine, however when you reach 5k rpm the motor will make a loud bang sound and CEL comes on very breifly than off and the motor still runs. also the car runs very sluggish and once you past 0 psi or a little higher almost immediately starts to back fire. in addition the stock boost gauge in the dash doesn't seem to work at all correctly. we have installed a aftermarket boost gauge.

it seems like fuel cut symptoms, but how can fuel cut occur at such low boost. and we also disconnected the hose going to the TPS, to eliminate fuel cut, and that didn't work either it still back fires. also timing has been checked and it looks good. and compression has been triple checked and its good all across. switched ecu's with a usdm and same thing happens. so we are definitely out of ideas... :shrug:

You may want to make sure that when he extended the wires he used shielded wire for the o2 sensor, knock sensor and anything else that has it stock. I know that when people do 1jz swaps in the mk3 supra they extend their harness. They have to re-run any shielded wires completly from the sensor to the ecu or the car will sometimes have problems. I'm not sure if this applies in your case, but you may want to look into it.

Boost leak?

You say your factory boost gauge does not work correctly? Hmm.....
I am wondering if the sensor (or wiring) is defective? Is your gauge stuck at it's lowest point or at it's highest point? Maybe the sensor is sending back it's highest value regardless of the true pressure? That in turn is setting off the fuel cut?

I dunno..I'm just throwing that out there

yea the stock boost gauge seems to just move around as it wishes, like at idle sometimes it will slowly creep towards full boost, and sometimes when boost is there it will read vac. we're thinking the problem could be the wire harness. but praying that its not.

however when you reach 5k rpm the motor will make a loud bang sound and CEL comes on very breifly than off and the motor still runs. also the car runs very sluggish and once you past 0 psi or a little higher almost immediately starts to back fire.

Click to expand...

Hey Lor,

I had a similar problem a while ago when I changed my injectors, when putting everything back together I put the metal gasket inbetween the throttle body on the wrong way this inturn left a gap so way too much air got in lots of back firing when boost got to 0 psi, loss of power.

maybe an idea or not.

its worth a shot, i'll check that out. maybe my buddy put that in wrong as he does do some dumb things sometimes, like forgetting to tighten the tb hose and thinking that it won't matter, and not pluggin in the ecu when we first tried to start it up. hahahah.... do you guys think a bad O2 sensor could do something like this?

I do not think this is a bad O2 sensor problem.

Can you double check that wire that is getting the signal from the pressure sensor? I am thinking something got crossed up when you guys lengthened the harness. I mean...you guys had to cut and resolder all the wires right? You did solder them right? I hope you guys didn't use those butt crimp-on connectors.

yea we did soldered each and every one of them. and when we extended, we only cut and extended one wire at a time, solder, then taped it up as to not get it messed up with any other wires. but yea we will try to measure the signal. also the tachometer use to not work very well either and would sometimes move very slowly or not move at all. but lately its been working perfectly for some odd reason. i dunno we're thinking maybe a connection in the harness is not good enough.

On the 1st gen All-tracs you can completely remove the boost gauge and the car runs fine. If your boost gauge is acting that strange, it is a connection some where, start with the boost sensor that feeds info to the ECU and the factory boost gauge.

If the factory boost gauge is seeing huge variations like that, chances are the ECU is seeing the same thing.