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4E-FE Overfuelling - broken intake earth


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Hi guys I'm an ex-mechanic looking for a bit of help with my girlfriend and mines new purchase. 

'98 Starlet GLS, 4E-FE N/A 

So the car runs fine except that it's haemorrhaging fuel like there's a hole in the tank (checked, it's not that lol).

the dude who owned it before us stuck a K&N cone filter on it and in his infinite wisdom tagged up the plugged in Air Temp Sensor (THA) to the back of the engine bay in a plastic bag. after noticed the oil rebreather was pretty clean I replaced the pipe with the old sensor grommet and fitted it to the filter. this sorted out the fueling and we had about 3 weeks driving on a single tank, life was good.

about a week ago now I noticed it was sitting at 1500 idle rpm again when cold, drank half a tank in a 50 mile trip, started hunting for revs when the clutch is down and you can smell the richness. it's lost a lot of power too but again I guess this is due to the incorrect fuelling. 

I've shorted the diag pins TE1 and E1 and checked for any known issues. none picked up. I've also checked the continuity in the wiring between the intake air sensor and the ECU pin 5, and that is fine. the resistance in the detached sensor is 3 Ohm, I dunno if that's correct but didn't seem too wrong from past experience. 

I'm running out of ideas fast, can any of you guys and girls suggest anything I've missed before I start trial and error testing? 

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Edited by Bliitzer
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okay I've sussed it now to be a break in the ground wiring for the sensor as the ECU common ground shows continuity to the battery but the brown with white stripe wire (sensor earth) has an open circuit between it and earth. I checked a really good wiring diagram found on this forum and it looks like there's a common intake earthing point, does anyone know where it is located? the wire goes through the bulkhead and I don't have time now to strip out the glovebox and dash. thanks in advance

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I've snipped the earth and connected the sensor earth to ground at the point where the gearbox is earthed in the engine bay. I'm really unsure as to whether this has solved the problem as it still smells rich but doesn't sound as aggressive. can anyone suggest if this should sort this issue or if there's something I'm missing here? going to leave it as is for today and see if the fuel is still being guzzled or not

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There are two earth wiring arrays in most 4e/5e looms.  One is for power grounds which supplies to things the ecu switches (injectors, idle ups etc) and is common to chassis ground, its the E1 pin on the ecu from memory.  There is also a sensor earth array that runs to the map sensor, water temp sensor, IAT, knock sensor shield, TPS, and narrow band sensor shield, from memory its in E21 on the ecu.  The sensor earth array doesn't connect to the chassis ground and the two should be kept seperate.  The ecu supplies the earth to the sensors and filters out electrical noise which is why the two are not connected together.  

Its very hard to work out what effect the AIT sensor has on a factory ecu as you can't plug a laptop in and see the tables, but its my theory that at higher air temps it pulls some timing out to avoid detonation, and it doesn't effect the fueling map all that much - maybe a small % trim if anything.  Thats generally how an AIT sensor is utilised in an aftermarket ecu. The sensor that can make a huge difference to fuel economy is the water temp sensor so I'd be checking that first.  You can dump it in a pot of water on the stove and work out that the resistance is changing as water temp increases from cold to hot, it should be relatively linear output - you may actually be able to find that data if you google it.  Anyway, if the ecu thinks the engine is cold all the time then it will be dumping a metric shit ton of fuel and there goes your mileage.  

Often with these sort of things its a combination of a couple of different faults that all compound to causing a significant change in tune, and often the ecu just thinks the engine is at idle/cold for example so wont throw a code. So id check that water temp sensor for starters and its wiring back to the ecu - although if there was an issue with the wiring the ecu would show a code and in my experience with 4e/5e's its also causes very hard starting.  Id also double check the TPS is outputting idle and WOT to the ecu. Plus check the map sensor is outputting the a variance and is plumbed right.  

 

Good luck.

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thanks a bunch for your reply mate, there's a lot of good info in there for me to work off. It's still got the standard ecu and it seems better now than before but it had a blip earlier, that may however be because I've earthed it back to the chassis so it's not filtering the noise correctly like you previously mentioned so I'll take a look at that too. I'll update this with my findings

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for reference, these are the pin references I eluded to earlier in the thread - I tested the earth at the THA connector back to pin E2 on the above map - single brown wire on the 26 pin plug - and got no continuity, again making me believe it's the earth for that sensor. I will be double-checking the rest of the sensor loom now too after reading your reply

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Edited by Bliitzer
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