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L28 getting horrible gas mileage.


ComicArtist

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I've exhausted all my ideas as to why my L28 is getting such shitty mileage. The only thing I haven't done is have the injectors flow checked to see if they're dumping too much fuel.

 

It's basically a stock motor, with a Pallnet fuel rail, stock injectors, an Aeromotive A1000 fpr, idling fuel pressure is set to 28 and gets to about 33-34 under throttle.

 

Besides basic mods like EGR delete, shaved N42 intake manifold, straight pipe exhaust, ect, the motor is all stock.

 

As to how bad of mileage it's getting, it took $15 bucks to make 36 mile round trip last night, which comes out to about 7mpg.

 

Any ideas? The car seems to run fine, it's not bogging from being rich or anything.

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The most common cause seems to be high resistance on the coolant temperature sensor circuit. TPS stuck on WOT and CSV leaking seem to crop up occasionally.

 

I don't have the cold start valve any more, I'm running the TPS from a KA24 throttle body and have never had problems with it before, but it's funny you mention the coolant temp sensor unit. It came unplugged when I was driving the other day and flooded the car instantly, it took a minute to figure it out but ever since I plugged it back up it's been getting horrible mileage.

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The cold start valve only squirts a little extra fuel when the car is started cold, say for 100 revolutions of the engine.  The main fuel injectors run off of a temperature corrected map that depends on the temp sensor.  (IIRC)

Edited by ttodhunter
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  • 2 weeks later...

I have three sensors on my thermostat housing. One is the single wire sensor for the temp gauge, one is the thermotime switch for the cold start valve, which I don't have so it's useless.

 

WHAT IS THE THIRD? I'd have to assume this is the bad one, it's a two wire plug and it's on the drivers side of the housing. When I unplug it the car dies immidiately and it won't start.

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Comic,

 

The amount of fuel is pretty sensitive to that CLT sensor. You noticed how it floods the car instantly when it comes off (you said it just dies, but it dies because the AFR tanks). This is because the ECU all of a sudden sees and infinitely large resistance which it determines to be a really damn cold engine & starts dumping lots of fuel to it. It will also run your car very rich if the connections are just corroded. The corrossion causes the ECU to read a higher resistance than is really there & then it thinks the engine is colder than it is, thus it dumps more fuel to it, not as much as when the connector fell off, but still too much. This could easily explain your poor mileage. Your plugs may be somewhat fouled now as well depending on how long it's been running rich.

 

The fact that the mileage problem occurred right after the connector falling off and being reconnected is pretty smoking gun like. Pull that connector, clean & then see how it does.

 

Len

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I think some of the KA24 TPSs have both an analog connection as well as a IDLE/WOT switch connection similar to the stock 280z tps. He may be running that. I'm not 100% sure on the dual connection but I think that is the case.

 

FYI My HellFire digital ECU accepts the bone stock 280Z tps switch or an analog TPS potentiometer using the same wires/pins. Just plug whichever you have in. I actually read the stock switch as an analog input & it just shows up as 0, mid range, full scale emulating IDLE/OFF IDLE/WOT.

 

Len

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Not quite, but close.  It doesn't send a signal, it just changes resistance with temperature.

Resistance changing with temperature is a signal ;) 

 

The CHTS is a critical sensor on the L-Jetronic setup.

 

The 240SX automatic has both a switch like the 280Z/ZX TPS (throttle position SWITCH in this case) and a pigtail for the TPS (throttle position SENSOR).  IIRC the three pin Bosch style connector is the switch on the automatic 240SX TPS - not sure if it's wired the same as the 280Z/ZX TPS.

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NewZed, on 10 Feb 2014 - 12:41 PM, said:snapback.png

Not quite, but close.  It doesn't send a signal, it just changes resistance with temperature.

Resistance changing with temperature is a signal ;)

But it's not sent.  Just to split the hair even finer...

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Haha... you guys are good. :)

 

I think for what it's worth, most of the time the signal (whether you think of it as read or sent) is usually wrong due to that pesky connection. If one were customizing the engine compartment anyway, relocating it may be something interesting to do. Depending on where it moves, it may change the way your EFI behaves slightly. However, it probably would only affect hot start operation.

 

Len

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