1992 AE100 High fuel consumption

[pavithra.kulasekara]

I have fuel consumption issue with 5A-FE engine. 1992 Corolla AE100
Manual transmission 5 speed
No check engine light or error codes

symptoms:
high fuel consumption. 8kmpl in low traffic, 10-12kmpl in long distance. without AC, light foot
idle warm engine - showing water steam out from tail pipe, accelerate - slight black smoke, cold start faint blue/black smoke present while pickup. no noticeable smoke after warm up and cruising

Tested:
Coolant temperature sensor (0.3V at 90C)
MAP sensor (3.6V at not running engine, 1.5V at idle)
TPS VTA-E2 600-Ohm when throttle closed, E2-IDL - on
Oxygen sensor - no any voltage detected even at fully warm and 2500rpm - replaced
Thermostat installed
Engine cold start around 1000rpm, warm idle settled around 700rpm.
PCV valve cleaned. there was some oil inside throttle body
Fuel pump recently replaced (no effect on fuel consumption)
engine oil - 4mm level dropped from H level for around 2000km

1. New oxygen sensor installed. but engine running rich.
   Oxygen sensor read 0.9V continuously while engine over 2000rpm
   connect T1-E1 in diagnostic port and VF1 voltage continuously 5V while keeping engine 2500rpm. no 8-10 times fluctuate as manual say.

2. Tried with heated oxygen sensor - same result as 1 wire sensor even at idle.
   if I increase engine to over 3000 and then release throttle quickly, sensor reading dropping to 0.2V range that means O2 sensors are working well and respond to lean condition.

O2 sensor showing that engine running rich even at idle.

I tried disconnect each injector power connector at idle and every time engine slowing down equally.
Fuel pressure regulator not tested. but no change in engine if I stop vacuum hose
ECU inspected for faulty capacitors or other faults - not found.

what should I check?
Can be leaking injector(s) ?
or fuel pressure regulator ?

[rajaahmed96]

First check map, if car is giving black smoke on high rpms. Also tps should be checked at idle and wot. Also if possible use 1 bottle of liqui moly fuel system cleaner in full tank. Ignition timing should be in factory spec. For blue smoke seems that valve seals need to replaced, also check engine compression. 5AFE MT in good nick should give atleast 10km/l in local and 13-14km/l on long with AC.

[pavithra.kulasekara]

Thank you for reply.
I tested MAP with meter and results are - 3.6V (ground & PIM) at ignition on engine stopped. and 1.5 ~1.6V at engine running idle.

TPS read 600-Ohm at close. wot - 5 k-ohm

Ignition timing is set on advanced side. it may be 15-20 degrees BTDC (Distributor turned clockwise fully)

[aziz.rehan]

Maybe your engine is weak.that's why average is poor.

[humayun.chughtai.7]

Few days back, I checked MAP sensor values on my 7afe & it was around 1.35V at idle, on full throttle it went upto 3.6V.

[ch.talal]

Check fuel pressure regulator and also the pump. Pump might be generating more pressure than required.

[pavithra.kulasekara]

is there a way to check fuel pressure regulator without a gauge?
Pump replaced 1 week ago. nothing changed

[pavithra.kulasekara]

Update:
I did trick to MAP sensor.
Connect MAP output wire via variable resistor and reduced voltage.

If output voltage decreased below 1.35V at idle, Oxygen sensor output start to swing 0.1V to 0.8V every 1 second this means correct mixture.

If output of MAP increased (1.5V - 1.6V as stock sensor output) Oxygen sensor output remain 0.9V continuously

[humayun.chughtai.7]

this means your MAP sensor is faulty.

[humayun.chughtai.7]

Do upload a pic of variable resistor & a little detail about it & how you added it in the wiring.

This will help many other members as well.

[pavithra.kulasekara]

I upload shematic.

I will clean sensor and vacuum hose , valve that fitted on manifold. and try again.

[pavithra.kulasekara]

Here how I connected variable resistor to change MAP output voltage. I used 50K VR. but 25K to 100K will work.
MAP output pin is middle pin of sensor.
This may be work for 90' toyota

[Hassan1991]

No you need gauge, and yes you have to check injectors.

[smiling.tulips11]

What is the price of oxygen sensor and from where did you get it? Do the sellers offer any warranty since it could either be checked through propane torch & meter or while being installed in exhaust pipe.

[xenerking]

improving fuel average has always been an uphill task, companies spent billions on R&D for that...

[pavithra.kulasekara]

It cost 3500rupees here in Sri lanka approximately 20$. used one but I tested it with gas stove and there are voltage reading. in exhaust pipe, sensor read 0.9V while 2500rpm.

[pavithra.kulasekara]

you said your one had 1.35V at idle. I got 1.5V at idle. (1.72 at cold start and then drop to 1.5v) may be something wrong with amount of vacuum
it seems MAP sensor output voltage is correct (3.6V at no vacuum)
I suspect that something can cause MAP to output higher voltage. - lower vacuum
there is no leaks
tomorrow I'm going to clean filter that connect MAP to manifold

Update:
Cleaning of map hose and filter not changed anything.
After long idle, oxygen sensor started to lowering voltage and start to feed back. 0.8V - 0.1v every second. but continues 0.9V while increased rpm over 800

I suspect about low intake vacuum. this can be worn engine?
I bought this car recently. so I don't know about history

[aziz.rehan]

Do dry and wet compression test on all cylinders