Car Trip Computer
Car Trip Computer

Car Trip Computer.

Parts required;

For a sequential multi-point or direct (triggers each injector nozzle independently) fuel injected engine (to tap into car computer connector for speed and cylinder one fuel injector).

Program will need to be modified for engine other than a four stroke.

Will not work on multi-point (they all spray their fuel at the same time or in groups) or single-point fuel injected engine unless program is modified .

Will not work on non fuel injected cars unless speed and fuel sensors are added and program modified.

Will only work on saturated injectors (one long pulse), will not work on peak hold injectors (main long pulse followed by multiple short pulses to keep injector open or normal current flow to open injector then reduce the current flow to hold injector open) unless a peak hold injector adaptor is used or program modified, otherwise readings will be erratic.
Refer to peak hold injector adaptor in Performance electronics for cars by Silicon Chip pages 108 - 111 (instead of 0.1 ohm 5w resistor, tap into injector lead in two places about 400 mm apart).

Will only work on fuel rails that run at a constant pressure, fuel pressure regulators that have a vacuum line fitted after the throttle body will not work as the fuel pressure will vary as the monifold air pressure varies (my 1988 Toyota 2.4L fuel injected 4Runner SR5 auto had this setup), a vacuum line fitted between the air cleaner and throttle body is OK unless the air cleaner is blocked or restricted. Fuel pressure will change slightly due to change in altitude or air pressure to maintain the same flow rate at the fuel injectors.
Unless you tap into the MAP (Manifold Absolute Pressure) sensor (output is a square wave/voltage in which the frequency/voltage is dependant on the vacuum in the intake manifold).

Car trip computer powers down after 10 sec when ignition is turned off to save car battery drain.

Over time the injectors may get dirty from carbon build up and or bad fuel, ECU (electronic control unit) should compensate for this by lengthening the pulse width (same fuel pressure and longer pulse width means fuel calibration will need to reduce to obtain the same flow rate).
Fuel injectors deposits can be cleaned by adding fuel injector cleaner to fuel tank or use fuel such as Shell V-Power or BP Ultimate or Caltex Vortex Premium (contains powerful cleaning agents to prevent the formation and remove existing deposits) or have them professionally cleaned.

Low fuel pressure due to blocked or restricted fuel filter/fuel lines/strainer in fuel tank or faulty fuel pressure regulator will cause erratic fuel use.

If MAF sensor located just after the air filter housing (mass air flow - a critical component for engine performance and power - determines the ratio of air and fuel together with other sensors) is dirty due to microscopic particles accumulating (buildup) on two exposed wires, (one wire is thin platinum and heats up when the car is running and the other is a thermistor (compensates for change in air temperature), the MAF measures the volume and density of the air by detecting how much the flow of air cools the temperature of the hot wire). Spraying these two wires and thermistor (IAT) clean carefully, do not touch, if the wires break the MAF is junk with MAF sensor cleaner (CRC) or electrical contact cleaner or electronic cleaning solvent (check if safe for plastics and leaves no residue) and leaving the MAF to air dry for 30 mins.

If cleaning the MAF makes no difference in performance and power, try unplugging the cable from the MAF (ECU uses calculated values from remaining sensors), engine check light may come on, if performance and power improves, MAF is faulty and needs to be replaced.

I had this problem, ECU fault codes 24 (inlet air temperature) and 31 (air flow meter), engine under load - poor engine performance/loss of power/hesitation, on filling petrol tank - calibration was erratic.
The two exposed wires inside one side of the tube and thermistor in the MAF were caked in red/brown bulldust from my trip to the Kimberley's and Pilbara plus has never been cleaned in 10 years. The bulldust is so incredibly fine that some of it gets through the air filter element. Cleaned up OK after a good spray with CRC MAF sensor cleaner. Removed the EFI fuse in the fuse box for 30 seconds to clear the codes and allow ECU to relearn. Hate to think how much bulldust the engine sucked in. May consider fitting a cyclonic pre-cleaner. The butterfly valve in the throttle body is caked in deposits and needs a good cleanout.
It will now be part of my maintenance routine each year to check and clean the MAF.


Toyota Prado MAF sensor, thermistor (IAT) on right.
Two exposed wires inside one side of the tube.
How it works
Thermistor (IAT) and two exposed wires need inspecting/cleaning every 20,000km or 5,000km in severe dusty conditions.

Other sensors to check if working correctly are (consult service and repair manual);

Functions
00 fuel mult, number of people in car, tolls, $ amount per person for fuel.
10 cents per km, tolls/number of people in car, $ amount for total km's.
20 distance travelled, time elapsed (motor running and/or car moving).
30 trip distance remainig, current time to destination, average time to destination.
40 battery voltage.
50 fuel used, distance travelled, current L/hr or L/100 km's (car stopped or moving), average L/100 km's.
60 km/hr, RPM, over speed value (when turned on).
70 over speed value and disable.
80 average injector on time, duty cycle %, ml/sec.
90 new or mod, cost/litre, amount left in fuel tank, display Range or current range (car stopped or moving), average range.
91 fuel used (used for calibration over 3 tank fulls).
92 fuel calibration value (start with 1600 for 4 cyl and 2400 for 6 cyl, my 1999 Prado is set to 2461, 3.4 litre 6 cyl engine).
93 fuel calibration on or off.
94 distance travelled (used for calibration).
95 distace calibration value (my 1999 Prado is set to 0255).
96 distance calibration on or off.
97 fuel reserve or unusable fuel.
98 not used.
99 not used.

Calibration
Distance:
Set distance calibration on (function 96).
Reset distance travelled (function 20).
Reset fuel used and distance travelled (function 50).
Set over speed value to 999 to disable (function 70).
Drive a few km's then enter km's traveled into function 94 then enter calibration result into fuction 95.
Set distance calibration off (function 96).

Fuel:
Start with 1600 for 4 cyl and 2400 for 6 cyl engine for fuel calibration value (function 92).
Set fuel calibration off (function 93).
Reset fuel used (function 50).
Fill your tank using the same pump at the same petrol station to avoid inaccuracies:

1. Fill at the maximum flow rate until the nozzle clicks off.
2. Wait 10 seconds, then continue filling at the minimum flow rate until the nozzle clicks off again.
3. Repeat step 2 two more times.

On consecutive refills, enter fuel used into function 91, which will give a correction/calibration value, which is then entered into function 92.
Reset fuel used (function 50).

Function 90, choose modify for consecutive refills, enter cost/litre and litres added to fuel tank.
Function 90, choose new for number of litres tank holds when full (very first fill or refill inaccuracies).

Fuel reserve:
Set functiom 97 to required fuel reserve value (mine is set to 10 litres).

Circuit diagram

Rear view

Diesel engine
In the near future will be updating 4WD to a common rail diesel, so will need to look into modifing circuit and firmware to work off the OBD II port with the ISO 9141 (10.4k bits/s) or CAN ISO 15765 (250k/500k bits/s, 11/29 bit) protocol.
Can use injection volume to obtain amount of fuel used, just need away of synchronising the readings.

Need to check injector seats and oil pickup.
Also need to check injection volume (5-12 mm3/stroke at idle) and injection feedback values for injectors 1 to 4 (+/- 3 mm3/sk or +/- 5 mm3/sk at idle) with techstream, take two readings, one with engine cold, and one with engine hot (observe readings for 40 seconds before noting result) to see if there going out of spec and need replacing and entering injector compensation code into ECU for each new injector.

The 90 series Prado, the OBD II port does not monitor engine parameters, only 120/150 series petrol Aug 2004+ (Jul 2005+?) and diesel Nov 2006+ (only models with 5 sp auto or 6 sp manual).

Or replace battery voltage input to sense fuel rail pressure.
Fuel rail pressure (OBD-II PID 23) can be as low as 20 MPa (2,900 psi) at idle and as high as 160 MPa (23,206 psi) at the highest rpm/load (redline), so fuel pressure will need to be included in calculations, read pressure sensor, range (0 - 190 MPa, 1.4 - 4.44v) at idle (18.75 - 50 MPa, 1.7 - 2.2V).
Subtract the voltage with engine not running (1.4v) from the voltage at idle and use that and voltage with engine not running as references.
Subtract 1.4v from pressure sensor voltage and multiply by 62.5 to get pressure. Subtract 1.4v from pressure sensor voltage and divide by reference to get change in pressure, multiply this by fuel used result to obtain total fuel used.
Injector duration (on time) varies only a small amount from idle to redline.
A "pilot" injection (to lower engine noise, vibration and NOx emissions) plus up to an additional two or more injections per stroke will also need to be included in calculations.

Alternatives.

AVR based fuel consumption gauge.
OBD II Car Computer.
Automon.
Detect 9 OBD protocols to RS232 interpreter chip.
Supported car OBDII protocols by manufacturer.
K-line communnication description (ISO 9141-2).
Vehicle on-board diagnostic.
Global OBD vehicle communication.
OBD-II PID's.
CAN protocol chip - controller area network.
Scangauge.
J1939 C library for CAN-enabled PICmicro.
OBD-II ELM327 compatible AllPro adapter.

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