adjustable map sensor

1/11/12 Adjustable MAP sensor

1/9jeep-xj.info/HowtoMAPsensor.htm

Adjustable MAP sensor

The MAP sensor ( Manifold Absolute Pressure ) has a 5 volt input and then reduces this

voltage according to throttle opening, altitude etc and sends the signal to the ECU. So by

supplying a higher or lower voltage, the return signal is varied by the same amount. This

can be used to fine tune the air fuel ratio at throttle openings over 80% and before the

engine is up to temperature. This can be an aid in getting better fuel economy under those

conditions but I have also used it to stop engine pinging after fitting a stroker engine for

a customer. Many stock engines also do this when running lower octane fuels. So what I

did was follow Dino's write up and take 12 volts and run in through a potentiometer and a

voltage regulator so I could adjust the input voltage to anything I wanted. If you supply a

higher voltage, the effect is that the injectors stay open longer so it is richer and vice a

versa. You can however only tune from 80% throttle opening to wide open throttle when

the ECU is in open loop mode. This is because below that the ECU will keep it as close as

it can to lambda 1 or stoichiometric which is 14.7 parts O2 to 1 part fuel. At WOT

throttle though it ignores the o2 sensor and relies on the map sensor to set the length of

time the injector is open. The other time is when you are running cold before the

thermostat has fully opened where the O2 sensor is not read either before it goes into

closed loop mode. This is a big reason not to run a lower temp thermostat as the ECU will

be waiting for it to reach operating temp but it never gets there if a lower one is used

effectively keeping the ECU in open loop mode where is does not read the o2 sensor.

Part list needed below which can be bought from Radio Shack, or here in Australia, Dick

Smith or Jaycar:

Multi-turn (15) Cermet Potentiometer 1000 ohm

LM317T Adjustable Voltage Regulator

220 ohm metal film resistor

Mini Terminal Strip

Heat sink for Regulator

SPDT toggle switch



You start with the regulator by bending the two pins outwards and then down so it can be

inserted into the terminal strip. I had to extend them by just soldering on some wire cut

from the resistor. It is then placed into the terminal block No. 2, 3 & 4. Only No.4 should

be screwed down at this point and the other two when the resistor is added.



The potentiometer is added using only the last two pins into No. 1 & 2. The resistor is

added to No. 2 & 3.

Time to add it to the box you wish to keep it in. I just used a small one sold at the

electronic store. I used a rivet from the other side to hold the heat sink to the regulator

and keeps the hold thing in place too. You will see also in the next shot the toggle switch

has also been put in place. The grey wire at No.1 goes to earth on the firewall. The purple

wire in No.3 runs the modified voltage to the switch. No.4 has the 12v input.



You can see here how the purple wire from above has been terminated at the switch. The

second purple wire to the centre of the switch is the return voltage be in modified or

stock voltage depending on the switches position. The dirty white wire on the last switch

position is the original 5v that supplied the MAP sensor.

Here above you can see how the stock purple wire with a white stripe has been cut that



went to the MAP sensor. This is the original 5v supply. The end coming from the harness

gets the white wire soldered to it and goes to one side of the switch. The other end going

to the MAP sensor has the purple wire from the middle of the switch soldered to it with

either the modified or stock voltage suppling the MAP sensor depending on the switches

position.

I found a good 12v source right next to the ECU and washer filler at a plug that was not

being used. This is the diagnostic port and is ignition only which is what you want and

already fused. The box I mounted next to the bonnet hinge to keep it away from heat and

water.



You can see above how it fits neatly there. Now for the tuning part. I had it easier than

most due to the fact that a good mate of mine Peter from FMA car and truck repairs lent

me his wide band exhaust gas analyser so I could test the voltage I needed to run a full

throttle. The picture shows what the ECU has it set at idle. I set mine at around 12.7 at

WOT which is considered safe and not wasting fuel or having it too lean to burn valves

etc. If you don't have a meter you could get very close just adjusting until pinging is just

heard and then richening it back up a bit more or vice a versa if trying to get rid of

pinging in the first place. Below will give you an idea of different settings.

A/F

RATIODESCRIPTION

6-9 EXTREMELY RICH: Black smoke and low power.

10-11VERY RICH: Some supercharged engines run in this range at full power as a

means of controlling detonation.

12-13 RICH: Best power A/F: Un-supercharged WOT.

14-15CHEMICALLY IDEAL: At 14.6 the A/F is at the theoretical ideal ratio with no

excess fuel or oxygen after combustion. Good A/F for part throttle cruise and



light to moderate acceleration.

16-17LEAN: Best economy A/F ratio. Borderline for part throttle drivability (worse

than borderline if EGR is used).

18-19 VERY LEAN: Usual lean limit (Driveability).

If you do not have one you can read the voltage from the O2 sensor to give you a rough

idea of the best setting. It will need you to have a nice long hill that you can hold it at

WOT to get some the voltages needed. It will fluctuate a lot so be prepared for it and it

is best to have some one read it for you. Remember you must have the engine up to full

temperature so that the ECU is in closed loop mode and reading the O2 sensor and that is

at the right temperature too.

These charts above will give you some idea of what to shoot for. All you have to do is

connect a wire long enough from the O2 plug to your digital multimeter inside where your

passenger can read it while testing. Just make sure that that wire never touches ground as

it will damage the sensor. The negative lead of the multimeter goes to ground so you can

read the voltage changes. You will find 4 wires on the O2 sensor. Two are for the heater +

& - and the other two are ground for the sensor and output to the ECU. The grounds will

be easy to measure and the two positives will have one at 12v for the heater and the other

will be at .450 volts when the engine is cold and in open loop mode will running. The chart

below shows a typical three wire sensor which uses the exhaust as the other ground.



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After reasoning operational temperature, the reading will begin to start changing from

near 0v to 1v. If this happens all is as it should. It should vary quickly and if not change

your sensor. If you do not get a change by the time your thermostat has opened, then

there is a problem and could be you are running too cold a one so it is staying in open loop

mode. The range is as follows: 3mV being the leanest, 1V being the richest. Part throttle

does not matter so much as the ECU always keeps it somewhere in this range depending

several on variables and inputs from other systems sensors. At Wide open throttle the

MOST desirable voltage should be somewhere between .850mV an .900mV Make

adjustments accordingly to reach the desired settings. Remember, the lower the voltage

the leaner, the higher the voltage the richer.

To give you some ideas of what to run here is mine and some others that have this setup

and what modifications that will influence the voltage:

Voltage Modifications Owner

4.25

Ford 24 lb injectors, 62mm T/B and spacer, Headers with

2.5" system, Ported head, 9.25:1 compression, JET stage 2

PCM, FIPK

Dr.

Dyno

4.65

2000 intake & injectors, T/B spacer, Adjustable fuel

pressure regulator at 47 psi, Headers with 2.5" system and

Bored 60mm T/B

Gojeep

5.15

Ford 24 lb injectors, 4.6L stroker, 65mm T/B and spacer,

Headers with 2.5" system, Ported head, 9.25:1 compression,

Crane 260/272, JET stage 2 PCM, FIPK

Dr.

Dyno

6.00

Ford 24 lb injectors, 4.6L Titan stroker, 60mm T/B,

Headers with 2.25" system, 9.25:1 compression, .420 lift

Clevite cam, Stock air box and filter, 1995 XJ with stock

39 psi fuel pressure. Engine only 200 miles old and stillGojeep



breaking in. Solved pinging problem.

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