Engine Tuning 


Disclaimer : - for stock EFI system, actually there is no such thing as "tuning".


The title is used just to attract readers :)


What is shown is simply making sure / setting back the "mechanicals" to their factory default position(s) so that the EFI program [software] (already stored / fixed and cannot be changed) in the ECU (electronic control unit) / EMS (engine management system) can function like new / as what the factory wants. 


Whether it is racing optimised or not is another story.


The mechanicals that the EFI system rely heavily on (in this case) are: - 

  1. Throttle Position Sensor (TPS) vs. reference Butterfly position (for multi throttles case).

  2. Distributor Timing.

  3. Vane Air Flow Meter's bypass valve (for AFM L-Jetronic EFI Systems only).

  4. 4 throttle butterflies' synchronization.

  5. 4 throttle's by-pass valve (4 throttle's vacuum synchronization by carburettor balancer) as shown by the red circles below.

  6. etc



The system or mechanicals had to be messed / disturbed because the engine idles around 1,500 rpm and is very noisy, fuel consumptious and annoying.

  1. Compensate using the AFM's by-pass valve to reduce to 900 rpm, but it will shoot back up again until around 1,500 rpm after actual driving or some times shoot back up immediately.

  2. Compensate using the 4 throttle's by-pass valve to reduce to 900 rpm, but it will shoot back up again until around 1,500 rpm after actual driving or some times shoot back up immediately.

  3. Compensate using distributor, cannot work as the engine must be set between 8 to 12 degrees. According to 4AG-E 20Valve Engine Repair Manual. Want to buy one from me?

  4. Suspect vacuum leak, but didn't find any.

  5. Suspect Idle Speed Control Valve (ISC Valve) damage but not sure.

  6. Suspect AFM damage but not sure (changed it, no difference).

  7. Force close ISC Valve, and set the AFM flap tension to fool the ECU, but not satisfied with this non-fundamental solution. It doesn't solve the problem from the root. 

Fuel economy is bad around 30 litres for 250km distance. Not good at all.


For this case: - Log of mechanical setting(s)




23rd March 2003 Sunday Evening - at Ah Leong's house (my mech.)

1) Ah Leong mess with the TPS position (without permission, I made a fuss of complain, but in matters of seconds, I stopped, cause it worked.)


Now idle better around 1,100-ish rpm instead of 1,500 rpm

When driving the car feels extremely dull, engine don't want to rev up no matter what, very slowly up anyway.


Come to think of it, may be when the car was sent to DMT Racing Development for the 4 butterflies' synchronization, they re-position the reference butterfly wrongly???. Regardless, before it was sent to them it was around 1,300 rpm and after was 1,500. Both was simply not acceptable.




24th March 2003 Monday Evening - at Ah Leong house (my mech.)

1) Ah Leong mess with TPS again, now idle perfect around 900-ish rpm

2) AFM's Flap tension reduce allot - until FLAP just about to open even without air flowing in. (i.e. one more notch and the flap will open)


The flap is now open. This would be the idling average position. 


Therefore imagine  the "Tension Lock" is released and turn the gear anti-clock-wise until OPEN, without air flow, then increase the tension gear one notch at one time until it is fully closed as shown in the bigger picture above.


3) AFM by-pass valve up around 3 rounds from bottom according to Ah Leong's statement.

Drive feel - engine rev up very light, effortless - AFM Flap tension too soft, when reach 80 km/h and shift to 5th gear and FLOOR it suddenly, the FLAP slams open and hit the end of the passage. The metal knocking / slamming sound can be heard within the car compartment at 80 km/h 5th gear. (i.e. it bottoms out)




Side Notes too long, so put in another page.




25th March 2003 Tuesday Morning

1) AFM's Flap tension increase 1 notch

Drive car - feels better, but AFM's Flap still slams open and bottom out, effortlessly. :(

26th March 2004 Wednesday Morning

1) AFM's Flap tension increase 1 notch

Drive car - AFM's Flap still bottom out but not often


29th March 2003 Saturday 5:40pm

1) AFM's Flap tension increase 1 notch (notice rpm drops a little below 900)

2) AFM's by-pass valve turn a little 1/2 or 1/4 to compensate (Oooppppsss, forgot upwards or downwards....sorry :)   )

Drive car - feels better, engine still heavy, rev slow up, power band only starting around 3,000 rpm.

Notice engine suck in air fast, as accelerator slam down, but air suck in too fast, engine cannot coupe, like distributor timing degree not fast enough to catch up. 

Have a sudden power burst conditions, causing driver and passenger to jerk / plunge forward and backward. Completely not smooth driving, as if the driver is a dick head, although the accelerator is kept constant. (like sudden release of clutch or sudden increase and release of accelerator)




Side Notes 2 too long, so put in another page.




29th March 2003 Saturday 6pm (best setting)

1) AFM's Flap tension increase 1 notch

2) AFM's by-pass valve to compensate - seems very low, near the bottom, like 1 round to bottom.

3) Timing Light Gun on to the distributor - was low around 0 to 5 degrees but now average jumping around 8 to 10 degrees, suspect low 10 degrees should be 8 degrees.

On this day idling perfect at 900-ish rpm 

Done 6:30pm

Drive car - feels great, engine light, rev fast up, feels powerful, response well. 

Rev below 3,000 rpm still dull, rev above 5,500 rpm will slowly rise till 7,500 rpm. Not as fast as from 3,000 rpm to 5,500 rpm.

Power band 3,500 to 5,500 rpm (very quick and extremely responsive)

Sudden power burst still around a bit , no knocking / pinging.

30th March 2003 Sunday afternoon

1) Increase AFM's Flap tension 1 notch

Start car - rpm drop to 500 and going to die, choking and coughing, but never die

2) Increase 4 throttle's by-pass valve around 2.5 rounds up from bottom to increase rpm

3) AFM's by-pass valve lock down 1/4 round

Idling rpm was 800 after compensation and start engine + rev a little.


Drive car - feels dull, rev slow up - must revert back to yesterday's settings.

Sudden power burst all gone

Can hear some knocking - must be AFM's Flap open not fast enough, while distributor timing advance too fast due to accelerator pedal (i.e. TPS signal) therefore not in sync with AFMs' response???


Same day evening


The settings were reverted / restored according to this log and was successful to achieve OPTIMUM feel of power as in 29th March 2003. Very lucky? no, this is systematic approach :)






Power feels fine and all, was not my major objective. Fuel economy now 40 liters get me a good 400km distance travel. This is around 75 to 100 km distance increase, which is a far cry from before. Now you know why I wanted to get fundamentals correct :).


The engine oil was changed  to Mobil 1 fully synthetic 5w-50 around early April 2003 - guess what? 


It took the sudden power burst away :) the car now feels very normal. However knocking is present a little. Can be provoked anytime. Good and bad.


According to Bill Sherwood, since local fuel are 97 octane only, they are bound to knock, since these engines were meant to use 100 octane. Reducing the distributor timing is one of the way to solve the knocking problem, but sacrificing early power output.




New findings is available in Engine Tuning Part 4,  not everything mentioned above is / are 100% correct / wrong.


  1. Engine Tuning

  2. Side Notes

  3. Engine Tuning Part 2

  4. Engine Tuning Part 3

  5. Engine Tuning Part 4

  6. Engine Tuning Part 5

  7. Engine Tuning Part 11



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