IGNITION TIMING IS RETARDED BEFORE IT IS ADVANCED ON 1.4 CIT

[Bronkie]

Hi guys - please help

I am trying to assist my son with his assumed standard 2006 1.4 citi golf. This is a carburettor model with a MAP sensor. In other words there is no vacuum or mechanical advance on the distributor.

The problem started off by the car not accelerating properly as well as being extremely heavy on fuel. I checked the timing with a normal timing light on the front pulley and discover that the timing gets retarded from idle up to about 1500 revs first before it is advanced as expected.

I connected a vacuum gauge on the manifold and a voltmeter on the MAP sensor output and it seems that the MAP sensor output follows the vacuum as indicated on the vacuum gage. At idle the vacuum gauge indicated approximately 19” of vacuum which I interpret as an indication that there are no vacuum leaks.

I read somewhere that the ECU is also using the temperature as an input and repeated the test after disconnecting the temperature sensor but the results were the same.

Does this means the ECU is faulty or can it still be due to a faulty MAP sensor?

[panic-mechanic]

This going to be very hard to explain without being able to draw pictures on a white board.
So firstly let's just be clear on 1 thing.
If you are using a true vaccum gauge you will see the 19" of vacuum at idle with the throttle closed and if you suddenly snap the throttle open you will see the vacuum gauge drop to 0 immediately in other words there is NO vacuum at full throttle and only then the motor will react as the vacuum action happens faster than the engines ability to pick up rpm.(you open the throttle faster than the rpm picks up)

So this ecu is basically coded in the following way.
1- it has a rpm based map which maps engine rpm to an advance curve
2- it compensates for temperature and commonly it will only add timing when the engine is very cold and from around 50 degrees or so will not do any compensation anymore. Ie that map is zero from there onwards
3- it has a vacuum advance compensation curve which adds timing numbers when there is vacuum.(which you should know now only exists when the throttle butterfly is closed or partially closed - no vaccum at full throttle right!!)

So let's build a little lookup table quickly.

Rpm curve might look like this.
Rpm - timing value lookup.
600 - 3
800 - 4
1000 - 5
1100 - 7
1300 - 12
1500 - 15
1800 - 18
2000 - 21
2500 - 23
2800 - 25

Now there is another lookup that says vaccum compensation but it would usually be absolute manifold pressure compensation but I will use vaccum as you seem to be comfortable with that. So basically all engines will have advance when there is vacuum and no advance when no vacuum. (Hence the term vaccum advance)

So that table looks like this.

Vaccum value in inches - timing advance value
0 - 0
2 - 0
4 - 1
6 - 2
8 - 3
10- 4
12 - 5
15 - 6
18 - 6
20 - 7

Ok so we take your sample. The engine sits idling at 1000 rpm and 19 inches of vacuum.
So for my example that lookup will result in 5 degrees from the rpm table and 6 degrees from the vaccum table for a total of 11 degrees of timing. I am assuming the engine is hot and it gets no value from the temp table.
Now you suddenly snap the throttle open This results in th same rpm lookup which is still 5 degrees but the vacuum has now disappeared which results in a 0 value from that lookup table so the total timing has now dropped to 5 degrees. Now the rpm starts climbing but vacuum lookup is still 0(you are holding the throtlle open which, remembers results in no vacuum) and now it follows the rpm value lookup which advances according to then rest of the rpm advance table.
So basically in short - yes it is normal and correct for the timing to retard when you snap the throttle open and then to advance as the rpm climbs.
As for the temperature - all those sensors are called ntc ie. Negative temperature coefficient means that the voltage drops as the sensor gets warmer. So when you inplug the sensor it goes 0 volt which to the ecu looks like hot so it does not affect the timing value.

[RJ8V]

Okay Panic so what will the cause be or where can one look for the fault since it is heavy on fuel and struggling with acceleration. Reason I am askong is that I have the same system in my 2005 1.4 carb golf and exact same experience of problems.

[panic-mechanic]

Does the accelerator pump work, is the timing you are shooting is correct. So basically get it on a dyno, set timing for best power, check air/fuel ratios and make sure the accelerator pump on the carb is ok and that the second stage on the carb opens.

[RJ8V]

Just curious, where on earth is the map sensor located in a new carburettor model?

[panic-mechanic]

Sorry i can't help you there. There only ones I have seen has the sensor built into the ecu unit ie. It has a vacuum line going into the computer box. Have personally not seen one with an external map sensor on a carb.
But then I have not played with citis in a long time.

[Bronkie]

Thank you Panic for a very detail explanation. What is interesting is that the timing is retarded for quite a while and not only momentarily if the carburettor throttle is open. The accelerator pump is working but I am not sure as to the volume it is supposed to pump.

I would very much like to hear if anybody else experienced this problem before as well as what the solution was. The problem is that this car still needs a lot of attention and we need to save where we can.

RJ the MAP sensor on the 2006 1.4 citi is, if you stand in front of the vehicle, just to the left of the booster and connected with a 5mm pipe directly to the inlet manifold.

[Golfdriver]

directly to the inlet manifold.
The map sensor should not be connected directly to the inlet manifold it should be connected to the rear port on the carburetor. I have seen various people try to modify the timing curve on these motors by connecting the vacuum line to different places ie the manifold, the front port on the carb and to the brake booster vacuum line. The ECU doesn't like these mods and it throws the timing curve way off and the performance and fuel consumption suffers. My daughters 1400 carb is completely standard and she regularly gets 800 kms on 45 litres of fuel.

[Golfdriver]

Also forgot to mention, the timing curve on this engine does not retard before advancing. It sits at 6 degrees at idle (900 rpm) and rises in a very linear fashion as the RPM rises

[JohnnyVw]

Please correct me if Im wrong, but shouldn't a MAP sensor be connected to the manifold vacuum? ie brake booster pipe, or the manifold vacuum log, and not directly to the carb?

I have one of these carb citis and the MAP sensor is connected to the brake booster with no ignition curve problems or fueling issues, you also notice the change in the curve when the engine warms up to when it is cold. Sometimes when it is cold it advances the timing till the starter kicks back and then when you swing again it retards itself and starts up fine.

Check for vacuum leaks on the fan controls which are vacuum operated, it is connected to the brake booster pipe then to a black toilet float chamber thing sitting on the other side of the booster as the MAP sensor, then this runs inside the car and connects to two vacuum switches then to other brass colored vacuum switches which open the flaps for the various fan vents inside the car. You should hear the vacuum pull on these pipes when you turn the ignition on and hear the flaps move to whatever setting you have on the fan control.

I have had these pipes come disconnected and had a vacuum leak which made the car run very roughly, you could hear a hiss from the inside of the car, when reconnected everything returned to normal running.

Its a good system and very smooth idle and cold startup compared to the other carbed citis.

[Ralph180685]

Hi guys sorry to jump in...im having some problems with my citi....how doe i test this sensor you guys are speaking of ?

I pulled it out its a Bosch part with a part number of 261 230 037....