Ignition angle problem - Alfa Romeo Forum
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(Post Link) post #1 of 3 Old 27-04-08 Thread Starter
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Ignition angle problem

Hello to all, my car is:
146 1.6 TS, year: 98, 88kw, 120.000km, metal engine cover, ECU 2.10.4...

car is cold, starts without problems, idle is fine, 850-860rpm, very stable...
When I press accelerator rpm drops to 650-700, engine stalls, after half of second I hear detonation (small explosion in intake manifold, like small Chinese bomb used for New Year), and after that rpm starts to go up...

Car goes great, if engine is hot, there is a small gap after pressing accelerator but almost without detonations...

On highway, I reach 190km/h without problems.

Also, I have a venturi LPG, and when car goes on LPG, there is no problems, there is no gap, car accelerates great, no explosions...

I have tried to take off LPG (put everything to fabric settings), but situation is the same.

I have also tried to put sequential LPG, and on seq. the problem is the same, except that there is no explosion, only rpm goes to 650-700, and after that rpm starts to go up fine.

Also, for example, when I'm in 3rd gear, 60km/h, want to slow down, take leg from accelerator, car start to slow down, and when I press accelerator again, car doesn't accelerate smoothly, but in one small moment very rapidly (feels like someone pushed me from back).

What I have tried:
- check the timing belt with camshafts blocked
- changed air flow meter
- changed spark plugs
- changed ignition coils
- cleaned injectors with ultrasound
- changed air temp. sensor
- changed crankshaft sensor
- fuel and air filters are brand new.

I have VISA diagnostics, and everything is fine, without errors, but something is strange:

- on idle, ignition angle is 6 - 8 degrees
- after I step on accelerator, angle goes to -7.5 (minus) for less than second, and goes to +26..28 (when angle is in minus, engine stalls and detonate). This is also visible with stroboscope.
- if I step easily on accelerator, angle goes to +3 and goes to +26..28 (without stall or detonation)

I have checked on another car, and after fast step to accelerator, angle goes to +3 and up to +26..28 degrees. Doesn't go to minus.

Do you have ideas what to check next?

Someone told me to check the knock sensor (between pins 1 and 2 should be 1Mohm), but whatever I try, can not read anything with standard digital multimeter, and I have no access to oscilloscope.
How to check this sensor.
I'm not receiving any errors on diagnostics when I disconnect it.
Also, connected or disconnected, can't feel any difference.

Does the EGR valve can have influence to this problem?

If I'm not clear enough, please ask me for details, because, here in Serbia no one can fix this problem...

Thanks in advance,
Ivan, Serbia
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Re: Ignition angle problem

I have a similar problem but on a plastic topped engine without LPG. Mine started after I fitted a new Idle Stepper Motor but isn't as bad as the constantly bouncing revs problem that the old worn out ISM caused.

I have attached what the manual says about operation of the knock sensor so it probably isn't the cause of a sudden 26 degree change in ignition timing.

My suspicion for your problem would be:

1) That either the ECU is struggling to learn where it normally needs to look in it's map due to the different octane ratings of the 2 types of fuel you are using and is thus searching around the map.
2) If all your problems are below 1800 RPM then you may have a faulty Idle Stepper Motor and need to replace it and then get the ECU reset / recalibrated
3) If it happens at all RPM check the Throttle Position Sensor.

Hope this helps
Attached Images
File Type: gif Knock Sensor.GIF (12.9 KB, 3 views)
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Re: Ignition angle problem

I have just thought of another possible problem. The fuel fume system is designed to vent petrol vapours into the inlet manifold when they build up in the petrol tank and lean off the injectors when it does so.

Did whoever fitted your LPG conversion take account of this? Or is the valve still allowing petrol in while you are in LPG mode? The system is shown in another thread "Think I've sorted idle problems". This is what the manual says:
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