My Thema does not like full accelerations

[Miquel]

Hello everybody

I'm writing from Spain with the hope that some of you have experienced and solved next issue with my Thema i.e.turbo (first series, so 8 valves). It has 287000 kilometers in the counter and 21 years of age.

As you know, this model has an obervoost system. Since several months from now, the car is having problems when the obervoost is working: during the full acceleration process, there are several brief but very notorious strokes. The sensation in each stroke is similar than no fuel is reaching the cylinders. In fact, in each stroke the revolutions counter fall down for a brief period of time (less than 0,5 seconds) and then it rises again until next stroke, when the cycle is repeated.

In the last weeks, the problem has become more serious: now, the strokes appear even without the obervoost system working. For exemple, if you press the pedal until the middle of its tour, the obervosst is off but the strokes appear. The only way to avoid them is to accelerate very softly.

Have any of you experienced this kind of problems?. If yes, do you know the solution?

Thank you very much in advance.
Best regards.

Miquel

[Angle Grinder]

The dipping of the Revs Counter suggests that you have a faulty RPM sensor at the flywheel and this may cause the engine management to be momentarily fooled into thinking that the engine is not turning.

Under these circumstances, you might find that the spark plugs are not sparking so no combustion occurs in the cylinder, hence the jerkiness.

The rpm sensor is mounted on the front of the gearbox near to the join between the gearbox and the engine block. It is located so that it generates a pulse from the passing of the teeth of the flywheel. I think it has a white and a brown wire coming out of it.

It should be easy to remove.

[Miquel]

Angle Grinder, thank you a lot for you answer. I will check this sensor asap.

Thinking about this possibility, ¿do you know what is the reason why this just occurs during hard accelerations and not during soft accelerations?

Thank you again

[Miquel]

Hello again.

Just more data about the problem. Today I was testing again the car. With the third gear inserted and the engine around 3500 rpm, I pressed totally the accelerator and I could detect this:
-The engine started to increase its rpm but, around 4000 rpm, the strokes appeared.
-The strokes are rythmic: around three per second.
-The strokes appeared even if obervoost is off. So I can think that the problem is not related with obervoost.

Thank you everybody

[Pol]

Miquel, i think you should check your fuel pump and pressure regulator,
when you accellerate your engine needs more fuel and if the pump cannot deliver full flow you get the effect you describe.

Pol

[Miquel]

Thank you for your answer Pol

Best regards

[David Hanson]

It may also be a problem with the knock sensor - or poor quality petrol causing a knock which would cause the computer to cut the spark

[Miquel]

Thank you to everybody. I have took note of all your comments and after holidays I will put the car into the garage in order to make a look into all those components.

I will keep you updated

Best regards

[Paul]

Did you find the problem?

[Miquel]

Not yet. Money reasons postpone the check

I just want to add a comment. Probably the rpm sensor is working well because the obervoost starts working just when the counter reaches 2500 rpm, never before.

Cheers