Thema now dedra

[St Volumex]

My beloved Thema 16V turbo ie started jerking at low speeds recently, then stalled at stop signs, and finally refuses to start at all. 

I've checked all the usual electronic suspects with a multimeter, and everything seems 'normal'.

There's also spark, and fuel that at least reaches the filter.

Any suggestions please?

[thecolonel]

I'd try cleaning out the idle air control valve they get carboned up and
can cause low speed problems (as far as I remember)

Geoff

[St Volumex]

My Dear Colonel,

Thank you for your suggestion, but where might such an incendiary device be located please?

I'm a relatively new recruit to this equipment, having used it only on a 'need to know' basis - when it works I don't need to know!

I've also heard that a faulty engine temperature sensor can cause the same type of thing, but unfortunately I would need a compass to locate that too.

Toodle pip,

Col. Bloodnok (retd.)

[thecolonel]

Bloodnok you old devil, do what I do get your batman to do it.......

top of engine you should find an item that looks a bit like a 3 inch aluminium tube
about an inch in diameter, flanged at one end and attached by a pipe to the induction,
other end will have a plug, not actually an easy item to describe. I'm sure other could
do better.

Geoff

[Angle Grinder]

Hi Guy,

I originally suggested the TDC or even RPM sensor (which is located on the front of the bell housing of the Gearbox).

However, when I read your symptoms it makes me wonder if you might have a sudden and massive air leak in the rubber pipes between the Airflow Meter and the throttle.

The engines can usually still run even with a major air leak, but maybe the climatic conditions of SA make it harder for the engine to get enough fuel at start-up?

A common fault that is easy to miss is the short rubber Crankshaft Breather Pipe that feeds into the underside of the Throttle Body near the Throttle switch. When this pipe splits, the engine sucks in enough "un-metered" air to cause the fuel mix to go all to pot and this would certainly cause poor running at low revs and stalling at stop signs. I've not known it to be bad enough to cause a car not to attempt to fire up though.

The TDC and RPM sensors tend to either work or not work, so I don't understand why you were getting the lumpy low rev conditions and the cut out at stop signs.

Of course, it could be the Airflow Meter and I have one that I think will fit your car if you can let me know what part number yours is.


Mike

[Angle Grinder]

Forgot to say, If you have a spark then it won't be a TDC or RPM sensor fault (at least not in my experience).

I'm more inclined towards the air leak problem since you seem to have fuel and a spark.

The S2 Thema Turbo has a relatively simple Auxiliary Air Bypass Valve (Idle Control Valve) that seldom fails and it would need a lot of gunk to foul it up bad enough to cause an engine not to start. If this was the problem you could try starting the car on a hot day and it would at least attempt to fire.

[chugga boom]

i had a thema turbo once that had slipped a couple of teath out on the timing belt that gave similar symptoms, maybe worth a check, a piece of broken platic off the belt cover had dropped down and gone around the crank pulley making the timing slip, nothing hit as in valves or pistons i was very very lucky!!, i totally agree that air flow meter and air leaks is also a good port of call to check out good luck and hope you sort it soon

[fay66]

I'd try cleaning out the idle air control valve they get carboned up and
can cause low speed problems (as far as I remember)

Geoff

Hello Guy,
I suspect the idle control valve as well, I would expect it to be similar to the Dedra item which is the first place to look on a Dedra with the symptoms your Thema is showing, I'd also go along with Mike on the small bore pipe to the throttle body, these perish and split, typical sods law, on the underside of the pipe where you can't see it, once again Dedra has a similar problem, but on Dedra the pipe comes off of the intake manifold and goes to the absolute pressure sensor.
If your still having problems locating the idle control valve despite the Colonels graphic description of it's whereabouts, let me know, and I'll scan the details from the workshop manual, but please remind me again what series your Thema is.

Brian
8227

[St Volumex]

Many thanks to everyone for their valued input.

Is the 'idle air control valve' whereof we speak the offending item below?

It didn't have any visible dirt inside, but also doesn't react to any power applied to the electric terminals, although the gate (valve) can be moved gently with a tool to open or close it - i.e. it's not jammed closed.

I removed it from the system altogether (reconnected the pipes without it) but no change.  

I heard from a colleague that the RPM and TDC sensors get magnetised over time and need to be demagnetised?  (On checking the RPM sensor I found it to be magnetic.)  What do you think?

A cursory look around the air piping seems all is well, but I'm trying to avoid removing a tonne of stuff just to look properly.  

[JohnMillham]

That thing might be the device which only permits full boost from the turbo when in top gear. Barry Waterhouse used to bypass them to allow full boost in all gears, which is possibly why my engine blew up soon after one of my brothers bought the Thema from me! I got blamed for that and I'm still not very popular with one of my sister-in-laws! Regards, John

[fay66]

Many thanks to everyone for their valued input.

Is the 'idle air control valve' whereof we speak the offending item below?

It didn't have any visible dirt inside, but also doesn't react to any power applied to the electric terminals, although the gate (valve) can be moved gently with a tool to open or close it - i.e. it's not jammed closed.

I removed it from the system altogether (reconnected the pipes without it) but no change.  

I heard from a colleague that the RPM and TDC sensors get magnetised over time and need to be demagnetised?  (On checking the RPM sensor I found it to be magnetic.)  What do you think?

A cursive look around the air piping seems all is well, but I'm trying to avoid removing a tonne of stuff just to look properly.  

Guy,
This is what the Dedra one looks like.

Brian
8227

[St Volumex]

Guy,
This is what the Dedra one looks like.

Brian
8227

Brian, you are right.  ePer shows a couple of different ones (including your Dedra one and mine) which is how they have 'e-valved' over time I suppose. 

[fay66]

Guy,
That's a terrible Pun!!
Hope the picture helped.

Brian
8227

[Angle Grinder]

The device pictured is the Auxiliary air bypass valve. It has a bi-metallic strip that rotates a circular disc with a "pie slice" cut out section that slowly closes the opening that allows extra air past the throttle flap during cold starts. Hence it is effectively an auto choke.

It moves very slowly so you may not see any movement upon the appliance of a current. If you put it in the freezer you should see the opening get larger and then it will gradually close as the valve warms up. Since it is mounted above the thermostat, it also will close slowly with the heat off the engine.

Very unlikely that this would prevent a car from starting. I don't think your TDC / RPM sensors will be at fault if you are getting a spark at the plugs. I would certainly be looking at the fuel injection side of things and if the TDC / RPM sensors are OK you really only have the Air Flow Meter, Throttle potentiometer and coolant temp sensor giving info to the fuel injection. The coolant temp sensor is mounted on the thermostat (there are 2, one for the gauge and one for the Fuel injection). From memory you want the one with the blue connector.

[St Volumex]

Mike, you have been fantastically helpful so far, but I have yet to get around to trying all your suggestions.  

I'll report back with military dispatches from the front line trenches in due course.

(P.S. Colonel Holmes, my batman doesn't take kindly these days to doing automotive repairs, but prefers secret black ops in his black rubber night camouflage in the company of Lance Corporal Robin, rather than black hands and fingernails.)

Tally ho,
Col. Bloodnok (retd.)