Help please! - 924 Turbo Series II 1981

[Guest]

Car starts and runs very well apart from what seems to be very short duration (approx 0.5 sec) complete ignition cutouts. It happens very occasionally after reaching operating temperature, most frequently after approximately 30 miles or 30 minutes running at any throttle position.

I have kept the revs below 3,500 since the first severe jerk whilst accelerating. On one occasion, when I presume it happened on the over-run, it was followed by 3 explosions in the exhaust (now 'straight through') on reapplying the throttle.

Any ideas before I start replacing the entire electrical system? - everything seems to look as it should.

 

[924nutter]

Easy thing first. Borrow an ignition coil. The pet catalogue list two for the 924, one up to 1980, and 1981 onwards, and does not list a special coil for the turbo, so if you have a friend with a lux of the right year, ask a favour.

There is a securing bolt for the pressure pipe, which secures it to the throttle body, and this doubles as an earth point for several earth wires. If there is a loose connection there the engine will cut, misfire and splutter with varying degrees of severity every time you go over a bump. Check this, and the overboost switch, the electrical connection on the right hand side of the pressure pipe.

Check the vacuum connection to the ecu, on the lower right hand side of the throtle body (looking into the engine from the nearside wing) for an air leak. Check the plug on the ecu for condensation.

Check the distributor cap segments, and look for tracking marks inside the cap indicating a crack. Try a new set of plugs and leads.

In desperation you could try checking for a broken valve spring or a sticking valve, but dont worry about the explosion, this is most likely caused by the unburnt fuel, which accumulates in the exhaust due to the momentum of the engine during ignition cut. When the cylinders chime in again the fuel will bang as soon as the fuel /air ratio in the rear of the exhaust is correct and a nice red hot piece of carbon comes along to ignite it.

If this fails to rectify the fault, it starts to get expensive. The flywheel sensor is made from unobtanium, ie no longer available, but I believe that these either work or fail; the engine runs, Q.E.D., and the only thing left to check is the ecu itself but you would have to be very unlucky indeed to have one of these fail.

 

[Guest]

Thanks, Mr. Nutter, for such a quick reply!

The earth connection which, on my car has been relocated to a coil clamp mounting bolt, is fine.

The overboost switch connection also seems fine, the switch itself appears OK when tested with a footpump with pressure guage and continuity tester.

The ECU vacuum connection also seems fine with no traces of condensation at all.

I'm unable to quickly borrow another coil so next I suppose is a new one (it could possibly be the original) plus a distributor cap, rotor and lead set. I would probably go to Berlyn Services though they don't seem to be advertising in the more recent club mags.

I don't think valve or valve springs are really likely as I have previously experienced sticky valves and they don't give this feel of complete loss of power as the engine still runs on the good cylinders.

I'd be grateful for any further thoughts you might have.

 

[924nutter]

Try your local OPC for prices you might be surprised. Until it was discontinued, my local OPC was the best price for a 924 waterpump. They also offered to beat the price of Porscheshop on a rear bumper insert.

Here is the link to Bert's.
http://www.partsforporsche.co.uk. Also try http://www.porscheshop.co.uk

To add a bit of interest, I 've just done my head gasket and if all goes according to plan you should be able to read about it soon.

I bought Elsie (LC W 487 W) from ebay as a series one, a write off and I repaired it, but then, when the head was off, I found that I had high (8.5:1) compression pistons and the engine number corresponds to a series two engine as well.

 

[937carrera]

I had a similar problem on my 924GT road car recently. It turns out that the screw which covers the mixture adjustment hole was missing. Put a screw back in & voila problem solved ! ( and yes it was popping & banging on overrun, but happened much more regularly)

I have also seen strange problems with the DITC pickup sensor. To be honest it does seem like a poor electrical connection issue. I would check the connections and wiring to the DITC box.

Can you give any more info about the circumstances when you have the problem........ braking, accelerating, cornering, any particular gear etc.

I wouldn't worry about trying to keep the revs below 3500rpm, ubless that causes the problem to become more severe.

Eliminate the boost switch from the equation by simply shorting the electrical connection to earth.

 

[Guest]

Good news! - the problem appears to have gone!

I changed the coil, distributor cap and rotor and the problem appears to have been eliminated, and although I've not done a lot of miles it feels 'crisper' with so far no hiccups.

Next weekend's 50 mile trip to Brands Hatch will hopefully be the proof we need.

We're HYP 735W (a red Carrera GT replica sprint and hillclimb car) and have tickets for Sunday. If you're there, come and say hello.

Thanks to both 924nutter and 937carrera for your help.