924 firing on 3 from cold start

[unclemark123]

my dads 924 only fires on 3 cylinders from a cold start, for about 5 minutes, once it starts to warm up it then runs on 4 sweet as a nut. any thoughts anyone? he suspects the thermo time switch at the back of the engine but cannot get a new one anywhere. help!!!!

[wildoliver]

Quite a lot of 924's have hot/cold start issues, I just wired a flick swith on to the fifth injector to assist it much in the same way as a manual choke, worked fine.

[zcacogp]

New plugs and HT leads, and think about a replacement dizzy cap and rotor arm. How old are these parts anyway?


Oli.

[hartech]

It could also be the first signs of a head gasket leaking at the rear that closes up when the head expands.

Baz Hartech

[zcacogp]

Thermotime switch ... if you are convinced this is the problem, would a VW one fit? These are readily available from ECP/GSF/dealers etc.


Oli.

[hartech]

If I remember correctly the thermo time switch only operates for a few seconds (how long depending upon the temperature) and the fuel pressure regulator has an influence on the fuel pressure operating on the air flow arm - to richen up the mixture from cold and works for longer - but they usually have an influence on the whole engine not one cylinder - so I suggest take out the rear spark plug (after leaving the car overnight and having just started it before is clears) to see if it is a different colour to the rest or damp (it is always the rear cylinder that goes first if it is a head gasket because there is no support for the gasket accross the rear of the block). Dry it with a lighter flame and put it back (or turn the engine over to see if collant comes out of the plug hole). They can run for quite a few months with this problem before the gasket completely blows.

The HT leads can be poor so also check which cylinder is the problem (run for 20 seconds - turn off and touch all the exhausts to find the cool one) then swap leads (remove from dizzy, replace) to see if the problem moves. Inside the HT lead spark plug cap there is a copper wire joining the wire to the spark plug inside the actual cap which corrodes away requiring the spark to jump a big gap which it finds hard from cold with soi much fuel to burn).

If it is the head gasket it will have distorted upwards at the back (no longer be flat) so it must be re-skimmed - however clean it may look.

Baz Hartech

[wildoliver]

Plug leads are a very likely cause of this problem, the 924 must have the original spec leads fitted with the metal shields over the plugs, as they run so close to the exhaust manifold, if they are not fitted the heat will degrade the leads and they will fail.