If it only happens when warm, you could have valve gaps closing up: easy enough to check them... After 120k miles, I expect the valve seats are knackered as presumably the car was designed to be run on leaded petrol?

Either that or it may need decarbonising : when warm the carbon glows red and you get pre ignition...

Take out a spark plug and shine a light down the aperture when the piston is at the top of its stroke and see how it looks...

(a cheap way of decarbonsing is to dribble water into the the inlet manifold : the steam removes the carbon..Put too much in and you break a conrod.)

Or the inlet manifold/throttle valve are carbonned up and require cleaning...

Or you need new plugs and plug leads

Or the timing has slipped.

or...Or as you say a new distributor cap..

or..or..

Edited by madf on 10/08/2010 at 14:41

We have worked on many Pug 205 Gti in our time and found that the VAF, Volume Air Flow Sensor and the ECT, engine coolant temp sensor are commonly known for this kind of problem.

The first thing to do is a compression test, cylinder balance test and check for air/vacuum leaks. As long as the engine is in sound working condition, then i would put it down to the fuel injection side. Try the two things i have mentioned above. You really need someone to carry out the correct tests to pin point the problems. We use storage scopes that can measure up to 80 million signal/second and will pin point the slightest errors in systems.

Good luck and let me know how you get on.

Edited by WellKnownSid on 24/08/2010 at 20:55

thanks for your help. will give those a try

We do smoke test without a smoke machine. We buy and use smoke pallets which are also use the check flues and chimneys. Light a few under the car whilst the engine is running and see if the smoke is sucked in anywhere.

Sounds familiar. I would get the engine management system checked out but if this is OK it might be the injectors.

I have a 1988 1.9 [not a 1.6] and it has suffered from uneven running over the years. I eventually took the injectors to a specialist cleaning firm [don't bother with your average garage Peugeot or otherwise] and they discovered one blocked, one dirty, and 2 clean. They cleaned them up [not cheap] and all seems well at present.

I believe the problem lies with the petrol. Cheap supermarket petrol often contains ethenol, so I only use premium grade 98 octane petrol {Shell V Power seems to be good] and a shot of Castrol Valvemaster [without octane boost] additive to replace the lead. Oh, and I also found it worth flushing out the cooling system and refilling with the correct anti freeze for an aluminium block, which also seemed to help.

But the conclusion is, I am afraid, that the problem you speak off is not uncommon, but the solution is eventually trial and error. Not very scientific!