Intermittent Ignition Fault/misfire Investigation

[Mac Crash 78 1 Cars]

8V 1.9 (1990) Starts first turn and idles as normal, I always allow engine to idle for a minute or so until the common lumpiness goes and the idle becomes consistent.... the last 3 or 4 times I have used the car the fault has become consistent, after driving off from start only a hundred yards or so the ignition seems to die just a for a second or so (engine has cut out/stopped once only) then clears and is okay for a hundred yards and does the same thing again, the rev's drop but engine keeps running, like it is misfiring, coughing, spluttering, then back in to life, clears itself and is fine after this, runs perfectly for a while over different loads, then suddenly just cuts out/misfires but keeps running just for a second or two and is fine again. Seems always to be a low road speeds?

 

Any advice on where to start investigating... ignition amp or coil? also the idle of the engine is good, the SAD works and idle is higher when cold and sits perfectly when warmed up, around 850-900rpm... could the SAD cause this type of issue? not exactly sure what the SAD does excactly apart from allow more air and hence fuel for cold starting...

 

My instinct tells me it's an ignition fault though...

[scbond 15 1 Cars]

When you first start from cold and the SAD is working how high are the revs?

[Mac Crash 78 1 Cars]

around 1300-1350 rpm

[Slo 283 1 Cars]

I would say its the dizzy failing in some way, have you checked the dizzy lead and the advance diaphram thing? Centrifugal advance inside could explain the rev range its happening.

 

My old engine used to just die when it was very hot (summer) or very cold (winter) and would spring back to life on the last few turns of the dying battery.

 

Could also be the ignition amp, It's more than likely temperature related anyway.

[Mac Crash 78 1 Cars]

The vacuum advance pipe isn't there, has been like this since I've had the car but the ignition fault has just appeared recently in last few weeks, presume you mean the king lead from the dizzy, what's the best way to check this lead? I'm going to check the spark plugs and replace with new HT leads but wanted to isolate the fault first.

I'll have a look at the ignition amp and connections too (paste etc) as I have a genuine spare one known to be working that I can try. The centrifugal advance is obviously mechanical, yes? which to me suggests if it was this then it'd be happening much more often than it is.

[Slo 283 1 Cars]

Yes the king lead sorry couldn't remember its name and yes the centrifugal mechanism is mechanical inside the dizzy.

 

Best way to test the king lead is to try a working lead and a multimeter on the suspect one while wiggling it.

[jackherer 543]

You mean the signal lead, the king lead is the HT lead that goes between the distributor cap and the coil.

 

The signal wire is the small black wire that comes out of the distributor itself, not the cap.

[DamirGTI 327]

Wont be the SAD issue that's for sure :

 

http://www.205gtidrivers.com/articles.html/_/articles-guides/engine-related/supplementary-air-device-sad-r33

 

It'll be something within ignition system , so try first with simple stuff like - plugs/leads/dizzy cap and rotor .. if that doesn't fix it , would be best if someone can lend you an working/known good coil and ignition amp. to try it out on yours ..

 

Dizzy as a unit is pretty trouble free on a 205's , only issues are with the springs which weakens/loose tension over the years of use and that effects power/torque output a bit .. other than that they're reliable as a unit except the signal cable which is common breakdown problem nowadays and this the cable is the main engine moving/rotation sensing part which supply's signal to the ignition amp which then fires the coil and the tachymetric/fuel pump relay ..

 

Vacuum advance , the capsule or the actual disc/plate inside the dizzy , if it fails it wont cause misfiring/coughing/spluttering - it'll cause fluctuations of the ignition timing , and this'll manifest as a too early or intermittent (it'll depend on the engine load) appearance of the ignition pinking sound (it'll over-advance the timing for a few seconds and then stabilize back as it was ..)

 

D

[Mac Crash 78 1 Cars]

Thanks Guys,

 

The signal wire is on my list now, I'm aware of it.... thanks for SAD link, hadn't seen that before. I have a spare coil and amp (both common to the 1990 car - i.e blue Bosch and inlet mounted coil pack from a known good working motor...

[scbond 15 1 Cars]

For how long does it cut out and is it doing it when you come to a stop? Doesn't sound like the SAD but it's always worth checking that before you go spending any money on anything new. An incorrectly adjusted SAD will cause a misfire when could and then stalling if the engine isn't left to warm up first. That said, the misfire will mainly be as you start it and the engine will struggle to start unless you use some throttle.

[DamirGTI 327]

It's not the SAD !! it can't make engine misfiring !

 

Ignition primary and secondary is first to check when dealing with misfiring issues , then the fueling , the cam-to-crank timing and HG sealing .

 

D

Edited July 6, 2014 by DamirGTI

[scbond 15 1 Cars]

It's not the SAD !! it can't make engine misfiring !

 

Ignition primary and secondary is first to check when dealing with misfiring issues , then the fueling , the cam-to-crank timing and HG sealing .

 

D

 

Actually it can. The SAD on mine was badly adjusted and the hole was too small. As a result the car would start roughly with throttle added and would misfire quite badly for a while.

[Mac Crash 78 1 Cars]

Since last time the fault has gotten progressively worse, misfiring under different loads until this morning when after starting and moving off less than 50 yards it was missing on what sounded like at least 2 - 3 cylinders, tried to keep the engine going but this time the engine would clear for just a few seconds and then return to misfiring badly. During this time there was oil/half spent fuel not sure which, black smoke/fumes from the exhaust.

If misfiring then, would this have been fuel smoke? does that make sense?

 

Anyway pulled the spark plugs out and they were all completely black and heavily sooted which explains why engine wouldn't clear, on number 2 (No.1 is nearest the dizzy end) the plug came out with oil around the threads and body, now I'm not sure if the source was external, residual and laying in the plug bore hole, so looking more carefully when removing plug No.1 I noticed oil laying in the bore hole before removing the plug, this plug naturally came out with oil on it too, tried to see if there is a path coming from somewhere to explain the oil but the head is reasonably clean, it's possible though it has been there from before though as I have never changed the plugs on the car since I've owned it as it has always ran so well... so I cleaned all the plugs and put them back in and then removed them again, no oil, but at this point I'm thinking head gasket... but I wanted to check the ignition amp anyway as I suspected this is what had been causing the real source of the misfire in the first place and that the fouled plugs are a result of the misfiring.

 

I removed the original French ducillier (Grey) amp and replaced it with a Blue Bosch one, and cleaned up the yellow block connector that joins the amp to the coil pack, it was oily but the pins look clean.

 

The engine fired up instantly, cleanly... a short road test at different loads and the engine is behaving perfectly, no sign of the misfire which had been consistent after the first 50 yards - the plugs are all tea biscuit coloured and no oil on plugs or from exhaust. Coolant level is fine, unchanged. The misfire seems to have gone. I'll road test it later this afternoon from cold to 100% confirm.... does all this sound like the problems a faulty ignition amp could cause?

 

Also both amps when removed from the zinc/alloy heat plate had no transfer paste between the amps and plates, only white dried residue, how often from experience does the paste last for?

Edited July 20, 2014 by Mac Crash

[Slo 283 1 Cars]

All the paste does is force all the air out of the gap and then conducts any heat away from the part. It should pretty much last the lifetime of the part.

 

I'd pop the old amp back on temporarily and see how it runs then you can rule it out.