Intermittent Misfire
 

Having spent some time looking to down the 123 route I decided to have my CDI unit repaired and it was faulty and exit capacitor replaced and car now running.

The issue we now seem to have is an intermittent but frustrating misfire at small throttle openings.

We have been through the entire ignition system- cdi repaired, new plug leads, plugs , rotor and even swapped the coil just in case.

So we are left thinking fueling but why at light throttle openings?

I had the valves adjusted before all these issues started and now there is a hint of tappet noise from cold - could a bad valve adjustment cause a misfire ?

Any thoughts appreciated as the shop seem to be struggling to track this down

Cheers

Details about the engine??

Sorry its a 2.7s on CIS USA spec car originally.
6 pin cdi and standard cis. Black coil

Do a fuel injector delivery test. See if there is a significant difference between injector outputs.

Have you done a smoke test to check for vacuum leaks? DO that too. If it runs well at high throttle, and has an intermittent miss or slight hesitation at light throttle, intake air leaks are likely.

I personally expect a way too rich mixture due to vacuum leaks as others supposed too. Also check ignition timing incl. advance and retard if present on low and high rpms.
The only way to find vacuum leaks is a smoke generator. I wonder why the car has a 6pin CDI. Afaik the 911 2.7 were originally equipped with the 3pin CDI. Do you have the exact dizzy number?

Thomas

So after the CDI was "repaired", you now have a running engine but it's misfiring, right?

Who "repaired" the CDI?

Sorry my bad 3 pin cdi

Yes a Porsche specialist locally - I wondered this too but elsewhere the view is they either work or don't? - is that not the case ?

I am in the UK so not many options for CDI repair but if this is a thing then swapping for another may be a good idea ?

Yes, that's correct. The 3 pin CDIs are more reliable than 6 pins, because they "see" less engine heat and are less complex.
They typically fail because of battery polarity reversing while "jumping" the battery. So if they run the engine, they're probably OK.

It's very unlikely that a bad CDI would cause misfiring at low throttle, but run strong at open throttle. Look for vacuum leaks.

Yup next steps are injectors and vacuum leaks.
Cheers All

Make certain the plug to the CDI box is securely fastened. Not likely your issue, but you want to be sure it’s on securely. And an easy step.

Check your green signal wire to the distributor. De-pin it and make sure the female spades are crimped down enough to make contact on the male sides in the distributor. I was chasing the same issue recently.

So got the car back last Friday - injectors all fine and no vacuum leaks.

Turned out it was a combination of a dodgy valve adjustment and the warm up regulator requiring a rebuild.

So hopefully my European road trip is back on in September

What is the status of spark plugs? Did you replace them at the same time? Once after a plug change I had an intermittent misfire at trailing throttle only. Under full load it did fine. Beat my head against the wall for a week on this. Finally I put the old plugs back in; problem solved. So I ditched the whole new set as there was no easy way to isolate the bad one. Odd though it would misfire under light load/trailing throttle, but that’s the way it was.

So having spent some time with a car in a Porsche specialist it was presented as fixed after a wur rebuild and another valve timing adjustment. I brought it home and it has been in the garage whilst I replaced a driveshaft gaiter.

Took her out this morning and she seemed to run fine at low revs whilst I warmed everything up but on full load above 4000rpm she had quite a bad misfire and was lumpy on the overrun.

I came home and put my original coil back in the car and she seems to run better but there is still something amiss at high revs.

Could both the coils I have be defective?

I am getting a bit pissed as looked at everything now apart from the dizzy.

CDI rebuild
New plug wires
New points
New plugs
WUR rebuild
Injectors all checked
Fuel pressures verified
No Vacuum leaks....

Any thoughts - love the car but this is just a pain in the ass.......

Since you mentioned in your latest post that the distributor hasn't been looked at yet, you could look at it next. Post #5 also mentions to check ignition timing...

Yeah I guess - what I don't understand is why it's not consistently doing it - that's what's puzzling me. Ignition timing was checked also heads studs are fine.

Is it worth trying a new coil do we think ?

I checked the spade connector to the distributor and it was frayed and the spade was free to move. Sorting that and tightening the spade has made a big difference.

So I have lost the hight revs misfire but she still does some weird things at about 2100 rpm and on the overrun.

Anyways happier for now

The joys of classic cars......

Sounds ignition related.

What happened with the 123 ? Has some nifty features.