Dansvan

Hello all. Time to ask the collective. I am an ASE master tech with 30 years, however this is the first 928 Ive owned. Purchased with broken timing belt. Replaced 8 intake valves. The rest were inspected and lapped. 40k motor - looks pristine inside. New belt, Porken timing tensioner. Cam timing set with Porken tools. Consistent miss on #2 cylinder. very bad when cold, smooths out when warm, random cyl drop when warm, assuming #2 but no way of knowing.

New sparkplugs
New cap - rotors -wires
Injectors cleaned and spray pattern verified
Injectors swapped around and changed - no difference
Ignitors swapped around
Coils swapped around
Smoke tested - no leaks
Reg and dampers - no leaks fuel pressure verified running and off
Ignition timing changes with rpm
MAF inspected but not rebuilt (to my knowledge) passes all tests, runs like crap unplugged
Temp sensor II verified
Injector leads swapped between cyl 1 and 2 as its a batch fire system -no change
New Bosch knock sensors
Flapper valve verified
Cranking compression even all cylinders
Running compression even all cylinders
Leak down shows 3%
Havent driven one of these prior but this one spins tires when mashed and cruises nice
Has Porken timing chip installed but symptoms were both before and after
Plug caps and wires ohmed - consistent
Spark load checked with spark checker - jumps huge gap

Im thinking ignition issue as nothing Ive done injector wise has changed anything. Occasionaly will get the rotten egg smell. Its never shut down one side or the other of the injectors.
I have a new o2 sensor coming. however it starts missing immediately when cold. Starting cold - pulling into garage - turning off and pulling sparkplugs - #2 wet, #1,3 dry. Is there something that could affect 1 specific cylinder in the EZK or LH? Another thing - very faint clicking noise from under pass valve cover. Is knock sensor sensitive enough to pick up and reatrd timing on #2 to the point of misfire? Springs are not broke - suspect possibly worn cam timing chain pads. Or possibly a lifter - however they all fell the same and Ive read they rarely fail.

Thoughts?

The Forgotten On

Rotten egg smell means that your cat is giving out.

A growing issue is that the ignition leads fail internally. It happened to a friend and previous poster on here that had his 87 cook his cat because a ignition lead failed internally.

Verify continuity in the #2 wire and wiggle it while doing so to be sure it isn't intermittent.

Who made the wires you fitted:? Beru is the OEM and what a majority of us use.

Mrmerlin

I would replace the #2 ignition wire,
also swap the distributor caps,
do one thing at a time. either with the old ones or new ones side to side.
NOTE at one point some bogus Bosch caps have been sold.
I had one on an 89 it would short out after it got hot,
swapped out the cap and the engine ran perfect.

NOTE its possible to buy a cheap set of Beru ignition wires.
the good set is a plug and play with all the wire spacers and the wire protector shields,
and comes with 2 sets of coil wires it costs about 400.00.
NOTE post a few pictures of the engine bay.
NOTE make sure that the coil wires are not rubbing any part of their run,
any wire that was rubbing and had marks on the insulation has probably been damaged.
NOTE you should replace the temp 2 sensor and its connector.
NOTE at this time you should also have a freshly rebuilt MAF and the same for LH computer .
Both of these parts time out .
Disconnect the battery to reset the computers and the running parameters when you swap any of the engine electrical components.
It might also be prudent to install the stock chips.

Dansvan

Appreciate the reply’s so far. It has the good pre-made set of Berus on it. Coils, caps, rotors have all been swapped around and are new. Temp II tests fine hot and cold.

Keep in mind this is only a single cylinder acting up. Cylinder #2.

I do plan on having the MAF and both computers gone through this winter

I know internet diagnosis is tricky - appreciate it

gazfish

You swapped injector leads between 1 and 2 but might be worth swapping the injectors themselves?

FredR

The knock control system retards the specific cylinder[s] that are knocking and will do so in up to three increments of 3 degrees of retard up to a total of 9 degrees total retard but that will not cause what you describe.

Considering the info you provided it is clear you are looking for a cylinder specific problem. On the ignition side as suggested already you may have a faulty distributor cap - you can check for this by swapping over the two items and check to see if the problem modulates at all. The other possibility is the fuel injection signal- the pigtails on the wiring loom get into an awful mess under the junior powertimer two pin connector weather sheath so check that out.

Injectrors can also fail - if the pintle is jammed partially opened I suspect that could cause what you describe.

Petza914

Pull the fuel pump relay and do a compression test on all 8 cylinders with the engine cold and start with the #2 cylinder.

Mrmerlin

If you follow everything i suggested and no JOY .
I would also suspect a bent valve or damaged lifter bore in that cylinder
NOTE bent valves dont have to be very bent just ever so much,
NOTE if its intake this will result in a stumbling idle,
if its exhaust then the offbeat cylinder will be heard at the tailpipe.
Also Im not sure you ever replaced the #2 injector try that also.

Geo55

This is quite the conumdrom. Have you checkerd #2 with a noid or scope? No lifter noise? I think I would pull #2 injector and test myself. With the compression and leak down testing good...But then you said #2 was wet. Know anyone with a Bosch Hammer? (Engine bay pictures please)

Dansvan

Injector has been swapped with another cylinder - no change.

Injector connector has been tested with noid light and I have run it with cylinder 1&2 injector connectors swapped between cylinders. #1 firing cylinder #2 and vise versa.

Compression has been tested hot- cold- running - cranking and is dead even across the board. 3-4% leak down cold and hot.

How smooth do these idle when running perfect?

Spark will jump a 3/4 inch gap at idle and high rpm using a spark strength tester. And does not drop out

jschiller

Are you sure you didn't crack a sparkplug then installing them? I'd check the exhaust manifolds with an IR thermometer when it is cold and you have the worst miss and see which is running the coolest, then pull that plug and look for damage.

Dansvan

Plugs have been swapped cylinder to cylinder and also the old plug installed. Infra red always points to #2. Wet on start up, clears up and runs fairly smooth when warm. No perceptable issues under load accelerating or cruising. Occaisonal miss when sitting at a light. but no way of knowing if its that cylinder when that happens.

Thanks for all the ideas guys

h2pmr

how badly bent were the valves in that cylinder?
bad enough to damage a valve guide, making a new valve stick?
new genuine Porsche valves fitted, or used versions ?
any recent service invoices prior to the belt snapping, relating to misfire diagnosis?
might have had this misfire fault before you bought it.

just thinking outside the box
cheers
Phil

Geo55

Did you say that you did an upper end refresh? If not, I would do one over the winter. May give you a chance to look at things from a diffwerent perspective. Getting to the point where we're throwing darts.

giddyupp

Check the Ground wires at the back of the V. Each injector has its own ground and they all connect into one on the passenger side of the V. I had a terrible misfire a couple of months ago. Spent weeks checking and re checking the intake, MAF, LH etc to find that the ground bolt was completely loose. As soon as I tightened it up the engine ran perfect. If its not the whole connection maybe just the one wire from that #2 cylinder. Should be worth a check at least.