bukki79

Hello all at rennlist -
so to he case of euro 928 s4 cat 5.0 from 1990
car has been standing over the winter in a warm garage. and was running fine when put in.

now the engine is running rough and first thought is a misfire, it starts and idle okay, but you can hear that its not fully right, and as soon as you try put the revs up it looks and feels like misfire.
i found a plug cap that had been glued and a broken cap on the rotor, both exchanced now with new original parts. when i try and pull the caps i can hear it sparking on all cylinders. and engine noise change.
i have checked vacumm on the fuel regulators(or what the name is) and checked that they hold vacum with a vac-gun.
i thought a way to see if one cylinder wasnt firing i could check how hot it gets on each cylinder with laser gun on the exhaust. and this baffles me.
the right side manifold is nearly 100degrees warmer than left side. so i suppose one side burns better than the other.
i would say timing should be okay, after all it hasnt been fiddlet with since put up for winter.
my idea was a bad injector but 1 whole bank to be bad seems weird..
i am newbie in this classic porsche game and i have searched the internet up and down for a solution but nothing hits the nail..

hope someone might be able to point me in right direction to go from here.

and in advance i am danish so if i ask something that doesnt make sense, please let me know...

best regards Jacob

FredR

Cam timing is the logical starting point. It is possible that the cam belt has jumped on the 1/4 sprocket but highly unlikely I would think.

Blocked cat?

You are looking for a problem that hits one bank so that rules out ignition.

GregBBRD

Huh?

FredR

The OP says he checked the temperature at the exhaust on each cylinder and read 100 degrees C [assumed metric- the OP is from Denmark] lower per cylinder on one bank. If the problem was ignition failure on a coil or one of the distributors the problem would be spread across both banks or so my logic process suggests.

Thus we are looking for something that impacts one bank only assuming the temperature profile reported is correct and a scenario wherein combustion is taking place evenly but supressed on each cylinder on one side. if somehow the belt jumped several teeth on one of the sprockets that would retard the cam timing and at low rpms the cranking pressure would be significantly lower, it would run rich and thus conceivably explain the temperature dfference measured.

Presumably something similar can happen if the exhaust system is partially blocked and gas flow restricted.

karl ruiter

I would be wondering what went wrong while it was sitting and my first thought would be fuel. Fuel turning to gum in an injector? Fuel eating the rest of the way through a damper diaphram? My next thought would be electrical (depending on the environment-I'm in Hawaii where oxidation happens while you watch). I would first look at where the plug wires screw into the plug connectors. This is a favorite spot for oxidation. Another possibility is the old oil-in-the-plugwell trick. If the cam cover gaskets are not perfect then they can leak oil into the plug wells. Somehow this continues when the car is sitting, and I have come back to cars that have this problem after a sit. Just pull the plug connector out of the well and if the bottom is coated in oil the look closer.

Mrmerlin

Umm last i checked the 928 S4 engine is made to have 2 separate ignition systems,
and those include 2 cylinders on each bank.
That said for your issue I would put in a bottle of techron and do short runs so it has time to work.on the injectors
Also replace both of the coil wires.
NOTE if the MAF and the LH computer are original and have not been rebuilt,
then you should also have that done now.

NOTE it is possible you got bad new Bosch distributor caps ,
they had some that would work when cold and once hot the caps would internally short,
these could have been chyna copies

And Finally if an intake refresh has not been completed now is the time

mj1pate

These cars are old. Piecemealing refreshes that haven’t already been done is progressively unrealistic.
It’s the full run of all those things that Mr Merlin mentioned plus some that he and the other Gandalfs of the forum have suggested to me when my 86.5 decided to suddenly do nothing but sputter.
Among the first thing, rotor caps should definitely be examined for carbon traces. Ohm out all the spark plug wires for exact resistance.
Your MAF, LH, (and possibly) EZK should be temporarily replaced, one at a time with someone else’s loaned “known-good” units” for trouble shooting an electronic fault.
I would mention to anyone that w/r to their car, If a top-end refresh hasn’t ever been done, LH/EZK/MAF haven’t been rebuilt, temp sensors haven’t been replaced, fuel pressure/dampeners never replaced; they should seriously consider getting the part fixed that is most responsible for preventing running (and demonstrating), and selling to someone who has the deep pockets to do all the catch up work.I wish it were different, but prices for some of the above have sky-rocketed.

bukki79

Thank you all for suggestions.

with abit more fiddling and checking, i tried the good old - brake cleaner over intake and Could hear different engine Sound - so maybe try take the inlet off and see what condition all the parts are in -

Mrmerlin

if you recently worked on the injectors then its possible the lower O ring is creating an air leak in the manifold

worf928

Search this forum for IMS.

Zirconocene

@worf928 : I don't know how funny you'll find this but I had an interesting short circuit in my brain when I read your comment. You wrote "IMS" and my brain immediately went to engine failure, thinking "TBF". Then I had to go back to read it again and realized I also should look up "IMS", as I seem to be having a brain fart.

Maybe I need more coffee.

Cheers

worf928

Everyone with an ‘89+ must know what IMS means: Ignition Monitoring System. Red and Green LEDs on clear relay under ECU brain case. Fuel cutoff for 1/2 of injectors if EGTs detect side-to-side temperature imbalance due to misfires (or “no fires.”)

mj1pate

yes, we have no InterMediate Shaft failure concerns with our Porsches.

bukki79

Okay guys.
thanks for replys and heads up about the IMS !

i put new gaskets on the inlet and injectors, and have got it back together. Starts and runs, but still as rough as before…

checked the ims and theres no lights there…
so im abit lost where to go from here ?

compression test and timing check maybe ?

Someone maybe have a fuse/relay diagram ?

Zirconocene

https://928srus.com/pages/charts

Cheers