Cadillac art

1984 Porsche 928S Automatic

Bought on BAT in late November. Great car but arrived with fuel leak at one of the short loop hoses attached to the fuel dampers. Okay- time for new all lines in the engine compartment. Roger to the rescue. I end up doing the entire top end refresh with new vacuum lines and elbows, all new fuel lines, new intake gaskets, oil filler baffle, thermostat, distributor gasket, plugs, fuel injectors sent out- cleaned and balanced, new green wire. Passed emissions with flying colors after installing new cat- the original was toast and all broken up inside. 2 months or so of weekends and it all runs good, no leaks, but once fully warmed it has a miss at idle (like at a stoplight, foot on brake, so under some load from that).

So now, it runs well, can be plated and driven legally, except it has this troubling miss at idle under load. I don’t “sense” the miss when underway but possible its there and I do not feel it. The miss is significant- almost like it could stall-it’s that much of a “bump” but it recovers immediately. It is also intermittent- sometimes I can be at the light for a full minute and no miss. Other times it may miss after 5-10 seconds of idling, but then follow up misses could come 40 seconds later, then 5 seconds, then 15 etc. It doesn’t seem to follow a pattern. It does not seem to miss sitting in Park idling.

I ran it in the dark garage looking for arcing plug wires. Did not see anything.

Thoughts on where to look next?

ddire333

Does have all the symptoms of spark issue, if you have assess to a oscilloscope they are great for narrowing down, but simple test - add a spray water over the leads / distributor to your night test, adds path to ground for any loose spark, you can also run near the wires with a test light hooked to ground to see it it picks up any spark. ps great job on the engine bay looks well

Jason89s4

Nice looking engine bay.
Let me make sure I'm clear, you did the top end refresh and all of that other work and you didn't replace the cap, rotor, plugs and wires?
-Jason

Shark2626

I snipped your post to highlight the key points. I don’t know of what would cause a “miss” that is related to applying the brake and while in gear, but not while the car is just idling in park, perhaps someone here does though.

If I were to take a stab at it I would ask if the engine vibrates more when the car is in gear and brakes applied than it does when in park. Motor mounts. And I suppose that could give effect to bad spark plug wires being jiggled around a lot more, one (some) could be broken internally or not be securely attached at each end.

I know it’s a funny reply!

Speedtoys

Lets keep it simple. Grab the Volt Meter, ohm out each plug wire end to end.

This has solved this issue more than once...and it's a easy $0 start to diag the issue.

Cadillac art

Will ohm out the wires at first opportunity.

The Forgotten On

Other things that could possible cause this is dried out capacitors in the ignition/fuel computers.

Even though the computers rarely go bad, caps can easily dry out in 30+ years and cause all kinds of issues.

You could try swapping another L jet in and see what happens. Start with plug wires first though

Shark2626

Yes, failed wires or their connections will most likely be at fault here.

But failing motor mounts could be triggering your observations. I’m sure someone would have chimed in if there were other possible causes for the “miss” occurring only while in gear and stopped, brake applied, but not while in Park.

Lifting the hood, starting the car and observing engine movement while in Park, and then applying the brake and placing it in Drive and observing if there is change in the engine movement takes one minute and costs $0.

Mrmerlin

With the later engine mounts , the best way to determine MM failure is to look at the bottom of the oil pan and compare it to the cross member,
if the pan is sagging below the cross member then the mounts are done.
NOTE pan should be flush with the bottom of the cross member if they are good.

Next best way is to start the engine while in the drivers seat, feel for smooth operation of the engine.
If you feel vibration from the seat or steering wheel then the mounts have sagged.

Next start the engine and with the hood open observe engine operation and if the engine rocks back and forth freely when the throttle is opened.
If this is happening then the mounts are still working

With the early mounts put the car on a lift ,
then jack up the engine to see if the solid mounts have separated