Close engine cover - engine stumbles. Weird.

Just a shot in the dark, is a coil wire shorting to the cover when closed?

 

Does the stumble occur only when you "slam" the rear bonnet down, or does the stumble also occur if you very gently lower the rear bonnet down?

 

I would guess that you have a short happening, which a mallet wouldn't show......or maybe the slamming of the lid is affecting the Fuel Distributor? I would start by lowering it to almost closed, then getting a flashlight beam in there....looking for contact. The only circuit coming into the bonnet itself is the license plate lighting.

 

May be that something is loose in the ECU compartment which is close to the area of the latches and might get moved with the slamming. Like a loose ground..........

 

Then lower gently.

I place the decklid down and press into the latches.

I do agree that something electrical is loose causing the engine issue. Check all your connectors and ignition wires for proper connects.

 

Ask the tappet brothers

 

+1....Click & Clack rock. My favorite weekend radio I'll bet that they'd love to have a Ferrari call.

 

If it doesn't do this over bumps, I would speculate that it is something like a split hose on the intake manifold. The scenario in my imagination is that slamming the lid may momentarily disturb the split allowing the manifold vacuum to come to atmospheric pressure in the hose, causing the stumble until the line normalizes to intake manifold vacuum. Going over bumps doesn't cause the issue because there is no downward pressure on the hose to open the split as the car rides over bumps.

This is quite a story I'm telling, so my speculation comes with this disclaimer:
I am weighing in on this problem as an amateur shade-tree mechanic with no formal training or professional certifications. I offer my advice to further the interest in the thread you started and to selfishly glean some check of my ability to reason in a public forum. If you are interested in tinkering on your car, you may be interested in what I have to say. If you really want your problem solved you should consult a paid professional to diagnose and repair your problem.

Thanks -Glen

 

The air box would probable touch the lid before anything else. My best guess is the air flow sensor plate is bouncing open when you close the lid. Try removing the air box and with the engine running tap on the sensor plate and see if it resembles the stumble.

 

I know...

 

Any chance part of the engine lid itself is interrupting air flow from the air box?

 

Okay, out with it.

 

Don't slam the lid, and don't press down on the trailing edge of the rear deck.

Lower gently, then press down firmly (open palm) at the far end of the buttress panel before it ends at the up turn at the trailing edge. Do this on one side first, then the other to properly latch the deck lid without resorting to slamming or risking denting or bending it. The buttress is strong, so there is no risk of denting or bending like there is if you push down on the rear spoiler lip or the flat part of the deck lid.

 

OK...The 328 uses a single Microplex ignition unit mounted in the LH corner floor of the trunk. The multi-pin harness connector is retained on the ignition unit with a "funky" spring clip. I had a few cars where the clip had become "unclipped" and the connector was just held in by friction alone. Any sort of shock will move the connector. This is a five minutes exercise to check. As the car has no ECU, any rough running will likely be in the ignition system. To verify that there no problem with the Lambda control unit, just (temporarily) unplug the O2 sensor...