Marelli engine, idle stumble

XJ-S
21

I think it wise to stick to .025" on any marelli ignition car in particular.

22

Too wide a plug gap in a high CR engine can cause the spark to extinguish. I saw a very good video made by David Vizard IIRC, the engine was transparent as they were researching ignition and flame front propagation, When the plug gap was too wide the spark was extinguished.

O2 sensor will not cause this issue, when the car is cold or at full throttle the ECU is in open loop mode and ignores the O2 sensors.

Could be grounding…as a bad connection heats up the resistance also goes up causing a larger voltage drop.

23

step 1: control if trottle push rot are in sync. and that the bushing on bracket butterfly house are good
if not then replace them because they wil be out of sync and the engine wil not run good or idle
Step 2: check clearance buttefly and follow procedure to adjust them.
step 3: control if micro swich is in working order end engaged when engine is running idle on butterfly house
step 4: adjust the throttle position switch till than if aal okay she must run normal idle

24

All

thanks again for your suggestions. I will address spark plug gaps, butterfly adjustments, resistor pack and grounding in spring when I take my car out of storage. Some of the proposed actions I did before, but will address them again, if only for peace of mind.
Does anybody have experience with ECU faults? It seems that these units are quite reliable.

Regards
Stephan

25

These ECU are pritty bom proof and they are not like modern ecu, yours is a 90’so it das not store faults like and modern car. Make also sure that all vacuum hoses are tight and snuggly fixed and the air filters ar air tight when you put your hand on the trumpet the car must run quit or stall if so, She will run better.

if you need documentation how to adjust all I can send you and look also for the new sparkplug wrench I have posted today, it saves you an half day work and you don’t need to take away the A/C for A1 B1 cilinders

Cheers,

26

Werdy,

thanks again. I believe I have all required docs to make the adjustments. However, I am always grateful for more documentation (can one ever have enough?).
I will look up your wrench today.

Regards
Stephan

27

Update: As mentioned in my post of January 4, I took the car out of winter storage and addressed butterfly settings, resistor pack, grounding and spark plug gaps. The culprit, however, was a bad ignition coil, which apparently intermittently shorted on the primary coil side. Since replacement the car idles steady.
Thanks for all your assistance.

Stephan

28

Engine stumbling
#1 Classic problem with the timeing chain & tensioner. Timeing chain has stretched or the tensioner has worn, maybe both,

#2 The idle controller Valve located at the rear of the engine on left side is sticking causing the surge. (RPM Change) Once the engine warms up the surge goes away

Hope this helps.

Automan (an old mechanic)

29

Thanks! As I stated previously it was one of the ignition coils causing the stumble.

Stephan

30

I would definately check for a vacuum leak. What I have on is tak a piece of brake line, crimp on end so that there is only a small opening, attached the other end to a propane tank. With the tank opened, move it to any place you suspect a leak. You will see/hear a difference if there is a leak.