1st things 1st.
1990 XJS V12, Marrelli ign, cats in place, only "turbo type" mufflers in the tunnel, K+N filters with opened intakes. Items on the to-do list for next week are:
Clean+oil filters, change plugs, hook up narrow band A/F gauge.

I have some drivability issues: The motor is down on power, a small "miss" or sputtering at idle and odd throttle response on slow down. Other symptoms are: When I accelerate smoothly the motor develops power linearly but on heavy load it tends to loose a little more power and it makes an odd sound like some part of the exhaust is touching the body and resonating through the interior or maybe a bearing failure?It also feels like it might be dropping a cylinder or 2. This happens about 60% of the time. Other times it is very smooth and sounds fantastic! Could the transmission mount be shifting? If I "baby" the throttle( rarely for me!) it runs very smooth. This only happens under moderate to hard loads in the lower RPMs. The other noticeable issue is when I am backing off the throttle slowly there is a moment where it feels like I just "chopped" the throttle and no fuel is getting to the motor, followed by "gurgling" sound from the exhaust with light popping as well.
I will have more info when I remove the old plugs and read them.

Thanks in advance

 

The "could be" list is long....but you gotta start somehwere :-)

I'd consider a partially plugged fuel filter, a TPS fault, and a throttle linkage adjustment/throttle bushing problem.

Don't be the least be surprised if your multiple sysmptoms have multiple causes....a "stack up" of faults.





Fairly common



[quote]or maybe a bearing failure?[quote]


Which bearings are you thinking of here?



Could be an ignition misfire. Could be a brittle and decrepit injector harness isn't firing all the injectors correctly






Yes, fairly common.





As I recall (others will chime in here) there IS a closed throttle fuel cut-off where the ECU cuts off the injectors on closed throttle until engine rpm drops to a certain point. Sometimes this can feel a bit abrupt...sorta like being bumped from behind.

I'm wondering if there might also be a problem with the over-run valves...which essentially dump vacuum on closed throttle. The springs inside the valves can weaken. I'm sure there's some good info in Kirby's book and/or others will chime in.


Normal. It's just that you wouldn't hear it with the original exhaust.

Cheers
DD

 

How do I do individual quotes properly like you do?

[QUOTE=Doug;710530]The "could be" list is long....but you gotta start somehwere :-)

I'd consider a partially plugged fuel filter, a TPS fault, and a throttle linkage adjustment/throttle bushing problem.

Once the A/F gauge is installed would it give me an indication of a clogged filter by running lean at the moment the noise/power loss happens?

Don't be the least be surprised if your multiple sysmptoms have multiple causes....a "stack up" of faults.

I expect a stack up problem. It's just good to have that starting place to drill it down.











[quote]or maybe a bearing failure?