Weird throttle issues question
#1
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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
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
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#2
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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
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.
and it makes an odd sound like some part of the exhaust is touching the body and resonating through the interior
Fairly common
[quote]or maybe a bearing failure?[quote]
Which bearings are you thinking of here?
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 be an ignition misfire. Could be a brittle and decrepit injector harness isn't firing all the injectors correctly
Could the transmission mount be shifting?
Yes, fairly common.
. 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,
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.
followed by "gurgling" sound from the exhaust with light popping as well.
Normal. It's just that you wouldn't hear it with the original exhaust.
Cheers
DD
#3
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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?
Which bearings are you thinking of here?
I don't know. The "sound" just sounds a little like a "walking bearing".
Could be an ignition misfire. Could be a brittle and decrepit injector harness isn't firing all the injectors correctly
I'll check the condition of the harness and correct any issues I might find when I do the plugs.
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.
I'll check "The Book"
Normal. It's just that you wouldn't hear it with the original exhaust.
Good to hear. Now if I could only get it to shoot flames when this happens.....(yes I already know I'm a geek!)
Cheers
DD
[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?
Which bearings are you thinking of here?
I don't know. The "sound" just sounds a little like a "walking bearing".
Could be an ignition misfire. Could be a brittle and decrepit injector harness isn't firing all the injectors correctly
I'll check the condition of the harness and correct any issues I might find when I do the plugs.
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.
I'll check "The Book"
Normal. It's just that you wouldn't hear it with the original exhaust.
Good to hear. Now if I could only get it to shoot flames when this happens.....(yes I already know I'm a geek!)
Cheers
DD
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