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Discussion Starter · #1 · (Edited)
Hi there,
I need to get an issue sorted with my 04 manual GV. So far no mechanics have been able to sort it out.

Basically when I first start the car each day the revs sit quite high (2000rpm). Then as I change gears the revs kind of bounce on their way down rather than snapping back to the desired revs for any given gear. This only happens for the first 2-5 minutes that I drive the car then the revs return to normal. It’s not related to the temperature as far as I can tell, and the last mechanic seemed to think it’s isn’t a significant problem to do with the clutch or transmission. He said there was a clutch master cylinder weep but that is was really insignicant so I’m not sure if that could be contributing to the issue.
This only started in the last couple of months.

other notes-
Someone rebuilt clutch at 100,000 (now at 155,000)
Timing chain went a few months ago (issue started a little over a month after this was fixed/replaced)




Any help would be amazing as I’m deciding whether or not it is worth selling.

Thanks a heap, Jones
 

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Start the car from cold and leave it to idle - rpms immediately after start should be around 1800rpm, 2000 is on the high side. Rpms should drop quite rapidly and be around 1200 (or less) within 20~30 seconds, depending on the ambient temperature, and within another 30 seconds or so, it should be under 1000 rpm - if this is not happening - your problem is engine related, and has nothing to do with the clutch or transmission.

Now - as I change gears the revs kind of bounce on their way down rather than snapping back to the desired revs for any given gear - let's try to figure out what that means...

On a manual transmission vehicle, once the clutch is engaged (pedal up), the actual engine rpm is dictated by the gear selection and the road speed - sure you can vary it by opening or closing the throttle, but it takes a finite time to either speed up or slow down - if the rpm is "kind of bouncing" without the road speed also "kind of bouncing", then you have a problem in the drive train, and more than likely it's the clutch.

Please note that the preceding paragraph has a condition - once the clutch is engaged - I know how I shift gears, but not how you shift gears - I'm on & off the clutch pretty smartly, once the vehicle is rolling, which is not true for all drivers, you have to look at what you're doing.

You may have a slipping clutch, but I don't see why that would occur for only the first few minutes of operation - you can perform a fairly simple test for clutch slip, by finding a traffic free stretch of road and drive gently along at say 40 km/h in third gear and then floor the throttle and about 10 seconds later quickly depress the clutch & release it - the revs should spike & immediately return to where they were before - there will be a gradual increase as the vehicle picks up speed but the spike & drop should be sharp & distinct. If it takes longer than 1~2 seconds for the rpms to stabilize the clutch is slipping.

A clutch master weep on these is not uncommon, you will need to fix it at some point, and sooner than later as the hydraulic fluid damages the paint work, yes, there is paint under there, and corrosion will follow.
 

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Discussion Starter · #3 · (Edited)
Hi @fordem ,

Thanks for the informative reply :) Yes I’ve done that test in the past and everything definitely all works as it should (after that 2-5 minute interval at least). If I leave the engine on idle the engine sits at around 2000 revs and slowly goes down until that 2-5 minute period ends. (If I drive off the revs also bounce down until that period ends while I’m still changing gears) In my last service the mechanic also said the fuel filter was pretty blocked so I’ve had that replaced and just put in 98 and fuel injector cleaners so I’ll see if that makes any difference.

Backlash in driveline and clutch fork rattle was noted on one of my previous services so maybe it’s to do with that as well.

Cheers.
 

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Don't you think you might want to do that test during the time frame that the problem occurs? Bouncing revs, without bouncing road speeds is not possible with the clutch engaged, unless the clutch is slipping.
 

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Discussion Starter · #5 ·
Don't you think you might want to do that test during the time frame that the problem occurs? Bouncing revs, without bouncing road speeds is not possible with the clutch engaged, unless the clutch is slipping.
When I say bouncing I mean only in a downward fashion, the revs go down then stop periodically at random intervals. Anyway yeah I just tested it during that 2-5 minute period and upon release they snap back to the same bouncing down issue that as I see between gear changes. I also tested it after the 2 minute period and the car runs as normal (revs snap back to the desired amount)
 

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Doesn't matter if it bounces down or up, once a manual transmission is in gear and the clutch released, the engine rpms & the road speed are mechanically linked - any variation in one has to be matched in the other - a smooth change in road speed, up or down will correspond with a smooth change in rpm, a irregular change in rpm without an irregular change in road speed, which is unlikely, given the laws of physics, and the inertia of the vehicle, would suggest either a clutch/transmission issue, which I don't see disappearing after a few minutes of operation, or, possibly a defective tachometer.
 

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Discussion Starter · #7 ·
Doesn't matter if it bounces down or up, once a manual transmission is in gear and the clutch released, the engine rpms & the road speed are mechanically linked - any variation in one has to be matched in the other - a smooth change in road speed, up or down will correspond with a smooth change in rpm, a irregular change in rpm without an irregular change in road speed, which is unlikely, given the laws of physics, and the inertia of the vehicle, would suggest either a clutch/transmission issue, which I don't see disappearing after a few minutes of operation, or, possibly a defective tachometer.
Hmm weird. I can hear and feel the tacho so it’s not that. I am thinking it’s engine related as the rev bounce regardless of sitting at idle (neutral) or changing gears. I’ll see what the next mechanic thinks.
 

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FOUR posts into the discussion before we hear that the "rev bounce" occurs with the engine at idle - not that that makes any difference to my last comment - once the vehicle is in gear and the clutch released, the inertia of the vehicle will prevent any sudden changes in engine rpm - so, maybe you also forget to mention that the "rev bounce" only occurs when you have the clutch depressed during the down shift?

Question - have any of the mechanics driven the vehicle and experienced this "rev bounce"?
 

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Discussion Starter · #9 ·
FOUR posts into the discussion before we hear that the "rev bounce" occurs with the engine at idle - not that that makes any difference to my last comment - once the vehicle is in gear and the clutch released, the inertia of the vehicle will prevent any sudden changes in engine rpm - so, maybe you also forget to mention that the "rev bounce" only occurs when you have the clutch depressed during the down shift?

Question - have any of the mechanics driven the vehicle and experienced this "rev bounce"?
I did say 'If I leave the engine on idle the engine sits at around 2000 revs and slowly goes down until that 2-5 minute period ends.' Anyway its probably a little difficult to communicate this well over text so sorry about that. No I notice rev bounce both when the clutch is depressed (out of gear) and without it depressed (in gear).

There seems to not be much consistency with when its more or less prevalent. I think its tied to when I last drove it. Anyway when I first took it to the mechanic it was fine because I had been driving it already, so I insisted he keep it overnight and try in the morning. Some days its much worse (revs start higher and take longer to bounce back down) and other days its not as bad. I don't know if it would have seemed significant of a problem to him when he tried it the next day but he did notice it.
 

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From wikipedia...

Bounce

verb

1. (with reference to an object, especially a ball) move quickly up, back, or away from a surface after hitting it.
2. jump repeatedly up and down, typically on something springy.

noun

1. a rebound of a ball or other object.
2. an act of jumping or of moving up and down jerkily.


I have to assume that your definition of bounce is different to the norm - think about a rubber ball - when you drop them, they bounce when they strike the ground - down, up, down, up, down, up, with the height of the bounce gradually decaying - now compare that to your statement that the idling engine "sits at around 2000 revs and slowly goes down" - I see no suggestion in that statement of anything other than the normal drop in rpm as the engine warms up - no repetitive rise/fall is mentioned - so there is no "bounce", at least using the conventional definition of the word..

I have to conclude at this point that what you're trying to describe by "bounce down" is a slow decline in rpms rather than a sharp drop when the throttle pedal has been released - if this is the case, start by making sure the throttle cable has some free play when the throttle is closed, that the throttle plate closes properly, and then check the PCV system for leaks.
 

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Discussion Starter · #11 ·
From wikipedia...

Bounce

verb

1. (with reference to an object, especially a ball) move quickly up, back, or away from a surface after hitting it.
2. jump repeatedly up and down, typically on something springy.

noun

1. a rebound of a ball or other object.
2. an act of jumping or of moving up and down jerkily.


I have to assume that your definition of bounce is different to the norm - think about a rubber ball - when you drop them, they bounce when they strike the ground - down, up, down, up, down, up, with the height of the bounce gradually decaying - now compare that to your statement that the idling engine "sits at around 2000 revs and slowly goes down" - I see no suggestion in that statement of anything other than the normal drop in rpm as the engine warms up - no repetitive rise/fall is mentioned - so there is no "bounce", at least using the conventional definition of the word..

I have to conclude at this point that what you're trying to describe by "bounce down" is a slow decline in rpms rather than a sharp drop when the throttle pedal has been released - if this is the case, start by making sure the throttle cable has some free play when the throttle is closed, that the throttle plate closes properly, and then check the PCV system for leaks.
Yes bounce was probably a poor choice of word. I probably should have said periodically declining and pausing. Again, I did attempt to clarify that -‘When I say bouncing I mean only in a downward fashion, the revs go down then stop periodically at random intervals’ -therefore not going up again. Anyway thanks for the advice I’ll get it looked at.
 
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