Strange acceleration

[SwanJack]

Recently I have noticed that if I floor the accelerator in third of fourth with the engine at about 3,000 rpm, the engine seems to race, then the revs drop for a second, then the revs then pick up again and the car pulls away well. Doesn’t happen in first or second and gentle acceleration is fine. Anyone experienced this or have any ideas what’s going on (97 4.0L)

[thom]

Maybe something is wrong with the ECU ?

[Robin S]

Swanjack,
Could be slipping clutch. The revs rise while the slipping occurs then dip again as the clutch starts to bite. You'll see this most in the higher gears. Only a guess but sounds similar to mine ('98 450) - I've had my clutch checked at last service and renewal is unfortunately not far away!!

Robin

[GreenV8s]

quote:Recently I have noticed that if I floor the accelerator in third of fourth with the engine at about 3,000 rpm, the engine seems to race, then the revs drop for a second, then the revs then pick up again and the car pulls away well. Doesn’t happen in first or second and gentle acceleration is fine. Anyone experienced this or have any ideas what’s going on (97 4.0L)

Do the revs actually drop or just it just stop accelerating for a moment (flat spot/loss of power)?

[philr]

Yep sounds like to clutch slipping to me as well.
When my Chimaera 500 started to do this I was at about 28k miles and was told that anything after 25k was pretty good going.

[jacko500]

I had exactly the same thing in my Chimaera 500 at 28k. Had a new clutch fitted at Offord Motor Co in Cambridge. Excellent job, friendly service and fair prices. Also nice bit of road to get yourself reaquainted when you pick it up!

[SwanJack]

The revs do actually drop, no loss of power and no increase either, the closest i can describe it, is as if the car momentarily goes into some sort of suspended animation mode (!). I thought it might be the clutch but hoped for something less radical such as a flat spot. It's done 38K so sounds like the right time for it to go. Is the concensus of opinion to get it done asap, or to leave it until the clutch gives up the ghost completely. I don't usually drive the car very hard so I can live with it for the moment. It's going for a service before long so i'll get them to see what stage it's at.

>> Edited by SwanJack on Wednesday 27th March 17:27

>> Edited by SwanJack on Wednesday 27th March 17:32

[RCA]

I don't think it is the clutch mine does exactly the same as yours and I had a new clutch yesterday and it has not cured that, I am putting new plugs and leads on tomorrow, will let you know how I get on.

[GreenV8s]

quote:The revs do actually drop, no loss of power and no increase either, the closest i can describe it, is as if the car momentarily goes into some sort of suspended animation mode

Not sure I understand you - are you saying you keep constant throttle, the car is accelerating but the engine revs go down? Only way I can see that happening is if something was slipping and then gripped again. I guess you would notice if the wheels were spinning so that leaves the clutch. But in the past when I've had a clutch start to wear out, I found once it started to slip it would keep going until I eased off the power. So it doesn't sound quite like a normal worn out clutch.

[q405mb]

quote:
The revs do actually drop...

---

Correct me if I am wrong, but if the revs DROP, the clutch cant be slipping. If the clutch slipped, the revs would RISE...

Aitch

[koen]

I have a flat spot in my Chimaera 400 HC around 3000 rpm. This only happens when I gently accelerate (half throttle).
The car holds up for a fraction of a second, and then
starts to build up revs again.
When I floor the accelerator, it's going smooth till
6000 rpm. This seems to be the opposite of your problem.
IMHO, it doesn't have anything to do with the clutch, because when this
is worn, the revs will build up instead of going down.
It seems to be an extra valve in the air intake, that
doesn't always open smoothly at half throttle.

Any other suggestions ?

[shpub]

The cam I think starts to come on at 3000 rpm so the hesitation could be caused by a mismatch in mapping and cam profile (could even be worn). If you are desperate to solve it, a rolling road session with Mark Adams is probably the only way to resolve this.

Steve
www.tvrbooks.co.uk

[SwanJack]

I took the car out again today for a blast to fully investigate what was going on. I decided to ignore the rev counter completely. The car pulled fine all through the range of acceleration, although pulling from low revs in 4/5 seemed to be a bit sluggish and took a while to get going, (maybe i'm expecting too much from the 4.0). I then decided to do the same and look at the rev counter. The rev counter at times, drops for a second and then rises again quite quickly, other times it stays still and then picks up quite quickly. I'm now of the opinion that the rev counter is giving incorrect readings and this is having a psychological effect. Although accelerating, the rev counter is telling me that the car is doing otherwise. If there is a flat spot around here, then the situation is amplified I suppose. I have also fitted Magnecour HT leads recently and read somewhere that these may amplify pre-existing problems (revcounter reading/flat spot?). The trouble with TVR ownership is always expecting the worse, although I don't know why as the car hasn't really a beat in the year that i've had it and my 'S', after taking a few months to iron out the wrinkles, was fully reliable for over 5 yrs.

As an aside, when inspecting the rev counter I failed to notice the Polizia coming up behind me, sirens going and lights flashing . Luckily he was going somwhere else in a hurry and was quickly followed by two other traffic cops.

I have decided to rip out my revcounter and admit myself this weekend for a full psychiatric evaluation .

>> Edited by SwanJack on Thursday 28th March 15:09

>> Edited by SwanJack on Thursday 28th March 15:12