Engine speeds up briefly under mild load in top

[beershark]

Hi all,

I have a 98 C2 Tip which has recently started a small issue.

Whilst driving along at a steady pace in top gear, maybe 4th, and I apply gentle acceleration the revs climb steadily until around 3,200 then it very suddenly speeds up a few hundred revs (like the car has dropped down a gear) but unlike a slipping clutch, the car actually goes faster and then maintains the gentle acceleration all without changing my foot position.

It doesnt appear to stutter, cough or splutter on idle or hard acceleration.

This is a fairly consistent problem happening about 25% of the time. I dont know what it is, but is it something similar?

 

[steveoz32]

It's not uncommon for a performance engine to have secondaries that open up around this RPM or variable valve timing that changes at a set RPM giving you a sense that there has been a sudden surge.

I'm still learning about these engines, but I understand the variocam system could be responsible from reading other posts - could be wrong though...

My daily driver has a very similar feeling at 3200 rpm - in it's design it has VIS motors which opperate at a set point to open up the induction to the secondaries, and another balance valve which opens higher up the range for both intake plenums to regulate the flow. The result is a slight 'kick' at just over 3k. The odd thing is if I am heavy on the accelerator and at WOT I do not notice it, when the throttle body is only slightly open, I then notice the kick.

 

[Richard_Hamilton]

I wonder if it might be the torque converter lockup clutch playing up. A diagnostic check might show up if there is a fault with the torque converter or the solenoid valve which controls the clutch.