Help in Diagnosing Slip/"Clunk" When Driving Off!

Hello - I have a 2003 C4S Tip that is going in to the shop in 2 weeks for "open heart surgery" - I am having the IMS pre-emptively replaced, my transmission torque converter seals replaced (and yes, I was able to find replacement seals ), and a bunch of other stuff. I figure this is the perfect opportunity to have the shop do a bit more diagnosis on an issue that I have had since I got the car almost 6 months ago. Basically, the problem is this. If I am stopped at a light, or pulled over at stop at a yield sign or merge to get on to a freeway, when I push the pedal to the floor to accelerate hard there is a short "slip" that occurs, such that power does not get to the wheels from the engine right away. I don't hear a clank or anything metallic like that, but I feel a bit of a clunk of sorts when the traction finally engages. Basically, if I accelerate sharply from dead stop it’ll move forward as engine revs (kind of as if it is in neutral), but there is a momentary “slip” prior to true engagement of the wheels... A few more symptoms: - appears to do it when it is cold OR hot; - shifts in all other ranges (up or down) are always smooth when moving; - does not occur if I accelerate “responsibly” / normally - (I wouldn't say I have to drive like a little old lady, but in a manner that isn't going to up my 0-60 records); - only known physical symptom is that transmission has small leak at the torque converter seals (being replaced in 2 weeks), and the transmission mount had a crack (was replaced a few weeks ago but does not seem to have helped this issue). Mechanic also checked the engine mounts, said they were fine. Other possibilities I am considering (but which mechanic has not yet followed up on) are: - rear CV joints; - "guibo" (rubber disk); - sticking brakes; - traction control/PSM malfunction; - transmission/torque tube “coupler” (not sure if this is the same as the "guibo" above, I believe this coupler is a metal part); - cracked rear wheel bearing(s); - sway bar links; - shocks (i.e. mounts may need to be tightened); and - that the leak (mentioned above) may be causing the issue. I would really appreciate people's ideas as to what my guy can diagnose while my engine and tranny are out! Seems like the ideal time to solve this problem. This is for sure not the end of the world, but part of the fun of a car like this is laying rubber at a green light!

There are a mind blowing number of computer maps that the Tip can choose to follow. The way your transmission shifts is based on what it has learned from the way you drive. If you mostly drive very conservatively, the trans will shift like the little old lady is driving, even when you attempt an aggressive move. Drive it like a kid amped up on energy drinks and it will surpise you how responsive it will be. We've had a Tip Boxster for over 8 years and when I go long periods witout driving it I notice the shifting is pretty lethargic and unresponsive. I have replaced the battery and possibly reset the DME (dont' remember for sure) and drove the car aggressively, winding it out in each gear. Then when even accelerating moderately and punching it, it is ready and performs very different than before. It's possible your car is stuck in a "lazy" map. Talk to your tech about this. If your car actually has a momentarily slip, you could verify this by watching your Rs jump up on the tach. I think it's just a momentarily lag that most likely is not as long as at seems. Some of the other things you mention would or could create momentary lash between the power plant and the chassis but would most likely if bad enough to feel would be accompanied by a knock. The same would occur when hitting the brake. Wheel bearings will generally create a different pitch of noise when the car is sharply turned on direction or another. GL

Hi Nick - good points. Once I'm moving, I'd say I drive pretty aggressively - I'm typically at least 15-20 over the speed limit on non-residential roads where I can do it safely, often much more on the freeway/highway if it's clear and my radar detector has a clear line of sight... (there's no cops here, right?) The slip/clunk or whatever does remind me a bit of what would happen in my Lexus LS430, which actually had a big engine and was pretty fast on the acceleration given that it was a boat of a car. Basically, if the traction control in my Lexus felt my rear wheels slipping, it would cut the power to them - creating a sort of "blip" which made it difficult to floor it from a light. However, if I turned the traction control off on the Lexus, it would "burn rubber". On my Porsche, I think I've tried hitting the PSM switch, but don't believe it made any difference. I will try this again, however.

i had the same problem before, and paid $150 to the dealer to reset my TCU, solved.

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I like the sounds of this suggestion! So does the TCU actually need to be separately reset, or is this something that I can do by unhooking the battery for a weekend or something like that?

before i went to the dealer, i had tried this route--disconnected the battery for one day, then re-connected the battery on second day, and the car acts the same problem in my case. so i took it to the dealer.

Further to Kent's suggestion, I've tried launching with PSM turned off, and the "lurch" is definitely gone/minimized. So, there is clearly a connection. I'll have my mechanic reset the TCU and see how that works.

Well, interesting update on this after almost a year. Having found a new set of winter tires from the dealer (they had the best price on them, as unbelievable as that sounds), I brought it in this week to have them installed - the first time my car had ever been in for service at this dealer. As I was there anyway, I figured "what the heck" and spoke to the service manager about the "clunk". We went out for a drive so that he could see if he could replicate the "clunk" - which he could immediately. He then tried a couple of maneuvers (accelerating up hills, from stops, while rolling, etc.). By looking at the lights on the dash we noticed that the "clunk" I was feeling on the hard acceleration was actually the car downshifting from 2nd (in which it sits while stopped) to 1st and the resulting lag/load on the engine! I didn't feel the clunk when driving off slowly because the car was not downshifting in those instances. While he didn't know what the clunk was all about, he noted that the car shifted smoothly in all other circumstances and exhibited no other "trans trouble" symptoms whatsoever. As luck would have it, he had a Carrera 4 Tip in the shop that day. When he took it out for a drive under the same conditions, he found that it exhibited roughly similar characteristics. However, when I picked it up that night, I'll be darned if the clunk I felt wasn't gone! Further to what some people here have said above, I think I was feeling it because the car was in a map that tried to keep it in 2nd a touch too long. Now when I accelerate fast, it downshifts itself to 1st immediately, and no clunk is felt! I think that the service manager's driving around to test it in the morning reset or adjusted the self-learning settings on the TCU. Next time I feel the clunk again, I'm going to go to a quiet street early in the morning and do about 20 jackrabbit starts - and I think that'll self-adjust it. Only downside of my visit to the dealer was that they indicated that I need new rotors - I'll probably do the brakes myself with the videos that have been posted. Anyway, I now call the case on this mystery closed!

Thanks for this information. I have the same car and have felt the same issue under hard acceleration. You corroborating this gives some piece of mind.

I can reproduce the problem on mine somewhat reliably if I start the car uphill on a gradient. The thunk / thud is very pronounced soundwise and feels like a kick from behind. Will bring the car to trusty mechanic to see if its tranny mount bushings going south. I am in the San Jose area. Can compare notes with OGWGFIW.