Help: Strange "jerk" or "thunk" going up grade

Going up a grade today (20%+?), behind a bunch of slow traffic. Was in third, only tach'ing about 2100rpm (I know I should have downed to 2nd, but there's a reason why I didn't). It all happened so quick, I didn't have a chance to see any changes in speed or tach during the event. All of the sudden the car gave a quick jerk or lurch, like I got tapped from behind, but then recovered like it never happened. I thought it might have been something in the road, but it did it again (and made the CD skip) about 200ft later. I assume the CD skipped and that it wasn't a momentary loss of power. This feeling could also be described like quickly letting in the clutch with a mismatch in engine and road speed; sort of a slight shock and thunk. No CEL, no other noises, rpm's climb fine otherwise, and shifting seems smooth still (no shuddering). Engine miss? Never felt a miss like this before, though. Could the clutch do this, considering it still feels OK? Anyway to test? Anything in the trans? (I do have the second gear syncro issue, but no popping out). Ideas? Macster, any of your indepth-post insights would be appreciated too!

a slug of water in the gas? Just a guess...

Engine mount start to fail around 60K. They can give a good clunk.

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You had me at going up a grade... Not a clutch symptom I'm familar with. This assumes the clutch not exhibiting any other symptoms and shifting is without any drama. Was the event an acceleration event or a deacceleration event? That is did the car feel like it was indeed given a bump/nudge from behind or like the engine momentarily lost some power? Regardless absent any other behavior out of the norm probably (always the probably...) nothing to worry about, unless it starts happening more often, too often. How often is too often? One guideline I use is if something happens and I can remember when it last happened... that's often, and often too often, if what happened wasn't say winning the lottery. Mountain roads especially those that see alot of traffic can develop ripples as the tires each tend to push the pavement backwards as the tires push the car forward. Under a heavy load, even at 2100 in 3rd pushing a 3200+ lb car up a 20% grade is a heavy load, if the rear tires encounter these ripples the biggest ripple can cause the tires to momentarily lose grip and the car will experience an interruption in forward momentum that can feel like a lurch or a jerk. There can be a bit of tire spin but the tach may not be able to react fast enough and generally this happens so unexpectedly unless one is very familar with the road and the appearance of the symptom to be prepared for it he won't notice. Also, depending upon the recent weather the road can have a damp patch, sometimes from ground water making its way out from under the road bed through the pavement, water coming from what was a seep or even a bit of a spring before the pavement was put down. If the water has a high mineral content it can actually deposit some of this mineral on the road (there's some kind of reaction between the pavement material and the water and its mineral payload) which can make the pavement unusually slippery, at least until this thin layer is worn away again. If not a damp patch I've encountered areas where water crossing over the road can make it slick even if the road now dry cause there's always a bit of fine mud or even sand left that can very easily see the rear tires break loose momentarily when they reach this stuff. If the road spends a great deal of time in the shade it can be a bit slippery to due to some dew that had yet to evporate. Mountain roads can manifest a variety of rather uncommon conditions that roads located on flatter terrain and more stable ground and less exposed to severe weather and wider temperature extremes do not manifest. If you want a more mechanical explanation: The gas pump may have inhaled a bit of air, unless you tell me the gas tank level not exceptionally low. Could have been the engine controller dialing back the timing due to detecting the early signs of detonation. These engines in some cases run very close to detonation so it doesn't take much to push them over the edge. Low rpm. Maybe a good amount of throttle, and a heavy load. All these contribute to the perfect storm of detonation. When the lurch occurred you may have inadvertently lifted the throttle a bit -- a natural and even automatic reaction -- and the perfect storm of conditions went away. Could have been a marginal "spot" in the E-gas throttle potentiometers. Could be any one of these or something else. Depends upon if the symptom appears again or not or what if any other symptoms appear. Sincerely, Macster.

oh - water cooled .... I was in a differant frame of mind.... funny thing happened to me in a bug once... we were coming back from skiing and I had a rather large friend in the back seat, the road was a bit rough in a wavy rapid succession that made the car go up and down, up and down over and over.... everytime it went down the engine stoped, lights off, wiper ect... then on again???? over and over.... came to the realization that the metal springs on the bottom of the rear seat were contacting both battery terminals at the compression of the bumps !!!! HOLY SHEEEAAT Lift your *** up !! this doesnt solve your issue, but it shows how sometimes wierd things can cause wierder things! I would run it and watch for it to happen again, could be a coil pack, MAF, dying battery, or any number of other things... GL and be safe out there

Is that car drive-by-wire?

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Yes, but Toyota did not make the parts

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Regardless, the drive-by-wire cars will lurch a little in certain circumstances, always at low RPM. It's irritating.

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Thanks guys! I'll take all of that into account when I switch my brain to data acquisition mode while I drive. To give a bit more data: gas take was 3/4 full, ground dry, and it really did feel like a jolt rather than a hesitation. Enough for me to be like "WTF was that??", rather than just an innocuous bump that could be chalked up to any number of things. I take this hill every day to work, so I'll make a point to try to duplicate all the variables and see what happens. I'm glad it's not throwing up immediate alarms and that no one is saying, "oh yeah, that means the front main bearing is ready to spin, the IMS bearing is letting loose, and all the cam followers are ground to nothing." I'll keep you all posted. BTW, the trans is going to be swapped out with a new clutch and IMS upgrade at some point soon too (AFTER the wedding in July!). I've got 54k on the motor, so might as well toss in some new engine mounts too while I'm at it. Darren, actually now that I think about it, I do notice this lurching after startup and driving out of my complex at low RPM! It's nothing like the "jolt" this thread is about, but I sure do notice a surging at low RPM, even after letting the car warm up enough to allow the air pump to turn off like Macster does (I remember reading he tends to wait until the he hears the pump turn off before heading out, and I've adopted that same habit; can't argue with a dude who's got over 250k miles on an M96 motor!!)

heavy load miss, have you changed spark plugs and checked the coils for cracks yet?

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Nope, haven't gotten to plugs yet, but it's on my list. Maybe I'll get to that sooner than I thought. Around 54k on the original plugs and coil packs. When I buy plugs, should I also go ahead and buy a few (or all?) new o-rings, tubes, etc? Will new o-rings being needed be indicated by oil all over the tube? Are cracked coil packs easy to spot? Maybe toss some baby powder on it or something to make any cracks more prominent?

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tubes leak when the o rings are bad or the tube is cracked. your car is an 01'? you may not need to worry about them depending on when the design changed... newer cars dont have the tubes cracks are pretty easy to see, replace the ones that are cracked - leave em alone if not.

I've had some strange jerks in my car too. Never was sure what to do - any advice?

Get new friends?

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LOL!!!!

I can't get it to do it again! I guess that's good, but I don't like unsolved things. I did hit a pot hole going up the same grade this morning, but it didn't feel the same as before, and I distinctly remember before it felt odd, like someone hit me from behind. Like the clutch suddenly grabbed, or the tires suddenly grabbed. But if either of those analogies were true, they would be prefaced by slippage, which I don't think happened. The mystery stays unsolved. I'll keep my eyes and ears open...

Check your transmission mount.

Thx, will do. When I get the chance, gonna check trans mount, engine mounts, and do a plug change which I'm getting towards being do for anyway.

make sure all your wheel bolts are torqued properly.

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I believe you and I have no experience with 996s (save my Turbo) to dispute what you write. My experience with my 02 Boxster and its E-Gas system though has found its behavior at all engine speeds has been superb. The E-Gas system and the control it provides and coupled with the engine's ability to pull cleanly off idle -- well, maybe a bit more than idle, call it 1000 rpms or thereabouts -- when in 3rd gear is quite amazing. Of course I don't give it lots of throttle but 3rd works so fine I find I seldom shift into 2nd while driving on city streets. Based on my experience with my Boxster and its E-Gas system I'm a convert to drive by wire (at least throttle drive by wire) systems. There has been at least one time though when the car exhibited what is hard to accurately describe cause it appeared so suddenly but a brief cut out I think comes close. I had come across info that suggests this could arise from the E-Gas system, I guess a potentimoter encountering a dead spot, and tried to reproduce the symptom to eliminate or confirm E-Gas system needed attention. (230K miles on system so it could be wearing out.) Was unsuccessful and so far it has not happened again even when I wasn't trying to make it happen. Now my 996 Turbo doesn't like operating quite at as low an engine speed as the Boxster. We're talking under 1500 rpms and closer to 1200 or even lower now. The Turbo has a lower compression ratio and just doesn't have the same pick up at these lower engine speeds. Now of course the bigger engine still pulls strongly, but there's a bit of a difference in smoothness and the Turbo engine just doesn't pull quite as smooth, quite as readily, down low. No big deal and I can easily avoid operating the Turbo engine in the low rpm area where it indicates it prefers not to be. Also, I was feeling a kind of a mild sort of lurch and after some time with car and by viewing the boost gage found the lurch -- very mild lurch -- occurs with the appearance of boost or with boost going away. I'm talking about just 0.1 bar boost though not bigger amounts. With bigger amount of boost... well, that's a post for another day. I will say here with bigger levels of boost the car's behavior is quite pleasant to experience though and not bad in any way shape or form other than speed limits cause one to experience this very infrequently and for far too brief a time. Anyhow, back to 0.1 bar boost: I encounter this boost induced lurch generally when I'm driving the car up a grade like over the I-580 freeway between the bay area and the tri-valley area where the road over a distance of 4 or 5 miles climbs a grade from near sea level to IIRC around 1400 feet and the Turbo's boost gage will report some boost, just 0.1 bar, with just a steady pressure on the gas pedal with transmission in 6th (or even 5th) gear. Sincerely, Macster.