knfeparty

Always thought this was a product of reduced back pressure from my muffler bypass. I saw on another thread that there are a few things that can cause hesitation at 1500rpm. It's the only symptom I have; car runs like a scalded dog otherwise and idles smoothly even at cold or hot start. No codes (I have a durametric). Basically if I gently apply gas and hold the throttle steady and gently let out the clutch to smoothly ease away from a light, the car always feels like I lifted off the gas completely for a moment when it gets to 1500 rpm. Not sure if the clutch switch is involved, or Variocam, or O2 sensors...what should I log?

Nedster

I have the same thing on my 40th, at low RPM's if I am just barely pushing the gas pedal in, the car will kind of like slightly jerk forward (barely noticable but it is there) and it seems like a slight change in exhaust tone for that mili second. I also installed the FD mode mufflers on mine as well.

Macster

Well, I have to point out the change in the exhaust could be at the root cause of the symptom/behavior.

The stock engine/exhaust are designed to work together. A change to one part of the engine's total air/exhaust flow path probably will have a negative impact.

You do not state the model year of your car but it has VarioCam (variable intake valve timing) or possibly VarioCam Plus (variable intake valve timing plus low/high intake valve lift control).

My info is that at 1400 RPMs the engine controller varies intake valve timing (and if present intake valve lift).

(I know you wrote 1500 but my observation is the in dash tach reads a bit optimistic, thus 1500 reading is closer to 1400.)

At idle to 1400 intake valve timing is retarded (and low lift is selected if supported). The retarded timing (and low lift) is (are) engaged to improve low RPM throttle response and help reduce emissions. This action is based upon the rest of the engine's intake/exhaust system being intact and properly functioning.

(With my Turbo and its VarioCam Plus I can I believe "see" when the low lift is engaged, as shortly after I start the cold engine the engine experiences a very brief hesitation concurrent with a dip in oil pressure. I believe this is when the DME engages low lift.)

(My Boxster has VarioCam and while I know the valve timing is retarded at low RPMs and advanced at the 1400 RPMs threshold (and retarded again at a bit over 5K RPMs) I can't detect any indication of this. I do know the engine has a world of tractability at low RPMs in part to the timing being adjusted.)

If the exhaust back pressure has been modified from stock the expected, in fact necessary back pressure is not there, and the benefit of the valve timing/lift is not recognized. In fact there can be a negative action.

So, with that out of the way where do you go from here?

Let's assume the exhaust mods are not at the root cause of this behavior.

One suggestion I would make is to avoid doing what you are doing.

You are using the engine in such a way that it is having to go through a valve timing adjustment or possibly even a low to high lift transition under load.

Develop a starting out technique that gets the car rolling with fewer RPMs. 1500 is a bit above the RPMs I use and that are required to get either of my cars moving from a stop.

(Another aside: On flat ground I can get my Boxster moving from idle simply by smoothly and not too fast of course just letting the clutch out and letting the DME adjust idle speed. At no time doing this or then afterwards even gently applying the throttle does the engine give any indication the valve timing is being adjusted. The action is transparent to me behind the steering wheel.)

I have to ask: When was the last time you had the brake and clutch fluid flushed/bled? Believe it or not some clutch (and shifting issues) can arise from clutch fluid that is past it flush/bleed date. (I won't make this post any longer but I learned this recently with my Boxster.)

While on the subject of vital fluids, I assume the oil is of the proper viscosity grade (0w-40, 5w-40 or even 5w-50?) and reasonably fresh? I have to point out the VarioCam and VarioCam Plus systems are all operated using hydraulic pressure supplied by the engine's oiling system. Variable timing and lift action is at the hardware level the result of oil flowing under high pressure through very tight spaces. The oil has to not only flow but it must also lubricate the pins that slide in and out to disengage or engage the high lift portion of the intake lifters.

Let's see... what else.

Well, if you have a car equipped with e-Gas perform an e-Gas recalibration.

Turn the key off. With your foot off the gas pedal this whole process turn the key to on. Do not start the engine. Leave the key on for 60 seconds. (Use a watch to time this or the in dash clock. A few seconds over is not a problem.) After 60 seconds turn the key off. Leave it off for 10 seconds. Done.

With the engine warmed up and at idle check the oil filler tube cap for a leak. Try to move the cap around a bit and listen for any sounds of an air leak or if the engine reacts.

If the behavior persists or if it gets worse or if it is joined by other symptoms or odd behavior, then there are a number of things to consider. Near the top of the list would have to be the AOS.

But let's not get ahead of things.

knfeparty

It's a 40AE and is used almost entirely for autocross. My "normal" launch methods include a bit of tire spin (: and it's only on the street driving with passengers in the car that this problem arises. I use my motive bleeder pretty often since it is used for competition.

To be clear, the car is rolling and the clutch is fully engaged by the time I get to 1500rpm. Applying any amount of throttle gives the same result- not a lot of power, which is normal since the rpms are so low, the little bit of hesitation at 1400-1500rpm ish, and then once it gets to 1800 or so, whoooosh tons of power. I do think it is probably just the muffler bypass and I could always just bolt the mufflers back on to check. I checked e-gas with the durametric and it is working smoothly with no dead spots.

Doing the "clutch only" method with the gas at idle works, or using an insanely low amount of throttle until I get to about 1800 rpm also keeps the hesitation from happening, but...well why would I drive like that?

Oil is Joe Gibbs DT40; I used to have the same issue with Motul xcess 5w-40.

My plan was to set up the durametric data logger to take data as I did normal starts. I was going to log the gas pedal position, misfires, fuel trim, variocam actuation, etc, but wanted some recommendations on what parameters I should log when I did this.

It could be that it's just the muffler bypass causing this, but it might be a variocam solenoid on its way out. I'm trying to tell the difference.

D6lc

Maybe it is a 40 AE thing due to the X51 kit.

Mine stumbles a little at low revs, it used to bug me but now I out it down to character as teh car is fully maintained and no error codes, its definately worse with the Evoms intake, but still did it a little with the standard intake.

knfeparty

Thanks for the replies, everybody. I won't be troubleshooting for awhile, but will post results when I do.

Macster

While the e-Gas checks out ok with the Durametric with no dead spots I'd still do the recalibration thing. Dead spots and incorrect calibration are two different things.

What to log?

Since you are not getting a check engine light I do not think misfires are an issue here. However, I vaguely recall that Durametric can provide data that is used to arrive at a misfire determination and if so if you can view that data to see if the engine is possibly working its way to a misfire that could be useful to know.

I like VarioCam activity (timing adjustment/lift changes) or camshaft timing, ignition advance, load, short term fuel trims.

But you can log what you listed as well. Log as much as you can. Easier to after the fact discard what is not of any value.

D6lc

How do you do the recalibration/reset?

Nedster

You can have misfires and not get a check engine light. I know from experience.

Ahsai

Yes because the DME keeps a misfire counter for each cylinder with a sliding time window and the CEL will only be tripped when the misfire frequency exceeds some preset thresholds. IOW, you could have some misfires but yet not enough to trip the CEL.

You can see these misfire counters in Durametric.

Nedster

Yes, i've had a total misfire P0300 about a year ago, which i saw on the durametric, but no CEL. The other day, with my ongoing issue, i had misfires on all cylinders except #3, and the light was on.

Macster

Specifics should be in the owners manual.

But for my 02 Boxster (and 03 Turbo), with the ignition key off and your foot off the gas pedal (and through this whole process keep your foot off the gas pedal), turn the key to the on position -- not the start position -- and leave the key on for 60 seconds. I use the in-dash clock to time this (I do not have a watch), turning the key to the on position as soon as the minute increments, then turning the key off when the minute increments again.

Leave the key off for at least 10 seconds. Done.

knfeparty

After replacing the clutch and flywheel, I still didn't have the smoothness I was looking for. I replaced the clutch pedal micro switch (the maroon one for cruise control) and it seems better now. Maybe I'm just happy that I can drive it again.

I learned something about these switches. If the plunger is pressed too hard, it makes a clicking sound like a zip tie and recedes into the switch body. This happened on my clutch switch and my cruise control stopped working because the pedal was no longer activating the switch. Fortunately, you can simply pull the plunger back out and restore the original length.

If this switch also controls fuel mapping, I think it was the culprit.

knfeparty

For posterity, I really think this was caused by the switch. I can recreate the problem by pulling up on the clutch pedal, which compresses the switch. As soon as I pull the switch back out, the problem goes away. I believe this would be a non-issue with the clutch helper spring still installed.

balefire

my 40th runs muffler bypass year round. no 1500rpm hesitation for me.
sounds like you've narrowed the culprit.