MB66

Hello All! Long time lurker, newly registered member. I've put Many hours into researching the forums for my specific issue and could never really find another issue quite like mine, so here i am. Before I get into it, I want to say that I appreciate all the info that the Rennlist community has brought to the table, many of my past questions have been answered with a quick search here on the boards, thank you for that. Now for my 996 issue....

Car "mechanical background": My car is a 2002 911 4S 6 speed with a factory installed replacement motor ( replaced in 2006 and chassis had 20K miles at the time). Chassis now has 97K miles on it and I bought the car when the odometer was reading 75K miles. Car has no computer mods, engine is stock with the exception of the Cat bypass pipes. The front diff was taken out of the vehicle around 85K miles as the carrier bearing was going out and I just didn't reinstall the diff after replacing the bearing. Vehicle sitting on 19" aftermarket wheels.

My issue: One night I was out driving the car thru a long windy road. I was alone and decided to run thru some RPM and some gears like any responsible Porsche owner would! I kept the car in its upper power band thru 2nd, 3rd, and 4th gears, 3000 rpm and above, but never red lining. I jumped on it from a rolling start, so this was not a "dump the clutch and go" kind of approach. Once I got done with my run thru the hills, I came to a stop and proceeded to make a U turn. As I did this, I let off the clutch, began to accelerate regularly, and as soon as the vehicle hit 1300 rpm, it had a "massive hiccup". For my old school mechanics, it felt similar to a carburated muscle car that has the accelerator squirters plugged. As you romp the throttle, the car falls on its face hard and then recovers when the fuel catches up. This is as close as I can describe my Porsches problem, but it is a Much more violent drop out and accel. It falls on its face at 1300 and then almost bangs when it catches up.
The vehicle is my daily driver and it has been like this for some time now. I have learned to feather off the clutch and into the throttle when accelerating from a stop, creating a gentle acceleration thru 1300 and the hiccup is not present or detectable. There is no check engine light, and no codes. The vehicle idles fine, accelerators thru its power band normally (passed 1300 RPM), doesn't make any strange noises, no ticking, no clunking, etc. You would think that nothing is wrong until you hit 1300 RPM on moderate to heavy acceleration. For testing purposes, I've confirmed that it does this hiccup regardless of what gear you are in. The car ran fine with my mods (pipe, wheels, etc) for several thousands of miles prior to my run thru the hills. I do still feel and notice the very common 3200ish RPM "kick" and always have, that part hasn't changed.
I recently pulled the number 1 cylinder side Variocam Timing Solenoid wire and created a check engine light (as I would expect) for testing purposes, and the strange thing is, I drove the car with the timing solenoid unplugged and the "hiccup" was all but gone even when I tried to produce it. Once I plugged the solenoid back in, i was able to easily create the hiccup again.
I pulled the timing solenoid and screen, visually inspected and things "look ok". I manually energized the solenoid and it has movement. I only have a basic understanding from my internet research on variocam plus system function. My best guess, its seeming to me like something is either hanging up (sticking), or damaged with the variocam timing actuator or slack in the chain or something. It just gets me that I have no check engine light and the vehicle runs like a champ all the time, otherwise. I wasnt thinking to condemn the solenoid since it functions when I bench tested it, and was thinking that its engagement was causing my hiccup, but I dont know. What do the Rennlist pros think??

I'm sorry for my long windedness, its just in my nature, haha. I'm just trying to cover all details of my problem. Thanks in advance for any help I may receive!

9964runner

I'll be following this thread as I also have a hiccup around 1300-1500 rpm, its more pronounced with more aggressive right foot and feels like an ignition or fuel cut, mine started when I installed 200cell cats.
So far and with no affect, removed Sprintbooster, disconected the battery, done the key throttle body reset, cleaned the MAF, cleaned the throttle body, and no CEL's to date.

MB66

Yes, exactly as you described 9964runer. If mine had occured right after I installed the Cat bypass pipes, I would suspect the change in back pressure (or lack of) would be causing it, but for me it was definitely immediately after I got done "racing" thru the winding hillside. I had my bypass pipes on the car for about 7-10K miles prior to the hiccup showing up. I haven't tried the throttle body reset or any other potential fixes really, but I have cleared memory and no change after doing this.
I feel a little better that I'm not the only one with this low end "hiccup". Hopefully we find a cure!!!

DBJoe996

Hook up a laptop with Durametric and get some real time values on what is going on. Create a log. Recreate the problem and you should be able to pinpoint what is going on.

fpb111

durametric help
http://www.6speedonline.com/forums/9...fterwards.html

The Radium King

unplug the maf. see what happens.

charlieaf92

Is it consistent at 1300 and no other RPM? I had a similar issue with my Z4 and it turned out to be spark plugs - but it wasn't confined to a specific RPM. As Radium King mentioned, the MAF is a good place to start. I would also do a visual inspection and see if there are any vacuum lines that have come off. To be honest I don't even know if there are any vacuum actuated components that would be active at that RPM but it wouldn't hurt to give it a visual inspection.

MB66

Its ALWAYS at the same RPM, it could be plus or minus 50 rpm, but its right near 1300. The other interesting aspect that I didnt mention in the first post, and hopefully I can explain this clearly enough.... when you are decelerating in gear, you can feel the rolling resistance change when it gets to the same (1300) engine rpm. So if Im in 2rd gear slowing down, there is "heavy" engine resistance, then once the rpm drops to 1300, there is an obvious release in engine resistance, meaning a feeling of less compression of air. This is what was leading me to believe it was a mechanical issue rather than an electrical one.

Its been storming pretty hard here in sunny California, so I haven't had a chance to get out there and give the MAF a shot or really dive in for the vacuum line visual. I would think that with a vacuum line or MAF it would hopefully set a CEL and even alter the idle if a vacuum leak, but I know this isnt always the case. I'll definitely get in there and see what I can see as soon as possible

Bash Hat

Sounds like a fueling issue. When you let off the gas the injectors shut down so you feel the engine braking. The "release" you are feeling sounds like the injectors coming back on line at 1300rpm. Then when accelerating, the injectors are spraying too much fuel the car will stumble from being too rich. Wouldn't that point to a bad MAF?

MB66

Im not saying "No", just feeding the conversation and thoughts. If you accelerate "normally" its hardly noticeable, and once you get passed the 1300 RPM its smooth sailing no matter how hard you are on or off the pedal. I think the 1300 RPM consistency and the hard "bang" aren't consistent with a fuel misfire. Its not really a "stumble" it a pretty violent drop out and then "bang" back to power. I will still be experimenting with the MAF, but I don't feel that this is an overfuelig issue. No lean or rich codes ever presented themselves.
It just all seemed to align to too coincidentally with the variocam 1300 rpm shift, and then theres the interesting point that when I unplug the variocam timing solenoid on bank 1 the "hiccup" goes away and I can not recreate it even with a hard acceleration off the line. Ive felt many ignition and fuel related misfires, and this one just feels like no "drop out" ive ever experienced, tihs is why I'm still so strongly believing its a mechanical issue.
Thank you guys for the feedback so far!!!

I'll unplug the MAF today and give it a run, then report back.

Macster

VarioCam Plus switches from high lift to low lift at around 1300 RPMs. At this point as I under stand it the throttle is not used to control engine speed just the amount of fuel injected. The intake valves are in their low lift mode. This reduces pumping losses at low RPMs, and engine friction, and is intended to reduce emissions at low RPMs.

My Turbo has VarioCam Plus but the only time I am aware of it becoming active is shortly after a cold start when the low lift mode is switched on. The engine has a bit of a hiccup -- really not bad at all though -- as the low lift feature is engaged.

At other times when it should be engaged it is engaged in a manner that is transparent to me.

The mechanism to select between low lift and high lift if driven by oil pressure. It is important the oil be of the right type, multi-viscosity grade, and not have too many miles on it. I have found for my type of usage 5K miles is plenty long enough to run the oil.

If the oil is ok you can try a couple of things. I don't think the behavior is MAF related. But you can disconnect the MAF, clear the codes -- to reset all the fuel trims to their defaults -- then road test the car to see if the behavior is still present.

When the engine controller switches from low lift to high lift and the throttle again controls engine speed the transistion's smoothness can be affected by the E-Gas/throttle calibration. Thus I would recommand you perform an E-Gas calibration. (Foot off the gas pedal through the procedure: With key off turn key on. Leave on at least 60 seconds. Turn key off for at least 10 seconds. Done.)

Use a bottle of Techron. I buy a bottle that treats 20 gallons but dump the entire bottle in the fuel tank then fill up the fuel tank. The advice I receive is if there is any improvement from running Techron to do the treatment again. However, I would modify this to just suggest you run a 2nd bottle regardless. When the fuel tank with the first bottle gets low, say under 1/4 tank, dump in a 2nd bottle and fill up the fuel tank. When this tankful of gasoline is then low again, or soon after, it is advisable to perform an oil/filter service.

After you change the oil/filter you can try Swepco 502 Oil Improver. This is the stuff with (200ppm) micronized Moly in it. You need two bottles to treat the 9+ quarts in the engine. This stuff provides a bit more lubrication to bearing surfaces like valve lifter/cam lobe interfaces and to the pins that are moved via oil pressure to switch the intake lifters from high lift to low lift and vice versa.

MB66

UPDATE:
Ok, just got back from a run. Cleared memory before run, Put the vehicle thru about 6 key cycles and various driving conditions with the MAF disconnected and there was no change in the condition at all. Still brief yet aggessive "hiccup" right at the same RPM as well as the feeling of less "engine load" on deceleration at the same RPM spot as well.
I'm sure this is probably going to come down to scan data review as suggested, I was just hoping someone on the baords would see this and say "YES! I had that exact problem, and this is what fixed it!!"
Haha, there's still hope!!

MB66

I'll give this a looking, thank you for the info!
I did perform an engine and filter change promptly after this problem showed its face (days after I first noted the hiccup, after my hard run thru the hill) and I've always used the Porsche recommended grade and brand. I wanted to be sure that there were no metals in the oil after that run as I had a sick feeling in my stomach thinking the worst at that time. None were present.

KingKong_

Reviving this old thread. Has there been any resolutions to this hiccup outside of a tune? Or can anyone confirm a tune has resolved the issue for them?

Just installed a catless x pipe and experiencing the same issue now.

Thanks!

dan_189

I had a similar issue - very flat spot in first gear around the same maybe higher rpm.

A battery disconnect resolved my issue.