Dead spot on Acceleration 35-37 MPH 1999 996 C2

Kind of an unusual thing has happened three times today... In first gear normal acceleration...when I hit 35 mph holding steady on the gas pedal it feels like there's a big dead spot till 37 mph then everything is normal again. Acceleration is fine. Like I said, happened 3 times today in normal driving conditions at outside temp of 40 degrees no rain, clear. Just changed over from Chevron Supreme to Shell V-power? First tankful today. Do you think that could have anything to do with it. Thx... Jim

Car still cold all three times when that happened?

Whay RPM are you at when this occurs?

Had a similar issue with my car but in all gears at a specific RPM, turned out to be the hall sensors. Ciao, Johannes E.

What are hall sensors?

Rhamy, Hall sensors = Variocam sensors = camshaft rotation speed sensors Ciao, Johannes E.

ok I just mentally deducted 20,000 dirhams

Car war at temp--35 minutes into driving stop in go around town type traffic... Thx... Jim

3500-3700 rmp

Thx...That may be possible...but that normally wouldn't be seen as a "dead spot" with a Hall sensor on the fritz. When Halls are bad there is just more of a drop in RMP. The RPM didn't drop in this case it was more like just a big skip with no drop in RPM (at least after the first time when the problem came up. The first time it just threw me off guard, the second and third time I felt the flat spot and kept my eye on the Tach...no drop. Thx Johannes... Jim

3500-3700 rmp Were you going 35 MPH? You state that you were in first gear and that rpm range does not coincide with your speed and gear. RPM range is more important for these types of problems as some sensors and controls actuivate at specific engine speeds. If in fact it was 35 MPH in first gear, then you are very close to the activation RPM that opens the resonace valve. Let us know what the actual RPM is when this happens. You say that you can hold it steady at that RPM as the problem occurs so you should be able to see exactly the rpm when it is happening.

Jim, When my hall sensors were bad, it happened in Variocam stage 2 at around 6000 rpm in any gear and it was just a short hesitation and then the engine continued to pull. It was very difficult to locate the culprit as the hall sensors. The dealer workshop had to drive the car through all the rpm rage and connect it to the Bosch unit without switching off the engine, or the faults would not be accessible in the memory. They said they had never seen anything like it before. Ciao, Johannes E.

This is commonly known. It's the variocam program switching in at 3250 rpm. See this thread on renntech.org for more info: http://www.renntech.org/forums/index...+shift+install

I'm confused. I am just touching 30 MPH when I shift at 5000 RPMs. Do I have a problem???

Until Jim clarifies what RPM he is talking about, the is no sense in guessing. It's either 35 MPH in first gear, or 3500 rpm.....but NOT both. If it's 35 MPH, then what he is feeling might be the resonance valve opening up at the programmed 5100 RPM.

Ooops... Thx...my bad --- 4700-5200...speed now unkown because its happening constantly now at just over 4700 then again at about 4900, 5100+ then it smooths out... Thx... Jim

Thx for the link Berny... Jim

Like I said above...I goofed on the RPM Ooops.--- its happening in the range of 4700-5200...but now its happening constantly at just over 4700 then again at about 4900, 5100+ then it smooths out...

Ok Jim, try this. The stumble you feel MAY be the resonance valve opening up at 5100 rpm. This valve on your car is on the back intake tube that controls the length of the intake. The normal operation of this valve is to be open from idle - 3100 rpm, closed until 5100 and then open until redline. When the ignition is turned on but the engine is not started, the valve should cycle and remain closed. The valve (which is nothing more than a butterfly plate) is controlled by a vacuum/electric switch (changeover valve) located directly behind your alternator. The swich has an electrical cable to the left and then 2 vaccum ports coming out of the other side. One vacuum line goes to a "Y" connection straight back on the top of the engine and the other goes directly to the resonace valve on the back intake tube. The line to the "Y" connection always has vacuum and if you remove it from the crossover valve you should hear the vacum release (this is with the engine OFF) If you hear the vacuum escape, then this line is connected properly. To test the second vacuum line, disconnect it from the changeover valve and suck on it. You should hear the resonnace valve close. Release the suction and it will open again. You can observe the valve movement by using a flashlight and shining it on the valve. If those 2 tests of the vaccum line are ok, then the vacuum function of the system is working correctly. Remember that the line to the resonance valve is connected to the SIDE nipple on the changeover valve (the one that has a 90 degree rubber elbow). The other line is connected to the nipple that comes straight out the front of the valve. To test the crossover valve operation, start the car and observe the valve movement. It should be open at idle. Rev the engine up to about 3500 rpm and it should close. To see if the valave is giving you that hesitation you are getting, unplug the vacuum line from the changeover valve that goes to the "Y" connection, Again, this is the line that is connected to the straight nipple. Plug this line. You now have eliminated the operation of the resonance valve and maintained porper engine vacuum. Take the car out for a spin and see if you get the same hseitation. You WILL notice a difference in the power band as the resonance valve is open at all times, but if it is the cause of your hesitation, you wil no longer have a problem. Let me know if you need more info. I know on the surface this may seem complicated, but go look at the crossover valve and resonance valve and I think you'll see what I am talking about.

[QUOTE=1999Porsche911]Ok Jim, try this. The stumble you feel MAY be the resonance valve opening up at 5100 rpm. This valve on your car is on the back intake tube that controls the length of the intake. The normal operation of this valve is to be open from idle - 3100 rpm, closed until 5100 and then open until redline. When the ignition is turned on but the engine is not started, the valve should cycle and remain closed... OK... I'll give this a shot tomorrow morning and let you know. Thx for the help, Man. I appreciate... Jim

Thanks for the clarification ... I thought I was going in for a costly repair!!!!!

[QUOTE=1999Porsche911]Ok Jim, try this. The stumble you feel MAY be the resonance valve opening up at 5100 rpm. This valve on your car is on the back intake tube that controls the length of the intake. The normal operation of this valve is to be open from idle - 3100 rpm, closed until 5100 and then open until redline. When the ignition is turned on but the engine is not started, the valve should cycle and remain closed. With a little difficulty, because I didn�t know exactly what I was looking for, I found the Y connection on top drivers side behind alternator. I followed the line to the connection point and I noted that the thin plastic line to the changeover valve was just barely attached�like it was just touching the connection enough and almost totally coming loose. I figured that this could account for a slight vacuum leak especially if the line moved slightly or expanded when the engine was hot. I re-attached it firmly to the valve. �thinking maybe this was the problem�I took her for a spin. Got her warm. Never above 2800 for about 5-6 miles�then drove her the way I normally do, winding her up a bit more and went to 3100, 3500, 4000, all the way up to 5200 no hesitation. Did it several times. No hesitation. Seemed to be operating properly at this point�when I shut her down and then lit her back up about 20 minutes later the CEL flickered on and off a few times like it was trying to decide if there was a problem. Turned her off then back on five or six times and no CEL�Took her for another spin. Turned her off and on a few more times. No CEL. Got here to her crib then attached the OBD II code reader�no codes�drove her again at normal higher revs in first and second gear� Took her out for an ice cream cone and she seems to be just fine and happy! My man�I think I owe you one� Thx� Jim

Glad it worked out for you.