[quote]Originally posted by rjp951:
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I've had a hesitation or pulsing type of response in my acceleration for several months. In general, this blip/hesitation happens in the mid-rpm ranges and under medium to heavy throttle. The pulse/pause can't be seen on the tach, but it feels like a series of momentary pauses (2-3 per second) in an otherwise smooth acceleration. It seems that the harder the acceleration, the stronger the pulse. It's driving me nuts.

Thanks
Ron</strong><hr></blockquote>

Ron,
I am having the exact same problem, last night I took off my air filter and MAP body and hooked up a 3" plug with an air fitting drilled through it. Attached to that is a pressure gauge and a ball valve. I charged up my intake system with .5 bar per the factory manual, and whoosh all the pressure leaked under the manifold somewhere. I had replaced all the (outside of the manifold) clamps and checked all the vacuum lines and fittings, which all seemed fine. I am going to take the manifold off in the next few days and find that &#@%*&@#)% leak. I have a feeling that this might be your problem too.
Try charging your intake and listen.
HTH
Jason

PS- Despite the leak I can get full (15psi) boost too.

 

Jason, please post the results of your work once you track down the vacuum leak(s). I've already got a new FPR and damper on the way from Paragon. I'll try to get those installed in the next week and will report on progress as well, weather permitting.

Good luck,
Ron

 

Well, I replaced both the FPR and damper, and to no avail. The car seems to be running stronger and overall better with every one of these replacement projects, but the hesitation is still there.

Jason, any progress in your attempts with the vacuum lines?

Ron

 

rjp951,
It sounds a lot like the problem that I had with my '87 951. The Compressor Bypass Valve was bad and I was lossing boost under accel. If your car still has the stock one in it, it is definitely bad. The stock ones had rubber diaphrams that were good for 60K or 5 years. The new Porsche replacement one has an alluminum diaphram and only costs about $30; Paragon sells them. Reading your earlier posts, I didn't realize that you had a Turbo or I would have mentioned this sooner.

 

i replaced my bypass valve and it actually seemed to magnify problem....problem was actually always there but was under toned by lost boost bleeding out the bypass. if you have a vacuum pump w/ a gauge test the bypass to see if its holding vacuum, if it isnt the valves shot. i havent had much time to pook under the hood to try and find the source of the problem (wanted to today but its miserible outside) but im ordering new intake gaskets along w/ some other stuff from paragon tommorow so i can get under the manifold to replace vacuum lines. best of luck

 

I did replace my bypass valve awhile ago... one of the first attempts at addressing this. I've also replace ignition wires, plugs, rotor, distributor cap, coil, throttle position switch, O2 sensor, and cleaned the AFM, throttle body, replaced clamps on the intercooler pipes, etc.

Next on my list is to get under the intake manifold and replace all the vacuum lines. I've already got the gaskets for the manifold and throttle body, and just waiting for winter to settle in so I won't feel like I missed one last opportunity to take a drive...

Thanks
Ron

 

You mentioned that you cleaned the AFM. To what means did you do this? Did you clean the airflow side or did you remove the black plastic cover that gets you to the electrical side and clean the carbon strip? I cleaned the strip and noticed a better, more linear throttle response. Removing the cover is scary at first since you have to pry all the silicon sealant around the edge of the cover, but once you get past that 10 minute task, it is quite easy.

 

David, I cleaned both. The whole time I spent removing that black cover I kept thinking I was going to crack it and have to spend $400 for a replacement. In any case, I lightly "scrubbed" the contact strip with a clean pencil eraser. No real difference as far as I could tell.

Thanks for the tips.
Ron

 

Ron,
I finally got my car back together, and the hesitation/sputtering is GONE!!!!!!!!!!!!!!!!!!!!!! WHOO HOO!!!!
I replaced all the vacuum/intake associated hoses/lines and replaced the spark plug wires. Problem gone!!
I'm 99% sure it was a vacuum leak at that little "Y" shaped metal piece. I did notice when I took an ohm reading of each plug wire that one of them had a much high resistance than the other three, if that was part of it I don't know.