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Power Surge when Gas Pedal Released Under Boost?

I've noticed that if I suddenly release the gas pedal while under boost [usually while in the beginning stages of boost], the power continues for about one additional second - And, I continue to accelerate...

While only a second, it seems like an eternity... If I happen to push in the clutch to shift, the RPM's will jump [about 2k] like I still have my foot on the gas...

Is this normal? If so, what is the term/name for it????
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.
Old 11-15-2008, 08:12 PM
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I would check the compressor bypass valve. When you lift it should open and relieve boost pressure from the intake (the run-on you describe is what it's supposed to prevent). I don't know how the stock 930 bypass works, it's that big contraption beneath your intercooler.
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Old 11-15-2008, 10:31 PM
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I wouldn't think the car would continue to accellerate without fuel. Is your throttle sticking?
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Old 11-16-2008, 07:14 AM
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I'm guessing throttle linkage as well!! Especially since your motor was recently out. I would look at the throttle rod that sticks through the sheetmetal first.

Has your clutch cable been replaced recently?
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Old 11-16-2008, 07:25 AM
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I'll look at the linkage, however, I don't see this problem when I'm not under boost... Or, for that matter if the boost/rpm is in the higher stages.

Again, it seems if i release RIGHT WHEN THE BOOST IS COMING ON [maybe at 4k rpm], I get a continued surge for about 1 second...

Rarly: I see your point the fuel. I do have a 7th injector. Could this thing be firing some addl. fuel after throttle released?

I understand the 7th injector operates when it sees boost. Maybe it is still sensing boost, and therefore sending more fuel?
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Old 11-16-2008, 09:59 AM
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If it is the throttle linkage, you shoudl be able to observe by looking at the pedal while you work it. Sometimes the clutch cable can bind against the throttle and slow the return. Push down on the throttle pedal with your hand and observe. I think you would have noticed the pedal sticking though.
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Old 11-16-2008, 01:34 PM
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I had a similar problem when I put my engine back in. If I let off the gas right as I pushed the clutch in it the RPM's would stay up and then slowly drop. If I was in neutral everything was fine, RPM's would drop as normal. Turned out to be the clutch cable was binding with the accelerator cable. Adjusted them and all is good.
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Old 11-16-2008, 05:02 PM
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Mark & Les:

Thanks - I will definitely look at the clutch cable. This matches the symptoms I'm getting...

I'll report back by mid-week.
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Old 11-16-2008, 05:09 PM
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Quote:
Originally Posted by mooney265 View Post
Mark & Les:

Thanks - I will definitely look at the clutch cable. This matches the symptoms I'm getting...

I'll report back by mid-week.
Hi Shannon,
Did the manuals I sent you help?

Les
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Old 11-20-2008, 05:45 AM
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Quote:
Originally Posted by mooney265 View Post
I've noticed that if I suddenly release the gas pedal while under boost [usually while in the beginning stages of boost], the power continues for about one additional second - And, I continue to accelerate...
..............
I had a similar problem when my boost diverter valve hose (from IC to diverter valve) came off. It almost acts like a compressor surge symptom.
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Old 11-20-2008, 08:40 AM
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Quote:
Originally Posted by les_garten View Post
Hi Shannon,
Did the manuals I sent you help?

Les
Les,

I just received the manuals. Didn't fully install, yet. Was able to look thru the 930 manual and was very impressed/informed!

I'm going to a PCA event this weekend. Will have time early next week to install. I'm assuming I'll have to use the install disk first and then follow the prompts to put in the rest of the disks???

Thanks - Shannon
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Old 11-21-2008, 06:00 AM
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Quote:
Originally Posted by WERK-I View Post
I had a similar problem when my boost diverter valve hose (from IC to diverter valve) came off. It almost acts like a compressor surge symptom.
Dave,

I'll have my car at the track this weekend [i'm not racing, though]. And, my mechanic will be there all weekend. I'll have him look at that. Thanks
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.
Old 11-21-2008, 06:01 AM
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Quote:
Originally Posted by mooney265 View Post
Les,

I just received the manuals. Didn't fully install, yet. Was able to look thru the 930 manual and was very impressed/informed!

I'm going to a PCA event this weekend. Will have time early next week to install. I'm assuming I'll have to use the install disk first and then follow the prompts to put in the rest of the disks???

Thanks - Shannon
Just put in the Install disc and I have it setup pretty easy with instructions.Let me know if it is not clear, so I can work on it some more. The other 17 discs are not used unless they are called for. Porsche's are on DVD #8. You may want to label it (Porsche). If you are working on a Chevy, it would call for a different disc. BMW, different disc, etc. Some discs have multiple makes. When you pull up you year Porsche, it will prompt for that disc if it is not in the drive. Then just Pick maintenance, Diagnostics, Technical Service Bulletins, or whatever, and point and click away. Includes all tools needed, part numbers, and costs, instructions, you name it! You'll never understand how you got along without it! VERY easy to use.
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Last edited by les_garten; 11-21-2008 at 06:26 AM..
Old 11-21-2008, 06:22 AM
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I agree with the cable option. There is just not enough volume of air trapped in the intake if the throttle is closed to continue for any length of time. The BOV / recirculation valve dumps the air from the area between the turbo and the throttle body when the pressure drops in the region after the throttle butterfly. If that air is not dumped, the turbo will stall but it still can not get passed the throttle plate unless it is stuck open for some reason. But hey, I could be full of crap ;-).

The easy test is have someone watch the throttle body as you push and jump off the gas while the car is not running.
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Old 11-22-2008, 01:55 PM
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Quote:
Originally Posted by '79 930 Sean View Post
............... But hey, I could be full of crap ;-).
..........................
My vote goes to you being full of crap!!
Just kidding Sean.....welcome to the forum!!

I can duplicate the problem I described in my last post. In my case the leak is where the hose from the IC attaches to the diverter valve. So, I don't even know if the diverter is opening since the surge occurs just as soon as I start releasing pressure from the throttle. It's the weirdest feeling....you're letting up on the gas, and yet.....it starts accelerating more! The surge doesn't last long, maybe a second. It's disconcerting, none the less.
I suspect when I release the throttle, there's still kinetic energy in the turbocharger allowing it to continue providing boost, thus keeping the metering plate down, which in turn provides fuel to the injectors. The loose hose on the diverter valve is allowing this bleed off to occur, fooling the fuel delivery system.
I don't know if this is mooney265's problem, since we are using different IC's. I don't know if he has a long-neck setup or not. The Porsche factory setup has a different type of recirculating valve, but I suspect it could provide similar symptoms.
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Old 11-23-2008, 06:11 AM
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I've never experienced this in my '87 but 2 of my local mechanic friends that have worked on porsches for a living for 20-30 years have experinced it.
Sounds scarey.

They said it is caused by the deceleration valve in the early cars. Maybe it was a defective deceleration valve they were talking about, I don't know.

It is the large gold cad plated vacuum limiter that bypasses air around the throttle body and it's opened by intake manifold vacuum during deceleration.

Removing it makes the car more pleasent to drive.
Old 11-23-2008, 07:14 PM
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JFairman and Werk-I:

This is very good information that I'll bring back to my mechanic. I really need to get this problem fixed - - Like you said, it's pretty scary [for that addl. second].

I'll look for the part over the weekend. JFairman: What should I do if I find it?

Either way, I'll pass this info along to my mechanic to have him look into it.

Thanks - Shannon
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Old 11-25-2008, 03:42 PM
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Power Surge when Gas Pedal Released Under Boost?

Quote:
Originally Posted by mooney265 View Post
I've noticed that if I suddenly release the gas pedal while under boost [usually while in the beginning stages of boost], the power continues for about one additional second - And, I continue to accelerate...
I agree with AFM744, I would check the compressor bypass valve.
A friend purposefully disabled the compressor bypass valve on a track car (why I don't know). The driver damn near killed himself the first time he drove the car hard.
Old 11-25-2008, 04:12 PM
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