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1985 stumbles when letting off gas? (Read 1098 times)
bret.allen8
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1985 stumbles when letting off gas?
Nov 8th, 2019 at 6:53pm
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Hello, I recently purchased a 1985 NA 944. It's in perfect mechanical condition except one small thing. When I take the load off the engine, the RPMs drop dangerously low for just a second and then it recovers and idles correctly. I can be in neutral, tap on the throttle, and when it goes back down it drops too much, then goes back up to normal idle RPMs. I assume something controlling the idle speed is not functioning properly. Any help is appreciated, thanks! It's never died on me but gets pretty close to it.
  
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Dave W
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Re: 1985 stumbles when letting off gas?
Reply #1 - Nov 8th, 2019 at 7:12pm
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This is a minor problem in the programming.  There is a way to "fix" it.  You can adjust the TPS so that the idle switch never closes.
  

Dave - black 84 no longer exists. Sad 83 self destructed. All better now. Smiley
... Let's go hit some twisty roads! ...
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bret.allen8
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Re: 1985 stumbles when letting off gas?
Reply #2 - Nov 8th, 2019 at 7:24pm
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Dave W wrote on Nov 8th, 2019 at 7:12pm:
This is a minor problem in the programming.  There is a way to "fix" it.  You can adjust the TPS so that the idle switch never closes.


TPS gotcha. I'll check it out in a little and let you know the results. Thanks!
  
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bret.allen8
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Re: 1985 stumbles when letting off gas?
Reply #3 - Nov 8th, 2019 at 7:36pm
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I found out this issue is referred to as the "944 shudder". Must be common. I'll check TPS and anything else people recommending. I couldn't find the threads earlier because I had no idea how to word it lol.
  
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OhioTom
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Re: 1985 stumbles when letting off gas?
Reply #4 - Nov 15th, 2019 at 9:10pm
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Parking the throttle plate off off the TPS takes the idle air control out of circuit.
Idle rpms are no longer controlled. It will be slow when cold and fast when hot.
Also, While it will solve the "shudder" issue that all our cars have, it will make for a feeling of Run-on when letting off the gas. It will take a while to return to idle rpm now.
Lastly, it will backfire if letting off the throttle at higher rpms.

I tried it once, didn't like it at all.

Here is the real issue explained:
When you let off the gas pedal, the TPS tells the DME (ECU) to cut fuel and air at anything above 1500rpm. So as the motor spins down on a rev, it is seeing no air or fuel.
Once 1500rpm is reached the IAC (idle air control) valve starts to work and fuel is enabled.
however, our motors come down fast and it takes a second for the IAC to react and stabilize.

2 fixes possible:
1. buy an aftermarkt chip that has the "shudder fix" programming. This brings the IAC back on-line at 1800rpm so it can have a chance to react before hitting base idle rpm. It makes it a smoother transition.
2. Raise base idle rpm. This can be tricky as raising the base idle rpm too much can result in all kinds of hesitation issues when pulling away.


Have you Checked the base idle rpm?

It may solve your issue if it is not set right.
Pretty simple to do:
Dis-able the IAC with the motor at full operating temp. Idle should drop to 550rpm (and shudder like mad).
Adjust bypass air flow on throttle body to get rpm correct (550-600). Re-enable the IAC.

If this doesn't help you can try adjusting it up to 650rpm.
Don't go too much thou or other problems will happen.
  

Tom Simpson&&VW/Porsche freak.
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bret.allen8
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Re: 1985 stumbles when letting off gas?
Reply #5 - Dec 19th, 2019 at 5:39pm
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Sorry to revive this thread I lost my account lol. I wish I could chip my 944 but I've got an early DME and can't find much information on chipping one. I've been busy but when I find some time I'll adjust that idle and post my results. Thanks!
  
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OhioTom
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Re: 1985 stumbles when letting off gas?
Reply #6 - Dec 19th, 2019 at 8:07pm
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Most early DME's are not chip-able.
but in late 85' they did make a few DME's that can be chipped.
I manged to find one in a junk car I bought.
I had to solder a socket on the board, then I added the chip.
WOW, what a difference.
The early DME with a chip is a thing of beauty.
Perfect running 100% of the time.

Highly recommended to have a look inside your DME to see if it can be chipped..
  

Tom Simpson&&VW/Porsche freak.
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