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928 GT, accelerates fine, misses when cruising

 
Old 01-08-2019, 07:24 PM
  #31  
Kevin in Atlanta
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Before you toss more money please research how to bypass the exhaust sensor relay (assuming you haven't already). Last year while driving Kevin Wilson's GTS it would feel like it was missing at cruise going up slight inclines. This was an issue with the exhaust temp sensors sensing a problem on one of the banks. The result was the relay would trigger the 4 cylinder mode transiently - not fully going into 4 cylinder mode.
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Old 01-09-2019, 12:02 PM
  #32  
worf928
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Originally Posted by Kevin in Atlanta View Post
Before you toss more money please research how to bypass the exhaust sensor relay (assuming you haven't already). Last year while driving Kevin Wilson's GTS it would feel like it was missing at cruise going up slight inclines. This was an issue with the exhaust temp sensors sensing a problem on one of the banks. The result was the relay would trigger the 4 cylinder mode transiently - not fully going into 4 cylinder mode.
Interesting failure mode.
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Old 01-09-2019, 04:21 PM
  #33  
Kevin in Atlanta
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Originally Posted by worf928 View Post


Interesting failure mode.
Like everything I touch. It was the strangest feeling.
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Old 01-09-2019, 05:42 PM
  #34  
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Pull your caps and see if you are missing the center button.

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Old 01-09-2019, 07:44 PM
  #35  
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—^ And check the housings for cracks.

I ran into that once two decades ago: dropped a cap on the shop floor. Didn’t think anything of it at the time. The 928 went straight to limp mode. Wasted a lot of time finding that.
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Old 01-09-2019, 08:33 PM
  #36  
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Originally Posted by macerl View Post
- ran perfect for ~30 miles, then again started missing at cruise speeds when not accelerating.
- still performs fine when accelerating onto freeway and when passing.
- only issue when moving along with traffic at cruise speed.

My next thought is oxygen sensor.
I had the same symptoms. Turned out to be oil seeped into the plug wells from the round seals on the cam covers. The engine would miss at highway speeds, but accel hard.

Remove plugs and note if they are oily around the base and ceramic. Once the plugs are out, vac out the oil from the well, and install new plugs. Ultimately replace the seals on the cam cover. Worked for my 90GT.
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Old 01-09-2019, 09:06 PM
  #37  
macerl
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thank you all for the input.
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Old 01-10-2019, 09:08 AM
  #38  
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Originally Posted by worf928 View Post
—^ And check the housings for cracks.

I ran into that once two decades ago: dropped a cap on the shop floor. Didn’t think anything of it at the time. The 928 went straight to limp mode. Wasted a lot of time finding that.
This was a funny one. Car ran fantastic but as described in the OP there was a slight miss at cruise and you could hear a "burp" at idle that happened every 30 seconds or so. Drove the customer nuts and one day we took the cap off to see that. Problem solved.
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Old 01-10-2019, 03:26 PM
  #39  
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I checked my relays and I already have the bypass relay (928.615.175.00) instead of the real one (928.618.175.00). Must have been installed by previous owner or his mechanic.
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Old 01-10-2019, 03:53 PM
  #40  
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Originally Posted by SeanR View Post
This was a funny one. Car ran fantastic but as described in the OP there was a slight miss at cruise and you could hear a "burp" at idle that happened every 30 seconds or so. Drove the customer nuts and one day we took the cap off to see that. Problem solved.
I’m reminded of another ‘funny’ problem. Intermittent misfire. Plug wires had been replaced by ‘someone else.’

Yup, Used the wrong coil wires. Of course, you know why there are four coil wires in a set.
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Old 01-10-2019, 04:24 PM
  #41  
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So, since I already have a bypass relay installed:
1) this indicates that my current issue is not with the ECU detecting a problem with the exhaust temp sensors (although I do need to get the correct relay; which appears to be pretty expensive).
2) I need to proceed with the injectors.
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Old 01-10-2019, 07:31 PM
  #42  
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How long did it take previously to start missing, you said recently it takes 30mins?
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Old 01-10-2019, 07:48 PM
  #43  
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Before the LH rebuild, it was right away.
After the LH rebuild,on the first time I drove it, it took about 30 minutes.
On the second drive, it was apparent as soon as I got to cruising speed on the nearby freeway; 5 minutes.
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Old 01-10-2019, 10:10 PM
  #44  
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Check coil wire connectors for green corrosion.
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Old 01-11-2019, 04:53 AM
  #45  
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Originally Posted by macerl View Post
Before the LH rebuild, it was right away.
After the LH rebuild,on the first time I drove it, it took about 30 minutes.
On the second drive, it was apparent as soon as I got to cruising speed on the nearby freeway; 5 minutes.
Assuming the coils, leads and distribution caps are all fitted properly and functioning I would say don’t let the LH work become a red herring. It needed done to eliminate it and provide future reliability but the fact it is still running rough at part throttle and only once warmed up, in my mind eliminate electrics and injectors. I refer to my original reply (post 2 or 3) and say you have a small air leak into the manifold which becomes more apparent once the fuelling drops after warm up.

When you accelerate it goes away because the LH adds more fuel and it is not as detectable at warm up as the LH is adding more fuel as standard.

I had the same symptoms when the sealant provided by Carl (928 MS) failed on my intake manifold spacers. Small air passages formed through the sealant resulting in a lean running condition providing the same symptoms you are describing.

I diagnosed the problem and source quickly and replaced the sealant for something more suited to the job. My problem is fixed and the car has been running fine since.
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