Limp Home Mode - Solved!
04-03-2017, 09:37 PM
#1
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Limp Home Mode - Solved!
Kevin Wilson's car.
132k mile 1993 (Third to be imported to the states) GTS A/T with following symptoms:
Limp home mode within 5 miles of driving above 70MPH. (Ignition Monitoring Relay was green)
Idled @ 1400
Faint engine knock under mild acceleration
Engine smelled hot from the driver's seat. Temp ran near the third line always.
Seemed down on power. Not very GTS like.
Travelling down hill with foot off the gas tach would drop from 1500 RPM down to 1200 and then back up. (thought that was the rebuilt transmission acting up)
Did all the reading here about the possibilities. The high idle and knocking under light load nagged at me. I wondered if a failing computer could be the root cause. I have seen a couple of computers that failed in the idle circuit recently.
I swapped in my spare GT computer and every one of those symptoms went away. Runs at the midway mark temp wise, pulls like a train up hills, no knocking. Downhill is now fine. I assume that was the car trying to go into limp home mode. My bet is the engine was running excessively lean.
Kevin now has a proper GTS. All for less than an hour of shop time and a rebuilt GTS computer.
132k mile 1993 (Third to be imported to the states) GTS A/T with following symptoms:
Limp home mode within 5 miles of driving above 70MPH. (Ignition Monitoring Relay was green)
Idled @ 1400
Faint engine knock under mild acceleration
Engine smelled hot from the driver's seat. Temp ran near the third line always.
Seemed down on power. Not very GTS like.
Travelling down hill with foot off the gas tach would drop from 1500 RPM down to 1200 and then back up. (thought that was the rebuilt transmission acting up)
Did all the reading here about the possibilities. The high idle and knocking under light load nagged at me. I wondered if a failing computer could be the root cause. I have seen a couple of computers that failed in the idle circuit recently.
I swapped in my spare GT computer and every one of those symptoms went away. Runs at the midway mark temp wise, pulls like a train up hills, no knocking. Downhill is now fine. I assume that was the car trying to go into limp home mode. My bet is the engine was running excessively lean.
Kevin now has a proper GTS. All for less than an hour of shop time and a rebuilt GTS computer.
Last edited by Kevin in Atlanta; 04-04-2017 at 07:35 AM. Reason: Added ignition monitoring relay information
04-03-2017, 10:23 PM
#3
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Yes.
04-04-2017, 12:21 AM
#4
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That's not "limp mode", it's 'merely' the symptoms of a failed LH controller.
"Limp mode" offers a default fuel injector pulse width that's enough to get you off a highway when the the LH loses airflow signals from the MAF. All the wiring needs to be intact and the controller needs to be working. "Limp home" supports a small amount of load only, as the controller has no idea how much load you are demanding with the skinny pedal. It makes for a fat idle mixture, and not a lot more. You still get a little help via the FPR but nothing more.
"Limp mode" offers a default fuel injector pulse width that's enough to get you off a highway when the the LH loses airflow signals from the MAF. All the wiring needs to be intact and the controller needs to be working. "Limp home" supports a small amount of load only, as the controller has no idea how much load you are demanding with the skinny pedal. It makes for a fat idle mixture, and not a lot more. You still get a little help via the FPR but nothing more.
04-04-2017, 12:31 AM
#5
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Kevin Wilson's car.
132k mile 1993 (Third to be imported to the states) GTS A/T with following symptoms:
Limp home mode within 5 miles of driving above 70MPH.
Idled @ 1400
Faint engine knock under mild acceleration
Engine smelled hot from the driver's seat. Temp ran near the third line always.
Seemed down on power. Not very GTS like.
Travelling down hill with foot off the gas tach would drop from 1500 RPM down to 1200 and then back up. (thought that was the rebuilt transmission acting up)
Did all the reading here about the possibilities. The high idle and knocking under light load nagged at me. I wondered if a failing computer could be the root cause. I have seen a couple of computers that failed in the idle circuit recently.
I swapped in my spare GT computer and every one of those symptoms went away. Runs at the midway mark temp wise, pulls like a train up hills, no knocking. Downhill is now fine. I assume that was the car trying to go into limp home mode. My bet is the engine was running excessively lean.
Kevin now has a proper GTS. All for less than an hour of shop time and a rebuilt GTS computer.
132k mile 1993 (Third to be imported to the states) GTS A/T with following symptoms:
Limp home mode within 5 miles of driving above 70MPH.
Idled @ 1400
Faint engine knock under mild acceleration
Engine smelled hot from the driver's seat. Temp ran near the third line always.
Seemed down on power. Not very GTS like.
Travelling down hill with foot off the gas tach would drop from 1500 RPM down to 1200 and then back up. (thought that was the rebuilt transmission acting up)
Did all the reading here about the possibilities. The high idle and knocking under light load nagged at me. I wondered if a failing computer could be the root cause. I have seen a couple of computers that failed in the idle circuit recently.
I swapped in my spare GT computer and every one of those symptoms went away. Runs at the midway mark temp wise, pulls like a train up hills, no knocking. Downhill is now fine. I assume that was the car trying to go into limp home mode. My bet is the engine was running excessively lean.
Kevin now has a proper GTS. All for less than an hour of shop time and a rebuilt GTS computer.
04-04-2017, 07:26 AM
#6
Rennlist Member
Thread Starter
That car had a history of melted fuses, multiple fuel pump failures and CE panel hacks. Kevin replaced the whole panel with one of my GTS CE panel spares earlier this year. Kevin has the pictures. I recommended he go through all the grounds before this latest failure.
04-04-2017, 07:28 AM
#7
Rennlist Member
Thread Starter
That's not "limp mode", it's 'merely' the symptoms of a failed LH controller.
"Limp mode" offers a default fuel injector pulse width that's enough to get you off a highway when the the LH loses airflow signals from the MAF. All the wiring needs to be intact and the controller needs to be working. "Limp home" supports a small amount of load only, as the controller has no idea how much load you are demanding with the skinny pedal. It makes for a fat idle mixture, and not a lot more. You still get a little help via the FPR but nothing more.
"Limp mode" offers a default fuel injector pulse width that's enough to get you off a highway when the the LH loses airflow signals from the MAF. All the wiring needs to be intact and the controller needs to be working. "Limp home" supports a small amount of load only, as the controller has no idea how much load you are demanding with the skinny pedal. It makes for a fat idle mixture, and not a lot more. You still get a little help via the FPR but nothing more.
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04-04-2017, 10:13 AM
#9
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Question: Can Kevin use his 87 S4 LH computer while he waits on the rebuild?
04-04-2017, 01:46 PM
#11
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Again that is NOT limp mode !! The sensors on the two exhaust ports were reading too far apart on temperature and though there might have been a failure in one of the ignition systems.