Engine switches to 4 cylinder mode
05-31-2009, 08:51 PM
#1
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Thread Starter
Engine switches to 4 cylinder mode
1990 S4 Auto
While on the way to a friends for lunch today (2hr each way) my S4 had a change of engine note 20min into the trip near the bottom of a longish hill while under no accellerator pedal (engine braking) and then proceeded to run rough and with reduced power. Drove carefully to the next town (8km) and pulled over & looked at the ignition relay & the red LED was lit (not flashing). The morning was cool -1°C.
I let the car sit (had something to eat) and started it up again - everything was normal. I then drove home - again with no issues (or lit up LED's).
From my searching the red LED means the cylinder group incl no 1 is disabled.
Now that there is no problem, should I just pretend it diddn't happen or what?
Your advice / experiences please.
Myles
While on the way to a friends for lunch today (2hr each way) my S4 had a change of engine note 20min into the trip near the bottom of a longish hill while under no accellerator pedal (engine braking) and then proceeded to run rough and with reduced power. Drove carefully to the next town (8km) and pulled over & looked at the ignition relay & the red LED was lit (not flashing). The morning was cool -1°C.
I let the car sit (had something to eat) and started it up again - everything was normal. I then drove home - again with no issues (or lit up LED's).
From my searching the red LED means the cylinder group incl no 1 is disabled.
Now that there is no problem, should I just pretend it diddn't happen or what?
Your advice / experiences please.
Myles
06-02-2009, 12:12 AM
#2
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Thread Starter
Ah well, looks like there is no opinion out there either way so I guess I will just keep driving it untill it breaks down again.
Myles
Myles
06-02-2009, 01:04 AM
#3
Team Owner
you could remove the relay and check the pins for corrosion. Possibly open it to check the internal contacts, also look at the Ce panel for water tell tales if you see rusty screw threads then its leaking water, time for a blower box reseal also make sure that you have a hot post cover fitted
06-02-2009, 01:40 AM
#4
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I'll provide a system overview an then comment on Myles' question.
The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 is comprised of cylinders 2,3,5,8, the left side coil and distributor. If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system. When the raw fuel hits the hot catalytic converters a fire may occur – OUCH!
From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit. The inputs to the relay are two exhaust temperature sensors that provide input to the relay. If one of the ignition circuits fails, the “Ignition Circuit Monitoring Relay” shuts off the pulse signal to all of the fuel injectors of the affected circuit.
As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.
The first step is to quickly identify which circuit is affected. Look at the clear relay module that is fitted next to the EZK spark control unit in the passenger compartment.
When ignition circuit 1 (cylinders 1,4,6,7) is shut-down, a red LED is illuminated.
When ignition circuit 2 (cylinders 2,3,5,8) is shut-down, a green LED is illuminated.
A more detailed explanation is provided in the factory workshop manuals.
=====================
Myles mentioned that he was going down a long hill with the throttle closed. In this case, the fuel injection is cut off, so the exhaust will cool off. The exhaust gas sensors probably have a different output as the exhaust gas cooled, the fuel cut-off system sensed too large of a difference and shut one fuel circuit off. The next time the engine was run the sensors were within spec.
So, I don't see a problem if you keep driving and see if another 4 cylinder shut down cycle occurs. If not, it was an isolated problem. If you lose a lot of power don't continue driving. You'll need to determine if one of the ignition circuits failed or if you need to diagnose the fuel cut-off system. I hear that the temperature sensors are pretty expensive so make sure they're the problem.
The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 is comprised of cylinders 2,3,5,8, the left side coil and distributor. If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system. When the raw fuel hits the hot catalytic converters a fire may occur – OUCH!
From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit. The inputs to the relay are two exhaust temperature sensors that provide input to the relay. If one of the ignition circuits fails, the “Ignition Circuit Monitoring Relay” shuts off the pulse signal to all of the fuel injectors of the affected circuit.
As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.
The first step is to quickly identify which circuit is affected. Look at the clear relay module that is fitted next to the EZK spark control unit in the passenger compartment.
When ignition circuit 1 (cylinders 1,4,6,7) is shut-down, a red LED is illuminated.
When ignition circuit 2 (cylinders 2,3,5,8) is shut-down, a green LED is illuminated.
A more detailed explanation is provided in the factory workshop manuals.
=====================
Myles mentioned that he was going down a long hill with the throttle closed. In this case, the fuel injection is cut off, so the exhaust will cool off. The exhaust gas sensors probably have a different output as the exhaust gas cooled, the fuel cut-off system sensed too large of a difference and shut one fuel circuit off. The next time the engine was run the sensors were within spec.
So, I don't see a problem if you keep driving and see if another 4 cylinder shut down cycle occurs. If not, it was an isolated problem. If you lose a lot of power don't continue driving. You'll need to determine if one of the ignition circuits failed or if you need to diagnose the fuel cut-off system. I hear that the temperature sensors are pretty expensive so make sure they're the problem.
06-02-2009, 03:08 AM
#5
Rennlist Member
Thread Starter
Thanks for the answers.
Mrmerlin - I have checked the main panel for corrosion in the last year without any problems. I am pretty sure this will be the case for the relay as well and I am loathe to pull it unless this happens again.
Rich,
That was my feeling as well as the failure occured under no load on a cold morning - I obviously need to keep accelerating down these hills ;-).
I have had a chance to search through further posts and see that this can be a once in a while issue and now know to turn the car off asap and if it restarts & runs ok for a few minutes then it should be good to go.
Now that I know what to expect I will have more confidence in continuing on the journey if it happens again (AND self corrects).
Cheers,
Myles
Mrmerlin - I have checked the main panel for corrosion in the last year without any problems. I am pretty sure this will be the case for the relay as well and I am loathe to pull it unless this happens again.
Rich,
That was my feeling as well as the failure occured under no load on a cold morning - I obviously need to keep accelerating down these hills ;-).
I have had a chance to search through further posts and see that this can be a once in a while issue and now know to turn the car off asap and if it restarts & runs ok for a few minutes then it should be good to go.
Now that I know what to expect I will have more confidence in continuing on the journey if it happens again (AND self corrects).
Cheers,
Myles
06-02-2009, 06:31 AM
#6
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Myles
the other alternative as your in NZ and (unless they changed the rules since I lived there) with no emissions testing you could just ditch the cats, fit an X pipe instead, then you don't need the ignition monitoring relay. Should you decide to do this remove the relay and install a bridge across three of the pins in the relay socket - can't remember which ones but I can look it up if you need it.
the other alternative as your in NZ and (unless they changed the rules since I lived there) with no emissions testing you could just ditch the cats, fit an X pipe instead, then you don't need the ignition monitoring relay. Should you decide to do this remove the relay and install a bridge across three of the pins in the relay socket - can't remember which ones but I can look it up if you need it.
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06-02-2009, 07:42 AM
#8
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Thread Starter
Jon in OZ,
no emissions testing yet - I would love to fit an X pipe but they are expensive to ship & I still have some PM to catch up on - ie intake refresh et al.
John,
I will leave it alone at the moment. If it continues to play up I will consider jumpering. At the moment at least it provides peace of mind if a real problem occurs.
Cheers,
Myles
no emissions testing yet - I would love to fit an X pipe but they are expensive to ship & I still have some PM to catch up on - ie intake refresh et al.
John,
I will leave it alone at the moment. If it continues to play up I will consider jumpering. At the moment at least it provides peace of mind if a real problem occurs.
Cheers,
Myles
06-02-2009, 09:07 AM
#9
Team Owner
Rich , thanks for taking the time to spell things out on the ops of this relay.
06-02-2009, 09:23 AM
#10
Team Owner
From an operations stand point if your going down a long hill drop the shifter back to 3 to keep some drag on the driveline this will then slow you down some so you will need to push the gas pedal thus keeping the engine under power
06-02-2009, 02:11 PM
#12
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You can still have a car fire with no cats if the unburnt fuel ignites. So just ditching the cats isn't the solution if one of the ignition circuits fails! If the spark monitoring protection system fails, then it can be disconnected and jumpered "out" but the onus falls on the driver to NOT keep driving when the ignition fails.
