90 GT suddenly running very rough
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From: Wilbraham, MA
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From: Wilbraham, MA
Stan,
I have no experience with the S4s, but when something like this goes wrong I usually do the old 'pull one plug wire at a time' thing. It can knock you on your *** if you do it wrong, but it will tell you if you've got one (or more) cylinder consistently not firing and it's quick and easy to do if your problem is consistent. If the idle changes when you pull one or more, you've found the culprit, now you've got to figure out what's missing (fuel, spark, compression). Mostly likely not a compression issue, so fuel and spark are most likely candidates if one's not firing.
Also, you mentioned the wires, caps, rotors. Are the coils old? What about a loose coil ground? I know you said this happened after a restart, but maybe something worked its way loose.
Actually I just didn't want to say what everyone else said, even though vac leak is the most likely. Just some more possibilities.
I have no experience with the S4s, but when something like this goes wrong I usually do the old 'pull one plug wire at a time' thing. It can knock you on your *** if you do it wrong, but it will tell you if you've got one (or more) cylinder consistently not firing and it's quick and easy to do if your problem is consistent. If the idle changes when you pull one or more, you've found the culprit, now you've got to figure out what's missing (fuel, spark, compression). Mostly likely not a compression issue, so fuel and spark are most likely candidates if one's not firing.
Also, you mentioned the wires, caps, rotors. Are the coils old? What about a loose coil ground? I know you said this happened after a restart, but maybe something worked its way loose.
Actually I just didn't want to say what everyone else said, even though vac leak is the most likely. Just some more possibilities.
So, this leaves me no more certain as to what the problem is.
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Stan-
I pulled this from an older post by Rich Andrade...
Since you're getting a green light, that points to Cyl 2-3-5-8 which should be fed by the driver's side coil. Be sure to chek your coil and coil wire if you haven't already.
I should point out that in 89 the EGT sensors were in Cyl 4 & 8, but then later (91?) moved them to 3 & 7. I'm not sure if Porsche changed the color coding or not.... Can anyone confirm for Stan that green means 2-3-5-8 and which cylinders have the EGT's?
Anyway, where I'm going with this, is that the IMR really only measure 2 cylinders - either 4&8 or 3&7.
Further, it assumes the other 3 on that bank are behaving similarly. So, if your coil check's out, it could be something wrong with that cylinder, or the sensor itself. I would try Rich's suggestion to measure the resistance between the two on a cold engine and then maybe after it gets up to temp.
I pulled this from an older post by Rich Andrade...
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.
Check the ignition circuits and repair as necessary. If no problem is found with the ignition circuit, there may be a failure of the Ignition Circuit Monitoring Relay circuit, the signals to test are:
1. Terminal 31: ground
2. Ground must be present at terminal AL when the ignition switch is in the off position.
3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition is in the on position.
4. Battery voltage must be present at terminals AL and 61 when the engine is running.
5. A Voltage value of approximately 2.7 V must be present at both terminals E1 and E2 when the ignition is in the on position.
6. The resistance between E1 and E2 is approximately 5 – 10 Ohms (measured at the disconnected relay socket).
The temperature sensors that fit in the exhaust system should also be inspected.
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.
Check the ignition circuits and repair as necessary. If no problem is found with the ignition circuit, there may be a failure of the Ignition Circuit Monitoring Relay circuit, the signals to test are:
1. Terminal 31: ground
2. Ground must be present at terminal AL when the ignition switch is in the off position.
3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition is in the on position.
4. Battery voltage must be present at terminals AL and 61 when the engine is running.
5. A Voltage value of approximately 2.7 V must be present at both terminals E1 and E2 when the ignition is in the on position.
6. The resistance between E1 and E2 is approximately 5 – 10 Ohms (measured at the disconnected relay socket).
The temperature sensors that fit in the exhaust system should also be inspected.
I should point out that in 89 the EGT sensors were in Cyl 4 & 8, but then later (91?) moved them to 3 & 7. I'm not sure if Porsche changed the color coding or not.... Can anyone confirm for Stan that green means 2-3-5-8 and which cylinders have the EGT's?
Anyway, where I'm going with this, is that the IMR really only measure 2 cylinders - either 4&8 or 3&7.
Further, it assumes the other 3 on that bank are behaving similarly. So, if your coil check's out, it could be something wrong with that cylinder, or the sensor itself. I would try Rich's suggestion to measure the resistance between the two on a cold engine and then maybe after it gets up to temp.
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From: Wilbraham, MA
Stan-
I pulled this from an older post by Rich Andrade...
Since you're getting a green light, that points to Cyl 2-3-5-8 which should be fed by the driver's side coil. Be sure to chek your coil and coil wire if you haven't already.
I should point out that in 89 the EGT sensors were in Cyl 4 & 8, but then later (91?) moved them to 3 & 7. I'm not sure if Porsche changed the color coding or not.... Can anyone confirm for Stan that green means 2-3-5-8 and which cylinders have the EGT's?
Anyway, where I'm going with this, is that the IMR really only measure 2 cylinders - either 4&8 or 3&7.
Further, it assumes the other 3 on that bank are behaving similarly. So, if your coil check's out, it could be something wrong with that cylinder, or the sensor itself. I would try Rich's suggestion to measure the resistance between the two on a cold engine and then maybe after it gets up to temp.
I pulled this from an older post by Rich Andrade...
Since you're getting a green light, that points to Cyl 2-3-5-8 which should be fed by the driver's side coil. Be sure to chek your coil and coil wire if you haven't already.
I should point out that in 89 the EGT sensors were in Cyl 4 & 8, but then later (91?) moved them to 3 & 7. I'm not sure if Porsche changed the color coding or not.... Can anyone confirm for Stan that green means 2-3-5-8 and which cylinders have the EGT's?
Anyway, where I'm going with this, is that the IMR really only measure 2 cylinders - either 4&8 or 3&7.
Further, it assumes the other 3 on that bank are behaving similarly. So, if your coil check's out, it could be something wrong with that cylinder, or the sensor itself. I would try Rich's suggestion to measure the resistance between the two on a cold engine and then maybe after it gets up to temp.
With regards to testing the EGTs, is there a photo or diagram anywhere that I can review so I know what I am looking for? I am guessing this is accessed from the underside of the motor, correct?
Since I am seeing a spark to all cylinders, I am presuming the coil and spark plug wires can not be the issue. Am I missing something there?
Just another area I will end up with knowledge in, more reasons to always have 928s
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From: Anaheim California
Andrew it is NOT one bank !! it is the cylinders which are on the same coil , same distributor.....So yes you could pull all the wires on one distributor and the car IF ALREADY is running on the OTHER COIL, OTHER DISTRIBUTOR it could care less ! Same result if the special relay as killed the injection to four cylinders (two on each bank) pulling the plug wire on a cylinder that is getting no fuel is a none event.
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The egt's are easy to spot from the underside. There's a clip on the back of each head that holds a connector..and from there the wire goes to the respective cylinder..under the exhaust port..on the 89 and 90..it's 4 and 8...91 and up went to 3 and 7. If you unscrew the EGT..it is a thin metal rod that pokes just barely into the exhaust manifold. These ports are on all cylinders and the probes theoretically can be moved to any one....
I believe Mark A leaves them connected...but hangs them off to the side so they will not see a temperature difference between cylinders...
I believe Mark A leaves them connected...but hangs them off to the side so they will not see a temperature difference between cylinders...
Last edited by Tom. M; Jul 11, 2008 at 12:35 PM. Reason: correction on EGT probe year range
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From: Atlanta, GA
I don't have a pic handy, but the EGT sensors screw in just under the cylinder's exhaust port directly upwards. And yes, it was cylinders 4 & 8 (all the way to the firewall) for '89 and '90, then went to cylinders 3 & 7 as of '91.
Dan
'91 928GT S/C
Dan
'91 928GT S/C
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From: Back in Michigan - Full time!
Andrew it is NOT one bank !! it is the cylinders which are on the same coil , same distributor.....So yes you could pull all the wires on one distributor and the car IF ALREADY is running on the OTHER COIL, OTHER DISTRIBUTOR it could care less ! Same result if the special relay as killed the injection to four cylinders (two on each bank) pulling the plug wire on a cylinder that is getting no fuel is a none event.
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From: Wilbraham, MA
Andrew it is NOT one bank !! it is the cylinders which are on the same coil , same distributor.....So yes you could pull all the wires on one distributor and the car IF ALREADY is running on the OTHER COIL, OTHER DISTRIBUTOR it could care less ! Same result if the special relay as killed the injection to four cylinders (two on each bank) pulling the plug wire on a cylinder that is getting no fuel is a none event.
. Doesn't this confirm all the electrical components? In order for each plug to get an electrical impulse, doesn't this mean the coils, coil wires, ignition modules & spark plug wires can be all ruled out?
2. The ignition protection relay finds a fault on the driver's bank of cylinders after a short period of running, so puts the green light on and disables the fuel injectors for that bank of cylinders. Running (for a short time) with the bypass relay in place, the car runs better but still not right. With the bypass relay in place, I can pull spark plug wires from each dist cap (and see spark) but the motor seems to keep running about the same.
. Shouldn't there be a noticeable difference when a plug wire is removed?
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From: Atlanta, GA
I have confirmed that the spark plugs look good and using a timing light that all cylinders are receiving a spark.
. Doesn't this confirm all the electrical components? In order for each plug to get an electrical impulse, doesn't this mean the coils, coil wires, ignition modules & spark plug wires can be all ruled out?
2. The ignition protection relay finds a fault on the driver's bank of cylinders after a short period of running, so puts the green light on and disables the fuel injectors for that bank of cylinders. Running (for a short time) with the bypass relay in place, the car runs better but still not right. With the bypass relay in place, I can pull spark plug wires from each dist cap (and see spark) but the motor seems to keep running about the same.
. Shouldn't there be a noticeable difference when a plug wire is removed?
. Doesn't this confirm all the electrical components? In order for each plug to get an electrical impulse, doesn't this mean the coils, coil wires, ignition modules & spark plug wires can be all ruled out?
2. The ignition protection relay finds a fault on the driver's bank of cylinders after a short period of running, so puts the green light on and disables the fuel injectors for that bank of cylinders. Running (for a short time) with the bypass relay in place, the car runs better but still not right. With the bypass relay in place, I can pull spark plug wires from each dist cap (and see spark) but the motor seems to keep running about the same.
. Shouldn't there be a noticeable difference when a plug wire is removed?
Dan
'91 928GT S/C
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I think this problem has MAF written all over it. I cured a similar condition last year (very rich running, low instantaneous mpg, low power, extra-deep rumble to the exhaust sound, lugging engine at low rpm) on my '89 GT with the replacement of the MAF (rebuild from 928intl.com). Stan, check your MAF connections and test the MAF (although my mechanic says that it won't necessarily test bad).
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!
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From: Anaheim California
Note that ...When ignition monitoring circuit 2 (cylinders 2,3,5,8) is shut-down, a green LED is illuminated.... the FUEL injection is turned OFF on #2 and #3 cylinders on one SIDE of the engine #5 and #8 cylinders on the OTHER side . The spark however if the shut down is a false one (bad relay, bad sensors,injector issue ,air leak ) will continue to fire for all 8 spark plugs. If you happen to pull a lead for 1,4,6,7 the engine would go from 4 to 3 cylinders which should be noticable. BUT pull 2,3,5,8 plug wire and the running should be the same. There is a time delay built into the system so the car needs to run long enough for the diode to illuminate.
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From: Wilbraham, MA
I think this problem has MAF written all over it. I cured a similar condition last year (very rich running, low instantaneous mpg, low power, extra-deep rumble to the exhaust sound, lugging engine at low rpm) on my '89 GT with the replacement of the MAF (rebuild from 928intl.com). Stan, check your MAF connections and test the MAF (although my mechanic says that it won't necessarily test bad).
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!
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From: Wilbraham, MA
I think this problem has MAF written all over it. I cured a similar condition last year (very rich running, low instantaneous mpg, low power, extra-deep rumble to the exhaust sound, lugging engine at low rpm) on my '89 GT with the replacement of the MAF (rebuild from 928intl.com). Stan, check your MAF connections and test the MAF (although my mechanic says that it won't necessarily test bad).
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!
That said, I had a return of the problem last night on a short drive... it started fine, but as I got moving it suddenly switched back to this lugging performance (sounds like the "green light" issue being described). I stopped at the Kwik-e-mart, and it was fine and dandy when it restarted. Good thing I have a backup MAF on the way...
Good luck!!