Eng Prob - U never heard of this one
10-21-2013, 06:05 PM
#1
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Eng Prob - U never heard of this one
Some of you may remember that several months ago I had some extensive engine work done, but could not get my car out of the shop. Everyone here was very helpful with their suggestions. That is very much appreciated.
I got my car back about three months ago, and since that time I have put about 1,500 miles on the car. The engine runs perfect. That is until a couple of days ago. Here's what happened.
I was running some errands that day and the engine was running perfect, as normal. I stopped to talk to a friend and turned the engine off for no more than 60 seconds. When I started the engine it immediately, and I mean immediately, ran rough as if running on four cylinders in the limp home mode. I was not too far from home so I was able to get it into my driveway although I was doubtful all the time it would keep running.
Because of the bad experience I had with the previous repair shop I decided to have it towed to RennHouse in Sarasota, Florida. They come very highly recommended and advertise in the Porsche magazine. After a few hours inspection they gave me a call and said they found the spark plug gap on three sparkplugs to be completely closed. They regapped the plugs and took it for a test drive. The engine ran perfectly for 15 minutes and then transitioned into the previous rough running condition. They again removed the spark plugs and again the gap was completely closed on those three plugs.
So this is where we are now. No one has any idea why the gap on these plugs would be closed. My suggestion, which they are going to do, is to regap the plugs and install them using an additional washer. Hopefully that will raise the plug and solve the problem. This cure has not yet been accomplished, but it should be shortly.
I'm sure you have not heard of this problem before, but if anyone has a possible cause and cure I certainly would appreciate hearing from you.
I got my car back about three months ago, and since that time I have put about 1,500 miles on the car. The engine runs perfect. That is until a couple of days ago. Here's what happened.
I was running some errands that day and the engine was running perfect, as normal. I stopped to talk to a friend and turned the engine off for no more than 60 seconds. When I started the engine it immediately, and I mean immediately, ran rough as if running on four cylinders in the limp home mode. I was not too far from home so I was able to get it into my driveway although I was doubtful all the time it would keep running.
Because of the bad experience I had with the previous repair shop I decided to have it towed to RennHouse in Sarasota, Florida. They come very highly recommended and advertise in the Porsche magazine. After a few hours inspection they gave me a call and said they found the spark plug gap on three sparkplugs to be completely closed. They regapped the plugs and took it for a test drive. The engine ran perfectly for 15 minutes and then transitioned into the previous rough running condition. They again removed the spark plugs and again the gap was completely closed on those three plugs.
So this is where we are now. No one has any idea why the gap on these plugs would be closed. My suggestion, which they are going to do, is to regap the plugs and install them using an additional washer. Hopefully that will raise the plug and solve the problem. This cure has not yet been accomplished, but it should be shortly.
I'm sure you have not heard of this problem before, but if anyone has a possible cause and cure I certainly would appreciate hearing from you.
10-21-2013, 06:28 PM
#5
Rennlist Member
First: There is no such thing as a four cylinder "Limp Home Mode" so lets get that misnomer out of the discussion.
What you are describing is the CAT protection relay killing the fuel injection to four cylinders, based on which Ignition bank (1,6,7,4) or (5,2,3,8) it thinks is not working, it does this by sensing the Head temp at cyl 3 and 7.
The/a MAF failure has a "Limp Home mode" where as the engine will run without the MAF, won't run well but will run.
As for three spark plugs getting clobbered again and again, Carbon build up on pistons?
I would ask to see the parts, and I would suggest they install new all new plugs and number the ones they remove.
Sounds like bore scope time, any idea which three?
Edit: also I would wonder if any of the valves could also be bent, as I would think they would hit before the plugs.
What you are describing is the CAT protection relay killing the fuel injection to four cylinders, based on which Ignition bank (1,6,7,4) or (5,2,3,8) it thinks is not working, it does this by sensing the Head temp at cyl 3 and 7.
The/a MAF failure has a "Limp Home mode" where as the engine will run without the MAF, won't run well but will run.
As for three spark plugs getting clobbered again and again, Carbon build up on pistons?
I would ask to see the parts, and I would suggest they install new all new plugs and number the ones they remove.
Sounds like bore scope time, any idea which three?
Edit: also I would wonder if any of the valves could also be bent, as I would think they would hit before the plugs.
Last edited by davek9; 10-21-2013 at 06:53 PM.
10-21-2013, 06:35 PM
#6
Rennlist Member
Putting a washer under the plug head isn't a solution .... if the gaps are being hammered closed, and they're the correct ones, then there's a big problem somewhere else you need to find. I'd be checking the plugs are correct, and then using an endoscope to check every cylinder for piston damage or carbon buildup, and not running it again until I found a logical cause that I could correct.
BTW ... a spark plug should only be installed with the crush washer that is fitted to it, which is designed to not only give a seal, but also to transfer heat from the plug to the head. If you fit another washer you'll alter the heat range of the plug.
BTW ... a spark plug should only be installed with the crush washer that is fitted to it, which is designed to not only give a seal, but also to transfer heat from the plug to the head. If you fit another washer you'll alter the heat range of the plug.
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10-21-2013, 08:32 PM
#8
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Putting a washer under the plug head isn't a solution ....
BTW ... a spark plug should only be installed with the crush washer that is fitted to it, which is designed to not only give a seal, but also to transfer heat from the plug to the head. If you fit another washer you'll alter the heat range of the plug.
BTW ... a spark plug should only be installed with the crush washer that is fitted to it, which is designed to not only give a seal, but also to transfer heat from the plug to the head. If you fit another washer you'll alter the heat range of the plug.
At least you know the Cat protection system is working and keeping the car from burning to the ground by not pumping raw fuel into the cylinders that are not firing.
Please let us know what you find out on this one.
10-21-2013, 08:39 PM
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What is the "MAF" ? From your description that sounds like what is happening.
As for the sparkplugs, the mechanic says he has always used NGK plugs in all Porsches. He ordered a set, but said they are exactly the same length as the plugs he removed. However, I don't know the brand of plugs he removed.
Note that when he recapped the original plugs the engine ran perfectly for about 15 minutes. Would this happen if there is carbon build up on top of the Pistons? Also, as mentioned, would not this carbon build up interfere with the valves as well?
Whatever the problem is, it's like a light switch. It's either on or off. Either the engine runs perfectly or it runs on four cylinders. At least that's the way it appears to me.
As for the sparkplugs, the mechanic says he has always used NGK plugs in all Porsches. He ordered a set, but said they are exactly the same length as the plugs he removed. However, I don't know the brand of plugs he removed.
Note that when he recapped the original plugs the engine ran perfectly for about 15 minutes. Would this happen if there is carbon build up on top of the Pistons? Also, as mentioned, would not this carbon build up interfere with the valves as well?
Whatever the problem is, it's like a light switch. It's either on or off. Either the engine runs perfectly or it runs on four cylinders. At least that's the way it appears to me.
10-21-2013, 09:13 PM
#12
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The correct plug is an extended tip spark plug, with 3/4" threads....I can't think of a spark plug with more "reach" than that one...although my "spark plug" experience is certainly limited to Porsche.
I'm trying to imagine an engine with enough carbon build-up to close the gap on the spark plugs...can't do that.
There must be some little person inside the combustion chamber hammering them closed?
I think I'd get to the bottom of this, before driving it very far...it takes some major "piston slop" for the piston to hit the spark plug.....like there are no rod bearings left?
I'm trying to imagine an engine with enough carbon build-up to close the gap on the spark plugs...can't do that.
There must be some little person inside the combustion chamber hammering them closed?
I think I'd get to the bottom of this, before driving it very far...it takes some major "piston slop" for the piston to hit the spark plug.....like there are no rod bearings left?
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greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
10-21-2013, 09:34 PM
#14
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In my limited experience, detonation is the only reliable force that closes plug gaps, short of mechanical interference.
The protection relay mentioned above looks at a thermocouple on either side of the engine, screwed into the exhaust manifolds. Each distributor fires plugs on both sides of the engine, so a sensor is under a cylinder for the right distributor and and the other sensor is under the outlet from a cylinder fired by the other distributor. The sensors are thermocouples, and so generate a tiny voltage when heated. The relay looks at the imbalance between the two thermocouples after the exhausts should be hot, and shuts down fuel to the cylinders served by a distributor that are colder. There's a delay on cold start, any start actually, before the relay makes a shut-down decision. Most-common system failures are with the sensors. Most common causes of action are coil wires that have come loose, followed by ignition wire failures, coil failures and plug faults. I'd start by verifying that the coil wires on both sides are intact and seated properly. Look at the plugs obviously, and decide if the electrodes are bent as your shop describes, or something else like a fauked coil or cracked/broken distributor part is to blame.
An inductive timing light can be a big help with diagnosing. Test each cylinder while the engine is running, even if the protection relay has cut off fuel to half the cylinders. The ignition system should still send spark to all plugs. A fouled or shorted-electrode plug will show a weak spark pulse if at all.
You can see the bottom of each of the thermocouples under the exhaust on each side of the engine underneath, forward of the downpipe. There's a stiff cable shield running to each one. Somtimes the cable/shield gets bent during major work or it may not have been reinstalled tight enough. Do whatever you can to keep the system functioniong; too many cars have burned down in the driveway from cat fires as owners tried to diagnose the "miss".
The protection relay mentioned above looks at a thermocouple on either side of the engine, screwed into the exhaust manifolds. Each distributor fires plugs on both sides of the engine, so a sensor is under a cylinder for the right distributor and and the other sensor is under the outlet from a cylinder fired by the other distributor. The sensors are thermocouples, and so generate a tiny voltage when heated. The relay looks at the imbalance between the two thermocouples after the exhausts should be hot, and shuts down fuel to the cylinders served by a distributor that are colder. There's a delay on cold start, any start actually, before the relay makes a shut-down decision. Most-common system failures are with the sensors. Most common causes of action are coil wires that have come loose, followed by ignition wire failures, coil failures and plug faults. I'd start by verifying that the coil wires on both sides are intact and seated properly. Look at the plugs obviously, and decide if the electrodes are bent as your shop describes, or something else like a fauked coil or cracked/broken distributor part is to blame.
An inductive timing light can be a big help with diagnosing. Test each cylinder while the engine is running, even if the protection relay has cut off fuel to half the cylinders. The ignition system should still send spark to all plugs. A fouled or shorted-electrode plug will show a weak spark pulse if at all.
You can see the bottom of each of the thermocouples under the exhaust on each side of the engine underneath, forward of the downpipe. There's a stiff cable shield running to each one. Somtimes the cable/shield gets bent during major work or it may not have been reinstalled tight enough. Do whatever you can to keep the system functioniong; too many cars have burned down in the driveway from cat fires as owners tried to diagnose the "miss".
10-21-2013, 09:39 PM
#15
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In my limited experience, detonation is the only reliable force that closes plug gaps, short of mechanical interference.
The protection relay mentioned above looks at a thermocouple on either side of the engine, screwed into the exhaust manifolds. Each distributor fires plugs on both sides of the engine, so a sensor is under a cylinder for the right distributor and and the other sensor is under the outlet from a cylinder fired by the other distributor. The sensors are thermocouples, and so generate a tiny voltage when heated. The relay looks at the imbalance between the two thermocouples after the exhausts should be hot, and shuts down fuel to the cylinders served by a distributor that are colder. There's a delay on cold start, any start actually, before the relay makes a shut-down decision. Most-common system failures are with the sensors. Most common causes of action are coil wires that have come loose, followed by ignition wire failures, coil failures and plug faults. I'd start by verifying that the coil wires on both sides are intact and seated properly. Look at the plugs obviously, and decide if the electrodes are bent as your shop describes, or something else like a fauked coil or cracked/broken distributor part is to blame.
An inductive timing light can be a big help with diagnosing. Test each cylinder while the engine is running, even if the protection relay has cut off fuel to half the cylinders. The ignition system should still send spark to all plugs. A fouled or shorted-electrode plug will show a weak spark pulse if at all.
You can see the bottom of each of the thermocouples under the exhaust on each side of the engine underneath, forward of the downpipe. There's a stiff cable shield running to each one. Somtimes the cable/shield gets bent during major work or it may not have been reinstalled tight enough. Do whatever you can to keep the system functioniong; too many cars have burned down in the driveway from cat fires as owners tried to diagnose the "miss".
The protection relay mentioned above looks at a thermocouple on either side of the engine, screwed into the exhaust manifolds. Each distributor fires plugs on both sides of the engine, so a sensor is under a cylinder for the right distributor and and the other sensor is under the outlet from a cylinder fired by the other distributor. The sensors are thermocouples, and so generate a tiny voltage when heated. The relay looks at the imbalance between the two thermocouples after the exhausts should be hot, and shuts down fuel to the cylinders served by a distributor that are colder. There's a delay on cold start, any start actually, before the relay makes a shut-down decision. Most-common system failures are with the sensors. Most common causes of action are coil wires that have come loose, followed by ignition wire failures, coil failures and plug faults. I'd start by verifying that the coil wires on both sides are intact and seated properly. Look at the plugs obviously, and decide if the electrodes are bent as your shop describes, or something else like a fauked coil or cracked/broken distributor part is to blame.
An inductive timing light can be a big help with diagnosing. Test each cylinder while the engine is running, even if the protection relay has cut off fuel to half the cylinders. The ignition system should still send spark to all plugs. A fouled or shorted-electrode plug will show a weak spark pulse if at all.
You can see the bottom of each of the thermocouples under the exhaust on each side of the engine underneath, forward of the downpipe. There's a stiff cable shield running to each one. Somtimes the cable/shield gets bent during major work or it may not have been reinstalled tight enough. Do whatever you can to keep the system functioniong; too many cars have burned down in the driveway from cat fires as owners tried to diagnose the "miss".
Water can do it, also. I dismissed that, also.
