Strange engine phenomenon
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From: Dallas, TX
Daxmanian,
You have certainly gone to great lengths and expense to solve in the absence of quantitative data what may be a perceptual issue.
Lots of normal stuff results in a perceived loss of power. The alternator cycling on to charge the battery, the AC compressor kicking in. The 993 does not idle very smoothly sometimes the RPM is on the low side at idle when warm. I would have another 993 owner who is familiar with how a 993 should run observe your issue before you spend any more money. I say an owner because a mechanic, with your insistence, will jump right in and solve a problem that may not exist. You know the line, “accept the things I should not fix, have courage to fix the things I must and have the wisdom to know the difference”
Andy
Andy
You have certainly gone to great lengths and expense to solve in the absence of quantitative data what may be a perceptual issue.
Lots of normal stuff results in a perceived loss of power. The alternator cycling on to charge the battery, the AC compressor kicking in. The 993 does not idle very smoothly sometimes the RPM is on the low side at idle when warm. I would have another 993 owner who is familiar with how a 993 should run observe your issue before you spend any more money. I say an owner because a mechanic, with your insistence, will jump right in and solve a problem that may not exist. You know the line, “accept the things I should not fix, have courage to fix the things I must and have the wisdom to know the difference”
Andy
Andy
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<snip>
I've been plagued by a minor engine problem on my '94 Porsche 993 for more than a year now and have not found a solution yet.
When I start the engine it starts normal, but after about one or two minutes the engine tone changes and it loses a bit of power. Engine sounds like one of the cilinders is not getting fuel or not getting a spark. Engine makes a "put-put-put" sound, like an old VW beetle.
Duration of this phenomenon varies, sometimes a few seconds and sometimes up to two minutes. When the engine behaves normal again it doesn't come back the entire ride until I shut down the engine and restart it a few minutes later.
It only does this in the first minutes the engine runs. Never later than the first minutes. It idles solid and you only notice the problem on accellerating.
When the engine returns to normal again it's instant, like an on/off switch.
<snip>
I've been plagued by a minor engine problem on my '94 Porsche 993 for more than a year now and have not found a solution yet.
When I start the engine it starts normal, but after about one or two minutes the engine tone changes and it loses a bit of power. Engine sounds like one of the cilinders is not getting fuel or not getting a spark. Engine makes a "put-put-put" sound, like an old VW beetle.
Duration of this phenomenon varies, sometimes a few seconds and sometimes up to two minutes. When the engine behaves normal again it doesn't come back the entire ride until I shut down the engine and restart it a few minutes later.
It only does this in the first minutes the engine runs. Never later than the first minutes. It idles solid and you only notice the problem on accellerating.
When the engine returns to normal again it's instant, like an on/off switch.
<snip>
So I wonder if you might still have some grounding issue.
Since I have a '97 (OBD II car), the check engine light would come on.
You said that you checked the grounds. Were you able to get all of them (particularly engine compartment, under seats, battery)?
Good luck with this!
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From: Belgium
@pp000830: I've owned my 993 for about 3 years now and the first 2 years the engine behaved perfectly, it's only in the last year it behaves different and it is very noticible. A friend who also ownes a 993 drove my car and he also felt and heared the difference so there is definitly something going on I only have to find out what.
@Steve Weiner: I know you know a lot about these engines. Can you please tell me which phases the DME goes through when the engine is warming up and what changes (mixture, ignition timing etc.)
@BesideTheBox: Because I read your post about your engine troubles that's why I decided to clean my ground contacts. Every contact I could inspect was perfectly clean. I only cleaned the ones I could easily reach, under the seat, battery compartment. The ones in the engine compartment I only inspected visual without unbolting them. Maybe I should take the effort and clean them. It's because all the others looked perfectly clean that I did not do all of them.
@Steve Weiner: I know you know a lot about these engines. Can you please tell me which phases the DME goes through when the engine is warming up and what changes (mixture, ignition timing etc.)
@BesideTheBox: Because I read your post about your engine troubles that's why I decided to clean my ground contacts. Every contact I could inspect was perfectly clean. I only cleaned the ones I could easily reach, under the seat, battery compartment. The ones in the engine compartment I only inspected visual without unbolting them. Maybe I should take the effort and clean them. It's because all the others looked perfectly clean that I did not do all of them.
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If you haven't seen it, check this post.
Ground points XI, XII, XIV, XXI apply.
Warning - XI is the most difficult of these 4 (I suspect easier with a lift).
The other three aren't too bad.
Unbolt, then clean each wire ring, flat surface around the post, and the nut with steel wool.
Wipe clean and re-tighten very tight.
Time well spent.
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From: Belgium
Found some time today to clean the remaining ground contact points.
Number XI was a real PITA but succeeded in getting the bolt loose and cleaned everything with steel wool.
On the most left ground point in the engine bay there is no wire attached? Any idea if this is normal?
Anyway, cleaning did not solve the problem. Went for a few short drives and everytime the engine hesitates for a few seconds shortly after starting it.
I do believe the problem has to do with a bad contact somewhere because of the on/off nature of the problem. I even sometimes have it in intervals, like hesitating 5s, 2s normal driving than again a few seconds hesitating and so on until it stops and doesn't come back untill I shut the engine down.
When the problem occurs the engine doesn't stumble or bucks it just loses about 50hp(guessing) and changes exhaust note. Maybe an injector stops for a few seconds?
Any suggestions still welcome
Number XI was a real PITA but succeeded in getting the bolt loose and cleaned everything with steel wool.
On the most left ground point in the engine bay there is no wire attached? Any idea if this is normal?
Anyway, cleaning did not solve the problem. Went for a few short drives and everytime the engine hesitates for a few seconds shortly after starting it.
I do believe the problem has to do with a bad contact somewhere because of the on/off nature of the problem. I even sometimes have it in intervals, like hesitating 5s, 2s normal driving than again a few seconds hesitating and so on until it stops and doesn't come back untill I shut the engine down.
When the problem occurs the engine doesn't stumble or bucks it just loses about 50hp(guessing) and changes exhaust note. Maybe an injector stops for a few seconds?
Any suggestions still welcome
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From: Jacksonville Fl.
Knock retard system acting up? Would be audibly different, no knocking noise, loss in power. What would be the effect of disconnecting the knock sensors then driving? Is it even possible to do without pulling the air shroud? A reasonable test? Dont know what type of data is available through a scanner on these, or the PID data rate, but if it is for truly 5 seconds should be enough time to capture the data (if data available)
Curt
Curt
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Sorry to hear that. Sure sounds like a grounding issue somewhere.
Plug wires - Reseat all of them, perhaps use dielectric on them as well.
Knock sensor - On my '97 - one it easy to get to above cylinder 4. You could disconnect it and see if it makes any difference.
Hall sensor - you could try to disconnect it and see if it makes a difference either. The only way I would think this might be an issue is if the wiring to the hall sensor is intermittently bad. This would affect the engine under load, and I believe you said idle was fine.
Plug wires - Reseat all of them, perhaps use dielectric on them as well.
Knock sensor - On my '97 - one it easy to get to above cylinder 4. You could disconnect it and see if it makes any difference.
Hall sensor - you could try to disconnect it and see if it makes a difference either. The only way I would think this might be an issue is if the wiring to the hall sensor is intermittently bad. This would affect the engine under load, and I believe you said idle was fine.
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From: Belgium
Knock sensors are easily disconnected. Left one has connection behind hot air blower, I think it's the white connector, not sure.
If I drive without the knock sensors connected It won't harm the engine I hope?
I'll try to reseat the plug wires. Car had the problem also before I changed the plug wires so not sure if it's gonna cure the problem but will try. Not sure what you mean by using dielectric, is that a spray like contact spray?
If the Hall sensor acts up wouldn't that give a fault in the DME memory? Car idles rock solid but yesterday I let it warm up slowly while idling to see if the problem occurs as well and indeed after about 2 to 3 minutes I hear the exhaust note change. Drove off and felt it was happening, stops half a minute later and the rest of the drive no problem.
Could moisture be a problem? Car sleeps outside and Belgium weather is not the greatest.
If I drive without the knock sensors connected It won't harm the engine I hope?
I'll try to reseat the plug wires. Car had the problem also before I changed the plug wires so not sure if it's gonna cure the problem but will try. Not sure what you mean by using dielectric, is that a spray like contact spray?
If the Hall sensor acts up wouldn't that give a fault in the DME memory? Car idles rock solid but yesterday I let it warm up slowly while idling to see if the problem occurs as well and indeed after about 2 to 3 minutes I hear the exhaust note change. Drove off and felt it was happening, stops half a minute later and the rest of the drive no problem.
Could moisture be a problem? Car sleeps outside and Belgium weather is not the greatest.
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Joined: Apr 2003
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From: Jacksonville Fl.
Dielectric grease is actually an insulator, not a conductor. It is useful to prevent moisture from working its way into a electrical connection. I also use it to hold O rings and rubber gaskets in place during assembly.
The knock sensors being disconnected will prevent the timing from being retarded when engine knock and/or pre-ignition is detected (yes i know pre-ignition has nothing to do with ignition timing by definition, however the piezo knock sensor cant tell the difference). So if you hear "spark knock" with them disconnected don't accelerate that hard, and re-connect the sensors. In theory with your symptom being prior to full temp the propensity to knock should be reduced.
But then again, what do I know. Love to find out though.
Curt
The knock sensors being disconnected will prevent the timing from being retarded when engine knock and/or pre-ignition is detected (yes i know pre-ignition has nothing to do with ignition timing by definition, however the piezo knock sensor cant tell the difference). So if you hear "spark knock" with them disconnected don't accelerate that hard, and re-connect the sensors. In theory with your symptom being prior to full temp the propensity to knock should be reduced.
But then again, what do I know. Love to find out though.
Curt
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Knock sensors are easily disconnected. Left one has connection behind hot air blower, I think it's the white connector, not sure.
If I drive without the knock sensors connected It won't harm the engine I hope?
I'll try to reseat the plug wires. Car had the problem also before I changed the plug wires so not sure if it's gonna cure the problem but will try. Not sure what you mean by using dielectric, is that a spray like contact spray?
If the Hall sensor acts up wouldn't that give a fault in the DME memory? Car idles rock solid but yesterday I let it warm up slowly while idling to see if the problem occurs as well and indeed after about 2 to 3 minutes I hear the exhaust note change. Drove off and felt it was happening, stops half a minute later and the rest of the drive no problem.
Could moisture be a problem? Car sleeps outside and Belgium weather is not the greatest.
If I drive without the knock sensors connected It won't harm the engine I hope?
I'll try to reseat the plug wires. Car had the problem also before I changed the plug wires so not sure if it's gonna cure the problem but will try. Not sure what you mean by using dielectric, is that a spray like contact spray?
If the Hall sensor acts up wouldn't that give a fault in the DME memory? Car idles rock solid but yesterday I let it warm up slowly while idling to see if the problem occurs as well and indeed after about 2 to 3 minutes I hear the exhaust note change. Drove off and felt it was happening, stops half a minute later and the rest of the drive no problem.
Could moisture be a problem? Car sleeps outside and Belgium weather is not the greatest.
I doubt this is moisture as you say you've had it for such a long time. You could have an issue that might be aggravated by moisture. Check caps maybe.
I don't know how much harm might come to your engine if you drive without knock or hall sensor connected.
Most of the hall sensor error reports turn out to be related to ignition issues, poor plug wires, caps, wiring to the hall sensor, etc, that might be leaking. I'm basing this on the searching I did last night on hall sensors - most of the hits I got came from the 964 forum - the 964 also has twin ignition.
Dielectric grease is actually an insulator, not a conductor. It is useful to prevent moisture from working its way into a electrical connection. I also use it to hold O rings and rubber gaskets in place during assembly.
The knock sensors being disconnected will prevent the timing from being retarded when engine knock and/or pre-ignition is detected (yes i know pre-ignition has nothing to do with ignition timing by definition, however the piezo knock sensor cant tell the difference). So if you hear "spark knock" with them disconnected don't accelerate that hard, and re-connect the sensors. In theory with your symptom being prior to full temp the propensity to knock should be reduced.
But then again, what do I know. Love to find out though.
Curt
The knock sensors being disconnected will prevent the timing from being retarded when engine knock and/or pre-ignition is detected (yes i know pre-ignition has nothing to do with ignition timing by definition, however the piezo knock sensor cant tell the difference). So if you hear "spark knock" with them disconnected don't accelerate that hard, and re-connect the sensors. In theory with your symptom being prior to full temp the propensity to knock should be reduced.
But then again, what do I know. Love to find out though.
Curt
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From: Belgium
@BesideTheBox : I do indeed think the problems are the same I just have to find what contact point is acting up. I did every ground point in the list unless there is one I don't know of. I'll keep searching and will report any progress I make