1987 928S4 - RPM Probs, HELP!
05-18-2005, 04:19 PM
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1987 928S4 - RPM Probs, HELP!
Hi Folks!
I'm happy to report that I am a new member to the 928 family! After falling in love with my collector friend's '82 Euro S, and a year of looking, I bought a well-maintained '87 5-SPD that has everything I wanted. I drove it for several hundred perfect miles and then put it in the barn for the winter.
Here's the problem. I took it out of storage, cleaned it up, and it drove as well as when it went into hibernation. Took it to my Porsche guy...very knowledgeable and has owned 928s himself...to fix the sunroof, bleed the new clutch, get the inspection done, wash the engine, and other relatively cosmetic things. He metered the tachometer and found it to be off appx. 800 RPMs low and reset the gauge. When I picked it up, I was surprised to find that the car's RPMs shut down at appx. 4000! I mean, BAM...like flipping a switch, it drops right off to zero. It hitches but recovers when I up shift but dumps again at 4000.
HELLLPP. Any ideas? A particular computer module maybe? The car still has the problem when driven with the tach disconnected. Has anyone seen this before? It's back at the shop with him, but he's a little stymied. Which makes me nervous, and it's frustrating as the car was running great.
Thanks so much for any assistance...
Jeff
I'm happy to report that I am a new member to the 928 family! After falling in love with my collector friend's '82 Euro S, and a year of looking, I bought a well-maintained '87 5-SPD that has everything I wanted. I drove it for several hundred perfect miles and then put it in the barn for the winter.
Here's the problem. I took it out of storage, cleaned it up, and it drove as well as when it went into hibernation. Took it to my Porsche guy...very knowledgeable and has owned 928s himself...to fix the sunroof, bleed the new clutch, get the inspection done, wash the engine, and other relatively cosmetic things. He metered the tachometer and found it to be off appx. 800 RPMs low and reset the gauge. When I picked it up, I was surprised to find that the car's RPMs shut down at appx. 4000! I mean, BAM...like flipping a switch, it drops right off to zero. It hitches but recovers when I up shift but dumps again at 4000.
HELLLPP. Any ideas? A particular computer module maybe? The car still has the problem when driven with the tach disconnected. Has anyone seen this before? It's back at the shop with him, but he's a little stymied. Which makes me nervous, and it's frustrating as the car was running great.
Thanks so much for any assistance...
Jeff
05-18-2005, 04:44 PM
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There is no approved way to "reset the tach" that I know of.. Whatever he has done has clearly changed the rpm signal into the LH - this is why the fuel cutoff rpm limiter is coming in early.
Regards
Regards
05-18-2005, 04:56 PM
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Hi John,
GREAT! I love it when "small repairs" turn into big ones. Oh well, all part of the fun of collecting cars I suppose!
So the 928 DOES have a RPM limiter! That was my first thought from the way it's acting and as the fuel definitely is cutting off, but I couldn't find it when I pored through my service manuals and wire schematics...I found a reference to an "upshift indicator" but the write-up sounded more like a visual aid to a not-very-experienced driver.
So, I suppose the only way around this is a new tach?
Regards,
Jeff
GREAT! I love it when "small repairs" turn into big ones. Oh well, all part of the fun of collecting cars I suppose!
So the 928 DOES have a RPM limiter! That was my first thought from the way it's acting and as the fuel definitely is cutting off, but I couldn't find it when I pored through my service manuals and wire schematics...I found a reference to an "upshift indicator" but the write-up sounded more like a visual aid to a not-very-experienced driver.
So, I suppose the only way around this is a new tach?
Regards,
Jeff
05-18-2005, 05:06 PM
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Originally Posted by blueblooddude
... He metered the tachometer and found it to be off appx. 800 RPMs low and reset the gauge.
05-18-2005, 05:40 PM
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Originally Posted by John Speake
There is no approved way to "reset the tach" that I know of.. Whatever he has done has clearly changed the rpm signal into the LH - this is why the fuel cutoff rpm limiter is coming in early.
Regards
Regards
Maybe Mark Anderson could use that feature for his sons car!
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05-19-2005, 05:48 AM
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Jeff,
The rev limitier works on the LH ECU, and kills the fuel injector pulses. It is programable in the LH software.....
The 928 is complicated enough, without someone making "repairs" that are non-standard....
The rev limitier works on the LH ECU, and kills the fuel injector pulses. It is programable in the LH software.....
The 928 is complicated enough, without someone making "repairs" that are non-standard....
05-19-2005, 06:49 PM
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Hi Gents!
OKKKKKK, here's the scoop. I spoke with my tech today and got some more details.
First, he checked his notes and it wasn't 800 RPMs low, it was more like between 300-400 (I admire the quest for precision, but I could have probably lived with that but what the heck...hindsight is 20/20). He "reset the gauge" by manually pulling the indicator off the post and resetting it by hand...this is my first 928 so I don't know, but he sounded as if this was something he's done before as a standard manuever. So, we know that the adjustments were mechanical and not electronic. My first instinct is that this "repair" is what created the problem...does anyone know how or why? Or what actions should be taken now?
Since your helpful insight yesterday (i.e.-I now know that there IS a limiter), I did some more research on the rev limiter and it seems that it should be kicking in between 6300 - 6600 RPMs. It was kicking in on me between 4000 and 4200 when I discovered the problem the other day. Mathematically, this does not correlate to the mere 300 - 400 RPMs of the manual adjustment.
More bad news...the mechanic informed me today that there seems to be degradation in RPMs!! He said that that it seems the problem is getting worse and the limiter is kicking in closer to 3000 now!!! My hair is turning thin and my beard is graying as we speak...
He also discovered that the alternator wire and other ancillary wires on the positive battery terminal were a bit loose. There was no change in the function of the rev limiter when tightened properly...could a voltage spike or arcing when loose have damaged the sensors, LH, or something?
He spoke with another Porsche shop today and they suggested testing the two sensors on the flywheel housing. This is weird, what do you guys think about this?!? They said that there are two identical sensors and to pull one and use the other one for testing...if the car doesn't start with one of them in a particular order, then that one is bad. The tech looked and there are two sensor plugs on the housing BUT ONLY ONE SENSOR! My first thought was BINGO, one fell out or was not put back in. However, He said that from the condition of the receiving plug and the fact that there is absatootley no harness connectors near it, it appears that there never were two sensors on this car. Does this make sense? Is it a split production year thing or something that the one sensor handles everything?
Ug... Thanks again so much for your assistance guys! I am definitely going to be joining Rennlist as a member. Man, I feel like I'm back in my Lotus days! lol I wanted to stay away from electronic issues for a while. John, I checked out yoru website...very nice. You must have the patience of a saint though to work with auto electronics day in and day out my friend!
Regards,
Jeff
OKKKKKK, here's the scoop. I spoke with my tech today and got some more details.
First, he checked his notes and it wasn't 800 RPMs low, it was more like between 300-400 (I admire the quest for precision, but I could have probably lived with that but what the heck...hindsight is 20/20). He "reset the gauge" by manually pulling the indicator off the post and resetting it by hand...this is my first 928 so I don't know, but he sounded as if this was something he's done before as a standard manuever. So, we know that the adjustments were mechanical and not electronic. My first instinct is that this "repair" is what created the problem...does anyone know how or why? Or what actions should be taken now?
Since your helpful insight yesterday (i.e.-I now know that there IS a limiter), I did some more research on the rev limiter and it seems that it should be kicking in between 6300 - 6600 RPMs. It was kicking in on me between 4000 and 4200 when I discovered the problem the other day. Mathematically, this does not correlate to the mere 300 - 400 RPMs of the manual adjustment.
More bad news...the mechanic informed me today that there seems to be degradation in RPMs!! He said that that it seems the problem is getting worse and the limiter is kicking in closer to 3000 now!!! My hair is turning thin and my beard is graying as we speak...
He also discovered that the alternator wire and other ancillary wires on the positive battery terminal were a bit loose. There was no change in the function of the rev limiter when tightened properly...could a voltage spike or arcing when loose have damaged the sensors, LH, or something?
He spoke with another Porsche shop today and they suggested testing the two sensors on the flywheel housing. This is weird, what do you guys think about this?!? They said that there are two identical sensors and to pull one and use the other one for testing...if the car doesn't start with one of them in a particular order, then that one is bad. The tech looked and there are two sensor plugs on the housing BUT ONLY ONE SENSOR! My first thought was BINGO, one fell out or was not put back in. However, He said that from the condition of the receiving plug and the fact that there is absatootley no harness connectors near it, it appears that there never were two sensors on this car. Does this make sense? Is it a split production year thing or something that the one sensor handles everything?
Ug... Thanks again so much for your assistance guys! I am definitely going to be joining Rennlist as a member. Man, I feel like I'm back in my Lotus days! lol I wanted to stay away from electronic issues for a while. John, I checked out yoru website...very nice. You must have the patience of a saint though to work with auto electronics day in and day out my friend!
Regards,
Jeff
05-19-2005, 07:18 PM
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Originally Posted by John Speake
Jeff,
....................programable in the LH software.....
....................programable in the LH software.....
05-19-2005, 07:27 PM
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Folks, doesn't the '87 have a test port for the flywheel speed sensor attached to the driver's side of the intake? (IIRC '87 and '88 have this. It was deleted for MY '89.)
Also, Jeff, how was your tech monitoring the actual RPM? Hooked to a plug wire? Coil wire? And did he measure this 'difference' while idling? Or through an RPM range?
Also, Jeff, how was your tech monitoring the actual RPM? Hooked to a plug wire? Coil wire? And did he measure this 'difference' while idling? Or through an RPM range?
