Surging/shuddering around 3K
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From: Peoria, IL
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From: Cambridge England
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From: Abbotsford B.C.
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From: South Florida
OK I disconnected the O2 sensor (green wire barrel connector thing by CE panel, hopefully that's the correct place to do it) and it did NOT help. The shuddering/surging continued as usual. From what you said below, should I not bother with the voltage test? Or is it still valid?
A couple additional observations...it only happens under load. If it starts doing it and I let off the gas, it stops. If I give it more gas, sometimes it gets worse, sometimes it clears up immediately. Might be that I need to be higher in the rev range so that giving it gas gets it out of the "danger zone." Not sure about that. If it were something like the torque tube, wouldn't it likely continue even if I let off the gas?
A couple additional observations...it only happens under load. If it starts doing it and I let off the gas, it stops. If I give it more gas, sometimes it gets worse, sometimes it clears up immediately. Might be that I need to be higher in the rev range so that giving it gas gets it out of the "danger zone." Not sure about that. If it were something like the torque tube, wouldn't it likely continue even if I let off the gas?
Porsche ran these cars very lean at light throttle. I've had a few (one recently) of the '85/'86 vehicles that will "surge" between 2600 and 3100. Especially noticeable in the lower gears....like when driving the car in slow moving traffic.
As a test, try disconnecting the O2 sensor and see if it runs better in this rpm range.
If this helps, hook the O2 sensor back up and monitor the output from the O2 sensor to the brain. Pay special attention to the rpm range where it surges. If the O2 sensor fluctuates between .2 volts and .8 volts.....that is correct and tells you that the O2 sensor is correcting the mixture as it "thinks" it should. If it doesn't fluctuate and "locks in" at one end or the other....then you have a component failure, of some sort.
Report back with your results. I may have some additional ideas, depending on what voltage reading you are getting.
As a test, try disconnecting the O2 sensor and see if it runs better in this rpm range.
If this helps, hook the O2 sensor back up and monitor the output from the O2 sensor to the brain. Pay special attention to the rpm range where it surges. If the O2 sensor fluctuates between .2 volts and .8 volts.....that is correct and tells you that the O2 sensor is correcting the mixture as it "thinks" it should. If it doesn't fluctuate and "locks in" at one end or the other....then you have a component failure, of some sort.
Report back with your results. I may have some additional ideas, depending on what voltage reading you are getting.
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From: Anaheim
OK I disconnected the O2 sensor (green wire barrel connector thing by CE panel, hopefully that's the correct place to do it) and it did NOT help. The shuddering/surging continued as usual. From what you said below, should I not bother with the voltage test? Or is it still valid?
A couple additional observations...it only happens under load. If it starts doing it and I let off the gas, it stops. If I give it more gas, sometimes it gets worse, sometimes it clears up immediately. Might be that I need to be higher in the rev range so that giving it gas gets it out of the "danger zone." Not sure about that. If it were something like the torque tube, wouldn't it likely continue even if I let off the gas?
A couple additional observations...it only happens under load. If it starts doing it and I let off the gas, it stops. If I give it more gas, sometimes it gets worse, sometimes it clears up immediately. Might be that I need to be higher in the rev range so that giving it gas gets it out of the "danger zone." Not sure about that. If it were something like the torque tube, wouldn't it likely continue even if I let off the gas?
You need to start at the beginning, not "midway" somewhere.
There's a world of difference between a torque tube issue and a surge and it concerns me that you are considering the problem to be either/or. You need to distinguish what your specific problem is, before doing much of anything.
A torque tube vibration will show up sitting still.
Start the car, pull up the emergency brake, put it in neutral, let out the clutch and rev the engine up. If there's a vibration from the torque tube, it's going to happen then, also. It doesn't matter if you are moving or not....if there is a problem in the torque tube, it will shake no matter what.
Generally, if the front bearing has migrated or has had the plastic bushing fall apart, the higher the rpms you spin it, the worse the shake.
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From: Johnson City, TN
We had 86 with intermittent bad shudder under load, esp. if trying to kick-down with heavy throttle at say, 50 mph. Thought it was coming from driveline behind engine. Wrong. Had several clogged injectors. Replacing them solved it.
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From: South Florida
Understood. Based on your torque tube test below, my problem is not the torque tube.
I just don't know enough about these cars yet to know where the beginning is. I would bet money if you or one of the other experts drove the car, you'd have a lot better idea where to start but I unfortunately don't have that luxury. Thank god for the Rennlist experts like yourself or I likely would never get past square one on any of these issues.
I just don't know enough about these cars yet to know where the beginning is. I would bet money if you or one of the other experts drove the car, you'd have a lot better idea where to start but I unfortunately don't have that luxury. Thank god for the Rennlist experts like yourself or I likely would never get past square one on any of these issues.
No, no voltage test required, at this point.
You need to start at the beginning, not "midway" somewhere.
There's a world of difference between a torque tube issue and a surge and it concerns me that you are considering the problem to be either/or. You need to distinguish what your specific problem is, before doing much of anything.
A torque tube vibration will show up sitting still.
Start the car, pull up the emergency brake, put it in neutral, let out the clutch and rev the engine up. If there's a vibration from the torque tube, it's going to happen then, also. It doesn't matter if you are moving or not....if there is a problem in the torque tube, it will shake no matter what.
Generally, if the front bearing has migrated or has had the plastic bushing fall apart, the higher the rpms you spin it, the worse the shake.
You need to start at the beginning, not "midway" somewhere.
There's a world of difference between a torque tube issue and a surge and it concerns me that you are considering the problem to be either/or. You need to distinguish what your specific problem is, before doing much of anything.
A torque tube vibration will show up sitting still.
Start the car, pull up the emergency brake, put it in neutral, let out the clutch and rev the engine up. If there's a vibration from the torque tube, it's going to happen then, also. It doesn't matter if you are moving or not....if there is a problem in the torque tube, it will shake no matter what.
Generally, if the front bearing has migrated or has had the plastic bushing fall apart, the higher the rpms you spin it, the worse the shake.
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From: Norcal
I seem to have a similar issue on my 81.
described here : https://rennlist.com/forums/928-foru...ing-ideas.html
I know big differences here from the 81 to the 86.5 but there might be a relation.
described here : https://rennlist.com/forums/928-foru...ing-ideas.html
I know big differences here from the 81 to the 86.5 but there might be a relation.
Thread Starter
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From: South Florida
I did see your thread in my initial search through the archives...I wasn't sure how much was relevant considering the year differences, but it could end up being the same thing. You haven't solved your issue then?
Former Vendor
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From: Anaheim
I think the best starting place is to make sure that there are no vacuum leaks (for both of you.)
Beg, borrow, steal, rent a smoke machine. Go online an build your own...whatever it takes. I struggled for years trying to find intake leaks and finally gave in and bought a machine. Amazing tool. I have no idea how I functioned without one.
One thing I've learned since owning a smoke machine: Those boots that attach the intakes to the plenums/plenums on both of your cars are a real bitch to get to seal. Perfect looking boots with tight clamps pour smoke. You'd swear that there is no possible way for them to leak....yet they will. I've rotated them, retightened the crap out of them, only to still have them leak. The only solution seems to be new boots, when they leak.
At any rate, don't guess, don't throw parts at it. Do a smoke test, first.
Beg, borrow, steal, rent a smoke machine. Go online an build your own...whatever it takes. I struggled for years trying to find intake leaks and finally gave in and bought a machine. Amazing tool. I have no idea how I functioned without one.
One thing I've learned since owning a smoke machine: Those boots that attach the intakes to the plenums/plenums on both of your cars are a real bitch to get to seal. Perfect looking boots with tight clamps pour smoke. You'd swear that there is no possible way for them to leak....yet they will. I've rotated them, retightened the crap out of them, only to still have them leak. The only solution seems to be new boots, when they leak.
At any rate, don't guess, don't throw parts at it. Do a smoke test, first.
Former Vendor
Joined: Feb 2005
Posts: 15,228
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From: Anaheim
Or get someone that owns a Sharktuner to simply burn you a set of stock chips with the different latency numbers, if all you want to do is change injectors?
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From: South Florida
OK, minor update on my situation. I haven't found anyone who has a smoke machine...the homebrew solutions look pretty workable actually...I even saw a youtube video where a guy was doing this test with a cigar...but I haven't had a chance to put anything together yet. I did use a stethoscope to see if I could find anything obvious, but I couldn't (however I love hearing fluid rushing, gears turning, etc with that thing...motor sounds 
)
I think I was wrong about something I said earlier, however. I said it didn't seem temperature-dependent, and while true that it doesn't go away when the engine is warmer, it does in fact seem like it is far worse before the engine is up to operating temps. Just noting this for the record in case it spurs any ah-HAH moments for anyone...
)I think I was wrong about something I said earlier, however. I said it didn't seem temperature-dependent, and while true that it doesn't go away when the engine is warmer, it does in fact seem like it is far worse before the engine is up to operating temps. Just noting this for the record in case it spurs any ah-HAH moments for anyone...