86 headaches

I have an 86 with 2 annoying problems. The first is that there is a vibration in any gear at 1500-2300 rpm under light to medium loads. The vibration goes away when cruising or heavy acceleration. I've had the
u-joints replaced drive shaft balanced( was ok) and replaced u-joints on half shafts but to no avail. Last year I cracked the flex plate and changed it over the winter. Which leads to the second problem. It now has onlt 13-14" vacuum. I've sprayed carb cleaner and starting fluid all around the intake looking for a leak, with no change in idle. The car will sit at 14 and then drop to 6-7" (600 rpm) for a secon and then back to 14. I've adjusted the valves, retarded timing, adn tightened every bolt on the TPI system I can find.But I'm at a loss now on what to do.Should I take it all apart and start over or is there something else I can try? A friend has said that the computer will take over ny abnormalities in idle, and that it sounds like the idle air control motor is acting up. Any ideas on these 2 problems?
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I would vote for a defective ECU, but only after you are certain about continuity to all of the sensor inputs leading back to the ECU harness. To check for vacuum leaks you can also use propane (a lot less messy & easier to use than starting fluid). If you have a manual trans, I would consider whether there could be a problem with the clutch which is causing a vibration.
========================================On Sat, 04 Jun 2005 11:03:06 -0400, "86vette"

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As to the vibration, think an engine mount might be fatigued?
-- Vandervecken
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Could also be true. But when I hear that a vibration seem to only occur between 1500-2300 rpm in any gear, then I think of something in the direct driveline (from front to back): crankshaft, flywheel, clutch, input/output shaft, driveshaft (front & rear u-joints), half shafts etc.... all the things that require precision balancing... it sounds like something is out of balance.
======================================On Sat, 04 Jun 2005 22:38:22 -0500, Vandervecken

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I agree it's almost certainly a mechanical drivetrain / accessory vibration or a harmonic. But I was thinking that an aged engine mount, losing some of its damping, could allow a specific vibration that a fresher mount would damp out. I've seen this occur in more than a couple of older cars, very similarly to what's described here - including the fairly specific RPM range.
That the vibration occurs in an RPM range, not a speed range, suggests to me that the cause is forward of the transmission.
But as you point out there are lots of problems that cause perceptible vibration only in specific RPM ranges as some natural frequency is excited. In these cases you often get several speed or RPM ranges in which it happens, usually multiples of each other (example an old Buick with a driveshaft imbalance that shook at 21, 42, and 63 mph).
It'd be handy if we could get an idea of the frequency of the vibration. That usually pins down its origin pretty quickly.
-- Vandervecken
X--Eliminator wrote:

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Seems low RPM for a harmonic balancer but might be worth a look. -- Paul H.
Vandervecken wrote:

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Oops - I meant a harmonic vibration, not a harmonic balancer.
-- Vandervecken
Paul H. wrote:

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I had a similar vibration. It turned out to be the water pump.

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I assume that you replaced the flexplate with the correct one: All '86 engines are externally balanced which means the flexplate (and the harmonic balancer) must have a balancing weight. Have you checked for a clogged catalytic converter?

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Timing chain...
--

ZRiX (<>..<>)




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