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Erradict Ignition Timing


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#1 willmoodom

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Posted 30 July 2006 - 09:17 PM

While engine idling w/vac advance disconnected and plugged, timing will not stay at set number. Fluctuates between 10* and 5*(attempting to set at 10*) Only mod done to ignition system is rebuilt dist. w/ new mitsu pickup, and vac advance. Stock coil, ignitor box(89),(used) upgraded wires, and fairly new plugs gapped at 35. Vacuum at a steady 18, temp at 182-184, oil pressure 30-36, a/f 2-3 bars rich. When gas tapped, will go to last 2 bars stoich and back up to 2-3 bars rich. Brand new TPS and rebuilt ISC. Steady idle at 1000rpm
  

My main question is What could cause this erradict jumping of timing according to the timing light?





#2 MontyTSI

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Posted 30 July 2006 - 09:20 PM

my guess, you knock sensor is coming loose, whether it be at the connector or internally

if you pull  the knock sensor loose at idle it should drop about 5 degrees of timing

i bet if you disconnected it the fluctuation would stop and be dead on 5 degrees at idle.

monty
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#3 willmoodom

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Posted 30 July 2006 - 09:26 PM

So you believe the knock sensor is quitting on me?

#4 MontyTSI

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Posted 30 July 2006 - 11:15 PM

either the sensor or the connector..there is a lot of heat right there

id go ahead and replace it im sure its in need as most are

oh and your checking the timing with the vac adv. hose unhooked and plugged correct?
A CERTIFIED EXPERT IN SPOTTING THE OBVIOUS :)CTS-V, 1961 MGA MKII, Aprilia rsv1000 factory, ducati 900ss (not stock), 1988 mighty max 4x4 4.22 geared, 1987 TSI, 1987 flatty

#5 Indiana

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Posted 31 July 2006 - 01:43 AM

how stable is that idle when it's out of range for the isc, a little increase/decrease in rpms throw the mechanical weights in/out and would change your timing and I've seen worn out stators in the dist. that move side to side and have towers that hit the teeth on the governor base too and that was playing hell on that motor (Kurts concrap from Atlanta came that way)

#6 MontyTSI

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Posted 31 July 2006 - 10:29 AM

indiana has a great point there

the exactly 5 degrees is why i went towards the knock sensor..

i check mine by disconnecting it as it runs (ive added 6 inches of wiring to get the connector where i can reach it though)
A CERTIFIED EXPERT IN SPOTTING THE OBVIOUS :)CTS-V, 1961 MGA MKII, Aprilia rsv1000 factory, ducati 900ss (not stock), 1988 mighty max 4x4 4.22 geared, 1987 TSI, 1987 flatty

#7 Professor Quest

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Posted 31 July 2006 - 10:42 AM

"While engine idling w/vac advance disconnected and plugged, timing will not stay at set number."

If it moves around........your Dist has a problem with the mech. a**. PERIOD! :wink: Like......strechted out the springs? or WRONG springs.
There should be NO VACUUM at idle from THAT HOSE .........at all. :wink:

#8 MontyTSI

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Posted 31 July 2006 - 10:57 AM

swap distributers and see if the problem follows...
A CERTIFIED EXPERT IN SPOTTING THE OBVIOUS :)CTS-V, 1961 MGA MKII, Aprilia rsv1000 factory, ducati 900ss (not stock), 1988 mighty max 4x4 4.22 geared, 1987 TSI, 1987 flatty

#9 mikec

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Posted 31 July 2006 - 11:53 PM

There's a signal from the ECU that causes an extra +5 degrees of timing advance (from 10 deg BTDC to 15deg BTDC); if that wire is flakey you'll get bouncing timing.  There are a couple dead-end connectors (test connectors) hanging from the wiring harness between the air filter box and fender... find the one with more than one pin/wire going to it.  That's the ignition timing test terminal.  Put a voltmeter on the red+yellow and black wires.  See if it reads a constant voltage (around 6 volts) while your timing is doing it's jumping... if not, the ignitor is being told crap by the ECU or by a bad wiring harness.

The ECU commands this extra +5 degrees of timing for:
* cold engine startups
* operating at high altitudes (just a couple thousand feet)

So a bad ECU coolant temp sensor or a bad barometric pressure sensor (inside the airflow sensor assembly on 87-later cars, on the firewall for earlier cars) can confuse the ECU.  You can actually jumper those two wires together to force your car into "low alt" mode and kill that +5 degree timing advance.  See if your timing stabilizes - probably at only 5 deg BTDC now... I'll bet your base timing is 5 degrees off due to this bug.

mike c.

#10 MontyTSI

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Posted 01 August 2006 - 10:29 AM

awesome info MikeC, i reported this to the mods to build a FAQ

im going to install a junction block for the ignition and fuel pump test leads

1 dead end terminal for fuel pump, the ignition test , and a 12v keyed sample.all close to make troubleshooting a simple use of jumpers and voltmeter.

I already bought the box to enclose them in.
A CERTIFIED EXPERT IN SPOTTING THE OBVIOUS :)CTS-V, 1961 MGA MKII, Aprilia rsv1000 factory, ducati 900ss (not stock), 1988 mighty max 4x4 4.22 geared, 1987 TSI, 1987 flatty




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