308 QV DIGIPLEX,IDLE SPEED AND VACUUM ADVANCE-NEED HELP?

Peter,

Interesting problem...with a very more bizzare solution. Let's presume for a moment that he is correct, although I have my doubts.

A little background on on these spark boxes create "advance"....
-There's a single vacumuum line from the engine to two spark boxes, one for each bank, connected via a "T" fitting in the boot.
-Each spark box has its own vacuum port
-The vacuum line on said box connects the a clever, but crude variable inductor...in which, a small ferite slug moves within the inductor. As the relative position of this slug changes, the inductuce changes, resulting in a shift in the spark boxe's timing circuit. This shift is the timing advance.
-The system on a 308QV should be viewed as two separate ignition systems.

As you have observed first hand, no advance (as the vacuum line is now disconnected from one of the boxes), results in "flat" performance. Not a very good solution to "the problem".

Can what he described as the problem actually be the problem?
Maybe...and here's why:

If the ferite slug were to get "stuck" within the core of the variable inductor, the timing would not advance, or even remain advance, regardless of the incoming vacuum signal. I have withnessed this scanario first hand...However, the vehicle will typically run reasonable well, but fall flat at a specific RPM.

This is important: Based on your orignal statement "The car ran reasonably well...", I have my doubts that an internal problem with the spark box is really the problem.

Spark box testing: Yes...the advance curves can be extracted and verified. The easy way is with the use of the factory diagnostic port on the left rear wing in the engine compartment, using a Jodatron unit. This is a very old, but useful piece of factory test gear design exactly for this purpose. If he has one, any questions regarding the proper advance of either unit can be verified. However, if he doesn't have one, the timing curves of either box can be verified, albeit a bit more time consuming, using a programable digital advance type timing light, by simply clipping on to one of the coil leads. This would require removal of the cover on the bellhousing to reveal the timing marks on the flywheel...and recording the timing versus RPM.

I have the facility here in the States to test these units...and in some cases, am able to repair the faulty internal vacuum advance units. However, before you even contemplate sending your boxes, 99% of the boxes I receive that are diagnosed as faulty test perfect!!!!

So, here's how I would approach this problem:

In my opinion (25+ years working with these cars...), I'd be looking down this path:

-The spark boxes are ground through the wiring harness. This can prove to be troublesome over time. Make up two small jumper wires, with an eyelet connector on each end. Connect the mounting bolt from each spark boxes to a nearby chassis location, to serve as a "known good" ground connection. Your "Ferrari-tech" should a) be aware of this and b) done this.
-RPM specific misses. It usually comes down to these three issues 99.999% of the time:
***Bad ignition wires
***Faulty extendeds
***Worn out distributor caps and rotors

If the car was in my shop, Here's what I would do:

-Reconnect the vacuum line
-Add the addition grounds to each spark box
-Test and likely replace the ignition wires and extenders
-Verify the condition of the caps and rotors
-Verify that the carbon contact inside each of the distributor caps are indeed intact

If the car misses at 3,000 rpm, this should be detectable with an exhaust gas analyzer, as we'll see "a spike" in the HC levels. Personally, I'd find another tech who has better diagnostic skills...The mere thought of disconnecting the vacuum signal is just pure hack!

David

 

+1 -- that's no "solution" at all. (Hopefuly, he/she didn't readjust the injection system after disconnecting the vacuum lines so you can more easily unfix ).

 

Not to state the onvious, but before you start testing advance curves set your idle speed at the proper rpm. 1500 is way too high.

 

Too fast of an idle can be caused by a vacuum leak, like from an unplugged, disconnected vac advance line. Or other vac leak.

Now, someone that disconnects vac advance as a fix, has not the slightest F'ing clue how an engine spark system works, so I would be suspect that your initial static timing is even correct.

Too retarded timing can cause sluggishness. I suppose too advanced could also, but for a different reason.

Vacuum advance gives better part throttle response (on initial throttle opening, it doesn't do squat once the throttles have been opened) and better fuel economy. That is all it does, it does not affect WOT performance.

Doug

 

Here, read these in no particular order. They are for a US V8, but a motor, is a motor, is a motor, same principles apply.

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/How%20to%20Set%20Timing.pdf

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/MAPPING%20ADVANCE.pdf

Doug

 

On any 308QV, this would be the proper, original set-up. By "plastic hose", I presume you mean "rubber", but as long at the hose has integrity, you should (and need) vacuum to both spark boxes.

I'm not sure what he meant by "disconnected" other than that there may be "a disconnection" inside one of the spark boxxes in terms of a vacuum leak??? Not to defend anyone, but this is a fairly simple system to understand, particularly if one spends a moment looking at the Factory WSM. At this point, here are the test that I would perform:

-Connect a vacuum gauge before the "T", and see if you have any vacuum. If so, you know the line from the plenuum is in good shape. (This test is important, as the line from the plenuum to the "T" is long and is routed through the wheel well.
-Connect a vacuum gauge, momentarily, where each of the spark boxes connects, individually. This will verify that the small section of vacuum line from the "T" to each spark box is "good". A simple visual inspection likely will reveal that these hoses are fines.
-With a vacuum pump (MityVac)...and the car not running, apply vacuum to each spark box. They should "Hold" vacuum. This is not a definative test; it only show that the vacuum-based, variable inductor has mechanical intergrity.

A quick test to see if the spark boxes are advancing, at all: Do this momentarily, only!
-Disconnect and cap the vacuum port on the plenuum.
-With the engine running, apply vacuum to each port of the spark boxes, individually.
-When vacuum is applied to either box, the idle speed and engine note should change, as effective, you're addding vacuum advance to each bank individually. Do this briefly...

-Sticky throttle? Won't idle down?
This is a pretty common proble. Verify that the throotle plate and linkage (externally at the throttle body) can move freely and do indeed close with the throttle is released. Often times, a simple clean and lube will do the trick.

I would verify with the tech just exactly what the problem was...and what he really did "to fix" the problem. I spend alot of time with these cars and likely can help you get it sorted out.

Regards,
David

 

Centrifugal advance plate sticking can cause high idle, as it leaves the timing too advanced. It makes a real interesting problem on a two dizzy car where one plate is stuck and the other isn't.

Centrifugal advance plate sticking can also cause lack of power, if the advance won't advance far enough

Doug

 

Doug,

Right idea, wrong car...

The QV doesn't have any advance plates or anything even remotely close in the distributors. The advance is controled 100% by the Digiplex units. These units advance the timing by reading the vacuum in the plenuum. One vacuum line, connected by a "T" fitting to two totally independant spark boxes, each with their own crank triggers...sharing a single TDC reference trigger. There nothing mechanical about this system...

David

 

Ah. I guess that does put the kabosh on the stuck advance plate theory. I assume these digiplex units have an RPM related advance curve programmed into them then.

Thanks for the educational tidbit.

Doug

 

You're welcome...and yes, they do.

David