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Discussion Starter #1
Hello All. My 1988 Spider has an ignition timing problem. With the engine running at idle , the ignition timing is much advanced past the max advance point. TOO MUCH ADVANCE. The engine will throttle up but does not run smoothly. It will start every time. When running, it may be running a slightly rich mixture but no smoke from the exhaust.

Some info on what has been checked. The cam timing and distributor timing to crank has been double checked. The Reference point and RPM transmitters have been inspected and Ohms ck'd and checked for proper position. The flywheel has been checked for proper positioning. The Ignition and Fuel UCU's have been replaced but I wasn't sure the Ignition ECU was a reliable one so, I replaced it with another one but there was no change in the advanced ignition timing. All of the sensors located in the intake manifold were Ohm's ck'd and found OK. All the grounds were checked. The AAV, throttle switch, AFM, VVT, Altitude compensator and vacuum sensor were checked and found OK.

I'm lost and need help. Is there some mechanical part or electronic part that can cause this problem. Any info, no matter how far fetched it seems, I'll take it. Thanks, Chief.
 

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Did this problem just appear? Any recent work or changes made just prior to the timing problem? Sometimes a resistance check on the flywheel sensors is not conclusive. Do you have a spare sensor to swap with an existing one for testing purposes?
 

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The L-jet distributor has nothing inside it - just a rotor that 'distributes' coil output to the appropriate spark plug. (if there is anything else inside then someone must have replaced it & my knowledge ends)

The ignition computer adjusts the timing. It seems unlikely two computers would have the same failure mode so it would be more likely it is external to the computer.

The computer uses the MAP sensor to know when intake vacuum is highest and that is when it advances the timing. Perhaps the MAP sensor is sending erroneous signals and the computer is resonding to that? You can unplug the MAP sensor - the engine will (should) run fine. Fuel mileage may suffer because of the lack of timing advance under light load (e.g. highway cruising) but otherwise it should start/accelerate/run fine.
 

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MAP vs VSD

Hello Eric!

Just a clarification on your advice to Chief about his ignition advance issues.

Our Bosch L-Jets use a straight vacuum sensor (the Vacuum Sensor Device or VSD) which we know lives on the passenger sidewall, under the rear parcel shelf. The VSD is connected to the intake manifold via a tube and at closed throttle, reads a steady, maximum "vacuum" signal that, via a bellows in the VSD, changes the inductance of a variable inductor which in turn changes the frequency of an oscillator in the Ignition Computer which (wait for it :)) adds to the pre-programmed, RPM-based advance curve stored in the ignition module (whew!). At idle however, the Throttle Position Switch (TPS) tells the ignition module to ignore the VSD signal and advance is fixed at the base number, 10*.

The shortcoming of a VSD, or any other vacuum guage, is that unconnected, it reads "0" at sea level or the top of a mountain. Obviously, not knowing air density (a function of altitude) is an issue in regard to the rich/lean nature of the final fuel mixture. While we do have an altitude sensor on our parcel shelf, it only talks to the fuel computer, as does the air flow sensor.

A Manifold Absolute Pressure device (MAP) on the other hand, has an internal vacuum chamber, on the opposite side of a movable diaphram from an intake manifold pressure source. The output of the MAP therefore, is a difference signal between the vacuum side and the manifold pressure side, a much more useful value that accounts for altitude, and is used in all modern engine management systems.

OK, where were we?

Oh, right! Chief has a problem with "too much advance" at idle.

How about some numbers, good buddy?

Tell us how you are reading timing advance and does that number change when you blip the throttle? Have you performed the procedure for setting the TPS (see stickys on L-Jet). If the TPS is not set correctly, it may allow the Ignition Computer to respond to the VSD, when it should not. How is the idle?

Meanwhile, I'm going to check some notes and refresh my memory on our ignition system.

All the best.
 

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Discussion Starter #5
Hello again and thanks for the info.

Here are some answers to y'alls questions; The engine was running fine, then as time went on it got worse and worse. It was like something was changing at a slow rate. The RPM & Reference transmitters were Ohms ck'd and found to be good. Do not have one to swap to do another ck.

Using a standard timing light, the ignition timing at idle is so far past the max timing mark it is impossible to give an accurate reading in degrees. I've not marked the crank pulley past the max timing mark with anything (like paint marks) to see exactly what it is. I'll try marking the pulley and getting a better reading.

Let me put this out there also; during cranking of the engine (before it starts and runs) with only the starter, the timing light shows around 10 degrees advance. This I've marked with white paint. I've also marked the TDC and max advance with white paint. When the engine fires off, the white 10 deg timing mark disappears to that advanced timing.

Nothing has been changed or adjusted, this includes the crank pulley, cams, distributor, Ignition ECU, Air flow meter or Throttle position switch. All things are as they were.

The TPS was checked for proper adjustment and an Ohms ck accomplished, all ck'd good. Idle is poor but will run. When the throttle is advanced the timing shows to advance on the crank pulley, then timing will fall back as the throttle is brought back to idle. The timing is still at the advanced position.

Stevew writes, as I understand it, the Ignition ECU is pre-programmed with an RPM based advance curve. Also, the Vacuum Sensor and the Throttle Position Switch affect the ignition timing during different stages of engine speed and operation. So, if I read this right, any or all of these parts could cause my problem. Even when said part ck'd good?? Since my troubleshooting hasn't worked, I think I might be in the parts changing business soon.

I did my best to answer questions and list as much as I knew.
Thanks again for all your help, Chief
 

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This is literally a shot in the dark. Have you taken a close look at the distributor cap and rotor, ensuring that the cap is correctly positioned and seated on the distributor and the rotor is correctly located and seated on the shaft? If you pull the distributor cap off with the engine at TDC on the compression stroke for the #1 cylinder, is the rotor aligned (more or less) with the reference mark on the distributor body?
 

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Richard Jemison
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Timing

If the sensors/computer continue to be a problem, just convert the timing controls to a common trigger type distributor. It will control both spark and fuel delivery timing.
On the coil you will find a large white wire (computer ignition control). Take it loose and tape it up.
Connect a distributor (points or electronic type) to the neg side of the coil. Time the engine ...
No more dealing with the computer problem from a timing standpoint. It will still control flow rate as designed.
 

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Curiouser and curiouser...

Note to casual readers. The following is wonky and may require a score card to follow. Read further at your own risk :smile2:.

Hello again, Chief!

Just to be clear, you show a 10* advance while cranking, but upon engine start, your white timing mark moves off into the next zip code and even with insane advance, idle is fairly nominal. Correct?

"I'll try marking the pulley and getting a better reading."

At this point, I don't believe having an exact number is relevant. Just referred to the Factory Service Manual (FSM) test procedure as well as some notes I made for an ongoing electronic VSD conversion project and found that 30* (+/- 4*) is maximum advance from only the RPM based map in the Ignition Electronic Control Unit (IECU), with the VSD vacuum line disconnected. The FSM check is done at 2000 RPM and with the VSD disconnected, should show 16* of advance. I have observed around 30* of advance from 2500 to 3500 RPM, again, w/o VSD* (for some reason, I made no readings at higher RPM).

The Vacuum Sensor Device (VSD) will produce a significant advance signal at idle, but as I mentioned earlier, a properly adjusted TPS will tell the IECU to ignore the VSD signal at closed throttle and revert to the 10* you have observed during cranking (good catch!).

As soon as the throttle is cracked, manifold vacuum drops dramatically (goes toward atmospheric), the VSD bellows collapse and timing reverts to the IECU map. The VSD only serves to fine tune timing at very small throttle openings, when the mixture is lean and additional advance is required because of a slower mixture burn rate (the fire needs to start earlier).

I have typed and erased a highly unlikely scenario but before I start talking crazy, I'd like you to try these experiments:

1) Hold the engine at around 2000 RPM, observe your timing marks as best you can, then have a helper remove the vacuum line on the intake manifold that feeds the VSD (OK, a three person job, unless you have an external tach and can read it while operating the throttle, reading the timing marks and a helper pulls the VSD vacuum line). The resulting vacuum leak at the intake manifold will have little impact at that RPM. Do you see any change in timing?

2) This experiment is more of a pia. Access your VSD by removing the rear parcel shelf and side panel. Disconnect the electrical connector at the VSD which leads to the IECU. This will completely eliminate any VSD timing effect from the IECU and you will revert to the RPM timing map. Does start/idle timing change from your current "next zip code" situation?

3) Repeat step two, but use a hand vacuum pump to apply a fairly substantial vacuum to the port on the electrically connected VSD (or adapt to the VSD feed line at the intake manifold). Again, you can block the manifold fitting but at 2000 RPM, it won't have much effect. Do you see any change in RPM as the vacuum is applied and released?

I've got a bad feeling all my scribbling may be in vain and there is some simple issue being overlooked. Our ignition systems are reliable and robust and I don't believe I have heard the like of your problem (congrats on that, BTW:smile2:).

All the best

*The FSM test may seem contradictory since it is done under an artificial condition that never exists during normal engine operation, ie: 2400 RPM and high manifold vacuum.
 

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Discussion Starter #9
Hello, stevew and thanks for the testing info. It looks like a lot of reading and doing but I understand what your looking for. It shouldn't be too hard to perform the vacuum tests and I do have a stand alone RPM tester. As soon as this weather (SNOW) clears out, I'll do the tests and let you know the results. Chief
 

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We're missing something simple...

Roger that, Chief!

I may have made the check-out process seem much harder than it should be and as I know you understand, you just want to isolate the IECU and determine it's performance independent of external inputs.

Although you said you checked and ohmed out the TPS, I remain suspicious. As I pointed out earlier, at idle and off-throttle cruise, manifold vacuum will pull the core of the VSD coil out of the windings, resulting in a minimal inductance signal to the IECU. If a "closed" signal from the TPS is not present to override that signal, the IECU will respond by advancing timing another 16*. At off throttle/cruise, who cares, but at idle, the 10* base timing plus another 16* ordered up by the VSD means 26* at idle. Obviously way too much, but it should still be readable with your timing light (vs, moving out of sight, as you reported).

Take a look at Post 13 here for more info, pics and a page from the FSM:

http://www.alfabb.com/bb/forums/spider-1966-up/35224-reposting-missing-pictures.html

Standing by!
 

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Discussion Starter #11
Hi. Won't go into my medical problems but that's why I've not responded until now.

Stevew, I agree with you on the signal sent from the TPS to the IECU. I know I've ohm's checked the wiring from the TPS to the IECU connector. Didn't ohm's check at the IECU connector by moving the TPS to see if ohm's signal came and went. This is a check I plan to do very soon. If the ohm signal comes and goes, then I fear the IECU is defective. Like you said, it's pre-programmed with ignition timing.

I can't rule out the Vacuum Sensor Switch and vacuum line.

I'm pretty sure I'll get this figured out

Thanks for everything, Chief.
 
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