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Discussion Starter #21
Tom,
Over the years, I've gone through the guide and checked and replaced almost everything. The fact that the problem is only in the RPM range where the distrib is supposed to be advancing keeps bringing me back to it. I will certainly reset the distrib placement to insure that it's set correctly at TDC, and will get a timing light to see if there is any advancing taking place. Once that is done, I'll report back the results.
If there is not any advancing of the timing, where can I check the signal that should be causing it to advance? The signal comes from the Ignition computer on the right sidewall behind the passenger seat, yes? Is there a signal that causes the distrib to physically adjust, or is it all an electronic change that takes place in the computer?
Charlie
 

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Is there a signal that causes the distrib to physically adjust, or is it all an electronic change that takes place in the computer?
All done electronically. There is nothing in the distributor itself that affects changes in timing.
 

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interesting problem.

did your car ever drive properly?
did it start and drive before you 'adjusted' the AFM? (...not something I would have done, but still!)

on these cars timing advance is not mechanical, it's all electronic: so flywheel rpm signal/engine temperature signal/ECU
https://www.alfabb.com/bb/forums/767242-post7.html

you mention you replaced a bad upper flywheel sensor (rpm) in an old thread with a no start, and the car fired right up.
Maybe try replacing the other sensor?
you connected them up correctly, right? upper to black, lower to grey:

perhaps check the temperature sensor (CTS) as someone above mentioned..
failing that
the ECU, but that would be my last choice, as failure of ECU only at a certain rpm band is somehow hard to believe.

just a thought, is your Ignition ECU the correct one? (POs get up to all sorts of wierd things!) should be Bosch 0 227 400 003
 

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Discussion Starter #24
Well, that leads me to this....
If the Distrib is lined up correctly at TDC....
And, there is no advance when the RPMS are goosed through 1750-2400~....
THEN....
Because the advance is signaled by the increase in vacuum....
THAT....
either, the MAP sensor is not sending the correct input to the computer....
OR...
the computer is not advancing the timing as it should.

Right????

I'll check the position of the distrib and get a timing light to check if anything is moving.
I'll report back when finished.
Thanks,
Charlie
 

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Timing advance will depend on both MAP sensor and RPM sensor (flywheel sensor).

Even if the flywheel sensors test OK with the ohm meter it might be worth checking for any physical damage. I helped a guy out with a no-start condition. The flywheel sensors tested OK but when we removed them one had a dent in the end facing the flywheel. We replaced it with a used but known good sensor and the engine started right up. There have also been cases where the bolt threads stripped out allowing the sensor to slip out of place.
 

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There are two advance mechanisms. The electronic ignition advances the timing based on RPM, and the MAP sensor adds advance based on vacuum.

The MAP sensor is mostly a fuel economy thing, and advances the timing at cruise for better efficiency. The car will run fine without it, so like I said I doubt that's your problem.

Unplug the hose to the MAP sensor and plug it at the plenum. Then watch the timing mark with the light as you rev the throttle. It should advance smoothly with RPMs.
 

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Discussion Starter #27
Dom,
The car ran well, and then it didn't.
It wouldn't even start until I adjusted the AFM. It actually runs well now, except when you goose it at lower RPMs, it bogs down. Above 2200-2400~ it runs great. And it idles well. Starts right up. If you increase the RPM from idle up past 2400 slowly, then it's all OK. It's only when you require a rapid increase in power from lower RPMs that it bogs down.
I replaced both flywheel sensors at the same time, and hooked them up correctly.
Tested the CTS and it checked good.
None of these corrections, replacements, or tests have fixed the problem.
I will check the # on the ECU, as you've recommended, and will report back.
Thanks,
Charlie
 

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I don't know if the fact you had to fiddle with the AFM is a red herring or a huge clue. Normally the AFM is a 'leave it alone' item. I have read about wear in the resistor strip on the printed circuit board of the AFM. This usually reveals itself as poor running in the mid range (where the majority of the normal running activity occurs). Try googling 'repair of Bosch AFM' and see if any of that makes sense.
 

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personally I'd put the new old stock AFM that you have (mentioned in another thread somewhere) back in, and then try get the thing to start, idle and drive correctly...because a messed about AFM is always going to be an 'unknown'
bending the arm to ride on a new track is one thing, but that 'clicking adjustment' you did (was it 8 notches ?) is fiddling with something that is factory set.

have you checked fuel pressure?
what if the vacuum hose to FPR is slightly cracked, so that all works fine if you feather the throttle, but when you stomp on it, the crack opens and then pressure fails?
 

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MAP vs VSD

Hello gang!

Just a note to add to the discussion, and I know I’ve spoken of this before, but the Vacuum Sensor Device (VSD) in our L-Jets is not a Manifold Absolute Pressure (MAP) instrument. MAPs, found on all modern cars, are referenced to an internal, 0 psi reference and generate a voltage proportional to the difference between the internal reference pressure (0 psi) and pressure in the intake manifold, hence the absolute in its name. With so many cars being turbocharged, pressure in the intake can obviously be considerably higher that atmospheric (where a vacuum gauge reads zero) and the ECM needs that additional information. In L-Jets, not so much.

Our L-Jet VSD is designed to provide a factory adjusted output at standard atmospheric pressure (changes in altitude are handled separately by the...wait for it...Altitude Sensing Device) and as the throttle opens and closes, intake pressure obviously changes, with the lowest manifold pressure (highest reading on a vacuum gauge) off throttle, when cylinders are working against a closed throttle plate, and the highest manifold pressure (lowest reading on the vacuum gauge) at Wide Open Throttle (WOT), when the intake is open to the atmosphere.

Kinda non-intuitive, what with the high vacuum, low manifold pressure, low vacuum, high manifold pressure business, but I didn’t make the rules. :)

Our VSDs (the ones that still work, that is) use a moving rod inside a coil of wire to generate a variable inductance which is fed to the ECM and changes timing advance as a function of intake manifold vacuum which is of course determined by engine load.

Wish I could contribute to the OP’s rpm sensitive headache other than to say I agree, as others have noted, making adjustments to the air flow meter (even though necessary for starting) was ill advised and there are some as yet undiagnosed issues remaining, including the no-start problem.

Mark Twain is reputed to have said, “It’s not the things we think are true that get us in trouble. It’s the things we think are true, that just ain’t so.”

Do the L-Jet diagnostic procedure again, assuming nothing.

Best
 
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