Low RPM Low advance Ignition misfires

[16vboost]

Hi All,

I've been running Megasquirt II on my 16V VW Scirocco since April and have been making constant improvements to the tuning since. One such improvement was reducing the ignition timing from 20-ish degrees at idle to the manufacturer specified 6 degrees BTDC at idle. This improved low end torque and driveability enormously but the car now misfires if it idles below 1000 RPM. I can tell it's the ignition because when I check the timing, the timing light misses flashes when the engine misses. If I raise the idle above 1000 RPM I don't notice them as much. I've messed with the dwell but haven't seen much improvement either way. Driving the car is great, there's no perceptible misfire. Below are some details on my setup

General: MSII on 3.67 PCB V2.89 code, Hall sender pickup on distributor, LC1 wideband

Ignition: BIP373 ignition driver, Stock Ignition Coil
Trigger offset 6 BTDC, (verified)
Max Dwell 2.5ms
Spark Duration 2.0ms

Any ideas?

-Alex

[arnold]

coil might be KAPUTT? check if th emissing is to been in between the coil and the dizzy by connecting the timing on that HT lead

[16vboost]

Good call, I'll let you know how that goes in a few days when I get another chance to check it.

[16vboost]

So I finally got my car back together and was able to continue troubleshooting this problem. One of the things I changed while I was in there was the distributor cap and rotor. The misfire persists. I took Arnold's advice and hooked up the timing light between the coil and the distributor and I can see missed flashes there. I also hooked it up to each cylinder and saw misfires on all of them. So whatever is happening is pretty universal. I took out the plugs and cleaned them a bit since they had a bit of soot from running conservatively rich. This didn't help. I also increased the dwell to 8ms just to rule this out. The increased dwell seemed to help at 7ms or higher, but only initially. It's now back to "normal". I might try reducing the gap on the plugs ".032" currently to something a bit smaller. I also have a new set of plugs as backup. I kinda want to try this out before shelling out $50 for an OEM coil. Any thoughts? Should I get one of these instead? http://www.DIYAutoTune.com/catalog/ign1 ... p-444.html

-Alex

[16vboost]

I did some more troubleshooting today. Well I figured out what isn't wrong. I went ahead and installed the new plugs, gapped at .032 and the problem continues exactly the same. I also tried a new ignition coil and again, exactly the same. I can only imagine that it's the spark plug cables. The strange thing is that the car ran fine with these same cables when it was stock. However, that in 2005. Do cables go bad sitting in a box? I wouldn't think so. I'm guessing everyone else is as stumped as I am. I'll post back what I find for the sake of closure to the thread.

[trakkies]

I did some more troubleshooting today. Well I figured out what isn't wrong. I went ahead and installed the new plugs, gapped at .032 and the problem continues exactly the same. I also tried a new ignition coil and again, exactly the same. I can only imagine that it's the spark plug cables. The strange thing is that the car ran fine with these same cables when it was stock. However, that in 2005. Do cables go bad sitting in a box? I wouldn't think so. I'm guessing everyone else is as stumped as I am. I'll post back what I find for the sake of closure to the thread.

Assuming they are resistive core cables (pretty well all maker's ones are) they can be checked with a DVM set to ohms. The actual resistance varies by type and length but will be something like 2000 ohms per foot of length. A faulty one will read several times this. It's very unlikely all are faulty so it's easy to work out what the resistance per foot is. And don't forget to measure the king lead to the coil.

[Peter Florance]

Check distributor phasing. Make sure at ~ 25 degrees bTDC that rotor points to either the cap post of plug 1 or whatever the 3rd cylinder in your firing order.

You must satisfy the phasing condition before adjusting trigger angle

[flatbill]

I found that on my '56 Desoto that if I tried to lower the idle advance too much I got misfires. Just as a test, try richening up the cells where the motor idles. A lean mix requires more time to burn. You may be pulling out too much advance. 6 degrees seems like too little. Your original spec of 6 degrees didn't include disconnecting a vacuum advance did it? I'm used to the old tech motors with vacuum and centrifugal advance. Billk

[trakkies]

I found that on my '56 Desoto that if I tried to lower the idle advance too much I got misfires. Just as a test, try richening up the cells where the motor idles. A lean mix requires more time to burn. You may be pulling out too much advance. 6 degrees seems like too little. Your original spec of 6 degrees didn't include disconnecting a vacuum advance did it? I'm used to the old tech motors with vacuum and centrifugal advance. Billk

IIRC, the vacuum advance on a dizzy works rather differently to the sensor on the MS. The take off is usually above the throttle disc at rest so has little effect at idle. With the MS, the vacuum is near max at idle.

[16vboost]

Thanks for all the good advise. I double checked the resistance of the ignition wires and the four vary from 4.38 to 5.64 kohms, which is just below or within the 4600 to 7400 ohm spec. But the wire from the coil to the dizzy was at 2880, which was just above the 1200 to 2800 ohm spec. These are 8mm Magnecore wires btw.

I really like the idea of checking the distributor phasing. It would explain all my symptoms. I thought I put the distributor in more or less the factory position, but who knows. This will take me a while since as of last night I have a much more pressing "zero oil pressure" problem, but I'll report back here once I check that out.

As for the 6 degrees, yes that's with the engine idling, and no vacuum lines disconnected. I was running like 15 degrees before and have noticed MUCH better low end torque and throttle response since changing it (with the exception of these pesky misfires).

After reading the Mega Manual up and down, I have a couple of questions on the trigger offset. My distributor has a hall sensor. Right now I set my trigger offset at 6 degrees which means that for a timing of 6 degrees, there is no delay between the ignition event and the hall sensor signal. Is this right? When the car is running at say, 20 degrees, BTDC, then the ignition event is calculated off the previous hall sensor event and ends up happening 14 degrees before the hall sensor signal for that cycle? So the first question is based on the above, do I understand how megasquirt times ignition events and how trigger offset works? The second question is, during cranking, how is the ignition timing calculated? Does the ignition happen whenever the hall sensor sends a signal? If so, how could I run a trigger offset of say, 30 degrees?

Thanks again for all the help.
-Alex

[16vboost]

Finally, success!

I didn't move my distributor. I ended up changing the trigger offset to 77 BTDC, from 6 and changing the ignition capture to "falling edge" instead of "rising edge". I also had to change the Cranking trigger to "trigger return" from "trigger rise". This was a bit confusing since I wanted the ignition capture to happen at 77 BTDC when the wave falls, and the cranking to happen at ~6 BTDC when the wave rises. I guess trigger return just means the opposite of whatever you set as ignition capture.

When reading the megamanual, you get the idea that the trigger offset can be anything. It seems that it should be a value that is well before any advance that you'd want to run. It seems like there was some confusion when the trigger offset was set to 6 and the car wanted to trigger at 6. Maybe it didn't know when to start charging the coil, who knows.

I'm still going to double check the distributor phasing but for now I'm very happy. There are no misfires at idle no matter what the timing is! Thanks for all the advise!

-Alex