Cranking RPM?

[BCM]

Quick question. Trying to get my car running again after a year of sitting. I had a running and tuned setup, and have changed almost nothing related to the MS. I did install new injectors, and rebuilt the motor.

What's happening is my crank RPM is oscillating back and forth, a little bit randomly, between 0 and about 180 when I'm trying to get it running. The change is abrupt, like it's losing the VR signal and then regaining it. The Tuner Studio indicator light goes back and forth between cranking / not cranking. Just want to verify - but with a "proper" setup, the crank rpm should stay relatively constant / steady, and the indicator should say "cranking" the whole time, right?

MS3 / MS3X, Running Sequential with a 36-1 Crank Wheel / VR Sensor, and a Honeywell 1GT101DC Cam sensor triggering off a single bolt...

Any ideas? A little bit puzzling because this setup worked fine when I parked the car last year...

[billr]

Since it is rebuilt, is it now cranking noticeably slower? I have often found that to be the case on a "fresh engine", it may be too slow for your sensors to output the same as before (and sufficiently).

[BCM]

It's not cranking any slower that I noticed... Did a little poking around and it looks like the plug housing that connects to my VR sensor might need to be replaced. One of the pins no longer "Snaps" into place and pulled right out of the housing when I was checking the wires. My guess is I'm not getting a great contact because of that.

I figure I'll replace that first, then give it a try with the same resistor I have in-line right now. If that doesn't work I'll pull out the resistor and start fresh...

[billr]

Pushed pins/sockets like that are a very common and sneaky problem. When you disconnect the connector for inspection, there is just enough friction on the pins, or stiffness in the wires, to make the pin/socket move forward and look OK; but when the connector is mated again the bad pin/socket moves back again without giving firm pressure and long "wiping action" on the contact surfaces. Let's hope that is all it is! Unfortunately, if the contact won't snap in correctly, you may have to change the contact or housing... but I would try something like epoxy or hot-melt glue first. Sure, that will ruin the housing and that contact (at that position) for further R&R, but you would already be at that point anyway, right?

[BCM]

Replaced the VR Sensor Plug in question, and tried again. Still not getting a consistent RPM, although it did seem better. I'd be curious what people thought of the attached composite log. Also attaching a screen shot of the diagnostics window (Sorry, little hard to see because of the RPM scale)

Under cranking, should I be expecting a consistent RPM, and no drops back to zero? One of those things I can't remember exactly how it looked when everything was last working for me...

Maybe I should remove the in-line resistor in the VR Signal wire?

[BCM]

Took another tooth log and composite log. Also including a data log. Could use some help ID'ing if this is all that abnormal....

Starting with my working setup from last year, This is the same MS3 setup and wiring, new injectors and injector pigtails. New, higher compression engine (But no wiring or config changes). Now a new VR sensor plug.

Comments?

[jsmcortina]

New, higher compression engine

That's the problem...
the high compression is making the missing tooth indistinct. Take a look at the tooth log and compare it to the examples in the manual. By eye the tall missing tooth is quite unclear and is not double the length of the tooth that follows it due to the compression slow-down.

Two ideas, you might want to do both.
- rotate the trigger disc so the missing tooth region is 2-3 teeth earlier (that will put it in the high-speed "dip" in the tooth log)
- remove one more tooth to make it a -2 wheel (remove the tooth that would pass the sensor before the missing region)

For either of these changes you'll need to re-set your tooth#1 angle.

James

[BCM]

James, Good catch, I'd started thinking about that myself, but I'm wondering if it's more than that. I took another tooth log, and another composite log, this time with the plugs out (Which I don't think I've ever done before) which gets rid of engine slowdown due to compression as a factor. I'm still not getting a perfectly "clean" tooth log, and the composite log still shows a bunch of sync loss too...

What would you expect to see in this scenario? Want to make sure I'm not making mechanical changes before I'm sure...

Thanks for the help.

Brendan

[BCM]

I am still testing with the plugs out. I have gone from a 10K to a 20K to a 30K resistor in-line with my VR sensor. The tooth logs are getting better and better, but I did get a couple of Sync Loss Reason 17 codes when I switched to 30K which I hadn't seen before... I don't want to keep hiking up the resistor value, but so far it appears to have been helping.

Any comments? Keep going up? How would I know when I get too much? Should I switch to adjusting the trim pots instead?

Latest datalog and tooth log attached.

[BCM]

Well, I guess I'm getting better at debugging. My end change: New VR sensor plug, 10K resistor out, 30K resistor in, went from 0 turns on R52 trim pot to 6 turns on R52 trim pot.

Before (Cranking with Plugs Out):





After:






Cranking with Plugs In:




Still haven't tried to fire it up, but I'm feeling better about it.....

[jsmcortina]

A bit of re-phasing of the wheel could help for sure if you can achieve it, put the missing tooth in the middle of the dip.

James

[BCM]

James, I would like to. But my trigger wheel is welded to my crank pulley... So it's a bigger effort than I'd like. Remove radiator, remove crank pulley, remove trigger wheel, probably destroying it and my crank pulley in the process. Buy new crank pulley, new trigger wheel, etc, etc...

I've even given thought to making it a 36-2 wheel like you suggested, but this removes my TDC marker pin, which causes me some trouble finding a way to add an indicator back for that...

So, I'm going to give it a try and see how it works as is... I've been cranking on a battery charger, I'm hoping a fresher battery will help tone down the fluctuations some.

Thanks for the insight on the phasing and the 36-2 option, I'll keep those in mind. I might even look into a different mount for my VR sensor, if it's not easy to move the trigger wheel, maybe I could move the sensor for the same effect.