RichardFranks

I'm at the end of my LS1 swap into a '92 BMW 318is. I've been fighting this issue since I got it running. Engine is a '01 LS1 out of a Trans Am with 243 heads, BTR valve springs/push rods, and a BTR stage 2 cam. Wiring harness was done by Wiring Specialties, specifically for the e36 LS1 swap.

The conditions/symptoms are:

While parked, it fires right up when cold, and idles perfectly. If I turn it off when hot and start it back up again, it has a hard time finding idle. It might stay on for a few revolutions, then lose its idle and shut off. If I give it a tiny bit of gas, it will usually stay on, but the next time I rev it and it returns to idle it will hunt for it then shut off.

If I give it some power once it's warmed up while driving, the engine sometimes acts like it hits a rev limiter or fuel cut and the tach starts bouncing around, until it catches itself and comes back. This happens at around 5k RPM, but doesn't always happen at the exact same RPM. Once I come to a stop, it will usually not find idle again and die. If I give it a little gas with the pedal, it stays on.

My tuner also told me the injector duty cycle looked odd, and he was having trouble with the MAF showing odd readings. This lead me to redo all the grounds to eliminate that as an issue, but it didn't seem to make a difference.

Here is a video of the engine cutting out as I'm describing: The battery light also flashes in there like it's not charging under that condition? I have no idea what would cause that.

In my attempts to fix it I have:

• Had the vehicle tuned
• Plumbed a new fuel return line to ensure constant fuel pressure
• Cleaned all grounds and battery connections
• Added 3 new grounds on the cylinder heads/block for a total of 6

Right now I'm working on replacing the crank sensor, but to do that I had to take the headers off to remove the starter to get to the crank sensor. This is a shot in the dark, but I'm guessing it might be part of the problem because the RPM gauge is the only one that freaks out while the engine cut issue happens (as seen in the video). While I'm in there I plan on looking over the entirety of the wiring harness with more detail.

I would greatly appreciate any help. I'm not sure how to check for interference in the electrical system, or where it might be, but I'd guess that's my next place to look. I'm at a loss at this point.

Thanks!

LLLosingit

iTrader: (4)

I don't see a voltage gauge on the dash, I would hook one up and see if you have a voltage/electrical issue. With the voltage light coming on I would wait on the crank sensor and check the electrical side of things first. It almost seems like you have a short of some kind not just a no charge situation.

LS6427

iTrader: (2)

A failed or failing Crankshaft Position Sensor.....does not do any of the symptoms you have. It either cuts the engine off clean....or it starts and runs perfectly.

Your issue sounds like a MAF or worn out TPS issue. Also, could be a failing IAC valve....they act up when engines get hotter.

Next time your engine is dead cold...unplug your MAF, then start it and go drive it until the engine gets hot. See if you have the same issue.....If it runs normally, get a new MAF.

But all these items can be purchased.....installed one-by-one to see which one fixes the issue. Then just return all the others. Just tell the parts guy they do not fit your car because the last owner installed a custom wiring harness.....and that you never installed any of them because none of them had the correct plugs or connectors. Works every time...

.

wssix99

iTrader: (5)

It's hard to read your gauges because they are in German, but if your hesitation problem is only at 6000 rpm, then it's probably a different issue from your idle issue.

^ If that is true, it looks to me like your engine is getting starved of fuel or air at high RPM. First step to test this would be to check this in neutral. That will start to narrow things down.


The first things I would check:
- Scan the PCM for codes
- Watch an O2 scan (you may have a problem when the engine switches to closed loop - https://www.hotrodhandbooks.com.au/e...Line-03-2.html)
- Make sure you have a good coolant temperature input to the PCM. (If the engine is warm and the PCM thinks its cold, etc. things won't go well.)

LS6427

iTrader: (2)

As weird and crazy as our PCMs are....could be a battery thats failing too....that wigs out the PCM. Maybe have the battery checked too....

Also, possibly a failing 02 sensor.....

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RichardFranks

You were right, I already had the car half way ready to put a new one in so I just went ahead and did it anyway. Old one showed no weird signs of wear and the wiring up to it was fine as well.

I also opened up the PCM to look for popped capacitors or burnt traces on the PCB, or anything like that and found nothing. Looked great in there. The MAP sensor had a clip broken off of one side of it so I replaced that too, just in case, though I really doubt that was it. Removed both O2 sensors to look them over and they both looked fine, but that doesn't for sure rule out a bad O2 sensor I guess. The only weird thing I found was the nut holding the +12v from battery to the starter had come loose, but I doubt that had anything to do with all this.

A couple more things to note:

• Car is on speed density tune, so MAF isn't really used. Tuner said MAF was throwing weird readings when I took it there so we went speed density.
• Tuner also said injector duty cycle looked odd. It was modulating up and down rather than growing linearly.
• PCM currently throws no codes
• I tried a new IAC as part of troubleshooting, issues were still present with a new one installed
• Battery was tested by parts store a little while ago
• ECT sensor seems to be good, I monitor it with my OBDII reader/torque while driving and it doesn't spike around or anything.

I'm starting to think more that it might be an overarching electrical issue. The harness was done by Wiring Specialties and is a really nice piece, so I'm hesitant to blame it but anything's possible at this point.

I think next time I go back to work on it I'll get the volt meter out and start there. Still kind of at a loss as to what it could be.

Thanks everyone for the help so far. I'm determined to get this thing fixed so I can finally drive it reliably next summer.

LS6427

iTrader: (2)

You need to get it scanned with an OBDII scanner while the engine is running and already up to operating temp......Check the 02 sensors.

With a SD tune that changes normal diagnosis ideas a bit.....I'm not up on SD tunes and how they differ from a MAF tune.

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wssix99

iTrader: (5)

Is your regular fuel line sufficient for flow to a V8 engine? Is your cutout only at high RPM when you are pulling in a lot of air/fuel?


Does the problem happen in neutral?

RichardFranks

Fuel system is a Walbro 255 to a Wix corvette style fuel pressure regulator with -6 AN lines throughout. The return line fix was done when I saw a big restriction on the stock BMW barb fitting. Fuel pressure was a little high, so I just switched to a -6 bulkhead fitting to rule that out.

It is safe to say that the cutout happens under high RPM and when I'm pulling in a lot of air/fuel.

I haven't had a chance to get everything back together - I had to drain the damn coolant to get the coolant temp sensor out of the way to remove the headers so I could drop the starter so I could change the crank sensor lol!

I'm taking off work tomorrow to work on it all day, so I will check for the problem in neutral and report back. I also have a voltmeter to wire in so I can check voltages under load.

Also, @LS6427 - I have the bluetooth OBDII scanner so I will be sure to check that tomorrow or Saturday as well.


Thanks again for the replies, your help is much appreciated.

wssix99

iTrader: (5)

It would be interesting to watch your fuel pressure and MAF readings during this exercise, as well.

Even though your MAF may be taken out of the tune, it might give you an indication of air intake issues.

RichardFranks

Had a bit of a runaround this weekend, the first crank sensor was DOA so the car wouldn't start. After the whole runaround of doing that, the car blew an axle on the test drive. So I got it towed and spent today doing the axle.

I couldn't get the car to stutter or have an odd idle on startup before I took it out, and I let it warm up and cool down a couple times while refilling the coolant. It also ate all the way up to the rev limiter multiple times while I was flogging on it during the test drive. If it's fixed, it's either the crank sensor or tightening the battery cable on the starter, but I really have no faith in that being the case.

Going to try and burn my last 2 days off this week to work on it Thurs/Fri, and see where it gets me, and will update then. I have the torque app set up to do logging for the MAF, O2, voltage, and other things that might lead me to the root cause.

RichardFranks

Well, I've been driving the car for the past 3 days with no issues. I can't get it to mess up again. I've tried everything, and it seems to be fixed.

Since the start of this thread I've done the crank sensor, the MAP sensor, and tightened the +12v wire on the starter. I don't really know how it could be the crank sensor other than it feeding the PCM an errant high or low RPM signal. Either way, it seems to be fixed, if it comes back again I will update with fixes.

Thanks again to all who replied and hopefully this can help someone else out in the future.