M5 rough idle, wet #2 plug, backfire on throttle

Specific conversations and info for the BMW E28 M5 and M535i.
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gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

My S38 has been against me ever since I decided to rebuild it in 2004. From machine shops leaving glass beads in the head and using Volvo shims that cut my cam shaft lobes, to an idiot owner (me) who tried to start it on 14 year old fuel and gummed up the injectors, I have been trying to get this car running since 2014 without luck. Obviously, you can tell that it's on the back burner most of the time. I'll wrench on it for a few weeks here and there, get frustrated and leave it for months, or years, but I'm ready to get it going again, this time.

Before this most recent time (in 2018), I determined I must have a stuck injector or two, as I had one wet plug, and when I removed all plugs and cranked it over, one cyl was sending out gobs of fuel thru the plug bore, one was sending nothing, and the remaining three were sending out fine fog/mist as they should. I sent all six injectors to Rich at Cruzin Performance (highly recommended) to be serviced again, despite having been sealed up after their last service there. That second service, Rich said came unstuck after only 30 seconds on the test bench, and otherwise were still in good shape, so he cleaned them, replaced the inlet screens, back-flushed them and flow tested them again. All was good. That was in May of 2018.

Now, two years later, I got around to cleaning out the bad fuel, fuel tank and replacing the fuel filter. I reinstalled the injectors after sending a little bit of WD-40 through them first, and buttoned up the fuel rail and injector harness.

When I cranked it for the first time, it took about three tries, cranking it for 10 seconds each until the fuel system primed and the motor started coughing. Just as before it behaves as though one injector is opening too much. It cranks nice and strong until that #2 cyl is on the squeeze stroke, when the starter strains and sometimes the motor almost spins backward one in ten revs. Seem like a hyrdo-lock from the unburned fuel to me.

When it finally catches, it runs a little rough, and pulling the #2 plug wire results in no change. It's like the plug is too wet to fire, or isn't firing, so fuel builds up. I've tested the lead for resistance (5.5k Ohm) and put a spark test light on it while running to confirm it fires. However, when running, it is the only lead that doesn't shock the hell out of me if I pull it off the plug. The others all zap me good. Maybe it's shorting out inside the loom to another lead instead, but I am trying to keep things simple for now.

The #2 plug is wet when I pull it. The others are lightly fouled.

I thought I remembered reading here once (but can't find the post) that someone had a similar problem with a plug not firing, and pulling the ignition lead resulted in no change. I can't remember what their solution was, but IIRC, it wasn't related to the ignition system.

When I move the throttle off-idle more than a little bit, it starts backfiring, and will not rev over 3k RPM, and I try not to do it anyway to preserve the catalyst. The backfiring made me doubt that I timed it correctly, except that it sat on a stand, fully assembled with just the valve cover off for two years. Every time my gear-head friends came over to hang out, we would turn it over to watch the machine do its thing. No fewer than three different enthusiasts ranging from hobby-grade to professional automotive technician turned it over by hand and confirmed the timing was spot on.

Does anyone have any suggestions for me? I'm selling my house, and I hope to drive the car to the next house, as it's ridden a flatbed to six different homes since 2004.

Freude am Fahren
Posts: 340
Joined: Nov 23, 2014 1:51 AM
Location: Culver City, CA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Freude am Fahren »

Start with basics. What’s the fuel pressure inside the rail? Is your FPR working appropriately?

austin8753
Posts: 1090
Joined: May 16, 2010 1:37 AM
Location: Portland, Oregon, USA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by austin8753 »

you may have a bad distributor cap - it's not entirely uncommon.

MannyE
Posts: 175
Joined: Aug 21, 2008 8:08 PM
Location: San Diego, CA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by MannyE »

Check your dipstick for sign of fuel in the crankcase.

Philo
Posts: 1992
Joined: Feb 12, 2006 1:00 PM
Location: Long Beach, CA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Philo »

Start with basic basics, compression and leak-down check. Even though it's a fresh rebuild you need to verify it after the problems with the builder.

Preyupy
Posts: 218
Joined: Jun 21, 2012 12:48 AM
Location: Issaquah WA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Preyupy »

Like they said, Basics first. Cap, Rotor, Wires, Plugs, compression test (leak down isn't that important if the compression is good right now) Are the wires hooked up correctly?

It really sounds like a plug or wire problem but you should check everything.

ahab
Posts: 6080
Joined: Jun 11, 2006 9:12 AM
Location: Chalfont, PA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by ahab »

From where I'm sitting and reading this, an ignition wire that's not shocking you when all the others are, combined with a soaking wet plug sounds like ignition delivery to me. I would put the number two plug wire on the valve cover with a spare plug in it and verify spark there first. With all that fuel spraying out you don't want to pull that plug, use a spare. Then work back from there. Do you have a spare cap/rotor?

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

The distributor and rotor are the ones that last ran on the car in 2004. They had only about 400 miles on them, but I replaced them today. When I pulled the black cover off the cap, I did see what I thought was a smoking gun: black arc damage on the #2 post and some hairline cracks in the cover. I decided this meant the #2 ignition pulse was leaking all over. Also, the brass posts had some carbon build up, and the rotor was sort of less-than perfect as well.

I replaced these two items with high hopes, and then cranked it for a bit with the coil lead off by accident (more on this later). When I connected the coil again, the motor exhibited the same behavior. Right now it won't catch, but seemingly most important is that there is a cylinder that, when the plugs are firing, is stopping the rotation dead, and overheating my starter after only 10 sec of cranking. With the coil disconnected, the cranking is smooth and regular. Once the plugs are firing, it would seem one is being triggered to soon, before TDC.

I should probably mention that the rebuild used new forged Wiseco 10.25:1 pistons and total seal rings. That one unknown of slightly higher compression was one thing that makes me wonder if the ECM isn't up to the task with the old Dinan chip. I have the Miller MAF and WAR chip with a set of stock base maps for my stated setup to Brody years back. Maybe I should just try that instead, but I thought break-in should be on the old AFM.

I also had to replace the exhaust cam with one from an M6, due to damage induced by a careless machinist, and I am no longer able to recall or find my email between myself and the seller that indicate what motor the cam came from. I think it was an E24 M6, but since I don't know, I should entertain the possibility that it is affecting valve timing.

I only mention these things because they are on my mind, but in the past, I have learned that jumping to the most complex solution before staying basic is a waste of time. I sent my ECU to be run in another M5 thinking it was dead, but it ran fine. I thought my crank pulse ring was off somehow, or that the machinist who resurfaces the flywheel may have forgotten the timing pin, and started to pull the motor again before I found a ground that was causing all my no ignition/injector issues.

I just picked up a new compression tester today, since I can't find my original one. I'll be hooking up my fuel pressure kit again too, but last time I checked, I was well within spec there too. I'll have to wait until Sunday to crack the fuel system, as I am showing the house and can't have the whole place saturated in fuel. Plugs are brand new. Ignition leads were new less than 5,000 miles before disassembly in 2004, and still test out to spec (#1 = 4k Ohm, #6 = 13k Ohm) according to the info I can find. Speed/reference sensors are new.

I guess I'm more explaining where I'm at in case anything jumps out. I used to be an automotive tech for Saab/BMW but that was 20 years ago and my mind just isn't what it once was. I started this rebuild on a whim when I realized the crank nut was missing, but now I hesitate to anything slightly daunting because I have so much else going on in my life.

Maybe I am taking it for granted that the wires are good, and one is leaking into an adjacent one, causing that plug to fire too soon, but these wires have low miles, and were kept safe in a heated garage the entire time they were removed. I don't have endless money to toss into it with shotgun diagnosis, is the thing. Kids, divorce/child support, moving again, COVID-reduced pay...

I hope someone hears about the motor kicking back on cranking and thinks of something I am not.

If not, I'll keep posting my steps and results here, for reference, at least. Some day, this damned car will run smoothly again!

austin8753
Posts: 1090
Joined: May 16, 2010 1:37 AM
Location: Portland, Oregon, USA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by austin8753 »

it might be worth swapping the #1 and #2 injectors, to see if there is an issue with the fuel injector. i know you mentioned that you had them rebuilt, but it doesn't cost you anything but time.

is there any chance that you can borrow a spare ignition wire set, just for testing purposes?

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

austin8753 wrote:
Jun 11, 2020 12:56 PM
it might be worth swapping the #1 and #2 injectors, to see if there is an issue with the fuel injector. i know you mentioned that you had them rebuilt, but it doesn't cost you anything but time.

is there any chance that you can borrow a spare ignition wire set, just for testing purposes?
I'll definitely try messing with the injectors when I crack open the system to check fuel pressures, but I don't have any spare wire sets from a friend. I'd have to buy a new set.

Freude am Fahren
Posts: 340
Joined: Nov 23, 2014 1:51 AM
Location: Culver City, CA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Freude am Fahren »

If you are near Los Angeles you can borrow my wires etc

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

Freude am Fahren wrote:
Jun 12, 2020 1:15 AM
If you are near Los Angeles you can borrow my wires etc
That's a very kind offer, thank you. Unfortunately, I'm a Maine resident.

Perhaps I should install the WAR chip and MAF which is set with a closer to correct map based on higher compression. Maybe the Dinan chip is too far advanced for these pistons. Strange it's only one cylinder that sounds like that though, so still troubleshooting Sunday when I can.

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

After considering the possibilities, I ordered a set of ignition wires today. Rationale below:

If I pull the coil lead to the cap, the motor cranks super smooth
Reconnect it, and the motor tries to fire backwards at a certain point in it's rotation. Always seems like the same point. rinn-uh-nuh, rinn-uh-nuh, rinn-uh-nuh, ERRTK! rinn-uh-nuh, rinn-uh-nuh, rinn-uh-nuh, ERRTK!

However, after some testing, I did determine that although the rough idle does not get more rough when I disconnect #2 lead, the hard cranking persists no matter which single lead I disconnect. I am loather to keep trying to run it in this condition, so I won't test running it with other leads disconnected. Suffice to say I am betting the dielectric properties of the leads has broken down to the point where some plugs are firing too soon.

I still haven't ruled out the ignition map in the Dinan chip, however. I can't find my original chip, so short of installing the WAR chip,. I can't change that yet, and I thought I had read (ages ago) that others had upped compression beyond 10.25:1 without trouble. We'll see what new wires does for me...

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

It wasn't the wires.

It seems that ignition timing is advanced in some way. I have to find all of my paperwork from the past fifteen years, but maybe I installed the EVO exhaust cam gear and the unknown exhaust camshaft, and have hosed my timing. I seem to recall not installing the EVO cam gear since I was changing compression.

PSA: Don't let your S38 rebuild project take fifteen years!

Preyupy
Posts: 218
Joined: Jun 21, 2012 12:48 AM
Location: Issaquah WA

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Preyupy »

Have you put a timing light on it? What do you have at idle? Nothing you can do to camshaft or cam timing is going to change the actual ignition timing (this is controlled by the ECU and the crank sensors in the bell housing. I suppose if you left the alignment dowel out of the crankshaft and installed the flywheel so it is not aligned properly you could have a serious timing issue.

Randomg
Posts: 432
Joined: Jul 12, 2007 3:12 PM
Location: DC

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by Randomg »

I'd be really surprised if it is related to the ECU's chip. I have 10.5 pistons and they ran just fine on a stock map, and I've played around with +10 to -10 degrees advance with the war chip, and never had it do what you're describing. The likelihood that the dinan chip changed the idle map seems small. I don't see how you could install the cap or rotor on wrong with the dowel pins. You could have installed the exhaust cam sprocket off by one tooth, which I think would be enough to potentially have ignition advanced enough to fight the compression cycle. You did a few test turns after connecting the cams and the markings line up? I think even with the performance sprocket, that cam marking should still be within the marking on the head even though it's not perfectly lined up. If the cams line up, I'd take an extra close look at making sure the wire going to each piston is the one you want to be, and that you didn't somehow read 5 and 2 upside down and switch them.

I'd avoid adding the war chip until it runs ok, I think should be looking to remove variables, and it's possible the combination of a poorly tuned chip and whatever is causing your problem you could cause some real damage.

gcracker
Posts: 144
Joined: Jun 05, 2009 6:00 AM

Re: M5 rough idle, wet #2 plug, backfire on throttle

Post by gcracker »

Thanks for the replies. This is a tough time for me to try to spend hours working on the car, as I am selling and buying a house in the next 30 days. I always hoped I'd drive it to the next house, but sadly, I don't think that's going to be my fate.
You did a few test turns after connecting the cams and the markings line up?
My motor was on a stand, fully assembled (minus the valve cover) for at least three years before I was able to get into a place with enough height to use my engine hoist to install it. In that time, I must have put at least two miles o the motor by hand. I loved watching it move, and kept adding assembly lube to lobes and shims to keep it lubed. After almost every hand rotation, I verified the timing marks lined up.

Thing is, now, seven years later, I am doubting everything. I wish my life had panned out so that I could have just torn it apart, done the rebuild, and reassembled it, but instead, 10 years passed from disassembly to first crank, and that was five years ago.
I think even with the performance sprocket, that cam marking should still be within the marking on the head even though it's not perfectly lined up.
That's a relief! I still have to check my M5 Parts shelves to see if I installed the EVO gear, but may have and forgotten about it.
I'd take an extra close look at making sure the wire going to each piston is the one you want to be
I even checked with an ohm-meter to be double sure they weren't mis-labeled at the factory, and they are going where they are supposed to go. I'll have to do a more basic troubleshooting again when I can risk making the whole garage smell of fuel for days, swap out a few injectors between cylinders, and tear the cover off to verify timing.

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