Weak Spark

[LastCat]

Fixed: bad ECU.

Got a another 059 from a guy in Florida, installed and started. Nice strong white/blue spark. Checked with the timing light on the front dampner (this is only to check timing, no adjustments here) and the timing stays constant on the dampner. ECU just had an issue, where?, not sure, it's toast.

Smoothed out quite a bit, but still has a little rumble. I suspect the Injectors. Which I already cleaned, ohm tested at 16.0, new screens and O-Rings. Time to get new Injectors. Probably going to try the Ford 280 155 746 19lb injectors. Much quieter than these old style 714's that sound like rocker arm rattle. I'll post the fix when I swap out injectors. Big thanks to everyone that replies, shout out to DesktopDave.


I've worked on cars since the 70's. Don't know it all (who does) but can do most things, engine, electrical, paint, interior, etc...

Ok, here's what I got.

1985 635 Euro import. Car sat for 7 years with a blown head gasket. Finally last year I rebuilt the engine using a B35 block and head. Installed all my B34 Sensors. Car started right up, idles, full throttle, but rough at idle. So I started checking Sensors, wires, etc..

New Rotor, Cap, Wires, Coil, Plugs (Bosh Silvers WR9LS ?), Main and Fuel Relays, Battery (fully charged), Fuel Pump (single conversion, good pressure too). Checked the AFM, TPS, CTS, TTS all good. Car starts and runs, but like I stated, I can feel it has a small shudder/rumble/shake. So I pulled the plugs. Medium carbon deposits.

Did a Spark Test (ground the plug, pull the fuel pump relay, all plugs are out and cranked over engine) and the Spark is yellow/orange and not very strong as I would consider to be weak. I tried other plugs and same results. I also tried a different brand spark plug (champion) and same results, yellow/orange and weak. Car starts right up. Plenty of fuel, but this weak Spark is not going to cut it.

So my question is, the ECU going south? For the Coil to get the correct Timing, the Ground to Coil #1 from the ECU controls the Timing. #15 is the 12v source from Main Relay. #4 is the Coil output wire to the Cap/Rotor.

Is there a possible ground issue with this ECU sending the correct ground signal to the Coil that is giving a weak spark?
Or is it the Pos. side that is coming from the relay that is low on voltage? I've check this voltage while cranking, not sure this was a valid test. But goes from 12.5v to 10.5v while cranking. I've thought about a straight fused wire, on a switch, that I could send from the battery to the Coil and see of the Spark changes.

I've been into this ECU before in 2012, when I had a no start. Found this post that fixed my problem. Had a poor solder connection, removed old solder added new and car started right up. Opened it again last night and looked it over under a magnifying glass, nothing loose or corroded as I could tell. Even went back over the 3 solder joints again and reheated. Put back in car, and still a weak spark.

http://www.e38.org/e28/louielouie/dme_fix.html

I can get a used 059, but not sure it would work, since I would not be able to use the O2 (or just unplug it). I also thought about the service by these guys. They do a $95 test. Which isn't a bad idea, at least eliminate the ECU as the problem.

https://www.ecudoctors.com/pages/95-testing-service

The ECU could be a 261 200 059, 35 pin. Not sure about the Model, since it was an Import and the ECU was built by Performance Services in Redondo Beach, Ca. They replaced the Bosch Label and added their own (be calling them today, if they still exist). I have included a few pics. Any insight or help, type away.

u4ZYobX.jpg

wIu5Ltj.jpg

VEoalnN.jpg

[marc79euro635]

I love your car, looks very similar to mine (blacked out).
Does jumping the car with another battery help spark?
Low battery may cause slow cranking, which might affect spark strength. I know on my megasquirt the pulse led will not even blink (like it should), if battery is too low.

[LastCat]

I love your car, looks very similar to mine (blacked out).
Does jumping the car with another battery help spark?
Low battery may cause slow cranking, which might affect spark strength. I know on my megasquirt the pulse led will not even blink (like it should), if battery is too low.

Thanks Marc, wish I had the Mega. But, probably more time and wiring I would spend on this car at this time. Almost did that 15 yrs ago. Actually the battery is fully charged and it starts and runs no problem, except for the little misfire. Seems it has a little elec. short somewhere, gremlins, argh!!! I think it's the ECU.

[DesktopDave]

I believe the coil is directly driven from the battery via the ignition switch, so you should see a nice solid +12VDC there at all times. What's the resistance across the new HT plug wires? IIRC they're 6Kohm resistor wires, except for the 3Kohm coil HT wire.

You're using the original matching B34 injectors, right? I'd think the B35 injectors could cause some trouble.

I'd be somewhat strongly tempted to swap out the DME...given that it's clearly been worked over. A lot of the old Euro high-compression M90 & M30 conversions used spark timing to piggyback (hack) an emulated O2 sensor signal. Usually it was called a "Johnson box", but yours is something I haven't happened across before. I wonder if it's a US DME or a reworked Euro?

I'm assuming commonly problems have been addressed (as you seem to know what you're doing): e.g. the crank position sensor has a good strong signal, well isolated shielding, and the flywheel pin is intact, etc.. I'm not sure how you'd graph out spark timing, dwell or injector duration on these old 15-pin diag connectors.

I first thought it'd be a cold solder joint, but you've been there already. Cold solder joints in the DME are very common with Bosch high current transistors. Like you, I've flowed a bunch of them to resolve spark & injector problems like this.

These old Motronic 1.0 boxes are quite durable overall, but I'd also suspect that these transistors can't possibly live forever. I don't have a part number to suggest, but I have heard good things from the Megasquirt forums about the updated EcoSpark replacement for the Bosch BIP373.

[LastCat]

These old Motronic 1.0 boxes are quite durable overall, but I'd also suspect that these transistors can't possibly live forever.

Thanks Dave, my sentiments too. 36 yr old ECU.

Org 18# B34 injectors, same as the B35. Pin still on Dampner and rear Flywheel. I actually thought about it for a day and just went all in on the ECU being the problem. It had just a slight small shudder, I could feel the engine shake, no pattern at all, just a random shake every 1/2 sec, like a electrical short. Car starts, idles, etc... Just not right.

So talked Jorge at Spec. ECU Repair. Boxed the ECU and sent his way, after a lengthy phone call. Odd too, I actually called that company in Redondo Beach, CA and they have never worked on ECU's, they have been in biz since the 80's.

When this car got imported to Florida, back in 86', they had to smog control the car. So the ECU only had work inside for O2. They pulled the original sticker, who knows why. No other box in sight (Johnson Box). When I get this ECU back for Jorge, I'll see how it goes and when I figure this out, for sure come back here and post the fix.

I have been reading the last week about ECU, Weak Spark, Coils, etc.. and some Post go back to 2003 and Roadfly.

[LastCat]

Sent my ECU to Specialized ECU Repair in Oakland Park FL. Got a call today and all is good, they are sending it my way. Test cost $95. Starts in there test car. They found the ECU as the 059. Welp, plan C on the way. Some reason there is low voltage to the coil side post #15.

Going to check the Main Relay by using a fused link (25 amp fuse) as in this pic and see if my spark will increase. If nothing changes, might just pick up another 059 and go from there. Crossing fingers, more to come.

relay_jumpers_193_114.jpeg

[DesktopDave]

Check your ignition switch too. It's a far less likely scenario, but also a problem with our old E24s.
Positive battery terminal is connected to ignition switch pin 30 via a red wire.
Ignition switches +12VDC to pin 15 in RUN or START position, it's wired to coil terminal 15 via a green wire.
DME pin 1 grounds coil terminal 1 fire plugs/maintain ignition timing.
Coil pin 4 is the primary "king" HT lead to the distributor.

You have a copy of the BMW Electrical Troubleshooting Manual hadny? Here's the link for the '85 from Miller Performance. There might be a few minor wiring differences during model years, or perhaps with the Euro cars, but the ETM is an excellent resource to have on hand. I used to print up a hard copy and leave one in every BMW. Now I just keep the PDFs on my phone for roadside emergencies.

[LastCat]

Check your ignition switch too. It's a far less likely scenario, but also a problem with our old E24s.
Positive battery terminal is connected to ignition switch pin 30 via a red wire.
Ignition switches +12VDC to pin 15 in RUN or START position, it's wired to coil terminal 15 via a green wire.
DME pin 1 grounds coil terminal 1 fire plugs/maintain ignition timing.
Coil pin 4 is the primary "king" HT lead to the distributor.

You have a copy of the BMW Electrical Troubleshooting Manual hadny? Here's the link for the '85 from Miller Performance. There might be a few minor wiring differences during model years, or perhaps with the Euro cars, but the ETM is an excellent resource to have on hand. I used to print up a hard copy and leave one in every BMW. Now I just keep the PDFs on my phone for roadside emergencies.

Thanks Dave, think I found the problem. I have the ETM and understand the pins #1, #15 and #4. The ECU controls timing through Pin #1 at the harness/ECU to the Coil, which is a ground. Speed varies and timing varies as per input to the ECU. My problem has been a weak spark.

I checked that at Pin 15 and 12.5v. All good. I had my ECU sent out to be bench tested and car tested. Before I sent it off, I was going to run a switched jumper wire from the Battery Pos to pin 15, crank the engine and switch the voltage to pin 15. See if the spark would increase. Right now it's a limp yellow/orange. Hence the carbon fouled 6 plugs. But I called ECU Repair in FL before running the hot wire test and figured why not, it was only $114 and peace of mind. Sent it out the next day.

Got a call 2 days ago from them and its all good. So today and here is where I think (emphasis on think) I found the problem. I was testing the Main Relay 332 014 112 today and it had low Ohms reading of 10-11. Should be from 20-200. I applied pos/neg to pins 85 and 86. I could hear the switch activate. I did the same again a bit later and it died. Tried it again just a minute ago and now it works, but I had to bang it on the table, it's toast. So I have a 112 Relay and a 12 631 708 646 Relay (oem) on the way.

The ECU arrives in a week as do the Relay's. I'll stick with the 646 (which is OEM) and keep the 112 (can be used, but in a pinch) as a backup. I think the relay did work but was just not enough voltage at the switch to allow a full 12.5v to Pin 15. Hope this work, crossing finger. Will see in a week or so and also come back and report.

[DesktopDave]

Keep us posted, feels like you're getting very close. I've had quite a few dead main relays over the years. I strongly suspect that the relay load side contacts degrade over time, as they do switch a fairly high amount of current fairly frequently. I do like to carry a few spares with me, along with the universal BMW relay jumper wire kit that you'd posted earlier, LOL.

I've also found a large variety of relays in that socket over the years, I suspect many of them were not original, or had been swapped in from another BMW, etc.. There were a surprising variety of relays spec'ed over the years by BMW as well. Some versions weren't protected with resistors, some required diodes, etc..

[LastCat]

Keep us posted, feels like you're getting very close. I've had quite a few dead main relays over the years. I strongly suspect that the relay load side contacts degrade over time, as they do switch a fairly high amount of current fairly frequently. I do like to carry a few spares with me, along with the universal BMW relay jumper wire kit that you'd posted earlier, LOL.

I've also found a large variety of relays in that socket over the years, I suspect many of them were not original, or had been swapped in from another BMW, etc.. There were a surprising variety of relays spec'ed over the years by BMW as well. Some versions weren't protected with resistors, some required diodes, etc..

For sure, will post the fix.

Getting close is right. I tested the Main Relay at one time and Coil was getting 12.5v. I think under load that relay was failing. Car always started right up too, never hesitated. I could tell it had a slight misfire. I also went and ordered a new Fuel Injector Relay 0-332-019-456 as well, what the heck. Next Monday, parts will be in.

[LastCat]

Well, got the original ECU installed with new Main and Fuel Relays, nadda. Same slight miss. Checked the plugs and they are turning black...sonofabeach!!!

Try a different ECU next.

[jmpower]

Have you checked the spark from HT lead of the coil? Is it the same weak spark?
When I have a problem like yours I find that I need to step back and say that the last person who worked on this was an idiot and that includes myself. It is easy to go down the wrong road and get lost. Starting from scratch can often help.

Have you done any volt drop checks on the ignition circuit, or on the engine block to ground?

[DesktopDave]

If the car's running really rich, also check the coolant temp sensor for resistance, check inside the CLT connector boot for a broken resistor (BMW TSB fix back in the day), and check the cold start injector to be sure it's not stuck open. I'll apologize for that in advance, it's a giant PITA to get at the CSI.

I'm also wondering if the newer components you've installed are compatible with the older Euro engine. It should all work without any trouble, but BMW made a lot of changes during production. I suspect a few were not documented well. I recall having a lot of trouble getting my '81 Euro motor to run well, it was an early Motronic, but still had the external O2 controller. I gave up on it, went with the Motronic 1.3 upgrade instead.

[LastCat]

It's seems it's a weak spark issue which is leaving unburnt fuel going out the exhaust. I checked the CLT, all good. Pulled while running and idle goes up and ohm's checked out. Yep, that CSI could be an issue, but doesn't override the weak spark issue, yet.

I found another ECU 059 and going to see where that goes. Even though I had it checked I have a feeling old weak Transistors are not up to the task. Then Injectors and CSI at the same time. I'll figure it out, just takes a little work.

If I can solve the weak spark issue, that would be leaps and bounds in the right direction.

If all else fails and hate to got this route, Megasquirt.

[DesktopDave]

I'm not too clear on which version of the dist & wires you're running. Motronic 1.0 and 1.3 used different setups. I'm just asking because I had similar troubles when I had to get creative on my last engine, it worked but definitely not optimally.

I'm also confused about your injectors. You're still running the early low-impedance injectors, right?

[LastCat]

I'm not too clear on which version of the dist & wires you're running. Motronic 1.0 and 1.3 used different setups. I'm just asking because I had similar troubles when I had to get creative on my last engine, it worked but definitely not optimally.

I'm also confused about your injectors. You're still running the early low-impedance injectors, right?

The B34 and B35 use the same Dist and Rotor, but on Real OEM they list 2 different numbers for Dist and Rotor, going to check on this, could be a problem.

Also the same Injectors, different model number 280 150 203 for the 535/635. For the 735i 280 150 714. Both are high imp at 16 ohms. I am using the 203 injectors. I cleaned them using ultra sonic and 9v battery to open the valve. New O-rings and filter. I never did a bench test, these are still on the list of things that can go wrong.

I put a Timing Light on Cyl. #1, just to see how the ign. was working, nothing to set here, just a visual. The light hit the same spot when running, except when it mis-fired. The spot on the Front Dampner would hit about 10 degrees retard in conjunction with the mis-fire. In no rythmatic pattern, just random. So I found a 059 ECU, should be here in a week. See if this changes anything.

[RSheiman]

Just for the hell of it, pull the Idle Control Valve and clean it out, open the MAF and adjust the arm to swipe a slightly different location. Both easy to do and free. Has zero to do with a weak spark I know.

[LastCat]

Just for the hell of it, pull the Idle Control Valve and clean it out, open the MAF and adjust the arm to swipe a slightly different location. Both easy to do and free. Has zero to do with a weak spark I know.

Thanks Rob, done weeks ago. Also ohm checked MAF, carb cleaned the ICV, all working to spec.

[RSheiman]

We thank you for your follow-up which is so important for us to learn from.

[LastCat]

We thank you for your follow-up which is so important for us to learn from.

You are welcome. I searched for a week and found post on many BMW forums to the same problem. 4 pages deep, never found what the fix was.

"Historical Documents", Galaxy Quest

[DesktopDave]

The B34 and B35 use the same Dist and Rotor, but on Real OEM they list 2 different numbers for Dist and Rotor, going to check on this, could be a problem.

Also the same Injectors, different model number 280 150 203 for the 535/635. For the 735i 280 150 714. Both are high imp at 16 ohms. I am using the 203 injectors. I cleaned them using ultra sonic and 9v battery to open the valve. New O-rings and filter. I never did a bench test, these are still on the list of things that can go wrong.

I put a Timing Light on Cyl. #1, just to see how the ign. was working, nothing to set here, just a visual. The light hit the same spot when running, except when it mis-fired. The spot on the Front Dampner would hit about 10 degrees retard in conjunction with the mis-fire. In no rythmatic pattern, just random. So I found a 059 ECU, should be here in a week. See if this changes anything.

NICE! Thanks for keeping this thread updated. That timing jump is a really odd symptom, good catch!

I wasn't sure if you 'd run a resistance test on the crank & position senders. Be sure they're in good shape. The plug female connectors are those wire bail PITAs and quite delicate, be careful. You should see about 1Kohm between sender pins 1&2 and very high resistance between 1&3 and 2&3. Maybe even open circuit depending on your multimeter.

Not to kick a dead horse, but I'm suspecting a version mismatch in your hardware. Your car appears to have a mix of early & late parts, just like mine, LOL. No big deal, should run just fine. I ran the same yellow-top injectors on mine, they're standard B35 injectors. I swapped in a set of updated disc-type design III. The car runs a little smoother, the injectors are notably quieter. They're part number 0 280 155 746, common on V8 Fords, easy to buy refurbished. I frequently pick them off Volvo turbo I5s, since those cars are common in most PnP and the rail is incredibly very easy to pull.

Most likely all this is unrelated, but just FYI: there are two completely different ignition setups that look similar at a casual glance. The early ones were a lower-voltage setup. BMW dealers did a running TSB, upgrading them to the newer high kV parts across the lineup. To make it complicated, the M30 cam nose drives went through (at least) three different versions, but BMW made up some adapters to make the updated rotors fit the earlier cam drives. They're all easy enough to tell apart once you know. Early had a push-on rotor with a matching dist cap, thin coil wire tower posts & boots. Later style was a bolt-on rotor, different dist cap and thicker coil towers, wires & boots.

Likewise, BMW went through two completely different Motronics. B34s got 1.0, flywheel crank sensor, low-ohm injectors (Bosch 200 or 201 IIRC), low-power ignition, different AFM. Later B35 got 1.3, trigger wheel sensor, high-ohm injectors, high-power ignition.

[LastCat]

Yes, good to get somewhere in the right direction.

Yep, that timing jumping around was due to the old busted ECU. It actually would jump (retard 10 degrees or so) at the same time the engine would stumble at random.

I have considered testing the Reference and Speed Sensors at the Flywheel, but is a minor concern now. See how new Injectors change things.

Yes, I was looking into those 746 Injectors. I went to FiveO and input the 635 and 735i (B35 Engine) and Injectors they suggest are the 280 155 884 https://www.fiveomotorsport.com/bosc...4-ford-4-hole/ Looking now to get a set of 6. Worth the try, since these 37yr old injectors 280 150 714 can be cleaned and tested, the 884's are Gen III EV1 better upgrade.

The Rotor uses 3 bolts. I checked at a few online sites, RealOEM, Autohaus, Pelican, etc... and found 2 variants to the Rotor, but one was the old part and the new part was an upgrade. Couldn't find any difference in the Rotor/Cap for 1985-1989 BMW 6 or 7 Series. Seem to all be the same.

Since the different ECU fixed the weak spark issue, right now I have a feeling and the way it idles, I suspect fuel issue. Going to get the 884's for about $225 for 6. Worth the purchase. I have so many new parts and this B35 is stout for sure, see how that goes.

[foundsoul]

Thanks Marc, wish I had the Mega.

Dude, let us know if we can help! That MS3Pro featureset just freaking rocks!

Here's a feature comparison doc of the different MS3Pro models: https://www.ampefi.com/wp-content/up...Comparison.pdf

[RSheiman]

Thank You for the follow-up It is how we all learn!

[DesktopDave]

The Rotor uses 3 bolts. I checked at a few online sites, RealOEM, Autohaus, Pelican, etc... and found 2 variants to the Rotor, but one was the old part and the new part was an upgrade. Couldn't find any difference in the Rotor/Cap for 1985-1989 BMW 6 or 7 Series. Seem to all be the same.

I believe the dist cap is the same after the high-tension upgrade. The cap must match the rotor and the coil wires must match the cap. I've been using the late setup myself, the ring-type coil is supposed to be quite a bit more powerful than the older can type.