Mystery Misfire

Hey BMW owners! So my 1990 M20 525i has been taking a little bit too long to start, idling rough, and has an incredibly frustrating misfire, yet no check engine light. Well naturally I did some diagnostics and online investigating. After checking just about everything I came to a dead end and resorted to the replace random parts technique. Of course this changed nothing, so I searched online some more. A thought I had was the crankshaft position sensor, the main sensor used for ignition timing, makes sense. When I looked online at the symptoms of a failed or failing sensor, almost all said the call won't start at all. I found the resistance spec for the CPS (540 Ohms +-10%), then I tested it and read out 460 Ohms. Although technically out of spec, it wasn't far off, so I thought it was okay. It wasn't until today that I decided to check the crank sensor once more, now it was at 3 Ohms! It ran the same as it had before, and still no check engine light. That's when I got curious, I disconnected the sensor completely and STILL no check engine light. Not only that, the car ran EXACTLY as it did with the sensor connected. So from this experience I've come to the conclusion the check engine light is either completely useless, or my ECM doesn't know how to store codes. Anyway, I've ordered a new CPS for $37.61 at Eeuroparts.com, compared to Autozone which stocks it for $119.99. I should be getting my package soon and I will let you all know if my car magically runs perfect or if this is just another random part.

Originally Posted by Microtesties

Hey BMW owners! So my 1990 M20 525i has been taking a little bit too long to start, idling rough, and has an incredibly frustrating misfire, yet no check engine light. Well naturally I did some diagnostics and online investigating. After checking just about everything I came to a dead end and resorted to the replace random parts technique. Of course this changed nothing, so I searched online some more. A thought I had was the crankshaft position sensor, the main sensor used for ignition timing, makes sense. When I looked online at the symptoms of a failed or failing sensor, almost all said the call won't start at all. I found the resistance spec for the CPS (540 Ohms +-10%), then I tested it and read out 460 Ohms. Although technically out of spec, it wasn't far off, so I thought it was okay. It wasn't until today that I decided to check the crank sensor once more, now it was at 3 Ohms! It ran the same as it had before, and still no check engine light. That's when I got curious, I disconnected the sensor completely and STILL no check engine light. Not only that, the car ran EXACTLY as it did with the sensor connected. So from this experience I've come to the conclusion the check engine light is either completely useless, or my ECM doesn't know how to store codes. Anyway, I've ordered a new CPS for $37.61 at Eeuroparts.com, compared to Autozone which stocks it for $119.99. I should be getting my package soon and I will let you all know if my car magically runs perfect or if this is just another random part.

I don't know what you disconnected, but it wasn't the CPS. I rarely speak in absolutes, but, the car cannot run without a CPS ... period. Take another look at the connector you tested and trace it to whatever sensor it goes to.

Originally Posted by BMR_LVR

I don't know what you disconnected, but it wasn't the CPS. I rarely speak in absolutes, but, the car cannot run without a CPS ... period. Take another look at the connector you tested and trace it to whatever sensor it goes to.

Well... Unfortunately you're right, I didn't disconnect the CPS. It was the cylinder identification sensor.. Which in my defense has the exact same connector right next to the CPS connector in a very awkward location. I feel pretty stupid right now haha well my crank sensor is still out of spec so it isn't a total waste replacing it. Guess I got a little too excited with this one.

Originally Posted by Microtesties

Well... Unfortunately you're right, I didn't disconnect the CPS. It was the cylinder identification sensor.. Which in my defense has the exact same connector right next to the CPS connector in a very awkward location. I feel pretty stupid right now haha well my crank sensor is still out of spec so it isn't a total waste replacing it. Guess I got a little too excited with this one.

Easy mistake to make. A couple of things, I doubt your current CPS is causing your problems. Most of the time, again, ... most of the time, it either works normally or doesn't. It can cause intermittent problems infrequently. I would suggest you keep your new one for a spare. However, beware of non-oem CPSs.

As to your misfire, check your distributor cap, rotor button, plug wires and plugs very carefully. You may find the source of the misfire upon inspection. If you haven't replaced them in a long time, and your wallet allows, I would recommend replacing them if it has not been done as part of the routine maintenance.

Another possible cause of a misfire is a faulty fuel injector. I’m 99% positive that your 90 model DME does not have the capability of detecting a fuel injector fault. I would look into a faulty fuel injector as a possible cause.

Good luck.

Originally Posted by BMR_LVR

Easy mistake to make. A couple of things, I doubt your current CPS is causing your problems. Most of the time, again, ... most of the time, it either works normally or doesn't. It can cause intermittent problems infrequently. I would suggest you keep your new one for a spare. However, beware of non-oem CPSs.

As to your misfire, check your distributor cap, rotor button, plug wires and plugs very carefully. You may find the source of the misfire upon inspection. If you haven't replaced them in a long time, and your wallet allows, I would recommend replacing them if it has not been done as part of the routine maintenance.

Another possible cause of a misfire is a faulty fuel injector. I’m 99% positive that your 90 model DME does not have the capability of detecting a fuel injector fault. I would look into a faulty fuel injector as a possible cause.

Good luck.

I have replaced the plugs, wires, and coil. As far as ignition the distributor is the only thing I haven't opened up, mainly because of how annoying it is to get it off. I've also replaced the fuel filter and pressure regulator and took the injectors off and cleaned them. I checked the fuel pressure at idle and it was about 38 psi, I believe it's supposed to be 40-47, though I haven't gotten a clear answer as to what it should be at idle.

My plan was if this CPS doesn't solve my problem I would investigate the fuel pump some more. Also I checked the AFM and the reading wasn't as smooth as I would hope as I pushed the flap open. As for that I got the Miller MAF conversion kit, I just haven't gotten around to installing it.

I must be getting close to fixing it because I'm running out of parts to replace.

I would still recommend you check the dizzy cap. It still sounds electrical in nature. Hope you get it sorted soon.

Cold start problems on an M20 engine - strongly suspect distributor rotors and wires. Microcracks in the plastic and insulation permit moisture to enter and screw with charge generation and transmission. You've changed the wires - good. Please change the distributor and rotor etc. $150 for OEM. As Steve mentioned, this is a general maintenance item. And you do have a car problem. And your engine will run better in any case with new ignition equipment.

Also change the fuel pump, its regulator, and the crankshaft position sensor. Under $300 oem for all three. If not you'll have no start problems after this over the next 2 years.

Please search for and download the searchable bentley manual for E34s and the troublshooting tables from the bentley manual. All the information about fuel pressure etc can be found there, and this is regarded as an invaluable tool to understand your car with.

So I installed my new MAF conversion, which I must say, is absolutely fantastic. Along with power gains it seems like my misfire has gotten SLIGHTLY better, and fuel economy is back to normal, for now. Considering I don't know when the distributor was last replace I figure I'll probably pick up a new one anyway. Should I just replace the cap, or the rotor as well? It's really frustrating diagnosing a car with no check engine light.. The problem is intermittent, sometimes it drives perfectly, others I can hardly stand being in the car. The idle and drivability quality always vary together, if it purrs it runs great, if it idles rough it drives that way too. I believe it to be a fuel system problem, mostly because whenever I replace a fuel system component it seems to drive much better when I go out for a test drive. The next day it's usually back to it's normal misfiring state, crushing my hopes and sense of accomplishment. I'm thinking it may be the O2 sensor or TPS, even though I tested both and they seem to be alright. I really wish I knew someone who was BMW savvy who I could take it to, but I do have you guys, so here are ALL the symptoms I can think of. I'll add more if I forgot any.

-Poor fuel economy, was 14 but today I reset it and it was 20
-Economy gauge on instrument cluster won't go below 20mpg no matter what
-Fuel gauge erratic, sometimes dead on, sometimes it cuts out
-Idle isn't smooth, sounds like it has a wider camshaft
-Misfire, seen on tacho, heard from exhaust, and definitely felt by passengers, occurs most often with less than half throttle around 1500-2500RPM
-Lacking power
-Long cranks to start
-Occasional groaning sounds from engine that varies with RPM, I think water pump
-Soft clanging from transmission in neutral, I think clutch bearing
-Bogs down for half a second when clutch is let off, then takes off
-Randomly get a "COOLANT LEVEL" notification from dash when car is turned off or before starting
-Fuel pressure 38psi at idle, 47psi without vacuum

I'm planning on replacing the water pump, clutch, distributor, and crank sensor. Once I do that I'll have a 100% new ignition system. I think the fuel level sender may be messing with the fuel gauge and economy gauges on the dash, I may or may not replace it. I would be ever grateful if you would all share your wisdom with me!

how many more times would you like us to say dizzy cap? change the rotor at the same time. i replaced the one in my M40 and the two in my M70 engine, all up was around $500 so probably around $150 for just one set for your car

Originally Posted by Microtesties

-Poor fuel economy, was 14 but today I reset it and it was 20-Economy gauge on instrument cluster won't go below 20mpg no matter what
-Fuel gauge erratic, sometimes dead on, sometimes it cuts out
-Idle isn't smooth, sounds like it has a wider camshaft
-Misfire, seen on tacho, heard from exhaust, and definitely felt by passengers, occurs most often with less than half throttle around 1500-2500RPM
-Lacking power
-Long cranks to start
-Occasional groaning sounds from engine that varies with RPM, I think water pump
-Soft clanging from transmission in neutral, I think clutch bearing
-Bogs down for half a second when clutch is let off, then takes off
-Randomly get a "COOLANT LEVEL" notification from dash when car is turned off or before starting
-Fuel pressure 38psi at idle, 47psi without vacuum

I'm planning on replacing the water pump, clutch, distributor, and crank sensor. Once I do that I'll have a 100% new ignition system. I think the fuel level sender may be messing with the fuel gauge and economy gauges on the dash, I may or may not replace it. I would be ever grateful if you would all share your wisdom with me!

If the crank sensor tests good, there is no point in replacing it. Cap and rotor, as suggested. Plug wires? At least test them for resistance.

Sounds like you may have a vacuum leak somewhere as well, probably the cause of your bog.

Clean the idle control valve, very often the cause of irratic idle.

Valve adjustment, the M20 requires a valve adjustment right?

You just weren't done with the tune up

Originally Posted by snowsled7

If the crank sensor tests good, there is no point in replacing it. Cap and rotor, as suggested. Plug wires? At least test them for resistance.

Sounds like you may have a vacuum leak somewhere as well, probably the cause of your bog.

Clean the idle control valve, very often the cause of irratic idle.

You just weren't done with the tune up

Vacuum leak and needs a tuneup is spot on

I tested for vacuum leaks with propane and couldn't find any, so unless there's a tiny tiny leak in one of my intake gaskets then it's air tight. The vacuum line and all intake gaskets are new so I doubt it, but I'll do some further investigating if the problem persists. I also ran my fuel pump through some tests: operating pressure = 38-47, engine off pressure = 44, max pressure = >100, current draw = 3 amps. Since the fuel pump checks out you'll all be glad to know I'm finally ordering a new distributor cap and rotor, luckily only $75 for OEM. Even though it acted up today my average MPG is about 18.8, but I'm going to assume that's from the MAF conversion.

On another unrelated note, does anyone know what sensor(s) the fuel economy gauge uses? My fuel gauge is also temperamental so I'm thinking both require the fuel level sending unit. I suppose another possibility is the gauge itself has something blocking it from dropping below the 20 mark.

I believe that the MPG gauge operates off of vacuum, but I'm not 100% on this.

All I have to do to get my MPG to drop to 0 is to floor the accelerator

The MPG needle (the constant moving thing under the Speedo) operates off vacuum.
The fuel gauge works off the level sender

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Originally Posted by BMR_LVR

I believe that the MPG gauge operates off of vacuum, but I'm not 100% on this.

All I have to do to get my MPG to drop to 0 is to floor the accelerator

That's what I thought, but I'm fairly certain there are no vacuum leaks. Maybe I just need to take my instrument cluster off and give it a good shake

Do you know where the gauge get it's vacuum from by chance? like brake booster, intake manifold, throttle body?

Originally Posted by Microtesties

That's what I thought, but I'm fairly certain there are no vacuum leaks. Maybe I just need to take my instrument cluster off and give it a good shake

Do you know where the gauge get it's vacuum from by chance? like brake booster, intake manifold, throttle body?

Unfortunately, I don't. So if you're drviing along and floor the throttle, the MPG gauge doesn't go below 20?

Originally Posted by BMR_LVR

Unfortunately, I don't. So if you're drving along and floor the throttle, the MPG gauge doesn't go below 20?

...lol efficient!

Check this out

Post #4 in the following link:
MPG Gauge-How Does it Work?

Originally Posted by Jeff N.

The gauge is driven by the fuel map in the ECU. Essentially, the combination of the engine speed, air velocity (AFM) and a few miscellanous other things (engine temp, air temp, blah blah) locate a fuel value in the chip.

This value equals a calculated fuel pulse. The fuel pulse and the road speed combine to give you a calculated fuel MPG calculation.

Simple huh?

Originally Posted by Monsignor

...lol efficient!

Check this out

Post #4 in the following link:
MPG Gauge-How Does it Work?

Yeah, but I don't think he is asking about the MPG calculation of the OBC (which seems to be the explanation above but ), but the actual MPG guage on the dash just below the tachometer. Microtesties, can you clarify?

Originally Posted by BMR_LVR

Yeah, but I don't think he is asking about the MPG calculation of the OBC (which seems to be the explanation above but ), but the actual MPG guage on the dash just below the tachometer. Microtesties, can you clarify?

I think that the link i provided is explaining the active, moving needle below the spedometer.

Originally Posted by Monsignor

I think that the link i provided is explaining the active, moving needle below the spedometer.

Ok. I certainly don't have much knowledge about it, so I will take his word for it.

It's all Greek to me anyway. I never really cared how it got there, I just trusted that it was accurate, being engineer by BMW techs and all...

Originally Posted by BMR_LVR

Yeah, but I don't think he is asking about the MPG calculation of the OBC (which seems to be the explanation above but ), but the actual MPG guage on the dash just below the tachometer. Microtesties, can you clarify?

Yes I'm talking about the swinging needle below the tachometer, not the digital one from the clock. And no the needle will not drop below 20 MPG no matter what RPM or how much throttle.

So does anyone know where the gauge gets it's vacuum? Is there a physical hose or does it read the output from one of the sensors?

im pretty sure theres a physical vacuum line that goes behind the dash...

Originally Posted by Monsignor

im pretty sure theres a physical vacuum line that goes behind the dash...

I believe the whole instrument cluster is electronic , but I dont know for sure for a earlier model e34 . Your explanation earlier Joe was the correct one

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