1972 CSi missing a beat every few kilometres

[Memo]

Hello everyone,

My stock CSi recently (about 300-400km ago) began missing a beat or two every few kilometres. It begins when the engine is warm, after 10km or more. It seems to occur at speeds of 40km/h or more up to motorway speed, under fairly light throttle or when easing slightly. Not particularly under acceleration. There is the occasional, almost imperceptible nudge (1 beat) and then a stronger one (2-3 beats?).

The fuel pump, filter, and regulator, as well as the plugs were replaced about 3000km ago after the regulator diaphgram broke because of overpressure from the pump. Ignition tips and other components appear clean and in good working order, including those controlling the fuel injection. Timing was slightly late and has been adjusted, sharpening the car a bit, but the problem persists. The wiring is from when the car was rebuilt in 2018 some 8000km ago. Most of those km are from the summer of 2024 and this one.

The only things of note from a day or two before the problem appeared are a bit of wet weather and filling her up. I’ve filled her up again since, but there must have been 10-15l of the previous batch still in the tank.

We will have to go through the ignition, plugs and wiring again, but in the interest of saving time, any pointers to other potential problem sources would be greatly appreciated.

P.S. What could cause the tacho to give up at around 3500rpm under acceleration? It kind of drops and wavers.

 

[sfdon]

Notorious problem acknowledged by BMW head engineer at the time.

I can post a copy of the technical notice that was issued if you want?

 

[sfdon]

With the tachometer issue, I would certainly take another tachometer connect to the black wire and drive the car if the problem is solved it’s the tachometer if it’s still occurring, it’s from the coil to the tachometer?

 

[Memo]

Notorious problem acknowledged by BMW head engineer at the time.

I can post a copy of the technical notice that was issued if you want?

Yes please,

 

[bengal taiga]

Hello everyone,

My stock CSi recently (about 300-400km ago) began missing a beat or two every few kilometres. It begins when the engine is warm, after 10km or more. It seems to occur at speeds of 40km/h or more up to motorway speed, under fairly light throttle or when easing slightly. Not particularly under acceleration. There is the occasional, almost imperceptible nudge (1 beat) and then a stronger one (2-3 beats?).

The fuel pump, filter, and regulator, as well as the plugs were replaced about 3000km ago after the regulator diaphgram broke because of overpressure from the pump. Ignition tips and other components appear clean and in good working order, including those controlling the fuel injection. Timing was slightly late and has been adjusted, sharpening the car a bit, but the problem persists. The wiring is from when the car was rebuilt in 2018 some 8000km ago. Most of those km are from the summer of 2024 and this one.

The only things of note from a day or two before the problem appeared are a bit of wet weather and filling her up. I’ve filled her up again since, but there must have been 10-15l of the previous batch still in the tank.

We will have to go through the ignition, plugs and wiring again, but in the interest of saving time, any pointers to other potential problem sources would be greatly appreciated.

P.S. What could cause the tacho to give up at around 3500rpm under acceleration? It kind of drops and wavers.

You mention that your vehicle was “rebuilt” about eight years and 8,000KM ago. But unless you imply that the vehicle is in otherwise showroom condition, you don’t describe the baseline condition of your engine, including any compression/leakdown readings or last valve job/adjustment, among other things. Any or all of these things could conceivably contribute to an “occasional” hiccup or misfire, or a “nudge” as you describe it.

The fact that “it” intermittently occurs per kilometer when the engine is at operating temperature and not necessarily when the engine is under a load (acceleration) suggests a fuel mixture that is too lean and/or a misfire possibly induced by compromised/poorly adjusted ignition components. Since you recently replaced a fuel pump and pressure regulator, do you know the vehicle’s current operating fuel pressure? Fun tip: On a stock (originally equipped) engine, fuel pressure that is significantly lower or higher than the recommended setting (~2 bar+.2/28psi+2.84) likely results in altered fuel mixture - and this might account for your described symptom. As you know, ignition timing works in tandem with fuel delivery, so perhaps you are on the right track when you mention that advancing the ignition timing “sharpened” engine operation. The "intermittent" nature of your symptoms also makes me wonder about the . . . remote possibility . . . of your vehicle's cold start valve/injector leaking or occasionally "dribbling." As someone probably said: "An unintended dribble can spoil a perfectly good idle and part throttle operation."

You mention that ignition components look to be in clean and in good working order. This is only half the battle since outward appearance may not always disclose a greater-than-normal resistance. This might also include otherwise hidden abrasion or cracked ignition leads, improperly gapped spark plugs, or spark plugs of the wrong heat range. Similarly, if your “stock” djet still sports original equipment, the distributor has “three” sets of points, all of which could look good while being maladjusted or slightly dirty (resistance). The cap and rotor could also show signs of wear, corrosion (resistance) or simple fitment issues. You might as well check the function of the distributor's centrifugal advance weights to see that they are not sticking. The vacuum retard and the smooth movement of the plate upon which the ignition points are fixed is important to ignition operation. Your djet’s ignition coil might look perfect, but that would not necessarily indicate an occasional or intermittent malfunction when “warm/hot” and/or “wet.” What kind of rotor is on top of your distributor shaft? If it is a rev limiting rotor with a very weak spring and/or a sticking part, this might also contribute to an occasional misfire. As with so many things, a picture can be worth a thousand words.

You mention wet weather and presumably fueling the vehicle during that wet weather. There is no question that gas and water don’t mix (unless the gas is predominantly alcohol). However, I would guess that water in your fuel would not result in an occasional “almost imperceptible” nudge or misfire, as distinct from an engine on the verge of stalling. That does not mean what you describe could not be caused by contaminated fuel, but this does not seem to fit within the picture you painted. Again, pictures might be helpful.

 

[TomHom]

....the German Coupé-Club e.V. published this guidance:

 

[sfdon]

Hello everyone,

My stock CSi recently (about 300-400km ago) began missing a beat or two every few kilometres. It begins when the engine is warm, after 10km or more. It seems to occur at speeds of 40km/h or more up to motorway speed, under fairly light throttle or when easing slightly. Not particularly under acceleration. There is the occasional, almost imperceptible nudge (1 beat) and then a stronger one (2-3 beats?).

The fuel pump, filter, and regulator, as well as the plugs were replaced about 3000km ago after the regulator diaphgram broke because of overpressure from the pump. Ignition tips and other components appear clean and in good working order, including those controlling the fuel injection. Timing was slightly late and has been adjusted, sharpening the car a bit, but the problem persists. The wiring is from when the car was rebuilt in 2018 some 8000km ago. Most of those km are from the summer of 2024 and this one.

The only things of note from a day or two before the problem appeared are a bit of wet weather and filling her up. I’ve filled her up again since, but there must have been 10-15l of the previous batch still in the tank.

We will have to go through the ignition, plugs and wiring again, but in the interest of saving time, any pointers to other potential problem sources would be greatly appreciated.

P.S. What could cause the tacho to give up at around 3500rpm under acceleration? It kind of drops and wavers.

Pic of your distributor please?

 

[Memo]

You mention that your vehicle was “rebuilt” about eight years and 8,000KM ago. But unless you imply that the vehicle is in otherwise showroom condition, you don’t describe the baseline condition of your engine, including any compression/leakdown readings or last valve job/adjustment, among other things. Any or all of these things could conceivably contribute to an “occasional” hiccup or misfire, or a “nudge” as you describe it.

The fact that “it” intermittently occurs per kilometer when the engine is at operating temperature and not necessarily when the engine is under a load (acceleration) suggests a fuel mixture that is too lean and/or a misfire possibly induced by compromised/poorly adjusted ignition components. Since you recently replaced a fuel pump and pressure regulator, do you know the vehicle’s current operating fuel pressure? Fun tip: On a stock (originally equipped) engine, fuel pressure that is significantly lower or higher than the recommended setting (~2 bar+.2/28psi+2.84) likely results in altered fuel mixture - and this might account for your described symptom. As you know, ignition timing works in tandem with fuel delivery, so perhaps you are on the right track when you mention that advancing the ignition timing “sharpened” engine operation. The "intermittent" nature of your symptoms also makes me wonder about the . . . remote possibility . . . of your vehicle's cold start valve/injector leaking or occasionally "dribbling." As someone probably said: "An unintended dribble can spoil a perfectly good idle and part throttle operation."

You mention that ignition components look to be in clean and in good working order. This is only half the battle since outward appearance may not always disclose a greater-than-normal resistance. This might also include otherwise hidden abrasion or cracked ignition leads, improperly gapped spark plugs, or spark plugs of the wrong heat range. Similarly, if your “stock” djet still sports original equipment, the distributor has “three” sets of points, all of which could look good while being maladjusted or slightly dirty (resistance). The cap and rotor could also show signs of wear, corrosion (resistance) or simple fitment issues. You might as well check the function of the distributor's centrifugal advance weights to see that they are not sticking. The vacuum retard and the smooth movement of the plate upon which the ignition points are fixed is important to ignition operation. Your djet’s ignition coil might look perfect, but that would not necessarily indicate an occasional or intermittent malfunction when “warm/hot” and/or “wet.” What kind of rotor is on top of your distributor shaft? If it is a rev limiting rotor with a very weak spring and/or a sticking part, this might also contribute to an occasional misfire. As with so many things, a picture can be worth a thousand words.

You mention wet weather and presumably fueling the vehicle during that wet weather. There is no question that gas and water don’t mix (unless the gas is predominantly alcohol). However, I would guess that water in your fuel would not result in an occasional “almost imperceptible” nudge or misfire, as distinct from an engine on the verge of stalling. That does not mean what you describe could not be caused by contaminated fuel, but this does not seem to fit within the picture you painted. Again, pictures might be helpful.

Thanks.

My mechanic currently has the car and will go through the ignition. Therefore no current pictures. It's difficult to debug as you have to drive for a bit before the problem occurs. I will pass on every suggestion.

I agree on what you say about the fuel. The symptoms don't match and I didn't fill her up under the rain anyway. It's just the only thing that comes to mind that correlates more or less with the issue occurring.

A photo from a couple of days before the problem manifested itself and some form 2018.

 

[Memo]

Pic of your distributor please?

As soon as I get one.

 

[sfdon]

Service Information

 

[Memo]

Pic of your distributor please?

 

[sfdon]

You are also looking at a typical problem that can be caused by a worn TPS -

The new ones I have made have an additional diode installed to stop the dropping out signal issue as the wiper arm moves slightly between positions.

And also a typical problem caused by worn trigger points- the ones installed in side of the distributor.

Also look at shaft play in your distributor…

I will be in Paris at CDG airport in September on the 15th, Porto on 15-18, Frankfurt on the 18th and Copenhagen until the end of the month.

I have rebuilt units in stock.

How old/worn are yours?

 

[sfdon]

Just looking at your plug wires- they should not be touching grounds as they are and they look really old?

When replaced last?

 

[Memo]

You are also looking at a typical problem that can be caused by a worn TPS -

The new ones I have made have an additional diode installed to stop the dropping out signal issue as the wiper arm moves slightly between positions.

And also a typical problem caused by worn trigger points- the ones installed in side of the distributor.

Also look at shaft play in your distributor…

I will be in Paris at CDG airport in September on the 15th, Porto on 15-18, Frankfurt on the 18th and Copenhagen until the end of the month.

I have rebuilt units in stock.

How old/worn are yours?

Thx,

Looking into all this. The distributor is not new, but appears to be in good working order.

The diagnostic cable looks so-so. Will have to try with a plain wire.

What are you asking for TPS, rebuilt distributor?

 

[sfdon]

Did you remove and inspect the trigger points on the side of your distributor?

 

[Memo]

Did you remove and inspect the trigger points on the side of your distributor?

The trigger points were like new 3000km ago (when debugging what turned out to be the fuel pump/regulator issue).

How much for the TPS? If you have the price scribbled on it, I can't quite make it out. My mechanic thinks that's a possible source for the problem.

 

[Memo]

You are also looking at a typical problem that can be caused by a worn TPS -

The new ones I have made have an additional diode installed to stop the dropping out signal issue as the wiper arm moves slightly between positions.

And also a typical problem caused by worn trigger points- the ones installed in side of the distributor.

Also look at shaft play in your distributor…

I will be in Paris at CDG airport in September on the 15th, Porto on 15-18, Frankfurt on the 18th and Copenhagen until the end of the month.

I have rebuilt units in stock.

How old/worn are yours?

Shaft play sideways 0.11 mm and lengthwise 0.53 mm