W123 230E 1982 misfiring

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bikeman

New Member
Joined
May 5, 2023
Messages
27
Location
Bucks
Car
230E
Hi, I'm trying to get my Dad's 230E 1982 ready for sale. It's a 1 owner car with 23,000 miles. It's been unused for almost 10 years.

I've fully serviced it (all fluids, brakes etc) and it now starts easily. It's a little lumpy when cold but I've put that down to the auxilary air valve being a bit lethargic.

After a few minutes it's driveable. The problem is after 20 minutes or so driving it then misfires very badly if I give it anything more than a gentle throttle.

Here's a list of everything that has been done:

Ignition:
Replaced sparkplugs, ht leads, distributor cap. Cleaned rotor arm.
Verified coil and ignition module voltages and resistance values as best as I can with data from the manual.
Idle mixture was ok for MOT and sparkplug colour suggests mixture is correct.
Reset valve clearances.
Compression test cylinders - all 180 - 185 psi

Fuelling:
Drained fuel tank, verified fuel tank strainer was clean, replaced fuel pump & fuel filter, added new ethanol free fuel and injector cleaner.
Replaced fuel injectors.
Replaced fuel distributor inlet filter, inspected the fuel distributor injector outlets and other ports all clean, verified plunger not sticking.
Cleaned warm up regulator inlet filter.
Checked operation of cold start injector, WUR and Auxillary valve bi-metal strips - all ok.
Veried fuel accumulator holds pressure and injectors and cold start valve are not leaking.
Verified that fuel delivered to all injectors when pump engaged and venturi plate depressed.
Check fuel pump flow rate
Check k-jet fuel pressures

I took it to a mercedes specilist who verified that the fuel pump delivery rate was correct and the kjet control and system pressures were ok but he didnt drive the car with the gauge connected so didnt observe whether fuel pressure was being lost. The fuel pump is a little noisey (about the same as the one it replaced) and he wanted to change it but as it had already been changed and the delivery rate was ok I declined.

He did also suggest soaking the fuel distributor in meths but I didnt think that was a good idea for the internal orings. Besides all the ports look perfectly clear and the fuel seems to deliver equally to all ports.

So I'm now a bit stumped. I'm tempted to get a gauge and test the fuel pressures myself. I contacted KMI but apparently they dont test WUR and fuel distributors, they just rebuild them. Obviously I'm reluctant to fork out almost £1k to get these rebuilt without verifiying they are faulty first.

So if anybody can help with any of the following it would be appreciated:

1. Does a violent jerking misfire on acceleration sound like fuel starvation or an ignition problem?
2. What might cause a loss of fuel pressure only when hot?
3. Is there anyone in the UK who tests Kjet fuel distributors?

Thanks
 
Simple check before going any further. Check the inside of the dizzy cap is absolutely bone dry. These engines are notorious for harbouring damp there which then migrates to the top where the high voltage is as the engine warms and the moisture starts to evaporate .
 
Mercedes W123 dont like cloned parts , so unles the parts were genuine Bosch or Beru you cant trust them . Also they only use spark plugs with no R in them . Please tell us what numbers are on your spark plugs ?. Another point is to look at all vacuum pipes , large and the small white micro lines, along with the rubber ellbows and joints for splits and cracks . Return ASAP tell us what you found .
 
Simple check before going any further. Check the inside of the dizzy cap is absolutely bone dry. These engines are notorious for harbouring damp there which then migrates to the top where the high voltage is as the engine warms and the moistu
New dist cap. No damp inside.
 
1. Does a violent jerking misfire on acceleration sound like fuel starvation or an ignition problem?
I associate 'sudden death' misfires with ignition problems. Fuel related are usually a touch 'softer' as the preceding firing events weaken before disappearing. Ditto on return of firing.
Have you ruled out overly rich? I noticed you have checked the cold start enrichment system for operation. Is it tapering off as the engine warms?
 
Mercedes W123 dont like cloned parts , so unles the parts were genuine Bosch or Beru you cant trust them . Also they only use spark plugs with no R in them . Please tell us what numbers are on your spark plugs ?. Another point is to look at all vacuum pipes , large and the small white micro lines, along with the rubber ellbows and joints for splits and cracks . Return ASAP tell us what you found .
Plugs are NGK BP6EFS
All vacuum pipes appear sound. There aren't many just one for the dist advance, the rest are for the central locking and lights. The fuel distributor foot, crankcase breathers and auxilary idle valve pipes are all in good condition.
The Bosch fuel pump was swopped out for a Ridex and it's output has been verified. Both get a little noisy at times hence the reason it was swopped.
 
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I associate 'sudden death' misfires with ignition problems. Fuel related are usually a touch 'softer' as the preceding firing events weaken before disappearing. Ditto on return of firing.
Have you ruled out overly rich? I noticed you have checked the cold start enrichment system for operation. Is it tapering off as the engine warms?
I thought the same but the Mercedes specialist who looked at it was adamant that it was a fuel starvation issue. The tacho doesn't bounce around like I would expect with an ignition issue.

I don't think it's running rich because the plugs look normal and it passed the mot emissions test. The mixture screw is weakened as much as possible and then moved back a little for smooth idle. It's not exact but probably not far off.

The cold start injector probably doesn't do much as it's not cold enough. I just checked to make sure it's not leaking. The auxilary air valve moves enough to start the car and idle ok as it warms up. I don't think either would cause fuel starvation as they only work during cold start. The misfire happens after 20minutes/10 miles of driving.
 
I've been wondering about the fuel damper. It's the only thing between the tank and the pump. How do they work? Could it be restricting fuel? How can I check it, Its a £150 item so I'm not just going to swop it?
 
I thought the same but the Mercedes specialist who looked at it was adamant that it was a fuel starvation issue. The tacho doesn't bounce around like I would expect with an ignition issue.

I don't think it's running rich because the plugs look normal and it passed the mot emissions test. The mixture screw is weakened as much as possible and then moved back a little for smooth idle. It's not exact but probably not far off.

The cold start injector probably doesn't do much as it's not cold enough. I just checked to make sure it's not leaking. The auxilary air valve moves enough to start the car and idle ok as it warms up. I don't think either would cause fuel starvation as they only work during cold start. The misfire happens after 20minutes/10 miles of driving.
Rich should/would be obvious with puffs of black smoke at the tailpipe. Failure of cold start enrichment to taper off with warm-up would push it to richness. The system will be doing more than you are crediting it with I suspect even though the weather is relatively mild.
But, if there's no sign of black smoke or sooted plugs (as the misfiring occurs) then fuel starved is likely. Someone recently had a fuel starvation problem that was traced to a blocked tank breather - identified with a sudden inrush of air into the tank on opening the filler cap. If not something that simple I'm lost! Zero experience of K-Jet system.
 
Rich should/would be obvious with puffs of black smoke at the tailpipe. Failure of cold start enrichment to taper off with warm-up would push it to richness. The system will be doing more than you are crediting it with I suspect even though the weather is relatively mild.
But, if there's no sign of black smoke or sooted plugs (as the misfiring occurs) then fuel starved is likely. Someone recently had a fuel starvation problem that was traced to a blocked tank breather - identified with a sudden inrush of air into the tank on opening the filler cap. If not something that simple I'm lost! Zero experience of K-Jet system.
No smoke. The breather is clear, no vacuum in the tank.

The cold start valve is definitely not a problem. I've run the car with it removed and nothing changed.
 
With this age of MB experiencing running problems the ignition unit is frequently cited as the cause. Heat related, often it's failure of a heat conducting compound twixt it and the chassis. Others can better advise and verify if your dad's 230E has said unit.
 
If it’s a violent shutdown when hot I would be looking at the fuel pump relay. You could bridge this out to rule it out, it would explain why it does it with both fuel pumps. And if the old pump was seized it could have caused the motor to draw more current than it should have. Sometimes they get dry joints inside (it’s 40+ years old unless it’s been replaced) and sometimes you get cracks in the tracks on the PCB to the pins.

I also wonder how the potentiometer for the airflow plate is, could it be giving incorrect readings once the engine is fully warmed up? I wouldn’t expect that would cause a violent shutdown though.
 
I’d also do some diagnostics to ensure the ECU is powered up and functioning as it should. These old systems can run with sensors unplugged or even no power to the ECU and often many people will not be aware other than poor cold starting and mixture. It was an early fuel injection system designed to be able to run with several electrical failures in mind, included a limp operation that ignored airflow and temperatures etc hence cold start, idle etc can be poor as a result.
 
With this age of MB experiencing running problems the ignition unit is frequently cited as the cause. Heat related, often it's failure of a heat conducting compound twixt it and the chassis. Others can better advise and verify if your dad's 230E has said unit.
Interesting. I'm pretty sure I've checked that the ignition module isnt getting hot but I'll check again. I have done the measurements in the manual and they were all ok.
 
If it’s a violent shutdown when hot I would be looking at the fuel pump relay. You could bridge this out to rule it out, it would explain why it does it with both fuel pumps. And if the old pump was seized it could have caused the motor to draw more current than it should have. Sometimes they get dry joints inside (it’s 40+ years old unless it’s been replaced) and sometimes you get cracks in the tracks on the PCB to the pins.

I also wonder how the potentiometer for the airflow plate is, could it be giving incorrect readings once the engine is fully warmed up? I wouldn’t expect that would cause a violent shutdown though.
The old pump wasnt seized. It was fitted only 2000 miles ago. It was simply changed because it was sometimes noisy and it was an easy replace just to rule it out. But the new pump is behaving exactly the same. I think it unlilkely that they could both fail the same way.

Pretty confident the relay is ok. The pump is running when the misfire occurs. Like I said the car can be driven gently and only misfires when more than a gentle throttle is applied. I will try bridging it to rule it out.

The airflow plate doesn't have a potentiometer. There is no ECU.
 
Like I said the car can be driven gently and only misfires when more than a gentle throttle is applied.
That in itself is a symptom of ignition failure as the increase in in-cylinder pressure (with increased throttle) makes it harder to 'introduce' the spark. It has to fight against the pressure and a failing ignition system falters.
 
That in itself is a symptom of ignition failure as the increase in in-cylinder pressure (with increased throttle) makes it harder to 'introduce' the spark. It has to fight against the pressure and a failing ignition system falters.
Not sure how I would get to the bottom of that without going down the road of substituting expensive parts. All I know is the garage I took it to said they hooked up the ht leads to something and verified a good spark on each lead. I'm pretty sure they did this after the car had been driven and was misbehaving.
 
Not sure if your 230E is an affected model but there's info here >> MB Club EZL paste - Google Search
I think that's a problem for a much newer car. I dont think the ignition module even gets hot. The ignition module is bolted on the curved side of the inner wing. I'm pretty sure the wing isn't flat so cant be a heatsink for the module. When it stops raining I'll have a look.
 
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Not sure how I would get to the bottom of that without going down the road of substituting expensive parts. All I know is the garage I took it to said they hooked up the ht leads to something and verified a good spark on each lead. I'm pretty sure they did this after the car had been driven and was misbehaving.
Does it only misfire during driving or can the misfiring be replicated/provoked by throttle alone? If the latter, an in-line neon tester can be installed and observed. If its flashing falters....
 

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