Random single misfire when idling and randomly fails to start 1 in 20 starts.

[mrindiafromuk]

I have a 1991 500E imported from Germany. I have a random misfire when idling, it can happen some days and not on other days. Its a single mis/blip and continues to idle smooth. Also randomly the engine will not fire on the button. 1 in 20/25 starts. When the starting issue is there, often while the engine is cranking I can let go of the key and at the same time blip teh throttle and it fires. Or I can turn off and start again and it fires straight away.

I have had the plugs. cap, wires changed. I dont seem to have any oils leaks, since I had them addressed when I had the transmission done. which is when I had plugs, wires, cap rotor changed .

Any help diagnosing will be appreciated. I was thinking coils, buy @gsxr does not think so.

I also had 2 lifts installed. pretty chuffed about them

 

[gsxr]

I have a 1991 500E imported from Germany. I have a random misfire when idling, it can happen some days and not on other days. Its a single mis/blip and continues to idle smooth.

What brand plugs were installed, what type, and what were they gapped at? Should be non-resistor plugs (not fancy fine-wire elements), and the gap should be opened up to 1.0-1.1mm (not the standard spec of 0.8mm). This may help the hot-idle random misfire. If you already have F8DC4 non-resistors gapped at 1.0mm, that's odd.

Also randomly the engine will not fire on the button. 1 in 20/25 starts. When the starting issue is there, often while the engine is cranking I can let go of the key and at the same time blip teh throttle and it fires. Or I can turn off and start again and it fires straight away.

Does this only happen when the engine is warm or hot, and it always starts normally when cold? If the fuel pressure regulator old or original, it's top of the list as a suspect.

I have had the plugs. cap, wires changed. I dont seem to have any oils leaks, since I had them addressed when I had the transmission done. which is when I had plugs, wires, cap rotor changed.

What brand caps, wires, rotors were installed? It's unlikely these are the root cause, but if the caps don't have the additional vent slots added as discussed here, I'd add that to your project list.

If the cam advance solenoids are original, they are almost certainly leaking oil and contaminating the lower edge of the distributor cap. There won't be a drop of oil on the floor under the car when parked, just enough to get into the distributor since it's only a few inches away.

Any help diagnosing will be appreciated. I was thinking coils, buy @gsxr does not think so.

Yep... unlikely to be coils. You can install new Bosch coils, but don't expect much (if any) change.

I also had 2 lifts installed. pretty chuffed about them

Oooo! Those are pretty!

 

[gerryvz]

Your coils are fine.

Diagnosis is in order here.

HOW-TO on replacing fuel pressure regulator is below (please check fuel pressure first, per the beginning of the HOW-TO).

HOW-TO: Replacing M119 / M104 fuel pressure regulator | "HOW-TO" Tutorial Articles

GSXR edit: Click here for "How to" writeup on checking pressures. One issue that pops up from time to time with the M119 (LH injection) and M104 (HFM injection) is the weakening and/or failure of the fuel pressure regulator. This is located in-line on the fuel rail and is a round, silver unit...

www.500eboard.co

But, I do not believe your issue is fuel pressure. As said, you need to add the ventilation slots to your distributor caps. That should solve the issue.

 

[mrindiafromuk]

1. What brand plugs were installed, what type, and what were they gapped at? Should be non-resistor plugs (not fancy fine-wire elements), and the gap should be opened up to 1.0-1.1mm (not the standard spec of 0.8mm). This may help the hot-idle random misfire. If you already have F8DC4 non-resistors gapped at 1.0mm, that's odd.
A. Dont know since I did not do it, so I will get one out to check whats happening here. I am guessing they will be correct because the place I had this work done are ex MB techs. However I need to know for myself so i will pull one myself and check .


2. Does this only happen when the engine is warm or hot, and it always starts normally when cold? If the fuel pressure regulator old or original, it's top of the list as a suspect.
A I noticed is also when cold but not as frequent , I will just replace it, it does not seem to have any record of changing in the history book.


3. What brand caps, wires, rotors were installed? It's unlikely these are the root cause, but if the caps don't have the additional vent slots added as discussed here, I'd add that to your project list.
A. Dont know the invoices dont show what was installed . I will check and change. I am guessing the MB dealer will have the right ones,


4.If the cam advance solenoids are original, they are almost certainly leaking oil and contaminating the lower edge of the distributor cap. There won't be a drop of oil on the floor under the car when parked, just enough to get into the distributor since it's only a few inches away.
A I will check if there is any oil in the cap. teh whole engine was changeed 50k miles ago by Mercedes in Germany in 2007, so not as old as the rest of the car.


5.unlikely to be coils. You can install new Bosch coils, but don't expect much (if any) change.
A I will hold off this for now.


Thanks I will look at these over the week. BTW I was wring my leads have not been changed . any recommendations

 

[mrindiafromuk]

Yes. As said, you need to add the ventilation slots to your distributor caps. That should solve the issue.

HOW-TO on replacing fuel pressure regulator is below.

HOW-TO: Replacing M119 / M104 fuel pressure regulator | "HOW-TO" Tutorial Articles

GSXR edit: Click here for "How to" writeup on checking pressures. One issue that pops up from time to time with the M119 (LH injection) and M104 (HFM injection) is the weakening and/or failure of the fuel pressure regulator. This is located in-line on the fuel rail and is a round, silver unit...

www.500eboard.co

Awsome I am just going to change them

 

[gerryvz]

Awsome I am just going to change them

Okay, but before you install new ones (and BTW, they are $$$ each at the dealer), MAKE SURE you cut new slots into them per the write-up here. Otherwise, you are just going to have the same, exact issue in the future.

 

[gsxr]

A. Dont know since I did not do it, so I will get one out to check whats happening here. I am guessing they will be correct because the place I had this work done are ex MB techs. However I need to know for myself so i will pull one myself and check .

Good idea, easy to pull a single plug and see what brand / model / gap you find.

A I noticed is also when cold but not as frequent , I will just replace it, it does not seem to have any record of changing in the history book.

Difficulty cold starting is unusual, but start with the FPR and see if that helps.

A. Dont know the invoices dont show what was installed . I will check and change. I am guessing the MB dealer will have the right ones,

I wouldn't replace them unless they are non-OEM brands (Bremi, Facet, or similar). If they are Bosch or OE/Genuine, it would likely be wasting money to install new ones. If you have Bosch caps, just add the vent slots via Dremel as shown in the thread linked previously.

A I will check if there is any oil in the cap. teh whole engine was changeed 50k miles ago by Mercedes in Germany in 2007, so not as old as the rest of the car.

Ah, that is interesting! If the engine was a factory reman from 2007 the solenoids may be ok. If not you'll find a trace of oil at the lower edge of the cap when you unbolt it. This area should be 100% dry.

Thanks I will look at these over the week. BTW I was wring my leads have not been changed . any recommendations

Original wires can last a very, very long time. I generally don't recommend replacement unless there is a visible problem, or they test as defective. Resistance at the plug boot should be 2000 ohms, ±20%.

Best replacement I know if are the Beru wires (click here) as they are made from quality components and, more importantly, are the correct length... however a couple people have reported either a defective wire out of the package, or a failure shortly after installation. YMMV, etc.

 

[Alphasud40]

Probably not your issue but has the LH ECU been updated with new capacitors? I find particularly on the pre 1992 LH modules the capacitors are bad. While these modules are robust bad capacitors even when open circuit can cause glitchy operation and ones that re shorted take out the fuel pump driver circuit.

 

[mrindiafromuk]

What brand plugs were installed, what type, and what were they gapped at? Should be non-resistor plugs (not fancy fine-wire elements), and the gap should be opened up to 1.0-1.1mm (not the standard spec of 0.8mm). This may help the hot-idle random misfire. If you already have F8DC4 non-resistors gapped at 1.0mm, that's odd.



Does this only happen when the engine is warm or hot, and it always starts normally when cold? If the fuel pressure regulator old or original, it's top of the list as a suspect.




What brand caps, wires, rotors were installed? It's unlikely these are the root cause, but if the caps don't have the additional vent slots added as discussed here, I'd add that to your project list.

If the cam advance solenoids are original, they are almost certainly leaking oil and contaminating the lower edge of the distributor cap. There won't be a drop of oil on the floor under the car when parked, just enough to get into the distributor since it's only a few inches away.




Yep... unlikely to be coils. You can install new Bosch coils, but don't expect much (if any) change.




Oooo! Those are pretty!

Can you take a look at this and let me know if this is worth me buying ? AAZ Preferred 119TUNEUPKIT Ignition Tune-Up Kit; KIT - Mercedes
I am ordering fuel regulator and coils as per post you gave me.

 

[gsxr]

Can you take a look at this and let me know if this is worth me buying ? AAZ Preferred 119TUNEUPKIT Ignition Tune-Up Kit; KIT - Mercedes
I am ordering fuel regulator and coils as per post you gave me.

That kit has Karlyn-STI wires... not recommended. Only wires I'd use are the Beru set linked previously.

 

[mrindiafromuk]

Ok Thanks this is super helpful

Going to do this first

1. Change FPR Just be sure
2. Check plugs for non resistor version and gap. ensure right gap and plugs are in .
3. Check dizzy cap for bosch and vent slots .
4. check to make sure no oil in dizzy.

 

[mrindiafromuk]

Ok “Boom” The FPR not only fixed the mis fire there has been some power gain. The good old 500E is accelerating like a rocket ship. Totally surprised.

Great advice and a cheep fix $40 for the Bosch replacement and 20 mins only because I never done it before. probably only a 10 min job .

Thanks you all

I will be checking the other bits on my list, plugs and dizzy for vents and oil

old FPR Original Mercedes Part

 

[mrindiafromuk]

Quick Update. Checked dizzy today , no oil in it and the chap who changed them did cut the slots in. However the plugs were new but incorrect . they had the FR8 with resistor plugs. I replaced with bosch without resistor as advised at 1.0 gap . went for test drive and the idle and power delivery improved even more super smooth

Thanks again

 

[mrindiafromuk]

quick question which is e mode and s mode bit confused if the s needs to show to be a mode or does e need to show to be a mode , my owners manual is in German lol

 

[gsxr]

When E is displayed it's in Economy mode, with early upshifts (lower RPM than normal).

When S is displayed it's in Standard mode, with normal upshifts.

 

[mrindiafromuk]

When E is displayed it's in Economy mode, with early upshifts (lower RPM than normal).

When S is displayed it's in Standard mode, with normal upshifts.

Thanks