Diagnosing misfire with new caps and insulators

[Matin]

ADMIN EDIT: Moved this thread out of "WTB" subforum, since the MAF was fine and Matin no longer is looking to buy one. Discussion below changes to misifring with new caps / insulators.

Hi Guys,

If anyone has a spare MAF for sale in very good condition, please do let me know.

I have a feeling that my MAF has backed up as the car miss fires extremely bad when the car is warm/hot.

I've checked the distrubutor caps and they are bone dry. I think the only other thing could be the MAF.

Thanks,
Matin

 

[gsxr]

Re: WTB: MAF for 92' 500E

Matin, what happens if you disconnect the MAF (for testing purposes), when the car is warm/hot?

 

[Matin]

Re: WTB: MAF for 92' 500E

I did not try it - is it supposed to stall? I’ll give it a try.

 

[gsxr]

Re: WTB: MAF for 92' 500E

I did not try it - is it supposed to stall? I’ll give it a try.

With the MAF disconnected, the engine management should go into a "fixed operating mode", where the engine may actually run better. If so, this pretty much confirms the MAF is defective. A bad MAF may not trigger any codes.

A more accurate method to diagnose would be to use a digital scanner that shows live data from the MAF where you can see if the reported airflow is grossly inaccurate. At idle, IIRC it should be ±20 kg/hr.

 

[Matin]

Re: WTB: MAF for 92' 500E

With the MAF disconnected, the engine management should go into a "fixed operating mode", where the engine may actually run better. If so, this pretty much confirms the MAF is defective. A bad MAF may not trigger any codes.

A more accurate method to diagnose would be to use a digital scanner that shows live data from the MAF where you can see if the reported airflow is grossly inaccurate. At idle, IIRC it should be ±20 kg/hr.

thanks! I’ll give it a try, can you disconnect it with the engine on?

 

[gsxr]

Re: WTB: MAF for 92' 500E

thanks! I’ll give it a try, can you disconnect it with the engine on?

I would turn off the engine before connecting/disconnecting the wires. Remember the MAF is a twist-lock round connector...

 

[Matin]

Re: WTB: MAF for 92' 500E

Just an update on this, I tested the MAF and it's working fine, I even swapped out the original MAF with a replacement that I had lying around (probably from my bro's old 500E) and the issue still happens.

The issue happens when I leave the car for the whole week and drive it for the first time (car left untouched during the weekdays). Once the car has reached 80c it'll start stumbling and miss firing quite alot.

The only thing that I have noticed is that the car takes a while to crank and start when I start it for the first time - could this issue be related to battery by any chance?? The battery is from Japan.

After this issue happens, I will leave the car for another day and start it the day after this issue happens and the car drives perfect, no miss fires at all.

So the problem only occurs when I start it and drive it for the first time since I left it for the week, it's fine after I leave it for another day.

I don't really know what to do, I've checked the caps, insulators, removed them all and they are bone dry, MAF is working fine, so not really sure what else can be the problem?

Help!

Matin

 

[gsxr]

Re: WTB: MAF for 92' 500E

The issue happens when I leave the car for the whole week and drive it for the first time (car left untouched during the weekdays). Once the car has reached 80c it'll start stumbling and miss firing quite alot.

After this issue happens, I will leave the car for another day and start it the day after this issue happens and the car drives perfect, no miss fires at all.

So the problem only occurs when I start it and drive it for the first time since I left it for the week, it's fine after I leave it for another day.

^^^ This sounds a LOT like the issue with the symptoms of liquid on the back side of the distributor cap insulators. I've you have removed the rotor+bracket and checked the back side of the insulator, and everything is dry, I'm stumped.

Are there any codes on any computer after the misfiring episode? If not, you would likely need to connect a digital scanner / SDS and view live data during the misfiring to get a better idea what's happening. Another long shot - what condition are the spark plug wires in? If they are old / original, I'd at least give them a closer inspection. Your description seems to indicate a secondary ignition fault of some sort.

 

[Matin]

Re: WTB: MAF for 92' 500E

^^^ This sounds a LOT like the issue with the symptoms of liquid on the back side of the distributor cap insulators. I've you have removed the rotor+bracket and checked the back side of the insulator, and everything is dry, I'm stumped.

Are there any codes on any computer after the misfiring episode? If not, you would likely need to connect a digital scanner / SDS and view live data during the misfiring to get a better idea what's happening. Another long shot - what condition are the spark plug wires in? If they are old / original, I'd at least give them a closer inspection. Your description seems to indicate a secondary ignition fault of some sort.

If there was liquid at the back side of the distributor insulators, why would the car work fine the next day? wouldn't it be the same issue everyday in the morning?

 

[gsxr]

Re: WTB: MAF for 92' 500E

If there was liquid at the back side of the distributor insulators, why would the car work fine the next day? wouldn't it be the same issue everyday in the morning?

Nope. It's been documented on many vehicles. In general, the car has to be parked for at least a week, if not longer, before the problem recurs. If driven daily, or even every few days, there is no problem. I can't explain it, but that's what happens.

 

[Matin]

Re: WTB: MAF for 92' 500E

Nope. It's been documented on many vehicles. In general, the car has to be parked for at least a week, if not longer, before the problem recurs. If driven daily, or even every few days, there is no problem. I can't explain it, but that's what happens.

Ok, I'll double check them again tomorrow...thank you!

 

[gsxr]

Re: WTB: MAF for 92' 500E

See photo & post numbers 13-17 here:
https://www.500eboard.com/forums/showthread.php?t=10710

Also, photo in post #7 here:
http://www.500eboard.com/forums/showthread.php?t=10825

That will give you an idea what to look for.

 

[sheward]

Re: WTB: MAF for 92' 500E

For the record, I've had this same thing happen and the cure was just the caps (with some arcing evident but no moisture present). I have not replaced the insulators and the problem has never returned even under infrequent use. I believe any arcing in the caps can be problematic.

drew

 

[Matin]

Re: WTB: MAF for 92' 500E

See photo & post numbers 13-17 here:
https://www.500eboard.com/forums/showthread.php?t=10710

Also, photo in post #7 here:
https://www.500eboard.com/forums/showthread.php?t=10825

That will give you an idea what to look for.

Thank you. I replaced the insulators with brand new Bosch ones. I'll give it another look tomorrow, but the last time I checked, they looked nothing like the ones in those pictures. Also the problem doesn't go away after 15-30 mins, it'll keep misfiring unless I leave the car to sit for something like 12 hrs/next day.

But I'll give it a shot tomorrow by opening the caps and insulators up.

 

[Matin]

Re: WTB: MAF for 92' 500E

For the record, I've had this same thing happen and the cure was just the caps (with some arcing evident but no moisture present). I have not replaced the insulators and the problem has never returned even under infrequent use. I believe any arcing in the caps can be problematic.

drew

Sorry, but what does arcing mean??

 

[sheward]

Re: WTB: MAF for 92' 500E

Voltage travelling along carbon tracks in the cap as seen in this picture although they are not always so apparent. https://www.500eboard.com/forums/showthread.php?t=319&highlight=Distributor+caps see post no. 41

drew

 

[gsxr]

Re: WTB: MAF for 92' 500E

...Also the problem doesn't go away after 15-30 mins, it'll keep misfiring unless I leave the car to sit for something like 12 hrs/next day.

Yours would be the first I've heard of with these symptoms, but the cause not being the insulators. I'm very curious what you find out.

BTW... I did encounter a defective Bosch distributor cap, brand new out of the box. It caused a misfire when hot, but only at idle. Took me a long time to figure that out, and the problem was present every time the car was at operating temp, which is not the same as your car. Again, live data would help diagnose this issue.

How old are the spark plug wires (and plugs)? What brand?

 

[JC220]

Re: WTB: MAF for 92' 500E

Matin is your 500E garaged indoors when not in use? There is incredible humidity at the moment in Ireland / UK. If I even open my garage door for a few minutes my cars start to steam up on the outside. It does sound like moisture in the caps to me too especially if your 500E sits out on a drive way at this time of year

 

[E500star]

Re: WTB: MAF for 92' 500E

It’s a vacuum loss. Put a cognac bottle on it

 

[Matin]

Re: WTB: MAF for 92' 500E

Matin is your 500E garaged indoors when not in use? There is incredible humidity at the moment in Ireland / UK. If I even open my garage door for a few minutes my cars start to steam up on the outside. It does sound like moisture in the caps to me too especially if your 500E sits out on a drive way at this time of year

The car is not garaged, it's most likely moisture as you guys are saying. I'm going to open it up next week I think, after the car has sat for the week to see if there's moisture build up.

It seems silly that the caps are so prone to moisture, and if you leave it for a week it builds up like crazy...

 

[JC220]

Re: WTB: MAF for 92' 500E

Yes the double distributor caps are the 500E’s Achilles heel IMO. Storing the car in a dry garage would vastly reduce the likelihood of issues due to moisture / humidity. One simple test for defective HT leads can be to observe the engine running in total darkness. Carefully visually inspect the HT leads all around but don’t touch them!! I have done this before and successfully diagnosed bad HT leads by seeing the blue arcing for myself. It’s worth a try since it costs nothing but a few minutes of time. (In the m119 most HT leads are pretty well concealed though) Even a carport makes a big difference to moisture levels of you can’t get it into a Garage at home.

 

[Stevester 500E]

Re: WTB: MAF for 92' 500E

I am curious, has anyone tried spraying WD40 in a dry distributor cap to see if WD40 keeps moisture away?

 

[gerryvz]

Re: WTB: MAF for 92' 500E

It seems to me that moisture of any kind (even WD-40) in a distributor cap would promote shorting and arcing of the spark. I wouldn’t do it, but that’s just me.


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[Matin]

Re: WTB: MAF for 92' 500E

Hi Guys,

You were right. There is moisture at the back of the caps.

How are we supposed to stop the moisture from getting into the caps??

UK owners, do you guys suffer from these problems at all?

Thanks,
Matin

 

[500AMM]

Re: WTB: MAF for 92' 500E

Hi Guys,

You were right. There is moisture at the back of the caps.

How are we supposed to stop the moisture from getting into the caps??

UK owners, do you guys suffer from these problems at all?

Thanks,
Matin

The never ending caps & moisture syndrome! ☺
You'll find some interesting reading about the insulators in post #6 and #15 here:
https://www.500eboard.com/forums/showthread.php?t=7506

Condense problem, post #66:
https://www.500eboard.com/forums/showthread.php?t=3363&p=55586#post55586

The additional ventilation did help, post #47:
https://www.500eboard.com/forums/showthread.php?t=4559

 

[gsxr]

Re: WTB: MAF for 92' 500E

You were right. There is moisture at the back of the caps.

How are we supposed to stop the moisture from getting into the caps??

Matin, did you find moisture / liquid inside the distributor caps, or on the back side of your brand-new insulators against the cylinder head cover?

 

[Jlaa]

Re: WTB: MAF for 92' 500E

Also -if you found liquid on the back side of the insulators, then may I ask what weight of motor oil are you using? I ask bc there is a theory out there that posits that the moisture on the insulators is not condensation but is abosorbed motor oil vapor from ever so slightly weaked cam seals or other seals which allows the oil vapor to be absorbed. Again, just a conjectural theory.

 

[Matin]

Re: WTB: MAF for 92' 500E

The never ending caps & moisture syndrome! ☺
You'll find some interesting reading about the insulators in post #6 and #15 here:
https://www.500eboard.com/forums/showthread.php?t=7506

Condense problem, post #66:
https://www.500eboard.com/forums/showthread.php?t=3363&p=55586#post55586

The additional ventilation did help, post #47:
https://www.500eboard.com/forums/showthread.php?t=4559

Thank you, I will take a look!

Matin, did you find moisture / liquid inside the distributor caps, or on the back side of your brand-new insulators against the cylinder head cover?

Moisture at the back of the insulators*, inside the caps were totally dry.

Also -if you found liquid on the back side of the insulators, then may I ask what weight of motor oil are you using? I ask bc there is a theory out there that posits that the moisture on the insulators is not condensation but is abosorbed motor oil vapor from ever so slightly weaked cam seals or other seals which allows the oil vapor to be absorbed. Again, just a conjectural theory.

At the time, I was using 10w40 [FONT=&quot]Liqui Moly, however, recently changed to MB 229.5.[/FONT]