Misfiring diagnosis

[ScotLandCruiser84]

Hi all - I'm new here and new to E500 ownership. I've had the car for 3 or 4 months but have been out the country so not been able to drive it much. I tried to take it for a run this morning and it was not driving smooth at all. Almost like not all cylinders were firing, felt hugely underpowered. Then all of a sudden it just died on me and I couldn't restart it. It would turn over but then it popped and banged out of the exhaust. Any ideas where I should start with this?

 

[gsxr]

Backfiring out the exhaust sounds like secondary ignition problems. How old are the caps/rotors? If fairly recent, what brand was installed? Also wouldn't hurt to check for liquid on the back of the insulator behind the rotor bracket, and check for oil leak from the cam seal while the insulator is out. If installing new caps, add ventilation slots as described in this thread.

Welcome to the forum!

 

[sheward]

Welcome and congratulations on acquiring that fine looking example.

What you are describing is very likely ignition problems resulting from distributor caps, rotors, and insulators. Do a search on the subject as this is something every owner should know.

GSXR obviously woke earlier than me this morning.

 

[dreaming.haze]

And a follow up on Dave's notes above, I would highlight Dan's great How To on replacing the dust shield and all the notes tools/torques/etc: HOW-TO: Replacing M119 Distributor Insulator/Dust Cover

 

[ScotLandCruiser84]

Thanks so much for the quick reply guys! And for the welcome to the forum. I come from a Land Cruiser 60 as a YouTube certified mechanic, where I started to feel quite confident about my skills, and I'm now entering the world of W124 mechanics.

It sounds like I've got some research to do on rotors, caps and insulators

 

[nocfn]

And please for the love of God, change out the cam seals as it would an absolute shame to do all this work again in 6 months. Lightly apply blue loctite in the rotor bracket bolts and such. This really can be a one and done issue. Unless you cam seals are leaking and on and on .....

 

[ScotLandCruiser84]

Just to add a confirmation to the diagnosis provided above. I pulled out the distributor caps and they look pretty worn so I think it's fair I need to replace these. From what I can read the Bosch ones are the best to get, and for UK delivery, the cheapest retailer is autodoc. So I've ordered:

2 x distributor caps Bosch 1 235 522 430
2 x rotor Bosch 1 234 332 422
2 x dust cover 1 230 500 240

I couldn't find the cam seals on autodoc so ordered them from classicMercedes.co.uk

I'll add a final post to confirm if this solved my problem.

On a separate note. Is there a service manual that I can download for this car? Do you guys tend to work on the cars yourselves or is it still best to get the service stamps and proof of maintenance for these cars?

 

[gerryvz]

No question that your caps, rotors and insulators are causing your issues. Looks to be a significant cam seal leak.

If you search online, you can find a couple of sites that have the W124 / M119 service manuals available for free.

 

[gsxr]

Just to add a confirmation to the diagnosis provided above. I pulled out the distributor caps and they look pretty worn so I think it's fair I need to replace these.

There is no significant wear visible on those components, but it would not hurt to replace them. Really bad ones look like this.

From what I can read the Bosch ones are the best to get, and for UK delivery, the cheapest retailer is autodoc. So I've ordered:

2 x distributor caps Bosch 1 235 522 430
2 x rotor Bosch 1 234 332 422
2 x dust cover 1 230 500 240

I couldn't find the cam seals on autodoc so ordered them from classicMercedes.co.uk

I'll add a final post to confirm if this solved my problem.

Remove the rotor/bracket/insulator and inspect the cam oil seal. There are other potential sources for the oil leak, in particular the cam advance solenoids are major leak points. You need to eliminate all the oil leaks, AND add vent slots to the brand new caps as mentioned in post #2 above, if you hope to solve this issue permanently.

On a separate note. Is there a service manual that I can download for this car?

Sample link to online FSM below. You can also download the manual (search for 124 chassis CD-ROM) and run this locally on your computer, no need for an internet connection:

Model 124 Maintenance Manual Index

Do you guys tend to work on the cars yourselves or is it still best to get the service stamps and proof of maintenance for these cars?

I do all my own work / DIY and document everything for future reference. It's getting harder by the day to find competent technicians who know how to diagnose and repair these 30-year old chassis/powertrains. If you are handy turning a wrench there's no reason you can't do your own work, the down side is you won't have a stack of inflated invoices to help boost the sale price when you resell in the future. The ignition parts listed above likely only cost about $300 USD total, while a typical shop invoice for replacement could easily be $1000-$1500 after the shop marks up the parts and tacks on a few hours of labor at $200-$300 per hour.

 

[ScotLandCruiser84]

If the cap looks ok is it the oil leakage that could cause the failed start and backfire? I guess I thought these contact points looked a bit past their best, and that was the reason it won't start now

 

[gsxr]

Do you mean the engine will not start at all now? That could be a different issue.

Running normally for 10-20 minutes after a cold start, then misfiring and refusing to re-start after a shutdown, is definitely secondary ignition. The oil leak is likely exacerbating the problem when the high voltage spark interacts with oil vapor, when there is not adequate ventilation.

 

[ScotLandCruiser84]

I drove it for about 20 minutes, it was lot as hell, then it died on me and I couldn't restart it. I left it about 30 mins after getting a lift home and tried restarting and it turns, didn't start but then it backfired loudly. I've not tried restarting it again because I was worried about damaging the exhaust system, but maybe if I clean the oil that's leaked and dry out the caps then try firing it up again I'll have a clearer idea if I've got a bigger issue

 

[sheward]

The contact points in your cap don't look that bad. Somewhat normal for this system. It's the moisture and/or oil and carbon tracking that causes problems. It's difficult to see but there appears to be carbon tracking in the cap. Once this appears you need to change caps as you indicated. Same with the insulators.

 

[gsxr]

I drove it for about 20 minutes ... then it died on me and I couldn't restart it. I left it about 30 mins after getting a lift home and tried restarting and it turns, didn't start but then it backfired loudly.

This is the textbook failure. Once it fully cools down, it will restart without any problem and run normally. And, it may run fine afterwards, until it's parked for X days/weeks, then the cycle repeats.

When you say it was "hot as hell", do you mean the engine coolant temp? What was the indicated temperature?

I've not tried restarting it again because I was worried about damaging the exhaust system, but maybe if I clean the oil that's leaked and dry out the caps then try firing it up again I'll have a clearer idea if I've got a bigger issue

It's almost guaranteed to start right up and run normally now, until the next time the problem occurs. Fixing the oil leak(s), and adding ventilation slots to the cap, should cure the problem permanently.

 

[ScotLandCruiser84]

"lot as hell" . I've been spell checked from "lumpy" to "lot". I meant to write it was lumpy as hell. I'm sure that makes more sense in this context

Thanks again for the advice

 

[Efraim]

Hi,

Let me share my thoughts about the misfiring issue. I think one reason behind the issue is spark plugs with resistors. Today it is common to use resistor ones while the high voltage current more easily finds unplanned routes as seen under distributer cap.

I have had similar misfires time to time, but not after replacing the factory specified non resistor plugs. So I recommend checking the spark plugs and replacing if necessary.

 

[195910]

If you are near Copenhagen, take the car to Kleemann. He already dealt with the issue with

 

[luckymike]

My miss turned out to be plug wires. Remove the covers on the valve covers and carefully observe the plug wires while running the engine in a dark area. I was able to see a spark jumping from two wires to the valve cover. On another set of wires, I could see light flashing internally along the length of the plug wire.