Misfire at idle, ignition components replaced

[JDAMG]

Hi everyone.

My 1992 400E developed a misfire it idle seemingly overnight. It runs fine otherwise once any sort of throttle is applied. The car had been sitting for a few days but nothing out of the ordinary. The temps did get a bit colder but I am in Florida so not a drastic drop or anything the vehicle has not experienced before. I had already replaced most of the ignition components in 2019 when I replaced the car but figured it couldn't hurt to refresh as the spark plugs are on a short interval anyway.

So i started with caps and rotors (OEM). No change.

Then performed a compression test, all good. 145-155psi in all cylinders.

Smoked it for vacuum leaks and nothing.

Replaced the spark plugs with OEM plugs from the dealer, same as last time. No change.

Replaced the ignition wires (one was open with movement and they were old beck-arnley wires that came with the car). I replaced then with the appropriate BERU wires. The ignition wires definitely were in need, they were reading high when tested and I thought that would be my culprit. NO change....

So now I am brainstorming as to where to go next.

I did a search but nothing quite the same as my symptoms that weren't solved with ignition components. As always, any suggestions are much appreciated!

 

[gsxr]

Have you removed the insulators and checked for liquid on the back side?

How much of a misfire at idle? A slight hiccup that goes away when you drive it, or a major misfire that causes drivability problems?

 

[JDAMG]

Have you removed the insulators and checked for liquid on the back side?

How much of a misfire at idle? A slight hiccup that goes away when you drive it, or a major misfire that causes drivability problems?

I have not checked the insulators. The car did endure about 24 hours of heavy wind and rain over the weekend... not sure if that could cause the issue.

It is on the rougher side for a misfire but only at idle, and seems to be limited to the passenger side bank, which also has me baffled. It does not cause drivability concerns, it does not stall or have an issue with acceleration.

 

[gsxr]

The passenger side, for whatever reason, seems to be the culprit when there is a significant misfire. However if the issues goes away under load / acceleration, it may not be the usual problem discussed on the forum. Still, it wouldn't hurt to check the insulators (must remove & look at the back side). And, cutting the additional ventilation slots as described in other threads may help too.

Gapping the plugs (non-resistor, F8DC4, right?) to 1.0mm instead of 0.8mm might help a little.

Bosch caps & rotors, I presume? There's a tiny chance of defective parts out of the box, but this gets difficult to diagnose without a lot of part swapping. Live data is helpful as it will show you the specific cylinders at fault.

 

[JDAMG]

Interesting. I did see the article you had posted in a previous thread regarding the insulators. I will check the insulators, as they have not been replaced under my ownership and are most likely original. The moisture affect has me thinking that could be my culprit now.

I will also check gap on plugs, they came straight from dealer, ordered with VIN as I did last time, so I assume they are correct but will also confirm. Same with caps and rotors.

I do have a spare EZL, not sure if that is worth trying or my symptoms match something that could be related to that.

Also thinking about taking a listen to injectors? Maybe a fuel delivery issue?

 

[gsxr]

The EZL is almost certainly not the issue, based on your description... don't mess with that for now. Fuel delivery isn't likely either, but verify the pumps sound normal and that the relay under the back seat isn't clicking at idle.

 

[JDAMG]

The EZL is almost certainly not the issue, based on your description... don't mess with that for now. Fuel delivery isn't likely either, but verify the pumps sound normal and that the relay under the back seat isn't clicking at idle.

Thank you. Just been knocking my head against the wall today, at least i feel like i have some direction. I have a hunch about the insulators, but it may just be desperation setting in. I will report back, gonna spend some more time on it tomorrow.

 

[JDAMG]

So took a listen to the fuel pumps/relay. All sounds normal. Also just listened to injectors, they are activating but not sure about spray pattern, etc. Have not had the opportunity to pull the caps/rotors again to check insulators. Hopefully tomorrow. The misfire seems to be getting worse, effecting acceleration at lower speeds now.

 

[JDAMG]

Replaced the insulators today. No change. They were a bit moldy, but otherwise fine. Sprayed some carb cleaner around the intake manifold and i now suspect that to be my culprit. Going to make a shopping list next week, get all parts needed for proper repair and give that a try. Frustrated but moving forward. Fingers crossed.

 

[JDAMG]

 

[gsxr]

Front of the insulator (Doduco, btw) looks ok... was there any trace of liquid on the rear, or was the back side completely dry?

 

[nbvcxw8000]

Hi JDAMG,

The color of the deposits on the insulator (motor oil) would seem to indicate a big conductive vaporization coming from the engine behind the rotors.

Didier

 

[JDAMG]

Front of the insulator (Doduco, btw) looks ok... was there any trace of liquid on the rear, or was the back side completely dry?

 

[JDAMG]

Hi JDAMG,

The color of the deposits on the insulator (motor oil) would seem to indicate a big conductive vaporization coming from the engine behind the rotors.

Didier

Just posted a picture of the backside of the insulator. What do you think?

 

[JDAMG]

Hi JDAMG,

The color of the deposits on the insulator (motor oil) would seem to indicate a big conductive vaporization coming from the engine behind the rotors.

Didier

Front of the insulator (Doduco, btw) looks ok... was there any trace of liquid on the rear, or was the back side completely dry?

On the motor side it looked fine. There was no moisture (see pic) just pitting? It wasnt wet.

 

[gsxr]

Just posted a picture of the backside of the insulator. What do you think?

Looks dry in the photo but you'd have to feel it. When liquid is present it will smear around the back easily. The front side is almost always dry.

I'd save the old ones as spares, they may be serviceable once cleaned up.

 

[nocfn]

If you have not done so, and no record of it being performed, change the cam seals when you do this. You will stop having to mess with this again for 5-10 years.

 

[JDAMG]

Y

Looks dry in the photo but you'd have to feel it. When liquid is present it will smear around the back easily. The front side is almost always dry.

I'd save the old ones as spares, they may be serviceable once cleaned up.

Yes i saved them. I did not change the insulators when i first purchased the car 2 1/2 years ago but I did change caps/ rotors. It had been sitting outside for almost 2 years prior.

 

[JDAMG]

Finally getting back to this… intake manifold seals and all the goodies with it. Fingers crossed.

 

[JDAMG]

 

[JDAMG]

Well still in the same boat and frustrated. Replaced the intake manifold gaskets and all the needed hoses and seals associated, still dealing with the same issue. A rough misfire at idle, goes away with throttle but will stumble a bit at low rpms. Once you are going it will drive fine for the most part, but since the most recent repair, it will develop a misfire while driving as well. This is new, but not sure if related.

I have replaced caps, rotors, insulators, ignition wires, spark plugs, and now this. Still no results. Any ideas?

 

[gerryvz]

Wiring harness replaced?

Pull codes.

Have you checked MAF?

ETA?

 

[sheward]

It seems over the years when this type of misfire persists after replacing the usual suspects that you have, a number of owners came to realize the problem was a coil going bad. Follow Gerry's advice on pulling codes and try a coil if you have a known good one. If I remember correctly you work at a shop and might have access to one. Watching the coils in the dark may also be helpful.

 

[JDAMG]

Wiring harness replaced?

Pull codes.

Have you checked MAF?

ETA?

Wiring harnesses are good, both were replaced by previous owner. I dont have the ability to pull codes, unfortunately we dont have the equipment.

I did unplug MAF with no change so ruled that out.

Havent checked ETA, not sure what to look for there…

 

[JDAMG]

It seems over the years when this type of misfire persists after replacing the usual suspects that you have, a number of owners came to realize the problem was a coil going bad. Follow Gerry's advice on pulling codes and try a coil if you have a known good one. If I remember correctly you work at a shop and might have access to one. Watching the coils in the dark may also be helpful.

I checked the coils, and they appeared to be strong, although didnt watch them in the dark. I may need to just throw a good one in and pray.

 

[gsxr]

I checked the coils, and they appeared to be strong, although didnt watch them in the dark. I may need to just throw a good one in and pray.

Two... there are two coils, different part numbers, different mounting orientation. New ones aren't a bad idea, but you'll want to pull the driver headlight. The swear jar will still receive contributions, but not as much as if you leave the headlight in place.

 

[JDAMG]

Two... there are two coils, different part numbers, different mounting orientation. New ones aren't a bad idea, but you'll want to pull the driver headlight. The swear jar will still receive contributions, but not as much as if you leave the headlight in place.

Thanks gsxr. Yes was just reading a long thread about that subject. Looks like i will need a couple of coils, a couple of hours, and a couple of beers. I think it might be worth a shot. Not sure what would be left after that. Everything ignition related will have been replaced. ETA, EZL, injectors?

 

[gerryvz]

Throwing a spare coil in and seeing if that works, and then on the other side, would be a good idea if you have a known-good spare. Remove the driver's side headlight to make this job much easier.

You don't have one of the eBay type blink code readers?

 

[JDAMG]

Throwing a spare coil in and seeing if that works, and then on the other side, would be a good idea if you have a known-good spare. Remove the driver's side headlight to make this job much easier.

You don't have one of the eBay type blink code readers?

No I do not, guess it might be time to get one. I used to have one in the shop but when my senior tech retired he took it with him.

I do some of my work, but I have another senior tech who helps me with the more difficult jobs. ( I am not a tech). I am an advisor.

We are stumped and he is constantly busy. So getting the time to play around with ideas usually falls on me. Unfortunately the 400e has been down almost three months now.

 

[JDAMG]

Throwing a spare coil in and seeing if that works, and then on the other side, would be a good idea if you have a known-good spare. Remove the driver's side headlight to make this job much easier.

You don't have one of the eBay type blink code readers?

Also i do not have a spare, would need to purchase one.

 

[JDAMG]

Ordered two new coils. Cant hurt right.

 

[nocfn]

while you wait for shipping, you can check the resistance of the wires to be sure they are not the culprit.

 

[JDAMG]

Changed the coils, still misfire at idle. As soon as i touch the throttle is goes away. Not sure where to go from here. Thinking its time for a blink code reader. Any recommendations?

 

[maw1124]

Wild guess here, but have your engine mounts been changed? The one time I thought I had a hiccup at idle, a friend who's owned one for years was in the car when I pointed it out as something I needed to track down, flatly said "you just need engine mounts is all." @Klink that was TG.

It was engine mounts, not a misfire at all. If what you have is something you can feel but the RPM needle isn't jumping, after all you've done if the mounts haven't been changed...

I still can't tell if you're tracking down something rough or subtle.

maw

 

[szvook]

Has a leak down test been done?

 

[Bogeyman]

Did you ever regap the plugs to 1.0mm?

 

[sheward]

Wild guess here, but have your engine mounts been changed? The one time I thought I had a hiccup at idle, a friend who's owned one for years was in the car when I pointed it out as something I needed to track down, flatly said "you just need engine mounts is all."

It was engine mounts, not a misfire at all. If what you have is something you can feel but the RPM needle isn't jumping, after all you've done if the mounts haven't been changed...

I still can't tell if you're tracking down something rough or subtle.

maw

I remember Klink saying as much. Many folks mistake engine mounts for bad idle.

It's hard to assess something subtle without feeling it. Don't feel as if what you have done is without merit. Having the spare and known good ignition parts for future spares and diagnosis is beneficial. I'd like to do the intake rings as well on my own.

 

[JDAMG]

It is a hard misfire, and it occurred while driving at one point, so motor mounts are out although I have seen this type of issue been mistaken for that. I have not regapped the plugs, but this seems a bit severe for that, but worth a shot. I have not performed a leakdown, only compression test.

 

[szvook]

FWIW, when my motor mounts were in need of replacement, I did not have any misfire symptoms at all. Same with the plugs prior to being re-gapped. The only slight misfire I did have was due to the corrosion on one of the plugs wire. And the misfire was only at idle and when the car was still a bit cold. New wire set cured the misfire. But my shop did a leak down test first, which prompted them to identify which cylinder to address, which led them to check the plug wire. And while at it, I had all new injectors replaced as well, as they were original from 1992.

 

[sheward]

I agree. After further clarification on the misfire, it's doubtful either of the things you mentioned would be the source. This is where my knowledge runs dry. You should get a blink code reader so the illuminati can guide you to the solution.

 

[JDAMG]

So got to spend some time on the 400 today with actual diagnosis equipment, and some help from Pierre. It has been determined that all work so far has been correct as far as installation (coil wires, plugs, no more vacuum leaks, etc) Injectors are also putting out the proper values. But the passenger side cylinder bank is not producing enough vacuum. The motor is producing about 600 bar. So we suspect the cam on the passenger side has jumped it timing.... more work ahead.

 

[landgreen22]

@JDAMG I have a 1995 w140 S420 and long story short, I have gone through the exact same symptoms and have followed a similar journey on the parts and testing you have. Still working to identify my issue since I don't have any codes. How did you test the vacuum pressure on each cylinder bank? Have you been able to confirm timing has jumped?

Thanks

 

[JDAMG]

@JDAMG I have a 1995 w140 S420 and long story short, I have gone through the exact same symptoms and have followed a similar journey on the parts and testing you have. Still working to identify my issue since I don't have any codes. How did you test the vacuum pressure on each cylinder bank? Have you been able to confirm timing has jumped?

Thanks

I have yet to pull the covers, but I am fairly certain that is the issue. Have it on my schedule for a couple of weeks from now and will be updating with the findings.

 

[JDAMG]

In the picture above you can see what we used for testing, and how it hooks up. Then we run motor and unhook an injector at a time to look for the fluctuations in values. My passenger side had almost no fluctuation at all when an injector was taken out of the equation. And the overall vacuum pressure on that side was lower.

 

[JC220]

Has the engine timing been checked? And upper chain rails etc inspected for breakage?

As mentioned in this thread already by others a Leakdown test is really a must at this stage.

Not sure what the vacuum test above will prove or disprove to be honest.

Can you have the fuel injectors tested on a flow bench or swap in a known good set for testing purposes?

 

[JC220]

Thread 'M119 Cylinder leak down test results - lessons learned!' M119 Cylinder leak down test results - lessons learned! | M119 Engine

 

[JDAMG]

Has the engine timing been checked? And upper chain rails etc inspected for breakage?

As mentioned in this thread already by others a Leakdown test is really a must at this stage.

Not sure what the vacuum test above will prove or disprove to be honest.

Can you have the fuel injectors tested on a flow bench or swap in a known good set for testing purposes?

Our next step is to check chain and guide rails. Injectors are putting out proper values. Will go on from there. Highly suspect broken chain guides, jumped chain. Misfire has progressed into a driving condition as well.

 

[JC220]

Our next step is to check chain and guide rails. Injectors are putting out proper values. Will go on from there. Highly suspect broken chain guides, jumped chain. Misfire has progressed into a driving condition as well.

Just to ask what do you mean by injectors putting out proper values? This is the only way to test and how I test mine with my rig:

 

[JDAMG]

I do not have a rig like that. But used SDS to determine they were at least functional by watching actual values. Also on the miss I am seeing, the pressures on one bank vs the other and the variations of the miss in different conditions. I have all but completely ruled out injectors. Not impossible, but doesnt seem likely.

 

[JC220]

I do not have a rig like that. But used SDS to determine they were at least functional by watching actual values. Also on the miss I am seeing, the pressures on one bank vs the other and the variations of the miss in different conditions. I have all but completely ruled out injectors. Not impossible, but doesnt seem likely.

I don't mean that you need to have a rig yourself to do it but just that it would be a useful test.

For example there will be plenty of injector cleaning / testing services near you or by postal service.

Chain and guides is a good next step yes