Looking for ideas regarding the source of my slight stumble at idle. This is for a 94 M104 E320. I have a slight misfire/stumble at idle, no surging or hunting, doesn't even register on the tach, but it does in the seat of my pants.

Just replaced sparkplugs properly gaped and torqued, new Bosch SP wire set as I noticed the old #2 wire had signs of shorting against the valve cover. Buttoned it up and still have the same misfire/stumble at idle.

All of the old plugs looked nice and tan, free of carbon and oil, so she's running right and under load she responds great, I only notice the issue at idle. Also I have no check engine light or any codes, all clear from that regard.

Although I couldn't initially spot any vacuum leaks I'll keep looking. The injectors seem tight fitting and no signs of a seal leak that I can see. Any ideas of what could cause this?

TIA

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

I take it, this is the one with the 80K mi on it.
The only thing that comes to my mind would be an engine compression test.
If it turns out to be consistent, start pouring valve cleaner into your tank..
Sometimes, dirty valve seats can cause a problem at idle by leaking some. At higher RPMs, this may not be apparent or the higher pressure may help seal the valve(s) better.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

I had that problem,it was a coil,but I replaced all three.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

That cured the same problem on our E320 wagon.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Thanks guys,
I have some valve cleaner on order so will be running that through the tank to see if it helps and as a preventive measure anyhow.

Regarding the coils, does anyone have any info on how to test these if possible? I recall on oil oil-filled coils you could test for resistance etc, but not sure about these solid coils.

Also tested FP reg and EGR Vac line per Arthur, both are fine.

Thanks

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

there maybe info on testing coils at Benzworld.org I noticed my coil,that was bad,had green corrosion on the place where wires plug in.But the metal was ruined and you could'nt clean it.Also I buy 99.9 % of my oem parts on ebay. Pelican sells alot of junk stuff now.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Not knowing what the other posts suggest on testing ignition coils, I think it can be very difficult to find a hair crack in the isolating housing.
Electro Technically, they don't go bad or lame or whatever. If the coil was sealed during manufacturing and it stayed that way, the only way you can kill that coil is to overload the secondary. On the secondary the wire is much thinner as it is on the primary e.g. low voltage side. A short spark plug could possibly overload the coil. But, I believe there is a firmware routine to detect that.
Real life, unfortunately nothing is 100% and there are always chances that contaminants, over time, can make it into areas they are unwanted. This in return can shorten the lifespan and maybe noticed in lower performance.
A coil manufacturer does specify the maximum output voltage, among other values, which in general, can range between 18,000 - 45,000 and more Volts. The coils for our cars are probably around 25,000 Volt. Here is the first problem, not everyone has a high voltage tester on hand. If you do and the voltage conforms with the specification of the manufacturer, it is usually safe to say that the coil is good. This excludes any cracks that could cause arching to any nearby metal that is attached to chassis ground or timing issues at high RPM's.
With that said, I believe any DIY testing is merely a guess and can never be 100% conclusive.

To test a coil conclusively one would have to do at least two tests.
1. High voltage output test (would require a pulse generator up to 7000 Hz or more and an Oscilloscope with high voltage probes to see if there are any misses)
2. Isolation test using a Hi Pot Tester (There are some extreme expensive ones on the market)

There are more tests that could be performed, but that is something a manufacturer would consider for QA.

Perhaps, a automotive electric shop maybe in the neighborhood and they maybe able to test the coils.

Here is something else that is overlooked. The Vehicle manufacturer does install ignition coils as a matched set. If one of them fails and needs to be replaced, the replacement may have different test values. Is this bad? possibly....
My point if you think or have confirmed that a coil or all are bad, replace them as a matched set. Bosch does test and match their coils in a set or their tolerances are very close, thats why they are more expensive.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

It's been a while but still have not been able to resolve the ongoing idle stumble most noticeable when in gear and standing at a stop, it is an irregular blip that happens regularly, like skipping a heartbeat. Below is a list of things that were done and as such are being "temporarily" ruled out-of-scope as sources for the stumble, although open to ideas and suggestions.

Things done before I purchased the car:
New EA, (I ran diagnostics and opened it up and it checks out good)
New upper main wiring harness
Newish fuel pressure regulator (I tested for holding vac and no fuel in line)

Things I have done within the past 8 months:
New lower wiring harness and Bat cable
New ECU
New Spark plugs, gapped (Bosch copper, the correct ones)
New spark plug wiring and stubs
New coils (all 3)
Have run a few bottles of Lubro Molly valve and fuel injector cleaner
Cleaned out the EGR lines (were not very dirty or clogged)
Cleaned out and restored purge valve (MOT) yesterday, holds closed now was frozen open before, this helped a bit

Looked for other vac leaks and cant find any, my idle is about 500-550rpm so it does not seem abnormal. I have no CE light or codes on any of the three main DM leads.

Given that it has a regular pattern to the idle stumble I imagine the issue is electrical, possibly on the spark side but also on the fuel end like an injector being switched off momentarily.

Things that I have yet to do, but could possibly be the cause of the problem:
Fuel filter (will change soon)
O2 Sensors, have not replaced and not sure if original
Fuel Injectors are original and have not seen any signs of leakage, they are all tight and I'd be surprised if they have failed or are sticking and would have expected a more sustained running problem as a result rather than a skip.

Any suggestions?
TIA

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

ignition switch and related parts?K38?Rotten vac,and smog hoses?

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I had a hunting idle on my m104 that I could never sort out. The engine recently came out of the car. I found a bad vacuum line near the top of the intake manifold- there are two lines that attach to plastic nipples on the intake manifold. It looked fine till I grabbed it and realized that it was split down the middle. I am almost certain this had to do with my issue. We will know in a couple weeks when it goes back in the car.

http://www.*********.com/forums/showthread.php?t=5082&page=3

post #137 down at the bottom of it show the two vacuum lines on the intake manifold I am talking about

Hi guys, thanks for the suggestions, still unable to find a vac leak, spraying carb cleaner everywhere but no change in rpm, at least the engine is cleaner now

Gerry I also checked the hoses on the intake as you have shown in your other post and they look fine, cleaned them up and there are no signs of failure. Any experience with the plastic intake manifolds? Do they commonly warp or leak as well?

Picked up some MAF/MAS cleaner and will hit that again to be sure. I capped off the purge (MOT) valve at the canister side port and it smoothed out ever so slightly, but the stumble is still there.

Keep the suggestion coming.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

I have had the same exact issue with my w124. It only occurred when stopped and seemed to be more prevalent when the climate control had the ac running. My car recently failed to start and it turns out my OVP had failed. I am not sure if this was causing the intermittent stumbling issue at idle. So far though I haven't had the condition re-occur. I will have to give it a week or two more to see if the stumbling idle condition is gone.

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1999 SL500 87K
1994 E320 112K
1998 Ford Ranger - the daily driver 274K and no major repairs (an indestructible truck)
1978 Pontiac Trans Am 110K
1977 Pontiac Trans Am 130K
1999 SL500 87K (the garage queen, only out in the summer)

I have the same issue, slight beat skip when stopped on idle. Were you able to find the root cause? Thanks.