How to systematically troubleshoot rough idle
 

1992 190e 2.6 M103

I'll try to keep this post balanced .. too long, people gloss over the details, too short people ask for details.

Trying to troubleshoot a rough idle. Stutter is hardly noticeable in park or neutral, stutters excessively in drive or reverse. Car starts fine cold (albeit now with the temps really low it requires some effort) and idles fine cold. It's when it warms up that the problems arise. Car drives fine once RPM is up so good on the highway, cruising, whatever. Accelerates consistently on the highway, does not bog.

Things I've done in my 2 years of ownership:

- All ignition components including non-resistor copper core plugs (have not replaced coil however)
- O2 Sensor
- Fuel filter
- Air filter
- Coolant Temp Sensor
- OVP
- I've removed the ICV and watched it click open and closed with a 9V battery so I assume it works (just a couple of days ago)
- I have sprayed carb cleaner all over the damn place underneath the airbox and I couldn't really register any change in idle. I did not spray the EGR and now I'm thinking I should have, would that be a good test?

And probably a bunch of stuff I'm forgetting. The rough idle is a stutter, not consistent rough idling and really only when warm. Lately it's progressed to stalling when putting it into gear. Idles fine in park or neutral, idles like crap in drive or reverse, sometimes stalls when shifting into D or R, sometimes stalls at lights.

If my EGR was stuck open I've read that would be a massive vacuum leak. I keep reading references to vacuum testing the motor but I don't know where to plug a vacuum gauge (I have one). My car does not have an economy gauge. I have drawn a vacuum on the hose going up from the EGR (to the check valves or whatever along the firewall) and either there is a big leak or no vacuum is supposed to be present there. What does a vacuum leak here indicate?

IF my EGR was stuck open, would it affect the cold idle as well?

What vacuum port can I use and where is it, to test my engine vacuum?

My fuel economy has always been crappy on this car.
Compression is 180-185 across all cylinders.
I have read numerous threads on rough idle and there is always a lot of conjecture and "throw parts at it" suggestions and that's not what I'm trying to do, I would like to know how to go about troubleshooting this in a systematic way by eliminating variables.

There is a faint exhaust smell in the cabin when stopped at lights, is this a clue?

Thanks

I guess it's possible there could be a small leak in either the EGR itself or the tube that foes from it and any associated connections, that may cause your exhaust smell in the cabin when stopped, if theres no leak in your exhaust system. I have no idea what the vacuum value is supposed to be there.

How's the fuel injectors? If they're old they might not being spraying like they used to...

And of course check the condition of all the rubber bits, such as the hoses to/from the ICV, the boot between the air meter and the throttle body, and the other little elbows/hard lines. There's tons of stuff that could play a part in a rough idle.

Take your car for a drive tonight, then pull in to your driveway and look. Make sure it's as dark as it can be. I chased a problem like this for a long time before I noticed arcing between the wires and the cylinder head. Ironically, I. Was dumping washer fluid in when I saw the spark.

Generally speaking, when it is a consistent stumble, it is fuel related. When the miss is completely random, it is usually spark related.

Keep in mind that 80's vintage cars need new spark plugs regularly.

Thanks. Yes, I actually did this. Drove into my garage, closed the door behind me and turned off all lights .. it was pitch black and I didn't see anything. In the two years I've owned it I've only driven 16k miles and I must have changed the plugs, wires, distributor and rotor about 10 to 12k miles ago, if not less. I had the plugs out about six weeks ago when I did the compression test and they were a light tan. My duty cycle is 50%.

That is useful information. What are the characteristics of a vacuum leak?

Also, is there any significance to the "rough in gear, smoother in park/neutral" observation?

I had a similar rough in gear and smooth in neutral and it was solved with new injectors.

If you plan on keeping the car long term, I would go to auto zoo, and buy a mity vac for $30 or so bucks. You should read a minimum of 13 hg, 15hg being ideal. Make sure everything is off (climate control).

A cheap test to see if your injectors are clogged is to buy a bottle of redline fuel additive. It can be found on amazon. If you notice a significant difference, (positive) then I would do fuel filter, then new injectors. Injectors are pricey....

Thanks for the reply. I do have a mityvac I just don't know where on the engine to hook it up! Simple, I know, but I have to ask because I couldn't figure it out.

I don't think they sell redline here in Canada. Will seafoam accomplish the same thing?

No, in my opinion seafoam will make matters worse. You cant order it online?

with the air cleaner removed, there is a small vacuum line towards the rear. that one controls the climate control, and the economy gauge. Once you plug it in, and fire it up, it should give you an instant reading. 13-15hg.

The more I think about it, maybe you can try seafoam. If (perhaps when) it makes your car run worse, then you would know certainly it is fuel related, and most likely injectors.

I would still try and buy redline. That stuff is the best chit I ever used.

It's pretty cold up here these days so my laziness at properly troubleshooting the vacuum lines hasn't entirely been laziness, it's damn cold out in the garage! And I can't idle the car in the garage anyway. So I've made a proper effort now that I know where to hook up the mityvac, thanks Lorainfurniture.

The EGR does not hold vacuum, so I plugged it off.
I found that one of the lines going to the HVAC was not holding vacuum either. I thought, surely plugging two vacuum leaks would fix it. Sadly no, problems persist. I took some video.

The first part is just idling and shifting the transmission around to demonstrate the dropped idle and the stutter, which is hardly noticeable in the video but very pronounced in person. The oil pressure gauge is probably the thing to watch here. Second part is me flooring it from a slow crawl, major stutter doesn't want to go, threatens stalling. In fact, it stalled on me backing out of the driveway. I can floor it on the highway and it downshifts like it should and does not stutter or bog, acceleration seems normal. Third video is the vacuum state as I mess with the throttle. You can hear the engine bog and stutter much better here. I rev it slower to show that it performs normally when throttle is applied slowly. It's blurry, sorry, but it's at about 16/17 HG which I perceive to be within reasonable operating range for these motors based on what I've read around here. The engine was as warmed up as it's been getting lately (have another thread about engine temp, it's running cool).

http://www.youtube.com/watch?v=IANlAFwoEt8

So, what do I try next?
Again, all ignition components have been replaced with the exception of the coil. I don't know how to test a coil and they aren't exactly cheap so not sure how to eliminate that variable. Otherwise, the car has 195k on it and my guess is original injectors, but no idea really. Would a bad fuel pump cause this?

Pics are of the blocked off vacuum lines. The EGR is disconnected. Anyone have any idea what the red vacuum line that I disconnected is?

http://www.morejoy.ca/stuff/190e/IMG_3215.JPG

http://www.morejoy.ca/stuff/190e/IMG_3214.jpg

Your vacuum is excellent. Your YouTube video is private? Can't see it.

My bad. Should work now.

I would try the redline. If you truly can't get it, let me know and I'll try and ship it to you. If I had a farm, I'd probably bet it on clogged injectors.

The theory is if you could imagine your injectors peeing instead of spraying. At idle, the combustion chamber starts filling up with fuel bc it's just not combusting well. When you gun it, you almost flood it, but once you burn out all the extra fuel it's like turbo boost and you have full power. Until the next red light.

Sound about like your symptoms?

Yeah, about like. Also terrible fuel economy!!! But the fuel economy has been terrible since I have owned the car. I might as well be driving one of the 8 cylinder SELs!!!

Bad MPG definitely unlike an m103. I get 20+ in mixed New York driving. And thats even with all original fuel components at 235K

I own a 230 E -> basically your engine with 2 less cylinders, and no EGR, and I have some problems with idle aswell. Have you looked at the crankcase ventilation? The rubber gets very loose with age, and when I bought a new one and compared it to the old hose, there was a huge difference in diameter.

I know this sounds a bit like "throw parts at it", but you can examine them beforehand. There is no sure way of telling if they need replacement, but if they feel really soft you should consider it atleast. You might however run into problems when ordering parts, even here in germany the EPC does not list all the hoses, the only way to get the part number is comparing it to the old one.

I'll update if I end up replacing the rest of the hoses in the near future.

I threw a bottle of FI cleaner into the gas tank, full bottle to half a tank (directions say full tank, so kind of a stronger dose). I thought I noticed immediate improvement but it wasn't so, the stutter remained.

Last night I replaced my plug wires (bosch, installed a year to a year and a half ago) with the old Bremi wires that were on the car when I got them. I tested the resistance across all of the wires, the Bosch wires were roughly all about 1.3k and the Bremi was roughly .95 to 1k! I'm using the proper H9DCO non-resistor copper core plugs and I put the Bremi wires back on. Taking this a step at a time so I know what the problem was once I get this fixed.

All of the plugs looked good, a few had some buildup likely from burning some oil but nothing drastic. Number six however, was wet and smelled of fuel. What could be causing that? Bad spark? Bad injector?

If the stutter at idle doesn't go away I'll be inspecting the distributor and rotor next, again, installed when the plugs/wires were installed roughly a year to year and a half ago.

Any ideas about the wet plug? I'll also note that it showed carbonization right up past the threads onto the body of the plug! It was in there tight however so I'm not sure how that happened. Again, any ideas or suggestions much welcome. I might just replace all the plugs but finding the correct H9DC0s was hard enough the last time.

Most certainly a bad plug or wire termination ... Replace that plug alone - if necessary, get any fit replacement one, just to see if it solves the problem. Clean the inside of the wire connector to remove carbon. If this solves the problem, shop for the correct spec replacement plugs.

Steve

The problem with most other fi cleaners is that they are solvent based cleaners ie, petroleum based.

Redline is synthetic, thus does a much better job than any other product off the shelf.

You have to ask yourself: if I had an ignition problem, wouldn't the "miss" happen past 2k rpm too?

If you can't order the redline from Canada, let me know and I'll send it to you. You I have clogged injectors, period. If the redline does not work, then they need replacement.

Following that logic wouldn't clogged injectors affect my performance past 2k as well?

If you look at the tip of an injector, it resembles an old school brass water hose. If you can imagine it spraying at low volume(idle), it would just kind of pee out. Without it spraying out proper, you wont get proper combustion. When not clogged, it will spray with a conical spray pattern,(think windex spray bottle) when clogged, it squirts like a squirt gun.

As you increase pressure, (wot) the hose will start to spray like a mist or something along that line. The result is proper atomization of fuel= no random misfire.

Im willing to put my money where my mouth is. Ill send you 2 bottles of redline, ($10e) or ill send you a set of clean injectors. (new are $30e)

If it does not work, I won't ask for any money. If it does, I will ask for the cost of the redline, + shipping. The injectors I value at $20 as well.

Nobody else on here is as sure about your problem as I am. I have spent thousands chasing the same stupid problem, just to solve it with new injectors. I will warn, you may have to make a small adjustment to your duty cycle, however, my car cleaned itself up after a tank or two.

A wet plug means fuel is getting there and never burning. No matter the injector spray pattern, with a proper spark and compression, the mixture will fire - and thus no wet plug. So, you have either no compression - which you appear to have - or no spark. With the plug insulator surface carbonized, there is a lower resistance to ground for the high voltage than the spark plug tip, and it is OUTSIDE the combustion chamber. Possibilities are a cracked insulator, broken plug center conductor, a broken plugwire termination, or insulator contamination. Once carbon tracks have formed, you will not be able to clean the plug satisfactorily. Replace that plug, clean the plugwire end, and see what happens.

I have had injector, plug, AND plugwire failures over the years in our M102, and no matter what injector issues you may have, a wet plug is an ignition issue.

Steve

I've had a slightly rough idle since I got the car at 62k miles. Now 129k and never have figured it out. All ignition components, all fuel components minus pumps including rebuilt and flow tested fuel distributor. Perfect compression perfect leakdown. All vacuum hoses replaced. All engine + trans mounts....

I just gave up and feel like its just part of the CIS-E system's imperfections. Drives fine soon as on the gas and I get 18-20mpg average.

Checked the distributor cap and rotor yesterday, it appeared as though there might have been a bit more wear/corrosion on the number six wire .. I lightly sanded all the contacts in the cap and the brush on the rotor and put all back together. Driving to work today, engine got up to temp .. stop and go traffic .. smoooooooth idle! But, this could just be a fluke so I'll put the celebrations off for now and put some more miles down before calling it good. There is still an ever so slight stutter, but it's basically how the car has always run, certainly nothing like how it was. That slight stutter is hardly noticable and when it's absent you can't even tell the engine is running, this in drive too!

might seem like an obvious question.are you running 91 octane?

Yes. Only the best for the benz.

Ok, drive home stutter returned. boooooooooooooourns

Still, I didn't replace the distributor, I only cleaned it up. The stutter is definitely not nearly as bad as it was so i think I'm on the right track.

And thank you Lorainfurniture for the very generous offer but I wouldn't feel right it. I do believe that injectors would improve the car's performance and likely the fuel economy, which is really poor right now but I do think there's an ignition issue causing the stutter.

What I don't understand is why it runs good cold, but not hot. Does the resistance change in the cap or rotor? Tolerance? It's not an open/closed loop thing because the stutter is there when coolant temp is below 80 on a restart but not on a cold started engine. Warm start kind of situation where the coolant temp is not 80 but not cold, stutter is there.

Hey smp

Ok, I finally remembered my password, been since 2008. :-)

Watched your vid, that is not a stumble, damn she falls on her face. Just some ideas since we are systematically troubleshooting. :-) I read through what you have done so far, actually you're ahead the game, good going.

1. EGR - yup if it won't hold a Vacuum it's done. Don't know if cleaning would do any good. You can also pull the vac line, depress and then hold you finger over the vac line fitting and the diaphragm should hold open. But, the EGR only is working when fully warm. Disclaimer, I own a 2.3 and thing are the same for a 2.6 but the way they get there can be very different. I see yours attaches to a box on the firewall where as mine is fed over the valve cover and up front to a Vac/Temp valve. When cold, the Vac/temp valve is closed anyway. Test is to rev UP and quickly pull the vac line and you should hear the EGR slam shut, takes a few times to be sure. But for now, you have blocked it off.

2. 50% Duty Cycle - is that fluctuating around 50% or locked. She must fluctuate once full warm. Locked at 50 is an O2 fault. I know you replaced it, might have a bad connection??

3. Vac - 13-15 is low with a real vac gauge on a running engine. I can't relate to a static vac using a pump. Can you get a reading with the mity vac with the engine running, or it's only a pump. I don't see how a pump could even work, you have a great big leak at the Air Flow sensor so I must be misunderstanding how you are using it.
14-17 and a steady Needle implies a Timing Issue. Good is 17-22 Hg on a healthy engine and your compression numbers look real good.
Cool link on how to use a Vac gauge, old school stuff before OB2 code readers.
Intake Manifold Vacuum Tests


4. I don't think just one injector could cause this, meaning your #6. This is a major air/fuel issue.
Here is an interesting link almost exactly how you are going about it. He found the air flow boot under the Air Flow sensor bad. You can see how he was chasing just as you are. Good possibility. I dont' know what MB he has. The manual pics are really bad. You can see the part at ********az 2.6 fuel/air Air boot.
Hesitation, KE-Jetronic problem? - Page 2 - MBClub UK - Bringing together Mercedes Enthusiasts

I think I would start with a smoke test. $1 cigar and 2-3 feet of vac hose that will fit on the fitting you found on the intake manifold. Block off the intake, mmmm tape or saran wrap/rubber band or a surgical rubber glove. block off the hose/ valve cover to air cleaner.

You can build a smoke tester from a garden hose sprayer but a cigar works very well. Take 4,5 or 6 blows, don't have to blow hard just give it some time.

Next plan we already discussed the importance of a smooth air flow sensor, must be clean. I am thinking IF not an air leak then the air flow sensor and it's relationship with the potentiometer and even the EHA.

Seems by the vid your ICV is not functioning 100% either but it might just be this Air Boot and the ICV can't keep up.

A lot of ignition issues can change as the engine heats. I would still replace that bad plug - you have a spark leakage issue there. The contamination may be getting more conductive as the plug heats. Also, the ignition coil tends to weaken with age, and heat makes it worse. Worthwhile replacing if the plug alone doesn't solve it.

Steve

Sorry, no way one bad plug/wire/injector would cause the main issue of no acceleration. It's a air/fuel issue and not getting instant fuel enrichment. It starts OK, the idle doesn't seem all that bad at this point nor does is sound that bad on slow increase in RPM. If it's bad then of course change it but it won't help much.

IF you had a well running 6, you could pull a plug wire off , now a dead cylinder and it would rev up better than the video.

1. For a "to the floor" response. You also need to check the Throttle Switch function of full closed and WOT. And the Decel Switch wouldn't hurt either.
2. I would still Smoke test for VAC leaks.

3. Sticking Air Flow Sensor and then
a. The potentiometer or Air flow Position sensor-
b. The fuel distributor "Plunger" could be leaking or sticking. Not so easy save for last.
4 th item that controls acceleration is the fuel pressure regulator.

Although a single plug misfire is unlikely to cause the full range of the poster's issues, it is a contributing and obvious problem that should be addressed before pursuing subtler problems.

As far as things mentioned above, the sensor pot DOES affect acceleration enrichment, but it usually wears out in its default position - idle. The symptom is almost always erratic idle. Also the fuel pressure regulator holds a constant pressure - if performance at high speed is ok, then the regulator, fuel pump, fuel filter are all unlikely culprits.

Steve

Thanks all and thanks very much Logan_Bob. I've been sidetracked with the purchase of a 95 s124 so the 190e hasn't been getting any attention for a while.

RE: pulling vac on the intake manifold. The mity-vac is a pressure gauge .. I'm not using it to pull vacuum, I'm just using it as a gauge, not sure how to describe it exactly but if you hook up the business end whatever vacuum is being drawn registers on the guage. Am I making sense? I'm not using it to pump any vacuum, that's for static lines to test for leaks, I'm just checking vacuum pressure.

I have new hoses for the ICV which I plan to install at some point. When I do I will just dismantle the whole thing up there and check on that boot. It'll make the installation of the hoses easier and allow me to poke around. Stay tuned.

Follow up for anyone coming here from a search:

Injectors.
Just did them tonight and it made all the difference. Once that was complete I installed new plugs and adjusted the duty cycle. Car is awesome now, it's all sorted. Love my car.

So it wasn't the fuel distributor, EHA or anything else .. in my case it was the injectors. Now that the car is running smoothly again it's worth fixing the timing cover leak, that's next.

I called it.

Congrats on getting your car running well. Knowing you found the problem is a sweet victory. Your mpg will increase as well.

Two cents added to an old thread
 

In case anyone comes across this thread, another thing to consider is replacing the rubber seals around the injectors and the o-rings around the injector sleeves. This made all the difference in the world to my car's idle and performance. I know smp stated early in the thread that he had done the carb spray test around the engine at idle and found no change when spraying, and this would likely have turned up an issue with the seals, but I'm still curious to know if he also replaced the above mentioned seals and o-rings in the process.

If so, I wonder how much of his improvement was due to injector spray pattern and how much was to fixing air leaks. In my case I tested all six injectors into glass bottles and ended up replacing the weakest one with another used one that tested about on par with the remaining five. However, the difference in spray was not so great between the 'new' one and the rest of them, and I strongly believe that it was replacing the rock hard rubber bits on all six injectors that did the trick, not the minor improvement of one injector. I suppose we'll never know how badly smp's old injectors test out, but it sure would be nice to find out.