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M103 low idle speed, vacuum issues causing a misfire?
This is my first gas vehicle in over 10 years. It is a 1988 300E in very nice shape, body wise and interior wise. Yet it failed emissions so was put up for sale. Older lady owned it and put maybe 200 miles on it a year.. for the last few years.. The tank has been drained and refilled with fresh gas, and a injector cleaner.
I've done a tune up, consisting of a cap, rotor, plugs, wires, IAC valve hoses, and tested the other small hoses connected to the intake manifold. Runs ok at speed, but stoplights are not much fun. HOT idle is about 600RPM in neutral. Vacuum is about 13 inches, and at the moment the car is at 5600 feet above sea level. Headed out to pull the Over Voltage Protection Relay, and test for vacuum leaks ... as I am telling myself that low vacuum is part of the problem.. (I have all ready ruled out the brake booster, and 5 vacuum ports on the intake manifold) Testing the fuel injectors next. Guessing there is no idle speed setting on this motor.. and that the low idle speed is ether vacuum related, or something else is out of whack. I am open to suggestions, on this particular issue, as well as general instruction on how to come up to speed on the CIS fuel injection system/engine management... |
The most common cause of a rough idle in a gasser is a vacuum leak somewhere, but you really didn't say anything about being rough. In a fuel injected car, I believe that it can cause a low, but not necessarily rough, idle. Don't forget all of the vacuum under the dash, maybe you can pinch off the supply to the dash and see if it helps. good luck...Mark
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Oops... yes it has a somewhat rough idle. it does not usually stall, but acts as if it would... I had the vacuum disconnected to the cabin and did not find any improvement.
Just followed along here PeachPartsWiki: Isolating Vacuum Leaks That is a really neat way to find vacuum leaks... I doubt I would ever have found it any other way!!! And found the big funnel shaped rubber, or square gasket at the bottom of the (what is the throttle plate assembly called by Mercedes?) funnel where it connects to the intake manifold runners.... That is gonna be a bit of a job getting it all apart.... anyway when squirting some ether there, it takes a moment then the O2 sensor jumps from about .15 =>.9 and sometimes over 1 volt.. Does that sound like I have found the culprit? After lunch I will re test.. Considering that I am now likely taking the whole fuel distribution assembly off, are there other things I should do to while I am in there? |
Drat! Still have low vacuum, and a low idle. The results I posted earlier were a false positive, as squirting some ether on the intake funnel did nothing to EGR readings when the air cleaner housing was on the throttle body....
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Blue may be temp sensor is faulty .Telling computer its hot, and in doing so its weakning out the mixture.Making it run weak. Also it may be fuel related .So a fuel pressure test is needed .Low revs at the lights could be down to bad fuel filter .Have you dhecked the idle control valve .It sits under the filter box .Remove it to look .It as 2 hoses going off from it , these can split ,and a plug on the rear .Remove it from the engine and clean it out with white spirits.Dont sink it in the liquid as it holds electrical items.This can change the tick over if gummed up inside..But if your sure its vacuum then its teasting time ,dont forget the vacuum hose in the front lights.
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Blue ,This is the part to look for . Its the one with sadle over it and 2 bolts to hold it in there. Just right of the dip stick.Ican see you have done the hose .Did you clean it out inside? What plugs have you in there must be resistor less
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IAC valve is clean. Will test it today by unplugging and noticing the change in idle speed. Also toying with the idea of applying 12 volts momentarily to see if there is a "click" VACUUM to the headlights? I can only find vacuum ports going to the transmission, the vacuum gage, door locks and one to the ignition system. I am really toying with the idea of changing the intake manifold boot, and gasket... I do not want to... and I would really enjoy having an engine that gets the best fuel economy, and shifts smoothly. |
FWIW, I had a similar issue with my 82 R107 that I chased for months until I did a smoke test and found 7 large cracks in the Air Chamber which was letting way too much air into the system and telling the analog CPU to add more fuel. The part was cheap (under $90) but was about 4.5 hours to tear down and replace. This was after replacing the OVP, O2 sensor, oil temp sending unit, and temp sending unit, and a few other un-needed part replacements.
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Yes, a vacuum leak is a problem... It may not be my only problem, but it won't be right until that is fixed at the least...
Where that vacuum leak is coming from, I do not know yet.. It would be nice to be able to rent a smoke machine.. |
I thought long and hard about purchasing a smoke machine which runs about $900 to $1,000. If you can get in on a trailer most shops will charge $100.00 for the test. much cheaper than buying one...
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Once the vacuum leak is resolved, then I can focus on what is left.. if there is any thing left to be resolved. |
Cheap smoke machine: 1 cigar. Blow in smoke through one of the lines and see where the leak is.
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Of course it will. It's doing essentially the same thing. https://youtu.be/zMok2y05jNE
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It would really be helpful if you can post the emission test report. Various values give a good clue as to what the problem(s) may be. Also search for emission research posts started by me, which include easy ways to lower M103 emissions like shorting the R16/1 resistor socket, disablinlg the vacuum advance, and verifing that the installed spark plugs are non-resistor type.
My 190E 2.6 has a five-speed manual and normal idle in Neutral is 700 @ 15". An automatic idling is drive will show lower speed and vacuum, but I don't have any data as to what is correct. These engines have a pretty aggressive camshaft with a fair amount of overlap compared to most of that era, so idle may not be perfectly smooth and may contribute to relatively high HC emisssions. It's actually likely that the problem(s)may be minor, so don't start tearing everything apart until you research M103 emission test issues as recommended above. Duke |
You might be chasing your tail. Have someone hold 700 rpm while you check for vacuum. Low vacuum is a symptom of low idle, if you have 18 ish inches of vac then your system is ok.
Buy a bottle of redline fuel system cleaner and try that with an Italian tune up. If you already did the ignition tune up with quality parts, and the vacuum system holds ok, the next thing to change the fuel injectors. |
Update for anyone looking for answers in the future. In this particular case, injector holder "O rings" have improved vacuum while driving, I ordered the wrong injector seals, so the old ones are back in at the moment. I suspect that new injector seals will solve the rest of the vacuum issue.
I also replaced what I call the throttle body boot. I doubt that had any real effect, so my advice to those that happen upon this thread, would be to do the injector seals and O rings first, as well as the IAC valve hoses. I did search for the vacuum leak with ether, while having a multi meter hooked to the O2 sensor, My results did not show any change with ether, but it could have been errors on my part. Also guessing that some of the injectors are leaking down when the engine is shut off, as it sputters a bit upon a cold start. Replacing the injectors when the new injector seals arrive, and I expect the car to start and run well after. |
Update: Dirty injectors were the cause of the misfire...
Injector O rings solved a lot of the vacuum leak, but not all of it. I ordered the wrong injector seals so had to pull the injectors twice. Now that the injectors have the correct seals, (rather than the old hard ones) my vacuum leak has increased back to where it was.. drat! Do the white nylon (mine are yellow now) injector holders shrink or warp in a way that can be a source of vacuum loss? |
Please let us know of progress .
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Pull off the temperature sensor conecting plug .And try starting the car from cold with out it .
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No the temp sensor is in the same circuit as the ovp and cold start system .If temp sensor is faulty it can send wrong message to the ecu, so it weakens the mixture .It can work both ways if stuck , it can send a rich mixture .But if you sure its vacuum , them i would just keep looking till you find it .It can be any place .You have a vacuum holder under front fender next the charcoal filtter ...
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Blue have you got a vacuum reading yet .Is it low ,?if so what was the reading.
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Thinking about this .As you said the car failed on emissions .Now, did anyone play with the adjuster for mixture settings? .If you say its a vacuum problem ,then you must have some reason to say this . Did the engine ever run right ? thats after you had it. Do you have a vacuum reading.
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This was taken from this site we are on.
A lean condition can be traced back to a cracked or broken or torn boot in the system Take off the oil filler cap as the engine is running .If engine is in good shape it will drop engine speed and run rough.. No change is a sure sign of vacuum leak . |
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I did a vacuum test with ether, and found NO trace of a vacuum leak, still I tell myself their has to be one somewhere, based upon the dash gage. Also there is an occasional misfire at idle, and I am assuming it is a lean misfire. I replaced the cap-rotor-wires-plugs 200 miles ago. Actually when running with out the OVP relay in while looking for the vacuum leak with ether, the engine ran pretty well. I assume it was running rich, but I did not notice a misfire...? I will go looking for that vacuum canister on the front fender. Oh I also looked at the injector duty cycle, and made a small adjustment. That did help the vacuum reading somewhat while driving. I am assuming the previous mechanic made adjustments to correct for other issues. Still need to do more reading about the duty cycle, to understand what is going on. I was attempting to get it in the 44-49% range. I need to focus on aspects of an up and coming trip, so this post is incomplete and a bit rushed. |
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I will check vacuum at 650 RPM. |
Vacuum leak at the "throttle butterfly" bushing..
I'm not sure what it is called by Mercedes. I assume it is not rebuildable, but needs replaced?
AND smoke coming from the intake manifold Rubber boot. Obviously did not tighten that enough when I replaced it? Gonna pull both to re-seal the boot, and investigate the throttle butterfly. Is there anything I should be attending to "while I am there"? |
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I'm not saying that it's impossible that you have a leak, but your low numbers might be a symptom of your low idle. |
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I got out the multi meter and took some reading from x11. First readings were 4.2-4.7 Volts =70-65% if my math is correct, and suggesting a lean mix. I got it to the point where it is 5.8-6.2V, 12.5" vacuum, and 700RPM My question is: at 2500RPM the mixture goes to 8.4-9.5V 39-30%. Well outside the 10% window. Does that mean that my EHA is faulty? Is adjusting it an option? |
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Hey, (OP) what ever happened?
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John i dont know .But the economy pointer should read virtualy nothing on tick over and bang over to the left . Check vacuum pipe on the gearbox for a leak . I would eliminate all vacuum and to do this block it off at the source as it comes from ewngine and try again.
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I was just reading this thread as I am chasing the same problem on a 1990 300SEL with 313,000 miles.
I noticed that blue85 mentioned an improvement in how the car ran when he had the OVP disconnected, but that was not discussed further. Mine exhibits the same behavior. Slightly low rough idle but when I pull the fuse on the OVP the idle is much stronger and dead smooth. I think I can rule out ignition and fuel system and focus on sensor input. I'd appreciate anyone's thoughts on that theory. Thanks, Kurt |
Sounds like you probably have a bad or failing OVP/OVR relay... replace it now if it hasn't been replaced semi recently. They go bad very regularly it seems, especially the aftermarket ones. The cause is corrosion inside the relay due to shoddy weather sealing of the unit (moisture ingress). I've seen it more times than I can remember.
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I should have mentioned that I had recently replaced the OVP with a Mercedes part. The old one had and bad fuse holder. I did run some Seafoam in the tank and that definitely helped. I only have 60K miles on this set of injectors but the fuel distributor has never been touched since the car was new.
I also tested the throttle switches and both coolant and air temp sensors. The only anomaly was the coolant temp sensor was out of tolerance on the hot side (low resistance) so I ordered a new one. The air temp sensor was just barely within acceptable tolerances but it was reading on the hot side as well so I decided to install a new one. If any of that helps I'll pass on my results. Thanks for the feedback. Kurt |
Vacuum Fittings Manufacturer
Vacuum Fittings Manufacturer
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This morning I changed out the 4-pin coolant temp sensor. When I got the old one out I discovered that the connector housing was split all the way down to the metal part of the sensor and was loose. The plastic was also very brittle. After installing the new sensor the car ran great although the idle is still a little low between 550 and 600 in gear but it is a lot smoother.
I am learning that a lot of little things can contribute to CIS cars idling rough. So far things I have done that have contributed to a nice idle include: Repairing leaks in rubber hoses and tubing Replacing rubber fittings and plastic check valves Achieving maximum pressure difference between upper and lower chambers with EHA adjustment Adjusting the duty cycle to average about 40% Replacing old sensors that are on the edge or just our of tolerance It's a process but very rewarding when you get it right. |
I did tell you about this in post 22 . But i still think if your econo guage is not sitting right over to the left or very close to it that you still have vacuum problem some place .You will get there i can tell .Good luck Motorthead .
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Optimus,
Looking back you sure did! And after rereading some of your other posts I will pay close attention to the economy gauge when I start the car. I also got my new intake air temp sensor so I will install it tomorrow. Have you ever tested the altitude correction capsule? I haven't looked at mine yet but did postulate that, if this sensor told the ECU the altitude was high, there would be another correction factor. I live close to sea level so I would expect to see that reflected in the sensor. |
Worth checking if you can find it . I no nothing about this sensor so you can teach me on this one .I will keep looking in on this .
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I've given up on trying to get the car to run perfect... Have put 50K on it since I was asking all these questions, and never really figured out what the issue was. some of it was due to the engine not being driven. As I put 1500 miles on it the first summer, things got better. Not great, but better. That being said there were times when it would idle pretty well and display excellent vacuum on the economy gauge.. That said, it typically gets 20-25MPG depending upon your right foot.. Most of the time it seems like it has a misfire at idle. Hot or cold.. I'm still driving it, and it has been absolutely reliable... |
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I was doing some research yesterday about the altitude correction capsule. When I looked it up in the Mercedes parts system it says the part is only valid through 1989 for the 300SEL. Now I'm wondering where the altitude correction factor comes from for the CIS ECU. A little more digging and I found several comprehensive fault code listings for CIS vehicles. For my year, fault code 8 says altitude correction factor from EZL is illogical. So I looked at the EZL housing and there does appear to be some type of breather cap on it. It would be great to get some confirmation but I believe that the ACC was incorporated into the EZL on later CIS models. Hope that helps, Motorhead |
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