91 300e slight idle problem - PeachParts Mercedes-Benz Forum

sptt · #1 09-15-2009, 12:56 AM

Experts...I just acquired a 91 300e 3.0 in great shape. I've replaced the fan and fan clutch due to being frozen and the fan being chipped (if anyone needs to know how, let me know...did it in 10 minutes). I also had a fairly rough idle and read on here to replace plugs and O2 sensor which definitely improved idle consistency and stopped pulling me forward at stoplights. Only problem I still have is a slightly rough idle in drive while at stop (around 575 rpm and need 600-650 rpm). Ordered new OVP relay and still waiting for it to arrive. Car shifts great (although I don't see the modulator adjustment per documentation and discussions on my trasmission for hard 1st shift on the driver side adjustment underneath), everything else seems to be as good as can be for an old gal. Oh, also, after I replaced O2 sensor, my MPG almost doubled (20-24 mpg now and used 95 or earlier Ford Mustang O2 Sensor from Kragen). I see I do not have an idle control valve adjustment as it is computer controlled. Will the OVP possibly resolve the ide or what else should I check? Thanks guys! Awesome forum so far!

Rick Hall · #2 09-15-2009, 05:40 PM

I have owned one of these since 7/01 and 17miles. Now have 189k on it. I can only offer up my experiences as I have thought at times that I had a rough idle. First and foremost, it will NEVER idle REAL SMOOTH. I think all you can do is get it to idle AS SMOOTH AS POSSIBLE. To that end, you don't say how many miles are on the car, I would most definitely check for vacuum leaks. Also, on the passenger side, behind the washer fluid bottle are two solenoids that control vacuum, one of which is the vacuum advance. There are check valves in line with each of these solenoids. After 18 years,I checked this visually and found I had a cracked check valve, replaced it and it idled SMOOTHER. Replaced BOTH solenoids and all associated hose fittings and VOILA....a "smooth" idle. You might also want to take a look at your distributor cap and rotor and spark plug wires. I can almost guarantee that new Bosch COPPER plugs (H9DCO), distributor cap, BERU wires, and a fuel filter will also help tremendously. I have used Bosch wires but they don't last as long as I have found the Beru's to last. Also, if you have high miles and original fuel injectors, seals and o-rings, then they too can be suspect. Once you get all of this done you will be surprised at how smooth and powerful this thing can be. Good luck

sptt · #3 09-15-2009, 09:07 PM

Thanks Rick! I have 170K on the car. Today I noticed that the idle is perfect, even when in drive, until the temp gauge gets to just under the 80 mark. Then it only idles a little rough when engaged in gear at a stop leading me to believe it is possibly a temperature related issue maybe? Idles exactly the same with or without AC on. Put it in park or neutral and voila, perfect idle! I will check for those vacuum solenoids like you said. Haven't checked dist cap or rotor yet but sure it will help if new or cleaned up. The idle is totally liveable but I know it's capable of better due to the good "cold idle".
Steve

Ferdman · #4 09-16-2009, 06:36 AM

sptt, distributor cap and rotor should be replaced every 60,000 miles. Unless you have service records for this 1991 300E definitely check the condition of the cap and rotor. If the ignition wires appear old and brittle replace them too, and opt for Beru wires. As I'm sure you are aware that vintage MB automobile will run for hundreds of thousands of miles if maintained properly.

sptt · #5 09-16-2009, 12:51 PM

Thanks so much! The plug wires look great. Will check cap and rotor next day off. Good to hear that these cars run forever if maintained well. This thing is very solid. I also like the fact that you can fix many things yourself saving thousands. I've already saved over 2k from what mechanics quoted me to repair certain issues that I was able to do myself off this forum!

sptt · #6 09-20-2009, 10:30 PM

FYI....I replaced the voltage regulator today in my alternator and she now idles perfectly in drive and reverse, at a stop, even when very warm or hot. The brushes on my old regulator were so worn down I'm suprised they made any contact at all!!! Also, it was sooooo easy to get at it only took my about two minutes to get the old one out and to put the new one in. Best wishes!

bsmuwk · #7 09-20-2009, 10:51 PM

How did you manage to replace the fan clutch and fan in 10 minutes?

pawoSD · #8 09-21-2009, 12:20 AM

Quote:

Originally Posted by Rick Hall

I have owned one of these since 7/01 and 17miles. Now have 189k on it. I can only offer up my experiences as I have thought at times that I had a rough idle. First and foremost, it will NEVER idle REAL SMOOTH. I think all you can do is get it to idle AS SMOOTH AS POSSIBLE. To that end, you don't say how many miles are on the car, I would most definitely check for vacuum leaks. Also, on the passenger side, behind the washer fluid bottle are two solenoids that control vacuum, one of which is the vacuum advance. There are check valves in line with each of these solenoids. After 18 years,I checked this visually and found I had a cracked check valve, replaced it and it idled SMOOTHER. Replaced BOTH solenoids and all associated hose fittings and VOILA....a "smooth" idle. You might also want to take a look at your distributor cap and rotor and spark plug wires. I can almost guarantee that new Bosch COPPER plugs (H9DCO), distributor cap, BERU wires, and a fuel filter will also help tremendously. I have used Bosch wires but they don't last as long as I have found the Beru's to last. Also, if you have high miles and original fuel injectors, seals and o-rings, then they too can be suspect. Once you get all of this done you will be surprised at how smooth and powerful this thing can be. Good luck

A proper running and tuned M103 can idle so smooth you can't even tell its running besides looking at the tach. That is how mine idles. Maybe yours needs some more servicing.

Do you really think MB would have sold a gasser that vibrated at idle?

You might need new engine mounts too, that can make it have more vibration than normal.

sptt · #9 09-21-2009, 11:57 AM

I was thinking I do need new motor mounts but when it idles in park or neutral it is smooth. When it goes into drive at a stop, it drops down about 50 to 75 RPMs and struggles a bit. I'm usre there may be one or two more things that will correct this stah I still need to do.

As far as the Fan and Clutch taking ten minutes....removing the fan shroud took less than two minutes. Then, I just put my hex bolt and wrench into the front of the clutch, secured the pulley with a makeshift bar tool from the back into the pulley holes, cranked her off and put the new fan and clutch (my old fan was cracked) right back in the same way. Fan shroud back on in a snap....done in ten minutes...literally! Let me know if you have any questions I can help you with.

Bob G · #10 09-22-2009, 01:47 AM

specking of replacing so tune up items. I can not get the spark plug ends to loosen on the distributor cap on my 1992 300-E. I have 39,000 miles and have noticed a bit of a rough idle. Is there a special way to loosened the spark plug ends off the distributor so I can replace the cap and rotor.
My engine is an M 103 6 cylinder.

Sincerely
Bob Geco

sptt · #11 09-23-2009, 09:32 PM

I just checked on my car and the wires pop right off the distributor cap with a pull. Not sure why yours won't come off...

Ferdman · #12 09-24-2009, 06:46 AM

Bob, turn the boots slightly before attempting to pull them off. Before reinstalling the spark plug wires on the distributor cap terminals apply a light coating of dielectric grease inside the boots ... same for the spark plug boots.

optimusprime · #13 09-16-2017, 04:52 AM

Pull both rubber hoses off the valve cover , and then clean out the holes inside the valve cover .One is larger than the other , and the hole in the small one is very small so dont damage it . I had this problem , just as i was sitting on the stop lights it was just as if it was going to die on me ,, but it never did. And i only cured it after removing the valve cover and gave it a good clean out inside . I can see now its a late post ,,but it could help other members in the future ..