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Old 04-23-2000, 11:05 PM
Posts: n/a
I have a 1986 300E with 165,000 miles on it. The vehicle runs wonderfully with one exception: at idle only, the engine seems to have a hesitation to it. The idle does not, however, fluctuate, at any time.

The car has done this since I bought it at 150,000 miles a year ago. I put new spark plugs, a new air filter, and a new set of fuel injectors in the car when I bought it to try to solve the problem. I also cleaned the slide valve. None of these things helped. The cap and rotor and spark plug wires are all in good condition.

I put new spark plugs in again at 160,000 miles. There was a little bit of oil on the plugs put in at 150,000 miles, and the car was using about 1 qt of oil every 2000 miles. The guides did not have any play at all when I put the new seals in, but the old seals were somewhat dry. The car's oil consumption has now dropped to almost nothing.

Because the head gasket is BONE dry all the way around, and because the valve guides on the car had no play when I installed the new seals, my assumption is that the car had a valve job previously.

But the car still had a slight hesitation at idle...

My wife has driven the car for the last month. I had not driven it for almost a whole month, and I drove it yesterday. The idle problem was not present! I came to find out that she has been running 87 and 89 octane gas in the car (she didn't know that the car should have 91 or above).

Could there be a correlation between running 87-89 octane gas in the car and the newfound smoothness at idle?

All I know is that my car is now running better and I am afraid to switch back to 91 octane!

Please advise.

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Old 04-23-2000, 11:39 PM
MB Shop Retiree
Join Date: Sep 1999
Location: Hockessin, DE
Posts: 1,047
I know people who use to have that problem when they filled up with bad gas. Try another brand of premium. I use 94 ultra and love it.

P.S. where in West Chester are you, we use to live in Chadds Ford, PA and now Live in DE.

Chirag (Charlie) Patel

- 99 Mercedes-Benx ML 430 V8 w/custom TV, Video, UHF, speaker system
- 93 190E 2.6 (4-sale) w/ an amp, equalizer, and CD-changer, integrated w. stock stereo for integrated cellular phone speaker system
- had a 93 190E m103 3.2, 5-speed trans,
- had a 88 300E 2.6
- had a 84 300TD w/275k+ miles
- 2001 C-Class - Maybe (if 190E sells)
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Old 04-23-2000, 11:57 PM
Posts: n/a

I actually live in Exton, but my office is in West Chester. Chadds Ford is a nice area.

I don't think this is as simple as bad gas. Over the last month, the car has been filled up with at the same two Mobil stations to which I took it during my tenure with the car. The only difference is that it is now being run on 87-89 octane instead of 91 or above. It is now consitently running smoothly at idle, while before, when running on a diet of 91 or above octane, it was consitently hesitating at idle.

I am essentially questioning WHY the accidental use of LOWER octane gas appears to have produced a good side-effect, so that I can duplicate this "side-effect" without putting lower octane gasoline in the car than the manufactuerer recommends.

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Old 04-24-2000, 04:42 PM
Registered User
Join Date: Jan 2000
Location: New Bedford, MA USA
Posts: 1,583
Ed, the most likely explanation for your situation is that something was causing a problem and now it's not. Probably a partially plugged injector which has cleared itself. Probably doesn't have much to do with the gas. But, if you want to do some research, go back to the gas you were using before and see if the problem comes back, (doubt that it will).

Jeff L
1987 300e
1989 300e
1987 BMW 325
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Old 04-25-2000, 02:10 AM
chupr98's Avatar
Registered User
Join Date: Mar 2000
Location: Vancouver BC
Posts: 134
Hi edbardzik, seems like you have exactly what I am experiencing with my 300E as well, I am getting lots of help from this site, if you are interested, the title is 'how to clean carboned up oil rings' and is active. I have spent last 6 months and lots of $$$ on this slight annoyance. I will tell you if I get lucky and find the cause in the near future

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Old 04-25-2000, 09:58 PM
Registered User
Join Date: Jul 1999
Location: Youngstown, OH
Posts: 150
Here's a little info on octane that may help. Simply put, lower octane gas is more volatile than higher octane gas. The octane number gives you a 'guesstimate' of the relative number of gas molecules that are 8 carbon molecules long; thus the name octane. The 8-chain carbon burn more evenly and more predictably. Other length chain are more volatile and can ignite early or burn faster causing a preignation which is heard as a knock or ping. An experianced mechanic could probably run with this and give you a few reasons why this helps your situation.
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Old 04-26-2000, 12:05 AM
Join Date: Jan 2000
Location: Indpls.,IN. USA
Posts: 84
It sounds to me like you may have a bad plug connector. These can go bad on any Mercedes gas engine from 50K miles on. The easy way to find these is on a scope. Other than that you can carefully read plug color and often find the culprit, I find this particularly useful on multi-coil ignitions that are difficult to connect to a scope. Other than that a vacuum leak could be a problem but I can't see it going away. If it returns pour water over your running engine an look for a reaction.
Good Luck with the hunt
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Old 06-03-2000, 06:47 PM
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Join Date: Mar 2000
Location: Fairfax, VA
Posts: 1,210
I should have checked before posting this same topic elsewhere, but too late.

I have an 89 260E and started using 89 octane about a month ago. Presto: the shudder at idle is essentially gone.

In the past year, I have replaced my ignition coil, overvoltage and fuel pump relays (not for the rough idling though) but still had the idling problems. Switching to a lower octane to save money seems to have had a very happy, and unexpected, side affect.

Eric Silver
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