Hesitation at Idle 260E

Hi,
I need help on my 86 260E. When the car is at idle with A/C off, the engine seems to be hesitating, somewhat like a misfire. When driving, its feels normal. When the A/C is turned on and at idle, the car experience slighter hestitation. The car has just recently undergone head and valve job and plugs are new too. Would adjusting plug gaps as posted earlier help or changing ignition coil and plug cables help too?
Thnx in advance for any replys!

Terence, while I don't know your mileage on this car but since you just had head work done, I suspect 90K or more miles. So why don't you try checking the spark plug wires. You can do this with an ohm meter. They should have a resistance of no more than about 3 or 4 K ohms per cable. Frequently the metal ends go bad. They should be replaceable individually. I hope this helps.

Hi Euroman,
Thnx for your reply....
I've got the mechanic to adjust the CO mixture higher and now it's slighter better, though still slight hesistation. He told me my O2 sensor is missing???(guess previous owner removed it for some reasons). Would a missing O2 sensor cause the problem as described earlier?
Thnx for all replys!

[anthonyb]

Do check those wires. Our 300E had the same problem, which was eventually cured by replacing the No. 1 spark plug wire. They're inexpensive to replace, and a good place to start when you're having rough idle problems and everything else seems fine.

anthony

Hi Anthony,
Thnx for your reply, but if one of the plug wires is faulty, wouldn't it be obvious enough to feel the engine hesitating even if throttle is applied? As in my case, the hesistation disappears once the engine hits 'bout 900 rpm.(confirmed by MB mech)
Thnx anyhow!
I need more HELP!!!

[anthonyb]

From a previous post by Euroman:

"Try to isolate whether it is an ignition problem or fuel problem. Are the spark wires relatively new. If they are the originals, the metal connectors can go bad and give a rough idle but the ignition will provide enough spark to go through them at higher rpm. You could pull a plug and see if you can tell which one is discolored. Sometimes you get a bad one or it gets a cracked insulator on installation. If one is darker than the others replace it and see if that corrects the problem. If not check the spark wires with an ohm meter. You should have continuity with resistance of about 2K ohms or less."

The full thread is at: http://www.peachparts.com/ubb/Forum1/HTML/005266.html

I'm not a tech, but I did have a similar problem with a rough idle that smoothed out on acceleration, and a new plug wire fixed the problem, so take it for what it's worth I guess.

good luck,
anthony

Hi Anthonyb,
Thnx again for your reply. Just another question, did you experience less hesitation when A/C is turned on like mine?
Anyway, I'll try to see if i can get the plug wires replaced(all 7 of them)and do u think those electrical contact spray for plugs would work in this instance?
regards!

[EricSilver]

I have an '89 260E and was experiencing the same problem at idle.

What finally solved the problem was switching to 89 octane gas. After the first or second tankful, there was no more shuddering/hesitation. In fact, I often can't tell if the engine is running.

This has appeared a couple of times in this forum, each of us noting the change in idle behavior after doing it. I think this will solve your problem too.

Eric Silver

[anthonyb]

Terence,

I vaguely remember the idle smoothing out very slightly with the AC on, but at the time our AC wasn't working very well, and we almost never tried to use it.

I doubt you'll have to replace all the plug wires, just have them tested per above, and see if any of them are bad/marginal. I don't have any experience with electrical contact spray, so I can't offer any advice on that one.

anthony

[EricSilver]

I neglected to mention that the switch to 89 octance was from 93 octane.

[robi99]

Terence,
Do you know when your fuel filter was replaced last??? If the answer is "no", you might do it. I replaced mine recently and it made a significant improvement in idle smoothness.

Thnx alot to all who replied.
This forum is great for amateurs like me.
I'm gonna fixed the O2 sensor as suggested by my mech and will let you all know 'bout the results.
Anyway, there isn't any 89 octane gas in my country. The lowest I can go is 92 octane and I've tried it but still no results.
Thnx again