Last few cars I had that ran like ass at full throttle (shaking/vibrating/bucking) had ignition issues. Cracked/corroded caps, worn rotors, wrong plugs, excess variance in the wireset, burned wire ends, etc. I would think that if the car is missing at idle the vac will not be steady as the cylinder will not be cycling properly.

Vac leaks throw off the AF mix as the air is unmetered. My experience with older cars is HVAC problems and poor fuel economy. If the leaks are large enough difficult starting and hesitation at low RPM. Newer stuff throws a CEL almost always. Inspect all the rubber connectors by hand even if they look okay. I had nearly 1" loss due to a three way rubber 'Y' connector at the firewall leaking... it basically fell off when I touched it!

Just have to remember, low mile/rarely driven cars tend to demand a stricter maintenance schedule and that city driving (stop/go) is harder on parts as they wear even at low speeds (think hours versus miles traveled).

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Thanks again. I appreciate it.

I'll start looking at the vacuum connections. New ignition parts are on their way. I already have some new Beru spark plug connectors, so I might swap those now. The plugs have only a few thousand miles on them, but I'll look at them again. They're the Bosch H9DC0 non-resistor plugs. When I first installed them they really made a difference.

It must be addressed before chasing ignition problems. If you have a vac leak no amount of changing ignition components will make the car run well.

I am a believer in repairing/changing one thing at a time. If you replace your ignition components and fix your vac leak at the same time, you will not know what caused the problem.

Look for the right angle fitting at the intake manifold between the engine and the firewall. That fitting is hard to see and hard to access when you find it. It resides in an area of high temperature and may also be coated with oil. It is usually the first to go because of that but is also the most overlooked because it is so hard to see.

Please let us know your progress so we can add to the data base.

I have driven cars with vac leaks that do not cutout at full throttle.

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Thank you for your response. I will take a look at that tomorrow. I will remove the air cleaner assembly and take a good look around. I did follow a number of hoses today, checking them and the fittings, but found no problems. I'll certainly report back anything I find as I know these threads are helpful for future reference.

Well, I've finally gotten around to changing the ignition parts with the help of another very generous local Peachparts forum member. Today we installed new Beru wires, a new Bosch distributor cap, a new Bosch rotor and a new Bosch suppressor cover under the rotor. We also installed genuine Mercedes inner and outer distributor cap covers. And while we were in there we changed the valve cover gasket and timing cover gasket.

I had previously changed the oxygen sensor with a new genuine Bosch unit that is correct for my vehicle, not a one size fits all unit. I also changed the spark plugs 3k miles ago. I use Bosch H9DC0 copper core plugs.

After the oxygen sensor change I was hoping the sputtering upon hard acceleration would be eliminated. It wasn't.

After all of the ignition parts were changed I was hoping the sputtering upon hard acceleration would be eliminated. It wasn't.

Now, please understand that I'm not just "throwing" parts at the problem in hopes that it will be eliminated. I have changed all of those parts because I really wanted to. I just wanted new parts as I believe they had never been replaced on my vehicle. I purchased it last year with just 62k original one owner miles on the clock.

I didn't change the ignition coil, but have one on its way. I had just forgotten to order one. That's just another item that I want new.

I also have a new fuel filter that I will be installing next week. I need to get some of the rubber lines also as they're pretty brittle.

I'll report back after I change the ignition coil and fuel filter. Of course I'll test drive the car after installing just the coil to see if that was the problem. If that doesn't solve it then I'll report back after the fuel filter is changed.

In the meantime, any thoughts are appreciated.

Thank you in advance.

I would check the throttle position switch on the throttle body. It's pretty simple on the M103 cars. But it basically indicates to the ECU whether you are at idle or WOT. If it's stuck in the idle position, that could be causing your issues.

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http://i193.photobucket.com/albums/z...-RESIZED-1.jpg
1991 300E - 212K and rising fast...

Thanks.
I'll take a look at it tomorrow.

I installed a new Bosch # 00086 ignition coil today and it certainly seems to have eliminated the sputtering and backfiring upon hard acceleration. I tested the car for probably 20 miles today after the coil was installed. I drove it HARD and was unable to get it to sputter or backfire. It really responds to the demand for power now.

I will post an update hereon in a few days. I want to be absolutely sure the problem has been solved with the new coil before stating the same.

After the coil replacement what is the new position of the economy gauge needle ? Pegged to the left or a little off ?
regards

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mark

I'll have to take a look later and report back. I didn't think to check the economy gauge after the coil install. Are the two related? After having read so many threads I was thinking that I had two unrelated issues.

Thank you.

Well, I went back and re-read my original post which started this thread. I started this thread because I had noticed the economy gauge in my 1990 300SE had moved from normally being pegged to the left during idle in park to being about 1/8" to 3/16" from the left under the same circumstances.

I had read a number of posts regarding the economy gauge and vacuum loss, so when the gauge changed from its normal position of pegged left during idle in park I figured something was not right. I wanted to get ideas of what I should look for in order to resolve the vacuum problem. So, I posted the bit about the sputtering upon hard acceleration hoping that would lead to a quicker resolution of the problem. At the time I thought the sputtering may have been related to the vacuum loss.

It turns out the sputtering was absolutely due to a bad ignition coil. After the coil change I have been unable to reproduce the sputtering and backfiring upon hard acceleration. There is no longer any sputtering or backfiring from my engine.

I was able to isolate the problem and definitely state that it was the ignition coil which caused the sputtering because I tested the vehicle after nearly each new component was installed. I say nearly because I had installed new plug wires, a distributor rotor, distributor cap and inner and outer distributor cap covers and tested the car after all of those items were installed, not after each of them. That would have been some work. The sputtering and backfiring upon hard acceleration remained after those items were installed.

So, the last ignition related item that I installed was the ignition coil. As I stated previously, that took care of the sputtering and backfiring problem.

I installed a new fuel filter two days later because it's on my list of items that I want to change. It made no discernable change in performance. I really hadn't expected it to.

Now I need to track down that vacuum leak which precipitated this thread some 6 months ago.