300e engine "stumbling"

Hello,
My 89 300e 3.0 with ~130k miles on it just started what I call stumbling--not stalling or dying--just missing a beat or two after the engine has warmed up and although it's most noticeable at idle I suspect
it's happening at other times too. It's not just misfiring though. It feels like its not getting any spark to all the plugs for maybe half a second every 5 to 10 seconds and it has definitely gotten worse in the last 10 days.
Maybe water in the gas and I'm going to dump some drier in with the next tank to see if that helps and am also considering a new coil but I thought I'd put it to the group first to see if anyone had any ideas about what else to check. I have new plugs and wires, distributor cap and rotor, one of two fuel pumps and new fuel filter, and also a new EHA valve so I think I can safely eliminate those as culprits.
Any suggestions about what to check next would be greatly appreciated.
Best regards,
Mike
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Given the new ignition parts I'd suspect a fuel related problem: either a small vacuum leak or a dirty fuel injector.
A vacuum leak allows unmetered air to leak into the intake manifold and dilute the air / fuel ratio causing a stumble. Suggest you check the vacuum hoses attached to the intake manifold, one may be cracked or broken or disconnected.
A dirty fuel injector won't emit a clear stream of fuel. The "dirt" is usually deposits on the face of the nozzle and these often can be cleared by an additive to the fuel like Techron etc. Fuel injectors don't have to be replaced, they can be removed and sent to a shop for ultrasonic cleaning, flow testing etc. for about $20 each - if it comes to that.
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Thanks TG.
I'm not familiar with the fuel injection at all but it seems that rather than the problem being with one injector, the whole system would have to be failing to cause what feels like none of the cylinders firing at once, sort of. Perhaps I'm being overly sensitive and what I'm just feeling is one or two cylinders misfiring.
I'll check the vacuum hoses as you suggest though.
What do you think of adding injector cleaner to the fuel? Should the injectors be cleaned or replaced periodically anyway? I don't know the previous owner but based on other problems I've had with the car there seems to be a lot of deferred maintenance.
Mike

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ONE dirty injector can cause the stumble you describe. Add some Techron etc. to the fuel and drive it. If, after two weeks, that doesn't improve it consider removing the fuel injectors for professional cleaning. There are numerous shops that clean injectors, a Google search for "fuel injector cleaning" will find one in your area.
Always do the simple things first.
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I know the fuel injection system which is the KE Jetronic and you have a M103 engine.What I don't know is if you have a pre-pump before your fuel pump.If you do,the delivery rate should be about a pint in 12 seconds.A failing pre-pump causes stumbling and hesitation.
If no pre-pump,perhaps the plate on airflow sensor is binding.TG suggestions of "secondary air could also be the culprit.Check the vacumm hose that goes to system pressure regulator,and the rubber boot that connects the airflow sensor to the intake.
Ultrasonic injector cleaning is about as much as new injectors and IMHO,new is better!If you do a volume test,they should not differ from one another more than 10%.Fuel pump should deliver 0.5 gpm if you do not have a fuel injection pressure gauge.
You also stated you replaced ignition parts,did you by chance mix and match the cap and rotor?Is there carbon tracking on the inside of cap?Did you gap plugs accordingly?Are the plugs single electrode platinum type?These suck in Bosch fuel injection systems and create air-fuel errors.Sorry so many questions,but need a good idea so you don't confuse a ignition problem with a fuel problem,plus it's cheaper for you this way!!
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Hello Cap,
Thanks for all your comments.
As I just mentioned in my reply to TG I'm going to give the injector cleaner a try and hope that fixes the problem and then I'll start on the areas that you pointed out.
You mentioned the pre-pump--I'm assuming that's just "the other" of the two fuel pumps located with the filter and near the tank. Not sure if there's yet another hiding somewhere that I'm not familiar with. I only replaced one of them and then only because it was making so much racket I could hear it as I was driving along and worried it might let me down.
I didn't check the output so I couldn't say if I was getting what I should. That'll be pretty easy to check to I'll look at that next along with the hoses and boots you mentioned.
I think I'm good on the cap and rotor. There was a little carbon tracking in the old cap but not that bad. Two of my plugs were almost complete fouled and one reeked of gas to it probably wasn't firing at all. The new plugs are single electrode Bosch and I check them and they were all gapped correctly at .032". I don't know if they're platinum or not--I just got what they offered for my engine at Autohaus. Are the multiple electrode plugs remarkably better and if so which do you recommend.
Thanks again,
Mike

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mharness wrote:

there.
Not sure if there's

The new

have the Bosch Supers.
--I just got

I have always been happy with the Supers! I did ONCE buy the Bosch platinum +4,but didn't notice any real difference i.e. increase in fuel economy,power etc.....Always use Bosch spark plugs IMHO!

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mharness wrote:

Oh,missed that! Pre-pump very suspect!!1 pint in 12 seconds and if not,replace.
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