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Discussion Starter #1 (Edited)
My 98 420 has run perfectly until today and I'd appreciate any input on what may be the issue. Upon accelerating onto the interstate it hesitated and stumbled a bit but it seemed to straighten itself out for the 15 mile trip and no problems while on the highway. After exiting the freeway and accelerating up to about 45 it again stumbled and was very sluggish, had to give it a lot of pedal to keep going. When I stopped it nearly shut off but caught and rev'd up to about 1500 and then settled down to a normal idle. When I started out it didn't shift like it has before, stayed in first gear until about 2500rpm and then shifted but not at all smooth as it has always been. Thinking this may be an issue with bad gas I put 2 bottles of HEET in and let it sit for about an hour. Drove it again and the problem occured again but there seems to be NO regularity with it and when I step into it it runs like a scared rabbit through the gears. It seems to hit after I go over some bumps that slosh the tank around. 3/4 tank of fuel. Always use Premium of course. Has me baffled, any ideas? Car is in excellent condition, all service intervals met, I baby the car and keep everything up to snuff, etc. Has 166000 miles but normally runs like new. Appreciate any input and thanks in advance.
 

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I'd suggest cruising by autozone, they will read codes for free and that may help narrow things down. I have a few suspicions, but it's always best to start with codes instead of guesses. :)

Take care and enjoy the ride,
Greg
 

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I had the same problem and just fixed it today, although mine would hesistate on/off and the engine light would turn on only when it hesitates. I ran a diagnostics and it didn't show anything,rt I assume because it was not cutting off all the time. I replaced the spark plugs, but was still having the same issue. Then checked the ignition coils and spark plug cables (1 by 1) and determined I had to change one of the ignition coils...works like a charm. Seems the ignition coil was going bad and was intermitantly cutting off.
I tried the following steps:
1-Open the engine compartment to reach the spark plugs (Easier than you think if you haven't done it before).
2-While the car is running, unplug the ignition coil cable (with out removing the coil itself), if the car hesitates the you know the problem is not resulting from the coil/cable/spark plugs(either of them). Keep repeating the same step with all 4 coils. Once you unplug the coil that has no effect you know that the problem is with in the area. Then see if it is the coil, cable or sparkplug...hope this helps.
 

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Discussion Starter #5
Thanks to all who replied, really appreciate it. Took the car to my indie merc mechanic and checked everything he knew, no codes at all and apparently this car has 4 places to read codes? Fuel filter is new (I replaced 6 mo. ago) but he pulled it and checked anyway, pulled all 4 cats and mufflers and looked to see if anything loose there that could clog exh and all looked good, replaced mass air, no change so he put the old one back on - no charge thankfully as this is a $500 part, all coils are fine, plugs are 1 year old - I did that as preventative a year ago. Mech told me he was baffled and all he knew for me to do was take it to the stealer which he hated to recommend and I hate to do for obvious reasons. He had the car for two full days and is devastated he can't fix it. I picked the car up (no charge at all) and it hasn't acted up yet, still wondering if it may be bad fuel, the place I gassed up just prior to the issue surfacing is a Sunoco station but the pump ran VERY slow taking me about 20 minutess to pump 25 gallons of gas. Have heard that when pumps run slow like this it may be that the water separator on the tank is full or faulty. Anyway to drain these tanks? I'll look this evening and maybe there's a plug down there or something. What an adventure! Love the car though so we'll learn something more when we get it repaired and I'll advise with a post.
 

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Discussion Starter #6
Hey all, the computer finally threw a code and it was the MAF sensor. So shook the money tree and it's running like new again after replacing the original 167,000 mile sensor.
 

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Glad to hear it's fixed and thanks for following up with us!

Take care and enjoy the ride,
Greg
 
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