'86 420 - Intermittent Bogging / Throttle Response

Hi all, I've used this group to find tons of answers before, here is a first post after searching for some time.
1986 420 SEL
About 4 of the last 10 times I have started the car, the following has
occurred: After about 1 or 2 miles of driving the car begins to bog down. Pressing the accelerator down will cause the engine to struggle/bog down, then finally catch up to the throttle level I am giving it. The efficiency gauge goes all the way into the red until teh engine/car catches up to it. Giving it VERY gradual throttle is the only way to go from a stop smoothly, though it takes forever to get up to speed.
The real strange thing is why it happens only a few times while driving. Other times, it runs strong.
Other: - Injector nozzles replaced @ 60K ago - fuel filter changed - premium fuel (2 or 3 tanks used now) - Not driven below 1/3 tank of fuel
Thanks in advance on any thoughts you may have...
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The symptoms suggest a very lean air / fuel ratio - that's why it stumbles and why you must "feather" the throttle to accelerate. You've already done the usual remedials so the question becomes: Why does it run too lean after warm up?
I'd suggest the problem, being intermittent, is electrical: Air Flow Metering system or the O2 sensor is a possibility.
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Thank you for the response. I was thinking along the same lines, a car only needs three things to run, spark, fuel, and air. I thought the fuel part was ok in that the economy gauge (vacuum based i think) would run into the red meaning it had the fuel. I also thought the electrical was ok because usuinally the RPM gauge would indicate that when idling (move erratically), so then I went to the air. Air clearner is perfect.
If it is just the O2 sensor that would be great. Given that it is intermittent, that may be the thing if it is going. It has happened more in the past few times driving it than before.
Thanks again, let me know if you get any more thoughts on this.
Shannon
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wrote:

Keep in mind that the vacuum (economy) gauge doesn't give any indication of whether or not the engine is getting enough fuel. Whenever you open the throttle, the airflow through the throttle body will increase and the vacuum will decrease (causing the gauge indicator to move towards the red), whether or not there's any fuel in the mixture at all.
I agree that it sounds like you're not getting enough fuel. If it were persistent, I'd look at bad gas or clogged injectors, but since it's intermittent, you're probably looking at a computer control/feedback problem. The O2 sensor sounds like the perfect place to start.
--
Scott Gardner

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the economy gauge (vacuum based i think) would run into the red meaning it had the fuel
I respectively disagree with that conclusion. The "economy gauge" is simply a vacuum gauge; the manifold's vacuum falls as one opens the throttle (thus the "red").
The problem here seems to be that the fuel part of that equation is absent - thus the stumbling.
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On Mon, 19 Sep 2005 20:10:54 -0700, "T.G. Lambach"

vacuum gauge; the manifold's vacuum falls as one opens the throttle (thus the "red").

Yep, you can test the operation of the vacuum gauge by getting up to highway speeds and killing the engine by turning off the ignition (don't lock the steering wheel). Turn the ignition back on, but don't start the car. At this point, the fuel injectors shouldn't be working, but the "economy" gauge will still respond to throttle movements.
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Thank you all for your help. I will post here when I get the car into service.
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Hi, I'd like to suggest an ignition problem, when was the distributor cap/arm last replaced and whats the condition of the leads like? I had a similar problem with another type of vehicle a while back. I dont think it would be the air mass meter as it controls the fuel mechanically (if its the same as my '85)
Ben.
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Hi Ben, probably similar to my 89 300e. I just recently learned to put the car into '2' and then shift manually up to 'D'. You see in '2' the car will in fact start in first..... as a bonus you get to do a little shfiting. Of course it needs a little practice to get a smooth transition between gears. I get mine to actually peel rubber :-) in both first and second ..... well it IS fun sometimes.
cheers, guenter
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