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1991 560 SEL
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147 Posts
Discussion Starter · #1 ·
First, context before I dive into my question: I have a '91 U.S. (California) spec 560 SEL in solid shape. I'm an avid student of H.D.'s posts (printed out and annotated in my service notebook!) and have a good working understanding of the KE Jetronic system and components. I'm able to do just about all of the core KE test procedures that don't require super specialized tools, i.e., I can do fuel pressure testing; measuring duty cycle, current, and voltage for various components; read codes at the blinky-blinky box (aka X92); etc. I have all the service manuals.

I haven't been able to locate a root cause for the following issue: during the "warming-up" phase, roughly between 55-70 degrees C on the coolant gauge, the car shows significant hesitation during part-throttle acceleration. The car runs beautifully in the preceding and subsequent modes: starts easy, "start-up enrichment" phase is fine and the car drives normally till around 55 degrees, then falls off a cliff during about 55-70 degrees, and then drives right again when it hits 70 degrees (i.e. halfway between the 60c and 80c marks). When it hits that 70c mark, it's like a light switch - full power instantly comes roaring back and all is well. During the problematic 55-70c phase, the car idles normally and starts to roll fine, but then any moderate throttle application is met with crazy hesitation and a bit of surging. If I push through to near WOT during this 55-70c phase, the car wakes up and goes.

So, what's going on here? I haven't been able to find in the service manual a detailed description of the "warming-up" phase specifically, so I'm unclear on what's suspect. The light switch-like change back to normal when the car hits 70c tells me there's something electronic that's malfunctioning or getting a false signal, and whatever that thing is is shut off or enters a different mode at 70c.

I'm confident the following components are good as all have been recently tested: EHA, IACV, 4-pole KE temperature sensor, potentiometer. For fun, I also swapped in a spare ECU that I have and the problem persists. (Both ECUs could potentially have the same issue, I guess.) Fuel pressures are good. I recently did a full KE calibration too - duty cycle / EHA current is good at idle and 2500RPM, potentiometer is set to 0.75V at about 700RPM idle.

My best guess: a partial failure of the EZL ignition unit? I know the EZL gets a temperature signal and will retard timing under certain conditions. And, the difference between the problematic 55-70c and 70c+ phases feels almost like a timing one - super retarded during 55-70c, then normal after. I think I have one spare EZL, although it's from an '88 560 and has a different part number.

I'm open to all input - things I've missed, components I should re-check, thumbs up on the EZL path...really anything. Thanks everyone! I tried to do some searching on this but didn't hit anything specific for this issue. If I missed it, steer me that way...

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Premium Member
1991 560 SEL
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147 Posts
Discussion Starter · #3 ·
Around 70 C system goes to closed loop. That means computer starts correcting air fuel ratio based on O2 sensor output. Have you checked duty cycle at idle and 2400 RPM?
That tracks! Yes, in closed loop, the car drives great and the KE settings are solid. A while back I swapped fuel distributors, so had to do a full calibration. Duty cycle bounces right around 47% and is within 10% of that at 2500. Engine is tight vacuum-wise. So closed loop operation is good.

It's that 55-70c zone where there's a problem, and that's where I don't have a lot of information about what the car is supposed to be doing, and which components are involved.

I'm guessing that the 55-70c phase is the "warming up" phase. The service manual lists 6 phases, but I don't find details on "warming up". (For reference, the phases I see are: start-up enrichment, post-start enrichment, warming-up, lambda control, acceleration enrichment, and full-load enrichment / decel fuel cutoff.) Since cold start and cold-to-55c or so are both normal, I'm guessing it's this in-between "warming up" phase where I have a problem.
 

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1990 420 SEC
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1,073 Posts
Mine had same symptom. Increasing pressure difference by adjusting EHA did help but then pressure difference was out of spec. Same with adjusting lambda screw. Duty cycle at idle was off. You could test adjusting lambda screw clockwise a bit to test if it is a case of lean misfire or ignition problem. As you know where duty cycle is at now you know your way back.
 

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Outstanding Contributor
1991 560SEC (Federal), 1991 420SEL (California)
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2,879 Posts
I'm confident the following components are good as all have been recently tested: EHA, IACV, 4-pole KE temperature sensor, potentiometer.
How did you check the temp. sensor? Did you check it every (let's say) 5 degrees (as the engine's been warming up)? Is the resistance variation according to the chart?
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1991 560 SEL
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147 Posts
Discussion Starter · #8 ·
Thanks for the continuing input, guys!

Duty cycle at idle was off. You could test adjusting lambda screw clockwise a bit to test if it is a case of lean misfire or ignition problem. As you know where duty cycle is at now you know your way back.
That's true, I could use the lambda screw to test whether leaning it out or enriching it (in open loop mode), not necessarily to fix it but to develop more data. I.e., if I turn that screw to the right and the problem is less or goes away, that tells me something. I'm not sure how it would help identify an ignition problem (e.g. if the EZL is super retarding the timing for some reason) - can you clarify? Whatever data I get, I definitely want to find a solution that's not just adjusting the lambda / idle CO screw, since I believe now I actually have everything dialed in correctly for closed loop operation.


How did you check the temp. sensor? Did you check it every (let's say) 5 degrees (as the engine's been warming up)? Is the resistance variation according to the chart?
Yes, testing resistance at the temperature sensor itself against that chart is how I measured. You're right to point out the 5-degree (or similar) increment as the engine warms up - that I have not done, but I can. That said, playing a bit of devil's advocate here - since the preceding mode and the subsequent mode both are fine (and the latter occurs right at 70c), that would indicate to me the temperature signal is doing its thing.


Here's a twist that both of your posts made me think of: it might be interesting for me to wire up the EHA for measuring current (I have a little harness I made) and then measure it from cold start through this problematic phase. Just to know what the EHA is doing during that phase specifically. For example, if the current shows that the EHA is at full-enrich or full-lean during that 55-70c period, then something is telling it to do that for some reason. The difficulty is, I don't know what's supposed to happen during this phase - e.g. if zero current should flow to the EHA...
 

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1989 300SE 247k+ miles
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4,271 Posts
Have you looked at your thermo-vacuum sensor? They make two of them, which open at different temperatures. I installed the wrong one and had the same type of symptoms. Installing the proper one resolved this.
 

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Outstanding Contributor
1991 560SEC (Federal), 1991 420SEL (California)
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That said, playing a bit of devil's advocate here - since the preceding mode and the subsequent mode both are fine (and the latter occurs right at 70c), that would indicate to me the temperature signal is doing its thing.
Well, you can apply that assumption to everything involved in the system; yet something is still not doing its thing.

One other thing you can test for, is the duty cycle as the you drive the car. In between 50-70 degrees the duty cycle should change, trying to adjust for "something" abnormal. After the engine is warmed up, coming back to the same road but with the engine warmed up and driving as much as possible as before, you should see a difference.

The suggestions presented in previous posts are all good, but I am getting the impression that at idle you will not see much; your problem appears to occur as the engine warms up and you drive the car. However, measuring the duty cycle as the engine warms up at idle may not be a bad idea either; you never know. At any rate, I have always been a believer in tests first.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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Here's a very random suggestion for you.

My car had a stumble for a long time right when it was transitioning to closed loop. Quite by accident one day, I discovered either a loose or dirty (can't remember) ground strap connection between the ABS unit and the power steering bracket. Cleaned, tightened, and no more stumble. Weird!
 

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560SEL 560SEC E320 Cab. MB Metris Van ML 320 ML320CDI/gone 300TD 300TE 300SDL, 300D, Unimog 406
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Oooh, I had this exact problem and dumped it off to my MB mechanic who fixed it first time. I don't recall the solution but I'll see if I have it in archives. I recall it was counter intuitive.
 
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