W140 (1994 S420) Engine Issue - I'm Stumped

[LWB250]

I have a good idle, but the car has no power taking off from a stop. Regardless of throttle position, the engine will barely speed up under load and the car does move forward, but quite slowly. No missing, noises, etc., just no power.

One you get the car up to about 20-30 mph, it goes forward without a problem and seems to run just fine. You can drive it at freeway speeds without an issue. Slow down to a stop or below that 20-30 mph threshold and it acts like it has no power.

I'm satisfied that everything in the way of the MAF, ETA, plug wires, distributor caps/rotors are OK. The problem seems like there is no change in engine timing regardless of throttle position.

I'm getting a CEL all the time, yet the DAS shows random codes, such as knock sensors or ignition coils. I attribute these to occurrences at low speeds with the throttle at or near wide open. I can clear them without a problem and they'll come back again after I drive the car. Again, the codes are random and not at all repeatable. However, even if I clear them the CEL is on as soon as you restart the car. Strange.

So my question is, what exactly controls engine (spark) timing? I assume it's the EZL. I'm getting vacuum there, so I know that's right. I'm wondering if the camshaft or crankshaft sensor or pickup on the engine that is sending it information about the engine position that it might not be seeing.

I'm not in "limp home" mode that I can tell, as there is no limit on speed or RPM.

I'm about to have the car taken to a local shop for diagnosis, as I've pretty much hit the wall on this. Anything the collective wisdom of the group has to add would be deeply appreciated.

[Rick76]

Is the engine wiring harness the original or has it been changed?
If original, it will be bad.

[LWB250]

Wiring harnesses were changed years ago by the dealer. They're in good shape.

Thanks!

Dan

[nulu]

If you have 2 fuel pumps 1 may have failed causing a restriction to second pump, or dist caps may be carbon tracking = shorting under load, or crank sensor may have shorted due to coolant or water , last ezl ignition may be bad only firing 4 cylinders not all 8 , check test and report back

[LWB250]

One fuel pump, pressure is good at manifold during operation.

Distributor caps are relatively new (less than 10k) and have been inspected and cleaned.

Crank sensor was replaced about four months ago. I'm not sure what you mean by it having "shorted due to coolant or water" since it's a sealed unit and located on the flywheel housing away from any coolant or water.

Engine idles smoothly and takes off without knocking or roughness, just no power.

Thanks,

Dan

[LWB250]

I would add that this condition started after the car had been parked outside in a damp area (next to a large open body of water) for about 10 days, during which we had about 10-15 inches of rain, too.

Dan

[oldsinner111]

hopefully it was a Hella crank sensor.Bosch are not good ones.

[LWB250]

I think it was a Bosch.

That being the case, why am I not getting any codes thrown if it's acting up?

Thank you,

Dan

[nulu]

if its your airmass meter try unplugging it it should then be in fixed operating mode 50% setting ie computer will set to 50 percent mode and of course set code , but if it runs better at low speed may be that

[LWB250]

Thanks! I'll add that to the list.

Because it seems like an ignition timing issue, I'm going to disconnect the camshaft timing magnets and see if it runs the same. Since they don't kick in until engine speed increases, it would be interesting to see if it has any effect. If not I know that they aren't getting a signal.

Does that make sense?

Dan

[oldsinner111]

one time I had a power issue that lasted a year.It was my crank sensor plug,not fully plugged in.

[LWB250]

Yeah, I've already gone through everything to make sure it is connected and clean. That was my first thing to do, especially since this cropped up after the car being exposed to a really high humidity environment.

Dan

[Ferdman]

LWB250, when were the Oxygen sensors last replaced? I have had a sluggish acceleration problem with an inline 6-cylinder engine resolved by replacing the Oxygen sensor. Fortunately that engine has only 1 Oxygen sensor. Your engine has 2 Oxygen sensors because of the dual exhaust. The recommended replacement interval is 100,000 miles.

[TX76513]

Originally Posted by oldsinner111

one time I had a power issue that lasted a year.It was my crank sensor plug,not fully plugged in.

Yes the ones that plug in like an "RCA" audio plug are bad about coming unseated (and I guess that's why they changed to a snap plug).

[Frank Reiner]

LWB:

In the event that the car is equipped with ASR, consider that if the throttle controller has a fault that mimics a slippery road condition, or is receiving a signal from the transmission selector that indicates "winter", the throttle opening will be restricted until vehicle speed is above the threshold that you have indicated.