My 87 420SEL seems to have the same rpms as my 300SE does. The idle is smooth (except for a hesitation when accelerating from a stop), but the RPMs are pretty close to the rpms of our 300SE. Also, the car also seems to be underpowered compared to our other 420SEL.

Any ideas as to why?

Buy a bottle of RedLine SI1, pour it in the tank, report back in 100miles.

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Gone to the dark side

- Jeff

How would that effect the rpms?

Reading your post, I only understand that there is a hesitation and possibly low power issue. RPMs?

__________________

Gone to the dark side

- Jeff

I'll explain a further. When going 60 mph in this car, the RPMs are around 3000, which is the same as the RPMs in our 300SE. Quite different from our other, properly working 420SEL.

Could the previous owner have changed something out?

Any ideas?

Could there be any chance the transmission may be swapped from a 6 cylinder?

The transmissions are all 1:1 in 4th, so that wouldn't be the answer. It is possible that the differential is different, and that might be original/correct, I don't know.

The original differential ratio in the '87 is 2.47:1. Perhaps somebody will know what the differential ratio should be in the '91 and the 300SE so that we can do the math.

__________________

Gone to the dark side

- Jeff

Is it possible the transmission is not shifting into 4th? Try the good 420 in 3rd gear at 60 mph and see what RPM it is at. To the best of my knowledge, all 420's have the same rearend, but yes, it could have been changed but why would anyone want to do that to such a fine car?
Paul

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84 500 SEL (307,xxx miles)

Hey, just a thought hear maybe the lock up torque converter is not locking up.

It does has a fourth gear. If it's in fourth and downshifted, the RPMs go up to about 3500.

How can you check it to see if it's not working right?

They don't have a locking torque converter.


As for 300RPM at 60, that sounds a bit high......unless the W126 + M103 has a different differential ratio than a W124 does.

The 300SE will have less power than a 420 at comparable speeds/situations...its a much smaller motor.....

Like babymog said, running some Redline SI-1 through a M103 can really boost its performance and clean out the injectors.

I've done that, and have been running V-Power gas lately....and it seems to love that. (instead of the redline stuff) I'd start off with a cleaning dose of redline though.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

It sounds like either your tranny is slipping, or your RPM gauge is off.

Do you have any tools that will allow you to verify that your RPM is actually what is being shown?

I think there is an adjustment/calibration potentiometer in the RPM display, if I recall correctly. Might be off......


Check your trans fluid. If there is dark stuff in the fluid, then you might be slipping. If so, your tranny isn't long for this world.



-tp

The RPM gauge may be off, but we don't have any tools to verify it. Would this tool be available to rent from Autozone?

We've never changed the trans fluid on this car yet (only had it for 6 months). It most likely needs changing. Hopefully, it's not that bad of a problem if it is the transmission.