94 E500 low idle speed

[e500.org]

Hello all - my 94 E500 has always had a low idle showing on the speedo. I think the correct speed should be more like 600-700rpm, I’m at under 500rpm. I’ve had some light misfiring recently and it’s got me thinking about this. Would anyone be able to shed light on what controls idle speed etc? Many thanks.

 

[a777fan]

Was that pic taken in P/N or D?

 

[e500.org]

Was that pic taken in P/N or D?

Park

 

[gsxr]

When at operating temp, idle rpm should be 650 in P/N and 500-550 in gear. Try neutral and see what happens.

Also check the gear position registered by the E-GAS module in live data. It it doesn't match every gear exactly, every time, the NSS is either failing or requires adjustment.

Also, the tach can read slightly off, check that vs live data as well.

 

[e500.org]

Thanks Dave. I need to get it on STAR to check this. In the meantime my E-GAS module is dated 2003, it could still be faulty though. I have an old unit to try and see if it makes any difference.

I am pretty sure idle is low due to the misfire.

 

[gsxr]

I've never heard of a faulty E-GAS module *only* causing a low idle. It's almost certainly due to the gear position data. Try putting the shifter in Neutral and see if RPM's go up to 650.

Misfiring will also cause a low idle, but after the misfire clears up, the idle speed will always return to normal. Misfiring doesn't cause a permanent low-idle condition.

 

[e500.org]

Idle same in all gears

 

[gsxr]

Still check the live data for each gear position and confirm it is accurate. I'm also assuming there are no fault codes. AND, verify the actual RPM via live data, or with a separate digital tachometer; the analog dash tach can be off by 100+rpm.

If it's still not right the only possibilities are ETA or E-GAS. Has the ETA been replaced / rebuilt, and if so, how many miles are on it?

 

[e500.org]

Idle same in all gears

Still check the live data for each gear position and confirm it is accurate. I'm also assuming there are no fault codes. AND, verify the actual RPM via live data, or with a separate digital tachometer; the analog dash tach can be off by 100+rpm.

If it's still not right the only possibilities are ETA or E-GAS. Has the ETA been replaced / rebuilt, and if so, how many miles are on it?

In my ownership at least I didn’t touch the ETA. I do have a spare one though and it would not hurt to get that rebuilt. Alternatively I may just purchase a freshly rebuilt one already, I seem to recall a company in US that does these for c $500? If anyone has the details please share.

I will get the codes read and revert.

 

[anderzen]

I'd bet on NSS.. from experimenting first hand a bunch with it during manual conversion, the lever is very touchy and if slightly off due to worn linkage bushings or slowly failing it can register incorrectly even though it's in "gear"

 

[gsxr]

Don Roden at RFC Electronics does the good rebuilds. Search the forum for his contact info.

 

[e500.org]

Seems NSS is NLA from the dealership...does anyone else have this issue? My MB specialist does not accept parts from outside and they can't order from MB...so they're proposing a used part (which incidentally does not make sense to me). I can see some on eBay etc but Dubai MB dealer says they cannot order it, no longer produced.

 

[gsxr]

Seems NSS is NLA from the dealership...does anyone else have this issue? My MB specialist does not accept parts from outside and they can't order from MB...so they're proposing a used part (which incidentally does not make sense to me). I can see some on eBay etc but Dubai MB dealer says they cannot order it, no longer produced.

Yes, I think it is NLA from MB, but it is still available aftermarket as OEM. Search the forum, and tell your MB specialist to accept new parts "from the outside". Seriously, they suggested USED for the NSS?

 

[e500.org]

Ok I just ordered a OEM one from the US, thanks all

 

[036]

300 kph, very nice.

 

[e500.org]

Revisiting this again as it’s still not sorted (not that I’ve done anything!)

Ok so the mechanic said it may not be the NSS as the reverse lights worked with ignition off in gear N and the car would not start in any gear other than D or N, he said that should confirm a good NSS…

Any other signs of a bad NSS that I should be looking for? My idle is still low and I suffer rough idle because of this!

I also think my AC condenser has finally had it but will post more about that on my owner thread.

 

[gsxr]

Ok so the mechanic said it may not be the NSS as the reverse lights worked with ignition off in gear N and the car would not start in any gear other than D or N, he said that should confirm a good NSS…

This is not a valid test.

The M119 LH-SFI NSS has two completely separate functions. One is for the starter lockout and reverse lights. The other, using a separate set of pins, is a gear position indicators. Your mechanic tested the first function and it's good. But he did not test the second function. This requires SDS with HHT-Win to view live data from the E-GAS module in each gear position. See post #4 above. Normally this will store a fault code on the E-GAS module as well, but it may be possible to have the problem without a stored code.

Any other signs of a bad NSS that I should be looking for? My idle is still low and I suffer rough idle because of this!

See above for the full test, checking both gear position in live data AND also the engine RPM because the tach may not be accurate either. ALSO, the NSS must be adjusted properly. A good NSS won't work right if not adjusted per FSM procedure.

Did you install the new NSS yet?

I also think my AC condenser has finally had it but will post more about that on my owner thread.

UH-oh. Those are NLA, but you can probably locate a good used one.

 

[e500.org]

This is not a valid test.

The M119 LH-SFI NSS has two completely separate functions. One is for the starter lockout and reverse lights. The other, using a separate set of pins, is a gear position indicators. Your mechanic tested the first function and its good. But he did not test the second function. This requires SDS with HHT-Win to view live data from the E-GAS module in each gear position. See post #4 above. Normally this will store a fault code on the E-GAS module as well, but it may be possible to have the problem without a stored code.




See above for the full test, checking both gear position in live data AND also the engine RPM because the tach may not be accurate either. ALSO, the NSS must be adjusted properly. A good NSS wont work right if not adjusted per FSM procedure.

Did you install the new NSS yet?




UH-oh. Those are NLA, but you can probably locate a good used one.

Thanks Dave - next time I have the opportunity I will get the car hooked up to live data - although I am pretty sure the tach is showing the correct engine RPM as the engine does not run smooth as you would expect and you can see minor deviations on the tach.

I have not installed the NSS, I should have done so at the same time the shifter bushings were done (yesterday) but I forgot to tell the mechanic (!) - when I mentioned I wanted to change the NSS after he did say it would have been super easy to swap it out at the same time as the shifter bushings - that is when he did a few of the tests above and said the NSS appears OK.

Re the condenser - I *think* this is my issue with the AC for a number of reasons - 1) I had the AC regassed a few months ago to around 950g, I have not been using the AC much recently due to mild weather but I put it on yesterday and it would not work. Got it plugged up to the AC machine and it only had 200g of gas. So it appears to be leaking - no apparent leaks anywhere in the car but would need to fill it with dye to be sure. The AC compressor appears to be working OK (but not perfectly), I think they tested it by raising RPMs and there would still be pressure held in the system (but it did go down slightly the more you rev the car). Other than that the mechanic said the condenser is very hot to touch, even when the AC was on, which suggested it may be blocked and not working optimally.

Luckily in Dubai there are many shops that can test / replace radiators / condensers, although I would prefer a brand new one so I may hunt for one online.

 

[gsxr]

For the AC refrigerant leak, definitely add dye and recharge, and see if you can locate the leak. Also, make sure it's not a faulty service port, which does not normally show up with traditional leak testing or pressure testing - click here for details.

The hot condenser is odd, first thing would be to verify the twin electric fans are running correctly based on refrigerant pressure. No fans = hot condenser. After evacuation & recharge, verify the pressures are normal, per the video here. If so, it's unlikely the condenser is blocked.