the forum members pointed to this.... this has nothing to do with fuel...

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1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


[/SIGPIC]

Alright, my rpm-drop is getting serious.
It's coming now even when driving in the traffic, and just idling.
But very hectic. So here are the symptoms I'm experiencing:

1: Cold engine start: OK, idle: OK and runs OK.
2: Warm engine starts immediately, but RPM drops for a few seconds, then idle flattens, runs OK.
3: Hot engine (over ~90C) hard to start, starts only with full gas pedal, serious RPM drop for a few seconds, then flattens the idle. But sometimes during standing in the traffic it just drops the idle hectically, then I need to pump on the gas pedal, rev it, then it idles OK for a while.

I noticed that my oil pressure is also dropped when rpm drops. I don't know if it's a reason or a consequence... Normally its 3 when running, 2 when idling, but when the rpm drops it also drops to around 1.5.

I also noticed that it is somehow related to the engine temp, if its over the line that is between 80 and 120 it drops rpm, when its cooled back down under that line, its OK.

And also noticed that as it were better if the headlights are off, so if it were relating somehow to the electronic consumption. Im going slowly in high traffic, let the engine idling, and when I brake, the rpm drops down, as the breaking lights could also affect this...

Also noticed that when its hot and hard to start, then my RPM gauge arm goes up high during cranking with the starter motor, is this normal???

I'm afraid that the problem just got bigger as the weather got warmer here. When I bought it a month ago, it was a few C degrees outside, now it 15 and rising, so I guess I wouldn't be able to start it if it were 30 outside, that is common at summer here in Hungary...

The engine did not stop so far, but I always starts pumping gas when I feel the rpm drops, so I'm afraid it would stop if I let her. When I push the pedal it always revs, and never drops rpm when driving, only when the engine is idle.

Any ideas, please???

moire76, I would remove the voltage regulator on the back of the alternator and check the length of the brushes. It's likely the brushes are worn down to short nubs, and the voltage regulator needs to be replaced. The worn brushes prevent the alternator from recharging the battery fully when the engine is running. Either that, or you may need a new battery if it doesn't hold a charge.

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Fred Hoelzle

Hi Ferdman!
Thanks for your answer!

I 've just measured, the battery is now at 12.16v with engine off, and 14.25v when idle. However the engine is not warm now, so I may just read the values again right after driving to home.

Can the charging system be faulty even if I'm reading correct voltage at idling?
Wouldn't a failing charging system cause the battery warning lights on?

moire76, with those readings your alternator/voltage regulator appears to be functioning properly. One would expect the warning lights to function if there was a problem, but I have been stranded with worn voltage regulator brushes ... and the warning lights only lit a couple of miles before the engine died. Since then I check the voltage regulator brushes periodically.

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Fred Hoelzle

Thanks a lot, Ferdman, to be honest, I rushed to the car to check if my voltage regulator is ever replaced, I did not remove it, but it seems quite new, its much cleaner than my alternator, and it has a white plastic base plate that is still white and not dark grey from dust, so I assume that it has been replaced not very long ago.

Yesterday I drove home with high temp engine, and it did not ever dropped the idle rpm, so the only thing it does consequently is the rpm drop for 3-4 secs after hot start of the engine. The hectic low idle really comes on just occassionally. Might be a vacuum leak, or OVP relay contact failure???

I have just found a way to read diagnostic error codes with a multimeter (duty cycle function required)!!! It looks awesome, cannot wait the end of the day to check my error codes!

here's the link for your reference: Bosch Fault Code Table

UPDATE: Checked the error codes, bu I'm unsure about the results. First I measured with cold engine, and I got a near static duty cycle at around 50% which reads for an o2 sensor failure, but I guess it is because the o2 sensor is not working until is hot enough so i warmed up the engine and checked again. THen I got fluctuating value between 30 and 50. According to the guide, fluctuating value around 50 +/- 10 means no error codes. I don't know if my 30-50 fluctuation could mean the same, or I may have multiple error codes that I'll not be able to check by this method?

I will check out the VR tomorrow and see what I will find.

Ferdman, you must be right about the voltage regulator!!

Today I had the chance to monitor my idle while I've been in an hour traffic jam.
I was raining, and I noticed that whenever the wiper takes a round, my idle drops 1-2 hundreds, and also the oil pressure goes down from 2.2 to 1.8. The moment the wiper is steady again, it all goes back to normal. I could clearly see dashboard lights are faded away for that moment the wiper works, so I'was pretty sure that my charging at hot engine is low.
Stopped by and checked at the battery, and bingo, it was 13,6V with lights on, idling, and only 12,3V while the wiper's licking along the windscreen.

I'll check the voltage regulator brushes tomorrow, and hope for the bests. WIll post results here.

THanks again, Ferdman, you're awesome!

OK, I took out the VR, here it is.
It looks worn, but not deadly bad, I think.
I will replace it to a new, just to make sure its ok.
what do you think?

Attached Thumbnails

     

moire76, the brushes are fine as-is. No need to replace the voltage regulator at this time. I would estimate they have at least another 3/4" of wear left.

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Fred Hoelzle

OK, thank you, I will leave it then for now.

I would check my ground cables' contacts to vehicle body, but other than that, I don't know what else I could check.
Other than the alternator itself...
Can an alternator be faulty this way? Charges OK at 1000 RPM and above, but just not charge enough at idle?

OK, I have cleaned my ground strap at the battery negative, and checked again. Turned on all consumers, headlights, fog lights, rear windshield heater, blower motor, etc, and it charges only 11.9v idle.
At about 1200 or above it charges fine.

My ground strap however looks ancient, and it's torn halfway by aging, so its definitely need to be replaced. I also need to check the ground strap at the bottom, between transmission and vehicle body, as I remember from last week, it was quite full of oil and dirt, so it might worth a check.

Are there any other ground straps on this car I need to look after, before visiting az automotive electrician with my possibly faulty alternator?

what is just fine? sounds like a failing diode on the alternator.

if you have a battery that isn't fully charged the alternator should put out in the low 14's.

i had this problem too... alternator would only charge at 13.4-13.6 at 2000 rpm. Bad diode... oh and this was a brand new alternator... sent it back.. second one works fine.

__________________
1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


[/SIGPIC]

When the engine is started cold, the initial RPM is about 1500.
Then I have 14.25V at the battery.
Once warmed up, idles a little below 1000, then charge drops to 13.6V.
If I put on ALL consumers at the same time it drops to 11.9V.
I don't know what the factory specs are, but I guess it should charge more then 11.9 at idle, even with everything is turned on. (But please tell me, if I'm wrong, and these are acceptable values! )
If I rev it, charge goes up over 14v so, I think my alternator is doing its job.

I'm afraid I could have issues with my tachometer (too?), which could make the error-finding harder.
Symptoms:
- During cranking it jumps up to 4-5000 until the engine started.
- Sometimes, when I get this RPM-drop-issue, I hear that the RPM is low, engine almost dies, but according to the tacho, it is still around 800. It's like my tacho would show a higher value, than it is.
- Sometimes I start the engine, and the tacho does not move for the first couple of seconds. Then it jumps to its value. Other times, it jumps right away.
- I checked my friends' W124 with the same 2.0E engine, he's got flat 850 idle according to his tacho, and the engine sounds as smooth as mine at 1000 (according to my tacho)....

So is there a way to check if my tacho works ok? Where's his sensor, it is CPS or is there another for the tacho?

Thanks so much for any help!

Hello there!
I've just filled full the tank and the results: 440km using 49 l of gas which stands for 11l/100km or 21mpg 100% city drive. That's not that bad from a 25 year old 2.0 car. My Mondeo mk2 ate the same with its 1.8i 16v. And I think it will improve after replacing my ignition wires and maybe rotor and cap.

The other good news: since I have cleaned the ground contact between battery negative and the chassis, my idle was never dropped during driving. The few seconds rpm-drop right after hot start remained, so I will need to still to take care of it.

I do not worry about my charging anymore, it looks perfect, even if its charge only 12v with everything is turned on. It's really not realistic having that much consumer turned on for any long while the engine's just idling, so I consider my charging system as perfectly working.

Thanks for all for your help!!! I'll get back with the news regarding the hot start rpm drop, once I get there.