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90 420SE // Normal ICV Voltages? // High Idle

2K views 18 replies 6 participants last post by  Teddyen 
#1 ·
I've had a high idle problem on my '90 420SE (M116.965) since I bought it three years ago. The car has 210'000km (130'000 miles) on the odometer. I've gathered some information in an attempt to diagnose whether I should buy a new ICV, Idle Computer or OVP.

My theory:
The ICV doesn't move freely enough to adjust idle properly, and needs replacement.


Symptoms:
- High idle (900-1100rpm) when engine is at operating temperatures, or have been driven for more than a few minutes.
- Often revs high (up to 1800rpm) during startup, before immediately falling down to idle speed.
- Sometimes have normal idle (500-650rpm) if engine is cold, and sometimes if engine has reached operating temperature without being driven (idling for 10-15 minutes).

Testing done:
Sprayed start gas everywhere. Can't find any leaks.
The valve does make a click with the 9V-battery test, but the valve also looks kind of dirty inside.

ICV Voltages recorded, idle speed with P engaged on the transmission:
4,35v: Normal idle, 500rpm. Engine not at operating temperature (70C/160F). Air temp: 19C.
4,75v: Normal idle, 650rpm. Engine just below full operating temperature (80C/176F) after a few minutes of driving. Air temp: 11C
4,76v: Normal idle, 690rpm. Engine just below full operating temperature (83C/182F). Air temp: 14C.
4,83v: High idle, 1050rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 14,5C.
4,87v: High idle, 1000rpm. Engine at full operating temperature (87C/190F) after a long drive. Air temp: 17C.

Once, I measure the voltage while the idle speed was rising. This was interesting:
It seemed like the idle speed started rising as the viscous fan disconnected and slowed down, hence the engine got a slightly lighter load. The idle speed rose from 690rpm to 920rpm over the span of around 20 seconds. The voltage then gradually increased from 4,76 to 4,85. The interesting thing is that the idle speed rose first, then the voltage rose slowly and gradually a few seconds later. It seemed like the computer tried to adjust the idle speed, but that it didn't help because the valve didn't respond properly to the voltage changes.

Any thoughts?
 
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#2 ·
Your readings look ok. Clean the ICV in carburetor cleaner soaking the end with the air openings -not the one with the contacts- in a glass or similar for a while or even overnight. Make sure the cleaner covers the holes completely.

Rinse with blasts of cleaner to remove any crud left, reinstall.

This works on most valves but be aware they all fail eventually.

Finally, you could also (most likely) have vacuum leaks. You need a smoke test to confirm. Recently a member thought his engine had none until he did the test and found a bunch.

Finally, finally... this is a common issue with our old cars, use the search function to get more details.

Welcome to the institute of the very, very nervous. :)
 
#3 ·
Thanks for the reply!

I cleaned the ICV, soaked it overnight, and blasted out the rest the following day. My idle has now gone from bad to worse, and is now at 1400rpm. I cringe every time I put it in gear.

Can't really understand why that would happen. I used fast evaporating brake cleaner. Wondering about cleaning some more, and giving it some WD-40 oil in the end or something if that makes a difference.

I'm looking to get a used valve, a new one on eBay is over $600 ex taxes, and a new one in Norway is $1500 inc taxes...

My parts number is 0001411625, and is only for 87-91 models 420s and 560s. Could an ICV from a 380 work? The part number is different, but it looks roughly the same.
 
#5 ·
I gave it a thorough cleaning with a mascara brush as well, and got out some more crud. No change.

I finally tried a trick I read about on some forum, maybe this one. The ICV in Gen II cars can be adjusted, but I haven't found the procedure anywhere else. At the intake of the ICV, there is a small plastic cover in the center. I removed this with a screwdriver. I forced in a M5 screw that didn't fit properly (M4 was too small, M5 was kind of right), and stripped out some of the fine copper thread in the process. Anyhow, the screw got a grip of the threads. After inserting the screw, I carefully, but with force, pulled it out 2mm. Then I unscrewed the screw, and put the plastic cover back on.

My idle has been perfect since, both hot and cold! Can't believe it actually worked.
 
#7 ·
I can only guess. I wouldn't exactly call it a regular idle adjustment, but maybe rather giving the oscillating mechanism of the ICV a new work area. After all, the ICV has a service interval, I believe I've read it's about 100'000 miles or 160 000km.

As far as I read in the other thread (which I can't find right now), pushing the screw inwards will give you higher idle, and pulling out will give you lower idle. It was a very minor adjustment.

I can't remember if this image was taken before or after the adjustment, but here you can see the thread insert:
Imgur: The most awesome images on the Internet
 
#10 ·
I can only guess. I wouldn't exactly call it a regular idle adjustment, but maybe rather giving the oscillating mechanism of the ICV a new work area. ...
Actually on the contrary … by what you did you gave the ICV's oscillating mechanisms it‘s old (original) work area back. Currentless, the ICV‘s plunger is completely pushed back by a rebound spring to the “completely open“ position. With the time the plunger gets a little sluggish, and the ICV coil‘s magnetic force, which acts against that spring and the plunger‘s accumulating sluggishness, can not push the plunger - with a given voltage (respectively current) - forward to its original position (work area). That spring is supported by the threaded sleeve on the ICV‘s intake side, and by pulling that sleeve a little out you reduce the spring‘s preload, so that the work area of the plunger is, despite its sluggishness, in the accordance with the coil‘s magnetic force again. By pulling it out too far the spring's preload becomes to small and idle unstable, though.

Btw, the last sentence of your initial post was spot-on. … :wink_2:

H.D.
 
#11 ·
Thanks for the explanation! It's good to know my reasoning wasn't too far off either. ;)

It's not behaving as it should during startup, but that hasn't changed with the ICV. If I quit cranking the moment it fires, it often dies out at once, but fires straight up at the second try. If I don't want to crank it twice, I can just keep running the starter a second after it fires, in total just 3-4 seconds. Not very bothersome.

The RPM at tickover is also often high, up to 1600rpm, before it quicly falls down to regular idle. The noise the engine makes during the high RPM tickovers aren't always nice, as the engine is probably starved from oil. I'm kind of worried that the chain is slapping the guides hard, but it has been like this all the three years I've had it. Could this be related to the sluggish ICV? The engine always stumbles for a few seconds, high tickover or not, but I guess this is due to a leaking injector. Cylinder #3 needs a new spark plug every 4-5000km, as it burns a little oil and gets clogged, but it has behaved very well since the ICV was fixed. :p The car is in great shape though, and the engine runs good.

The cams, lifters, chain and upper guides are relatively new though. The cams, lifter and chain have 60 000km on them, the guides have around 40 000km. One of the old guides had broken in two and was hanging, my mechanic found them while replacing the valve stem seals right after I bought it.
 
#13 ·
Do the startup problems occur during cold or warm startup ?

For how long does the high idle speed last after startup ?

... The engine always stumbles for a few seconds, high tickover or not, but I guess this is due to a leaking injector. Cylinder #3 needs a new spark plug every 4-5000km, as it burns a little oil and gets clogged, but it has behaved very well since the ICV was fixed ...
"stumbles“ or “stumbled“ ? … The underlined passages in the quote seem to be mutually exclusive. … :wink_2:
 
#12 ·
The noise could be sticky lifters. Are they aftermarket units? There is hydraulic lifter additive fluid that can help clean them a bit, if that's what it is.

Or, it could be a slow tensioner. Was it replaced with the other bits? Unfortunately aftermarket versions of this part have a generally bad track record and the expensive MB part is the only really trustworthy one.
 
#14 ·
As far as I know, the tensioner has not been replaced.

It's not a distinct lifter tick, it's more of a deeper metal rattle at startup that goes away when there is oil pressure. The engine makes the same sound at first startup after an oil change, but then for several seconds. Now, I always unplug the coil and run the starter until I get oil pressure to avoid this.

I have thought about the tensioner. Maybe I should get it replaced?

When it's running, there is some ticking, but it's been very constant for three years / 40 000km, and I've heard other presumably good engines have it, so I just live with it.

"stumbles“ or “stumbled“ ? … The underlined passages in the quote seem to be mutually exclusive.
The engine still stumbles on tickover, but this problem is probably is separate from the cylinder 3 problem. Cylinder 3 gets so severe in the end, that it stumbles during driving. It behaves nicely for a long time after the plug is changed though. It gets full of a very hard, shiny black substance, which I guess is oil. It ultimately connects the anode and electrode (or whatever we call it) if I let it go too far. I've tried changing the injector on cyl 3, but no luck. I have a complete set, and intend to change all of them.
 
#15 ·
... The engine still stumbles on tickover, but this problem is probably is separate from the cylinder 3 problem. Cylinder 3 gets so severe in the end, that it stumbles during driving. It behaves nicely for a long time after the plug is changed though. It gets full of a very hard, shiny black substance, which I guess is oil. It ultimately connects the anode and electrode (or whatever we call it) if I let it go too far. I've tried changing the injector on cyl 3, but no luck. I have a complete set, and intend to change all of them.
What leads you to think that the stumbling at idle speed is probably not related to the cyl 3 problem ? … The cyl 3 problem may be ignition related (and not fuel injection related). … Did you check the distributor cap and the plug lead & connector for cyl 3 ?

Regarding the startup problems and the temporary high idle speed after startup you mentioned in post 11 … see the first two questions in my last post.

H.D.
 
#16 ·
I've misunderstood the meaning of "tick over" completely. I see that my posts seem very incoherent now that I know what it means, very sorry about that! Read my post again, but replace "tick over" with "directly after engine fires on startup".

The engine does not stumble during idle, other than after 4-5000km without a spark plug change on cyl 3. The engine always stumbles directly after firing up, but only for a few seconds until it evens out nicely.

On startup, the engine often goes directly to 1600rpm, but stays there only for one second, before falling down to regular idle. There seems to be no system for when it goes up high, or just stays low as intended.

The stumble and high rpm directly after startup, happen seemingly randomly, and is not much related with engine temperature.

I have new plugs, new and correct Bosch spark plug wires, distributor, and I've tried to change coil to another used one. The fuel pump relay is also new, the old one started to fail.

I think the problem with cylinder 3 started after I had it a mechanic who didn't properly attach the spark plug wire at cylinder 3. I think this gummed up the cylinder, as I didn't notice that the cylinder wasn't firing properly until it got really bad. Then I found the just partly connected spark plug wire. I guess some of the piston rings isn't moving freely, which makes it burn a little oil in that cylinder. I've tried Omega 907 and oil flushes, but with no luck. It's just my theory, it could be a vacuum leak at the injector or a bad fuel distributor as well. I would guess that the hard black crust is oil though, I've had to refill about 1 litre every 3000km, but it seems to use less and less in the past. Exhaust is fine. Before I changed the valve stem seals, it used 1 litre every 1000km, at that point the exhaust was blue after idling a few minutes...

I've done oil analysis, but have driven 20 000km since that. It pointed to the piston rings. Don't know what more to do with it after having tried an oil flush. Maybe spray water mist in the air intake, or pour some ATF or some carburettor cleaner into the cylinder, but I haven't gotten so desperate yet. I'll do another oil analysis now to see the development.

You can see it here: https://www.dropbox.com/s/9f85hgcvh5kkgv0/MB420SE-031716.pdf?dl=0
 
#18 ·
... The engine always stumbles directly after firing up, but only for a few seconds until it evens out nicely.
On startup, the engine often goes directly to 1600rpm, but stays there only for one second, before falling down to regular idle. There seems to be no system for when it goes up high, or just stays low as intended.
The stumble and high rpm directly after startup, happen seemingly randomly, and is not much related with engine temperature ...
There are several possible causes for that (often observable) behavior … deposits on the backside of the intake valves and in the last part of intake ports / a problem with the fuel system‘s holding pressure / worn injectors / faulty CSV control (due to a problem with the CTS, CIS-ECU, FPR) …

... I think the problem with cylinder 3 started after I had it a mechanic who didn't properly attach the spark plug wire at cylinder 3. I think this gummed up the cylinder, as I didn't notice that the cylinder wasn't firing properly until it got really bad. Then I found the just partly connected spark plug wire ...
Sounds like what I smelled in my last post:
… The cyl 3 problem may be ignition related (and not fuel injection related). … Did you check the distributor cap and the plug lead & connector for cyl 3 ? ...
… Don't know what more to do with it ...
Have you done a leak down test yet ?
 
#17 · (Edited)
On the surface, it sounds as though your engine, albeit surrounded by good W126 knowledge, needs a refresh?

As far as I know, the tensioner has not been replaced.

It's not a distinct lifter tick, it's more of a deeper metal rattle at startup that goes away when there is oil pressure. The engine makes the same sound at first startup after an oil change, but then for several seconds. Now, I always unplug the coil and run the starter until I get oil pressure to avoid this.

I have thought about the tensioner. Maybe I should get it replaced?


I have been alerted that the cam chain tenioner in the [US version at least] of the 380/420 was an issue as it 'bled down' all the time, causing the upper chain guides to fail as it [the chain] slapped about before oil pressure was attained on starting.

The 'later version' is different, I am assured.

A thought man,

MBL
 
#19 ·
On the surface, it sounds as though your engine, albeit surrounded by good W126 knowledge, needs a refresh?

I have been alerted that the cam chain tenioner in the [US version at least] of the 380/420 was an issue as it 'bled down' all the time, causing the upper chain guides to fail as it [the chain] slapped about before oil pressure was attained on starting.

The 'later version' is different, I am assured.

A thought man,

MBL
Sounds just like my problem. I'll get the tensioner replaced ASAP. I see that the expensive genuine Mercedes part is the only way to go.

I've seen a few writeups that change the tensioner while doing guide/chain replacement, but it looks like it can be changed by just undoing a few bolts on the side of the engine. Can I easily change the tensioner myself without even taking the top covers off, or is there more to it than what I think?

There are several possible causes for that (often observable) behavior … deposits on the backside of the intake valves and in the last part of intake ports / a problem with the fuel system‘s holding pressure / worn injectors / faulty CSV control (due to a problem with the CTS, CIS-ECU, FPR) …

Have you done a leak down test yet?
It was good three years ago when my mechanic changed the valve stem seals, but haven't checked it after that. I'll see if I can get it done somewhere. I don't have the equipment myself, and most of the workshops around here are totally incompetent when it comes to actually diagnosing older cars without OBD-II, so it's an hour drive to my mechanic. ;)

The plug connector for cyl 3 have been properly connected for two years now, and as mentioned, I've changed all high voltage ignitions parts after that. I had some stumbling on other random cylinders as well, due to a bad set of new Beru wires. When I finally changed wires again (to Bosch this time), I figured cylinder 3 was still a problem. Haven't been on to cylinder 3 for more than six months, now I change that plug freuently to keep if from stumbling, hoping it will eventually clear up some of the deposits and loosen the piston rings over time. I hope the engine can last a long time if cylinder 3 just gets sorted out.
 
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