Misfire on Mercedes 500 SE

[Mercv8]

Hello gents,

Unfortunately my 1990 Mercedes 500 SE has developed a slight misfire and has gone from super smooth to super lumpy..

The machanic has checked it and said its possibly an air leak or dodge sensor but not sure. He checked the spark plugs and ignition leads and these are ok. I have run some injector cleaner in there and thats made no difference.

Its very slightly leaking oil from the oil cap, but he said this would make no difference to the misfire. It appears to be random and not associated to any one cylinder.

My mechanic said theres some many hoses that it could be a nightmare trying to find it, and as hes busy can take it on for a couple weeks anyway.

So wondering what to check out, as dont wanna be messed about by some mechanic i cant trust.

Any advice much appreciated.

 

[television]

The distributor cap is prone to getting condensation. well known problem on these engines, give it a good clean,both sides ans wipe some silicon or teflon onto it.

Malcolm

 

[Mercv8]

Ok, new distributor cap and new K1 quad core ignition leads later, the car is running lovely with no misfire.

Dunno how much of a difference these leads have made but god the car pulls so much better then it did before the misfire even. These leads has made the economy setting feel like its in sport, and sport like.. Well.. Super sport. lol

Its great!

My only minor gripe though, is the car is idleing at 1100 when warm. When cold its at approx 900. Dont know what the problem is there, the mechanic said it maybe will adjust itself in time because of how badly it was running before, but im not sure if these can adjust themselfs?

Is there an idle control valve on these? Or anyone with any suggestion as to why im getting this high idle?

 

[television]

The idle on your car should be around 700 rpm,when you put it in D,does the speed change by 500 rpm.

I think that you have a rotory idle adjuster, that is electronicly controlled.
If the engine slows down by lifting the air flow sensor plate, the its possible that some one has altered somé linkage to compensate for your last fault.

Malcolm

 

[Mercv8]

Yes mate it is at around 700 rpm when in drive. I thought there was an adjuster, so should i try and find this and manually do it?

If so, what do you recommend the idle be at? 800 rpm reasonable? I take it its best to adjust it when its warm?

This doesnt change the fuelling at all does it?

Other then this im happy very happy with her but as you would know yourself its the little things that can get really annoying! Or the little things that turn into big problems..

This is the battleship herself..

 

[eric242340]

This is a jetronic system. if you remove the air filter casing, you will find an alluminium tube standing straight up. inside this tube is the adjuster for the air flow plate. it requires an allen key. insert the allen key and press down then turn it slowly. also this engine does have an Idle Speed control Valve, and they always need cleaning. Both these jobs should be done by a mechanic. Plus these engines are prone to intake manifold air leaks especially when hot.

 

[television]

This is a jetronic system. if you remove the air filter casing, you will find an alluminium tube standing straight up. inside this tube is the adjuster for the air flow plate. it requires an allen key. insert the allen key and press down then turn it slowly. also this engine does have an Idle Speed control Valve, and they always need cleaning. Both these jobs should be done by a mechanic. Plus these engines are prone to intake manifold air leaks especially when hot.

You cant adjust the mixture setting for the idle with the alen key, this must only be done when joined up to a CO meter and does not effect idle. If it is the rotary idle valve there is no adjustment as its electronicly controlled.

malcolm

 

[Mercv8]

You cant adjust the mixture setting for the idle with the alen key, this must only be done when joined up to a CO meter and does not effect idle. If it is the rotary idle valve there is no adjustment as its electronicly controlled.

malcolm

Bugger! So how do you suggest i get to the bottom of my high idle?

 

[The Gunslinger]

my 560SEL does the same thing, when i start her up cold it idles rough for a few seconds, between 4 and 6, then it smoothens right down. There is a bit of a rumble to it (vibrational, not audio rumble) that makes me suspect it has a fouled injector or its firing rich in one of the cylinders. I put bosch platinum 4 plugs in, new plug wires, and new distributor cap, and its helped greatly, but im unable to get rid of it still. Taking it to the MB dealer here in a few weeks to get that fixed....(looking at $1500.00, ouch!)

 

[eric242340]

I agree you cannot adjust the rotary idle valve. However they tend to stick when hot. All you can do is remove them and clean with carb cleaner. Then use a 12 v battery and two jump wires to test its operation.

 

[eric242340]

Normally idle speed problems occur with a cold engine, but he is saying that, when cold the idle speed is ok. but when hot the idle speed increases to 1100. This is the opposite to standard problems. Also I would go to the Jetronic Computer in the passengers footwell. Its behind an alliminium plate. Remove the carpet and you will see the plate, its held by two 10mm bolts. Disconnect the computer and clean the contacts with electrical cleaner. but dont open the ignition whilst this is disconnected.

 

[shayes]

I agree you cannot adjust the rotary idle valve. However they tend to stick when hot. All you can do is remove them and clean with carb cleaner. Then use a 12 v battery and two jump wires to test its operation.

I just tested a faulty (?) ICV with two tester lights to a battery and the lights lit up but nothing else happened. I have put some carb cleaner in the unit though not too sure this is wise it being an electrical solenoid presumeably. Will thoroughly dry with a hairdryer before testing again.

I tried this s/h ICV because my own, having driven to the Charente region for Xmas, being exactemont 1200 miles return, still suffered from occasional stalling on de-acceleration when approaching roundabouts or downhill. About seven times in all which is not too bad a problem. This S/h replacement although fine in terms of the idle speed, stalls more often as I have today discovered driving just fourteen miles and three stalls. Embarrasing sitivation.

So back to my old ICV but will see how the s/h one operates after a clean.

Didn't the old Girl do well for 22 years young. If only these leviathans could have a change of electrical guts for a token price they would be on the roads forever and surprisingly use less juice than a fourtrack!!.

 

[television]

Hey Stuart,I was thinking of you the other day.http://www.mercedesshop.com/Wikka/EngineControls


Take a read.

Malcolm

 

[shayes]

Hey Stuart,I was thinking of you the other day.http://www.mercedesshop.com/Wikka/EngineControls


Take a read.

Malcolm

Hi Malcolm, yes I've been away from the scene.

Before I read your thread, I can't resist sharing with Memebrs an interesting link to an item I have just seen on ebay.

Before you joined the Club a guy named "Talbir" was a great help with problems and it looks like he is selling one of his old thoroughbreds (he has 8MB's I read) .Anyone for a Stratton AMG?
http://cgi.ebay.co.uk/Mercedes-AMG-...oryZ9855QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[television]

Yes Stuart, I have seen the name,

I hope that you do not mind I have put a link into the front page of the forum.

So this car is only around the corner from you, very nice to own.


Malcolm

 

[shayes]

Ok, new distributor cap and new K1 quad core ignition leads later, the car is running lovely with no misfire.


My only minor gripe though, is the car is idleing at 1100 when warm. When cold its at approx 900. Dont know what the problem is there, the mechanic said it maybe will adjust itself in time because of how badly it was running before, but im not sure if these can adjust themselfs?



Expect by now you've resolved the issue or thrown in the towel.

If not, what I discovered was after Television kindly breathed on a s/h replacement Idle Control Unit (not valve) my high idle problem ceased to exist.
Cheekily I asked Malcolm to breath on my original - but many months later when I did put it back into the car to see if it worked properly, back came my high idle, so it may be the answer to your problem, but they are not always repairable. Circuitry hidden deterioration I guess.

Thanks by the way to Malcolm for the hotlink. I did replace my OVP with a brand new one but whos to say that the wiring beneath the contact points has not begun to deteriorate, dry solder etc. ? I'm unlikely to go there when it comes to problem solving, just don't have the time or patience, sadly.

 

[wscheffer]

Your 500SE has the same setup as my 1987 420SEL and Malcolm is right on this! The idle is controlled electronically by the “Idle Control Valve” you can clean them and check them as mentioned above, but when they’ re bad you can only replace them.:cry:

 

[television]

When the Idle circuit has a fault these cars do revert to High idle from the main ECU.

Malcolm