'86 MB 420 SEL Injector changeout neophyte

[randy]

Hi, regards to all and Merry Christmas. Santa brought an'86 420 SEL, a solid car with 143K miles, which I have known since new. I am changing out the injectors and seals, and could use advice from a knowledgable and experienced person. I bought injectors (Bosch), seals, and green o-rings, I assume the o-rings go under the injector holders. Is it necessary/advisable to remove the black holders, or just change out the injectors? If I need to remove the holder, how? and how much should I worry about the grit surrounding the outside of the holder? I blew off around them with an air hose, but wasn't satisfied with the cleanliness, so I decided to change the injectors and seals, unless otherwise advised.
Engine has high idle speed (1500 RPM), and I assumed it could be the idle control, but decided to change the injectors and seals, as they probably haven't been changed in a while, anyway. incidentally, the injector clamp on the right-front cylinder is difficult to access because of the throttle linkage bracket; will it hurt anything to loosen the two bracket bolts, which appear to hold the intake as well?
One might surmise that I am a beginner at Mercedes, though not at car repair in general. It's like doctoring on one's mother. :lol:

 

[Andy]

Hi

Not sure why you are changing the injectors or the seals. The seals won't cause a the engine to idle at 1500rpm. May cause lumpy running but really you need to check the emissions & check for excessive oxygen. But the idle problem, can be either idle control valve,relay or throttle valve switch.


Good luck


Regards

Andy@ www.mercedesservicing.com

 

[talbir]

Worn injector seals WILL cause a high idle - as will any air leak after the throttle body. I have seen this occur on a couple of W126's - after a step-by-step diagnosis, it was found to be worn injector seals that caused the high idle - with engine running, spraying WD40 around the seals momentsrily reduce the idle. Very easy to prove - take a W126 V8 with correct idle (550rpm +/- 50 in 'P'), remove one injector and replace the seal with a well worn one. Watch the idle jump to over 1000rpm.

IF there is an air leak in either the air hoses on the idle air valve, the engine will idle high.

Other causes of high idle are of course the idle air valve, idle control unit or the throttle valve switch. A sticking air metering plate will also cause high idle, as will a bad OVP relay and a leaking cold start injector.

Normal fuel injectors typically have a life span of 70k miles. Given that the car has done twice the mileage, I would say the injector change is well
overdue. Take an old injector and compare the fuel spray pattern to a new one and you'll understand why they should be changed every 70k miles. And so-called magical elixirs don't really clean these out.

Disconnect the 2 prong connector at the idle air valve (just in front of the air cleaner, has part no. part no 000 141 1625. written on it) and check that the right voltage is being delivered to the valve by the idle control relay - should be 1000mV at operating temp.

Disconnecting your idle air valve connector should send your idle upto approx 1800rpm - this confirms it is opening fully. However, it may not be closing fully (it is basically a solenoid driven gate which opens and closes) - you can try cleaning the valve with carburettor cleaner. But if it's bad it usually means buying a new one. On your car this part is approx. $200.

But before buying any parts, find out which part is causing the problem. Use parts from a identical model from either a breakers or a friends car before buying any new components. 9 times out of ten it'll be the idle air valve, but there are a number of possibilities.

Be sure to address this problem fast, before any damage is done to your gearbox. The gearbox is designed to engage 'D' or 'R' at 550rpm on the 126 V8, not 1500rpm.


Good luck.

 

[Francois]

Talbir,

Yes I know we have spoken about this issue until exhaustion, but my car still idles at 1k. The air valve is fine, although I should check the voltage. The idling computer is new.

The only thing left is to change injectors and seels I suppose, but how can I test the cold start injector and where is the throttle valve switch located?

Regards

 

[talbir]

Hi Francois,

If you are happy that your idle air valve and idle computer are fine, then check the following :


OVP relay - this is located in your fuse box (on pre-85 W126's) and has a red plastic lid with a fuse in it. Check that the fuse is not blown. When this fuse is blown, then car usually idles at bang on 1000k.

Cold start injector - this is situated at the front right side on the inlet manifold and has a loom connected at the side, with fuel pipe at top. The wiring loom is scured to the cold start injector by a small metal wire retaining clip. With car at operating temp, remove this clip using a thin flat face screwdriver and pull off the loom. If disconnecting the loom lowers the idle, then your thermo time switch is faulty, as it's not shutting off the cold start injector.


I reckon one of the above should solve the problem - do keep in touch, as I am keen as you to get to the bottom of this one !


cheers and Happy New Year

talbir

 

[Francois]

As ussual I can depend on you Talbir to have an idea of what is going on and what might be the problem.

My car is idling at exactly 1000k, so I shall definitely have a look at that fuse tonight!! Hopefulle this will at last solve the problem.

Thanks again!!

 

[Francois]

I had a look in the fuse box but could not find the red plastic lid. I did see a yellow square rellay and removed it but there was nothing under it.

There is a strange looking fuse between the fuse box and the cooling fluid reservoir. This is totally loose standing with rather thick wires attached. The fuse in there is just a metal plate. This was broken and I shorted it to see if this makes a difference but to no avail.

It would seem that the cold start injector is also functioning correctly.

Did I look in the wrong place? Maybe I should try to take a photo of the fuse box and surroundings and try to post it here?

 

[talbir]

Hi Francois,

The stand-alone fuse that you found is for the heated rear windscreen - with that fuse broken the heated rear screen will not function. If you press the heated rear screen switch, it will just shut off instantly - replace the fuse (it's a 50A fuse, avilable only from MB really) and the rear screen heater will work.

The yellow relay is also for the rear screen heater - it switches the heater off after approx 20 mins.

There should definitely be a silver relay inside the fuse box with a red plastic lid and a fuse inside the lid - because on pre-86 SEC's it was ALWAYS in the fuse box. On later models it was located midway between the battery and brake booster, just by the wiper motor.

Can you see an empty space in the fuse box, where there are pins, but no relay ?

Maybe your OVP relay is missing ??! You never know....this may ex[plain the high idle...........

If your OVP relay is missing, car will start and run fine (because unlike on later model W126's it doesn't control any of the injection system components) - but will definitely idle high.

Maybe this OVP relay is missing - using a torch, and with the bonnet lid upto max height, see if you can see a space with pin holes in the fuse box tray. If you can see 4 or 5 copper pin holes, then you know your OVP relay is missing for sure.

Please let me know....I'm dead curious now !

 

[Francois]

Talbir,

You would not believe me but it would seem that the earlier W126's in South Africa did not have an OVP!

I am taking the car to an independent garage this evening and we will hopefully get to the bottom of this!

Thanks for all the help!! I shall definitely post the result.

 

[talbir]

I think I believe you - it does appear that on the 83 380, there is no OVP.

The MBCD-Rom manual references it as a part associated with the Lambda Unit but, the reference diagrams do not list the OVP relay under Component Locations.


So we can rule that out - sincere apologies for misguiding you on that one.

However, i'm still keen to get to the bootom of this high idle. Can you remind me again as to what checks you have done on the following :

idle air valve and associated air hoses
idle controller
injector seals and air tubes
air metering plate
cold start injector
thermo time switch

Perhaps email me off list - my email address is : sharanbains at hotmail.com

Let's try and nail this problem.


cheers
talbir

 

[Francois]

Thanks Talbir!

Mailing you!

 

[mjtray]

As you have both gone offline..........don't forget to tell us if you fix the problem. I've been following this link and am keen to see what happens!

Cheers

 

[Francois]

Will do!

It is also handy to mention the results here since it will be kept in the archives for future searches.

 

[randy]

'86 420 Injector Changeout Neophyte

Hi to all; thought you might be interested in the results of my efforts: injector changeout was not difficult at all. There were several clues which, taken together, tell me these were probably never changed out. The only difficult parts were moving the throttle linkage bracket to access the injector for the right-front cylinder and some of the idle-control to access the left-front cylinder. the car idled perfectly when I completed this; however, it did not last. I ended up changing the idle-control circuit in the passenger footwell and now I have a really great, smooth idle. I think the injector change out was well-worth the effort and relatively small cost.
Regards to all, and thanks for the help, Talbir, as I got the advice from a post you made some time ago to another member about a W-126.

 

[Francois]

Nice to know it made a difference!

Most people do not realise that it is good idea to replace injectors, HT leads and dizzy cap and rotor every now and then. You will find cars that are 20 years old that would still have the originals.

 

[smokey]

check out fast idle on smokey

 

[Bumper]

A two year old post, I think the problem will be rectified by now.

 

[AndreaDC]

Radio Anti-Theft System Problem

I accidentally set off my car alarm recently, and in the process managed to activiate the radio's anti-theft system that renders the entire radio useless. I read in the manual that this would cause a straight line to appear across the display screen (which it did), but I was unable to find anything that told me how to unactivate it. I have an '86 420 SEL, and would really appreciate the help!