88 420SEL - sporadic engine start difficulty

[wandawoods]

the car is a 1988 420SEL w/166k miles, and this car has been thru a lot! in last 24-mont, we have replaced:
timing chaing
16 volves (were bent caused by broken timing chain)
power steering pump
starter/alternator/battery
motor mounts
front brakes and suspensions
flex joints at transmission
all 5 belts

(1) after engine has been heated up (about 90f coolant temp) after said 10~15 min worth of driving. sporatically, the engine refuses to start after short stops!

we have replaced starter, alternator, battery with new ones in last 30-day.

when it happens, we can hear the starter trying to crank up, and it feels like gas is flooding the ignition even with very little or no gas paddle depressing. we have to wait a few minutes, then it will start. it never happened when engine was cold (1st morning start)!

(2) the idle speed in 'drive' with brake pressed is 700rpm, but in 'park' it goes up to 1200rpm. pretty weird!

(3) AC will come out from windshield/left & right/rear seats, but it refuses to come out from 3x center positions above stereo, and floor

(4) while unlocking driver door, it may trigger factory alarm sporadically, then we have go around and use the key at passenger door to stop the alarm.


thanks in advace if someone can give me a diagnosis!

 

[talbir]

Firstly you haven't mentioned fuel injector change - I would reccomend doing this, since fuel injectors usually have a life pan of 70k miles. Your vehicle has done more than double that mileage. Each fuel injector is approx 21 punds plus VAT - 8 of them on your car. I reccomend changing them and the seals that they sit in. The seals are about 15pence each. Also check the condition of the air tubes on both banks - if they are hard and brittle, replace. The air tubes are also fairly cheap.

The 'rich' condition, i.e. flooding, could well be caused by leaky injectors. So cold starts will be great, since cold engine loves a rich mixture, but warm starts will be problematic.

Your idle speed at operating temp in 'Park' or 'N' should be 550 +/- 50 rpm, and in 'D' should be 500rpm, on the 126 V8 that you have. Your high idle could be caused by a number of things, but I would guess that you have either :

(1) a vacuum leak. This vacuum leak could be caused by bad fuel injector seals, or one of the injector air tubes leaking.

(2) Faulty idle control module - this is a relay which controls the idle air valve and is located in the upper part of the passenger footwell. Lift out carpets and in the footwell, you'll see a grey plastic panel held down up by 2 plastic clips. Remove the circular clips, pull away panel and you'll see the idle control relay. Tihs may have bad solder joints, but cannot be opened up to inspect since it is heat sealed.

(3) Faulty idle control valve - this is just underneath the front of your air filter. It is basically a solenoid operated slide gate that opens/closes to increase/decrease air flow. It may not be closing fully due to solenoids or extreme amount of dirt clogging gate. Remove and clean well with carb cleaner and try again. There is a bypass hos on this valve - worth checking that it is in place and is not leaking - if this little plastic pipe is damaged or dislodged, you will suffer high idle.

You'll need to test the above 3 items step-by-step - see if you can borrow the items from another correct car to test. The idle control relay will be specific - you will need one from another 420. The idle control valve can be used from any 126 V8, i.e. 420, 500 or 560. You cannot use the ones from pre-1986 126's though, as their idle control valves had slightly different dimension inlets and outlets.

I would definitely start with changing injectors, their seals and air tubes. Then go from there. You may find this job cures your hard starting and high vacuum. Otherwise, at least you have good new injectors which are delivering the right amount of fuel. Then investigate from there.


AC vents - the lever control mechanisms may be dislodged. You'll have to open up centre console panels and investigate. Do you have climate control or heatmatic system ?

Not sure about the alarm - is it the factory one or aftermarket ?

 

[wandawoods]

thanks a lot, talbir!

although lots of things you mentioned sounds foreign to me, but I will print it out and show it to my mechanic. fuel injectors first!

I just realized I accidently registered last nite on an UK site... but I am in Altanta, Georgia, USA. your kind response in details is greatly appreciated!

it looks like I will have to exchange pounds and pence to dollars and cents... LOL

 

[plink49]

No comment on your technical problems, but I too am from the USA (currently in NM, but lived 5 years in Lawrenceville, GA), and have found this UK forum to be the best!! The people here are very generous with their time and expertise....thanks all!
But to our Brit friends, I as well would like a current exchange rate between dollars and pounds (2:1?). Also, what are J and K registrations? and why do used MB seem to be so inexpensive there?

 

[wandawoods]

No comment on your technical problems, but I too am from the USA (currently in NM, but lived 5 years in Lawrenceville, GA), and have found this UK forum to be the best!! The people here are very generous with their time and expertise....thanks all!
But to our Brit friends, I as well would like a current exchange rate between dollars and pounds (2:1?). Also, what are J and K registrations? and why do used MB seem to be so inexpensive there?

you were close by! I've been living in Doraville since 88.

although this is my first post here, sorta by accident, but I do agree that UK forum looks very promising judging by the 1st UK response from Talbir. indeed generous with time as well as in technical details! outstanding!

 

[talbir]

Ex rate, USD to GBP = 1.58 to 1

 

[alec]

J and K

The J and K are part of the car registration number.

J relates to 1991 approx and K to 1992.

It gets more complicated after 2000!

Alec

 

[talbir]

BTW, fuel injectors, seals and tubes should cost in total about $250.

Should not take your mechanic more than 1.5 hrs to change all 8 injectors. I have done the job more than once, in less than an hour. So make sure he doesn't charge more than 2 hrs labour max. It's not a difficult job, just a little fiddly.

Good luck

 

[flagstaff]

plink -

used mercs over here in UK are relatively cheap because they are perceived to be expensive cars to maintain. your average used-car buyer would rather shell out (say) 2,000UKP (~3,000USD) on a four-year-old ford than an eight-year-old mercedes. without realising, naturally, that a merc of that vintage is a better car in every way than even a brand new ford.

 

[plink49]

flagstaff....unfortunately, that same perception exists here in the states as well, but your prices for MB autos seems to be WELL below what we must endure here.