SEC Idling Question

[Francois]

Hi,

I bought a '83 380 SEC. When in neutral it would ide at around 1500 rpm. I don't think this is correct, please advice me. When in drive with breaks applied it comes down to around 650 rpm.

Secondly, when the engine is hot, and is idling in drive with breaks applied, it would seem like it misses. I am not sure if this is just gearbox trying to engage every now and then or if it is really the engine miss firing.

This is a beautiful car though and in good condition, except for minor faults on some of the gadgets.

F.

 

[dieselman]

It sounds like there may be an air leak in the induction system.

 

[Francois]

Aha, it could be.

I think I'll change the plugs and wires and then have a look at the induction system too. Looks like the previous owner haven't serviced it for 2 years 25000 km's.

Would a stock K&N air filter be worth the money?

F.

 

[talbir]

Francois,

It is MOST likely to be the idle control valve or the Idle control unit that is faulty on your 380SEC. The idle control valve is located just in front of the air filter and the idle control unit next to the ignition ECU. These 2 are the most common causes of high idle on SEC's.

It could also be caused by a vacuum leak, most likely around the injector seals.

For the 126 V8’s, at operating temp, with selector in ‘P’ , idle should be
550 +/- 50rpm.

On cold start-up, idle will be around 850rpm for about 90seconds, since cold start injector is giving the extra shot of fuel. Once coolant temp reaches 15degC, the thermo time switch (or coolant sensor) cuts-out and breaks the circuit to the cold start injector. Then rpm should immediately drop to 550rpm….and stay there.


I give a detailed checklist below that I wrote a while back when I encountered the same problem on one of my SEC's.

I would reccomend that this problem is fixed ASAP, to prevent damage to the gearbox. The gearbox is designed to engage into gear at 550rpm, not 1500rpm. The idle air valve is in the region of £150 + VAT from MB, the idle control unit about £100 more. It's a step-by-step guide to elminate this problem of high idle as it can be caused by a number of things.

Checklist :


AIR METERING PLATE – ensure that this was closing fully. With the engine
switched off, if the plate is pressed down it should rise back up smoothly
and fully, without any contact whatsoever with the intake body. In any case, if there is contact, it will not shut fully and would also cause very very hard starting.

OVP RELAY – bad relay, or blown fuse, would give poor cold starting and an ABS light. Will also result in high idle.

THERMO TIME SWITCH – coolant sensor may not be cutting out. Disconnect wires to switch to see if this drops idle.

COLD START INJECTOR – a leaky injector can cause high idle of course, and will decrease mpg of course.

VACUUM LEAKS – check vacuum after throttle body – should be 23inHg
there. Also expriment by spraying WD40 around each fuel injector guide whilst engine running to assess any minor vacuum leaks here, but none whatsoever since idle didn’t drop with spray pattern around any of the injector guides.


THROTTLE VALVE SWITCH – The three pin adaptor is situated on the inlet
manifold, near the cold start injector. Check that the central earth wire
was good earth and with engine off but ignition in position II, check the
‘IDLE’ and ‘FULL THROTTLE’ contacts – at idle left pin is earthed and
moving linkage to full opening, right pin is earthed. If not ok, then bad throttle valve switch which means removing inlet manifold and matching up the throttle valve switch with the throttle body.


THROTTLE LINKAGE – check that linkages, including linkage with cruise
control actuator is not too tight and that throttle linkage is coming back
to full rest when released from full.

IDLE AIR VALVE (000 141 16 25 - post 85 only) – inlet and outlet hoses were fine and bypass pipe was good too. On removing the 2 pin connector, idle jumped to 1800rpm. just because it’s opening fully, doesn’t mean it’s closing fully as well. This means either idle air valve is bad or it is not receiving the correct voltage from the idle control unit to close it fully. Cleaning will NOT fix a bad valve. It helps clear it up and keep it smooth, but essentially the idle air slide valve is operated by 2 solenoid resistors and no amount of cleaning is going to rectify those if they are bad.


IDLE CONTROL UNIT– on post 85 cars, this idle control unit (held in a plastic bracket behind the big plastic plate behind the carpet in the passenger footwell) if faulty cannot be opened without special equipment, since they are solid state sealed units, very much like relays.
On pre 85 cars, the idle control unit is located on the nearside engine firewall, it can be easily opened and checked for bad solder joints, and is situated in the engine bay, near the ignition ECU.

 

[Francois]

Thank you very much for all the info!

This is going to keep me busy for a while!


Francois Schutte
380 SEC

 

[Francois]

I am going to give this car a good service, but I can not find the specifications for the spark plug gap size. Could someone help me with this?

(Metric if possible)

 

[Francois]

Found the problem at last. It was the computer controller. I have replaced it and things aer running smooth now. Thanks Talbir for all the help.

 

[Francois]

Replacing the control unit did solve the extremely high idling problem, but the engine is still idling at 1000 rpm's in Park. This drops to about 850 in Drive.

When I switch back to Park after putting it in Drive, the revs shoot up and takes about 2 seconds to come down to 1000 again. If I should drive at a speed that equals about 1000 rpm's and then put it in Neutral, the revs does not shoot up and stays at 1000.

Could this be the Thermo Time Switch or Cold Start Valve/Injector?

Anyway, where is the Thermo Time Switch located? (Yes I still don't know... )

 

[Francois]

Anyone?

 

[talbir]

Have you checked the throttle valve switch ?

Sounds like this is the most likely culprit.

What about the idle air valve ?




talbir

 

[Francois]

Talbir,

I have checked the air valve and it seems to word OK.

I looks like the throttle switch is fine. I am curious about the fact that the rpm's shootup when going from D to N or P, and then comes down to 1000. This only happens when the car is standing still. Should I let it move at 10 or so km/h then it doesn't shootup. It looks like something is trying to keep 1000 rpm's. Would the gearbox switch that tells the engine that it is in D be faulty?

B.T.W. Where is the Thermo Time Switch located?

 

[talbir]

The 'surge' that you describe points to the throttle valve switch or idle air valve, not the thermo time switch.

There is a large 3-pin adaptor on the inlet manifold -with ignition in pos II with throttle closed, one pin should be earth and with throtte fully open, other pin should be earth. The centre pin should be earth at all times.

How have you checked your idle air valve ?

What is the idle speed on cold start-up ?

After cold start-up do revsdrop after a couple of minutes when engine has warmed up ?

 

[Francois]

Hi,

To answer your questions:

1. How have you checked your idle air valve ?

While idling, I disconnected the electrical connection to the air valve. The revs emidiately went up to 1500 and stayed there. When reconnecting, it came down to 1000 again.

2.What is the idle speed on cold start-up ?

Immediately after the engine starts, it shoots up to around 1800 and immediately comes down to about 650 to 750, then it takes about 10 to 30 seconds to get to around 1000 rpms.

3. After cold start-up do revsdrop after a couple of minutes when engine has warmed up ?

No, the revs climb untill it's reached 1000 and stays there.

Thanks Talbir for all the help. I know this is getting drawn out. If it is easier, we could always correspond using e-mail and then place the answer in this thread when we have solved the problem...

 

[talbir]

Hi Francois,

Your answers for me confirm that it is the idle air valve :-

- the fact that the idle shoots up to 1500rpm when you disconnect the adaptor on the idle air valve only confirms that it is opening fully....it doesn't mean that it is closing fully

- the fact that immediately after cold starting your revs come down to 650 and then climb to 1000 as car reaches operating temp means that your thermo timeswitch is cutting out correctly. The fact that revs climb to 1000 as operating temp increases confirms that your idle air valve is not closing properly.

- as a final check, worht checking the small green bypass pipe on the idle air valve hose connection. The pipe is small, about 5cm or so- it may be loose or cracked, which would result in your high idle.

I am 99.99999% certain it's your idle air valve. But before you buy a new one (150 pounds from MB) I would reccomend you try a breakers. Ask if you can test the item on your car - if it solves the problem you'll buy it, otherwise you'll pay 10quid for test usage. I have good contacts with most Merc breakers and can help here if you require.

Where are you based ? I am in Heathrow and my 500SEC (1985) has an identical idle air valve to yours. You are welcome to pop round and I can plug my idle airvalve onto your car and see if it solves the problem.

 

[Francois]

Hi Talbir,

Thanks for the info. I shall certainly have another look at the air valve and maybe try a breakers or scrapyard. Would this account for the seemingly high fuel consumption?

Thanks for offering to help me test the valve, but unfortunately I am currently residing in Johannesburg, South Africa and would have to decline your offer. :wink:

I see more of these cars around these days and I am sure that I could find someone that would be able to help me with this. Otherwise I would just have to go to the MB dealership around the corner ask them if they could assist me.

I have been very reluctant to go to an BM dealership to help me because they are more interested in selling you a new car than to keep your car running, and techs there don't know much about the older engines.

Thanks for the help!!!

 

[talbir]

The high fuel comsumption is being caused by the high idle - your car is idling at 1800rpom to begin with, then after dropping to 650 or so for a few mins jumps upto 1000 and stays there. 1000rpm is 450rpm more than normal, so that's where all the fuel is going.

I would very much reccomend this problem is sorted asap - as your gearbox is suffering all the time you have a high idle. The gears are designed to be engaged at 550rpm - 1000rpm is putting unneccasary load on the tranny.

I think the idle air valve is worth changing at this point - it's seen 20yrs of service. A new one will do wonders to the idling and smoothness of the engine. It can sometimes be cleaned with carb cleaner/wd40, but if it's bad cleaning won't usually cure it.

BTW, did you check the bypass pipe for cracks ?

 

[Francois]

I had a look at the bypass pipe and it looked sound.

What would happend if I were to remove the valve and plug the two pipes it is connected to? (For testing?)

Would a '87 560 SEC's valve work? I can get hold of one for testing to make sure this is the problem. Over here a new valve costs around 250 British pounds!

 

[talbir]

the 560 valve is very slightly different but you could probably just about use it for testing. The only real difference is the size of the inlet and outlet pipes on the valve - but it's a very small difference.

The solenoids in the valve may have different voltage workings but I don't know for sure.

Try it - may work well.

Would you like me to see if I can get my breaker to sort you one for testing ? Post it backif it doesn't work...

 

[Francois]

Thanks for offering, but I think I would rather look around my area and see if I can come up with something. If all else fails, I shall let you know. I can borrow a 560 valve and have a look see.

Where does the throttle cable connect to the gearbox?. My car always starts off in 1st and I suspect this could be becuase of this cable being to tight. ( Although, at full throtlle it changes to second at 4K rpms.)

 

[Francois]

For the record,

I did manage to get hold of a idle control valve for testing. The results were the same.

While fiddling with about everything else in the engine bay, I found a loose vacume tube. (Yellow and White). It comes from a device on passenger side of the engine bay. As far as I know, this device lets the engine know when the aircon is switched on. I don't have clue where the other end of the tube has to plug in , since there aren't any obvious places it could go.

Any help with this shall be creately appreciated.