W126 500 SEC idle problem

[ironduke]

Hello!

New around here but on my third 126!

I recently bought a 500 SEC which is lovely and very loved (full and mostly MB history) However it's got a niggle... at idle it will occasionally cut out, always restarts fine and is fine when off idle. Sometimes it will be fine all week and then do the "death at idle" thing twice in the same day.

It's irritating as a) there's no pattern to it and b) it's embarrassing as due to the nature of the beast a lot of carpark speed driving or even just around town coming off roundabouts etc is done at very love revs at/around idle.

I've done a bit of digging and am I right in looking at idle control valve or relay?

What should I be looking at first basically, hints and tips welcome! thanks.

 

[ianrandom]

Yup, you can clean the idle valve. Yours might need a new fuel pump relay though. Try the idle valve first I would as it's very easy.

 

[grober]

If you have the motorised fast idle device and not the bimetal strip type then its driven off the OVP relay - so that's the one you need to check /resolder for dry joints. There's also a throttle closed microswitch it might be worth checking.

 

[jaymanek]

Does your car have ASR?

This causes issues...

Also worth changing the OVP relay as this causes issues with pretty much any electronic device linked to the engine.

 

[horatio]

I had the same problem on my 190e and it was just a split vacuum pipe (leading to the idle valve). The split wasn't visible until I removed it.

Have a good dig around, check they are all in good condition and everything is connected properly before splashing cash on parts!

The idle valve can be soaked overnight in meths to clean it out

 

[PACERC250T]

Another common reason for the SEC to cut out is if the mixture is too high. Although this usually happens (but not always) when slowing down at junctions; after over run

 

[ironduke]

Well an update. I THINK I've cured it. I took the Idle Control Valve off and gave it a good soak and clean with meths and then a blow through (LOTS of muck came out) and the problem seems to have gone away...fingers crossed!

 

[arfan786]

i have a similar problem with my sec just started doing this yesterday it will start up fine sometimes but once i have it started up then switch it off the car wont start again i will have to wait for abit give it a few tries before it eventually starts? is this fuel pump relay? or ovp?

any ideas?

 

[ironduke]

i have a similar problem with my sec just started doing this yesterday it will start up fine sometimes but once i have it started up then switch it off the car wont start again i will have to wait for abit give it a few tries before it eventually starts? is this fuel pump relay? or ovp?

any ideas?

Mine always started fine (and re-started fine after having a tizzy) It was just annoying having it die at idle speeds...which you end up doing a lot of with a big V8 auto and can't keep the rev's up "manually" in the same way you can with a manual.

 

[HowardP]

I am new to this forum but spotted this thread, which touches on a recent issue that has arisen with my 1989 420 SEC.

But first, I should mention that when I bought the car in 2016 it had the common high idle problem, and in the end I replaced the OVP relay with the newer twin fuse type, had the Idle Control Valve professionally rebuilt, had the Idle Speed Control Module tested (confirmed OK) and finally, had the mixture adjuster assembly on the throttle body replaced, because it had seized.

All has been OK for the last two years, although I admit to using the car very little because I was restoring the bodywork and various mechanical bits. However, now that I am using the car more I find that the revs drop to about 400 when D or R is engaged at idle, which, as the original poster said, makes low speed manouvres very tricky. Unfortunately the issue is not consistent and will sometimes behave perfectly normally, by which I mean the normal idle speed is maintained when sitting in traffic with D engaged (a bad habit, I know!).

From what I've read a leak in the vacuum system is more likely to cause a high idle than a low one, so is there any other key compoment in the idle circuit that I might have missed? Is the Throttle Position Switch potentially a factor perhaps?

I would add that the problem does not appear with a cold engine, only when it's warmed up, which leads me to think it might be mixture related, but I don't know enough to be sure.

I did recently get hold of a brand new Idle Speed Control module, but that made no difference whatsoever.

The idle is somewhat lumpy as well, with occasional surges to 1,100 rpm or so when idling, but the engine has done 170,000 miles, so I should not expect perfection I guess.

Any helpful advice would be gratefully received.

 

[alabbasi]

Mileage should not have anything to do with idle , especially if the problem is intermittent. A small vacuum leak will create a high idle issue , a big one will result in it not wanting to run at all. Check the vacuum distribution hoses that run on the inside of the cam covers. Because you don't run your car often, your fuel system may be gumming up. The fuel pressure to the injector is driven by the fuel distributor and metering device. If you're not getting adequate pressure to the injectors , it will run poorly at low speed. Increasing speed will increase pressure and over come the issue. You should consider removing your injectors and cleaning them. It's due.

 

[HowardP]

Thanks Al. I've checked the paperwork I got with the car and the fuel metering unit was overhauled in 2009, when the mileage was 133,000, and at the same time the injectors and throttle body were replaced, so I am ruling those out, at least for the time being. However, it probably wouldn't hurt to remove an injector to check the condition, including the seal. I'll put some Injector Cleaner in the fuel as well - it can't do any harm.

Normally I replace the ignition components as a matter of course on any old car I buy, but on this occassion I haven't, because I noticed the distributor cap and rotor were also replaced in 2009. However, just having checked the Bosch distributor cap thoroughly, I noticed there is a hairline crack running from one side to the other! Not good. Presumably because of this the central contact pin has started to wear away the plastic on the rotor arm, so both will have to be replaced. There is a moral here somewhere, apart from it being a schoolboy error.

I also checked the ignition leads, which are Cambiare (who?), and I will be replacing those as well, just to be sure, because there is no evidence of when they were installed, and they appear to be one of the cheaper brands. Just for good measure I may also splash on a good quality coil.

I am also toying with the idea of replacing the vacuum hoses for the idle system anyway, which don't seem to be expensive, at least in the USA. My main dealer in the UK might have other ideas though, although sometimes they can be more competive than aftermarket suppliers - all very hit and miss.

 

[alabbasi]

Sounds like a good plan. CIS injectors rarely need replacing but it's important to run the car often so that they remain clean. They are basically a brass nozzle so ethanol in sitting fuel will gum them up if the car sits for too long.

 

[HowardP]

I fitted the new distributor cap and rotor arm, which only resulted in a slightly smoother idle, but no change to the original issue.

Onward and upward!

 

[HowardP]

I've been doing some dismantling this morning and have discovered the following:

All of the vacuum hoses in the idling system have gone hard, unsurprising, since I imagine they are the originals. Although there is nothing obviously amiss, I might change tham anyway since the full set from M-B is less than £100. Peace of mind for the future.

The Air Temperature Sensor that fits in the right hand air intake looks past its best to me, so might also need replacing. Should it really look like this? (picture 1).

The ignition lead bracket on the right cam cover is broken, although the holes were covered with tape, so hopefully not significant, but still needs sorting out. (Picture 2)

I also noticed that the plastic piece that goes between the connections to the ICV was not seated correctly. What IS the point of that, since it sealed at one end? (Picture 3)

 

[alabbasi]

The holes for the lead holders won't cause a vacuum leak as they don't go all the way through. Certainly vacuum leaks will have an impact on the idle. You can use propane or carb cleaner to help sniff out leaks while the engine is running or find someone who has access to a smoke machine.

 

[HowardP]

Thanks. The injectors were supposed to have been replaced 9 years/35,000 miles ago, but I have my doubts. I'll try the method you suggest to check for leaks , although apparently WD40 can be used as well.

BY sheer luck I've found your article about cleaning the injectors - very useful! It's added to the list!

 

[HowardP]

I've now bought a few bits and pieces from my local M-B dealer, i.e. vacuum hoses, injector seals and air temperature sensor:



Vacuum hoses and injectors removed:



According to an invoice, the injectors were replaced in 2009 - I don't think so!:



Now to do a bit of cleaning up.

 

[HowardP]

Yesterday I finished putting everything back together, with a new set of vacuum hoses, injector seals and an air temperature sensor, which seems to have solved my idling problem, although I now have another, so I've started a new thread.