500SL R129 misfire - anyone got any ideas?

[jefft]

Hi All,

My R129 is up to no good again :sad:. A previous habit of the revs dropping/surging occasionally at idle - annoying but minor - has turned into a full-grown misfire. My local Merc dealer was offering free health checks, so gave it to them today... "Major misfire" was their conclusion, along with an estimate of at least 2 hours diags to start with.

Before I give it over to Merc or someone and pay a fortune for diagnosis, thought I'd check whether anyone has any ideas!

The problem manifests mostly when the engine is properly warmed-up (oil temp showing around 85~90degC). Then, we have symptoms of a bad misfire - rough idle, dying & surging, stalls, no power on pulling away, etc. Feels like it's idling on about five cylinders - might be enough for some cars, but this one likes all eight! Seems to run fairly smoothly until it's warmed up, although not 100% perfect.

For background - caps and rotors are looking ok, they're only about 4,000 miles old anyway. Plugs ditto, no signs of coking or overheating there. Tried the water spray trick and couldn't see any signs of tracking from the leads.

Any thoughts? Ideas gratefully received! :-D


Thanks in advance,

Jeff

 

[Richard Moakes]

You have ruled out my first thoughts, i.e. rotor arms and distributor caps, mine had similar problems and new caps/arms made all the difference.

The only other areas could be coils, EZL or sensors, unless this is a fueling issue which is masquerading as an ignition fault.

On the basis of looking at cheaper stuff first, then I would start looking at the coils, HT leads and cam sensor. Also consider if the fuel pump is failing when it warms up and is starving the engine of fuel?

Final thing to check/change is the EZL, but that is awfully expensive

 

[Alex M Grieve]

Hi All,



The problem manifests mostly when the engine is properly warmed-up (oil temp showing around 85~90degC). Then, we have symptoms of a bad misfire - rough idle, dying & surging, stalls, no power on pulling away, etc. Feels like it's idling on about five cylinders - might be enough for some cars, but this one likes all eight! Seems to run fairly smoothly until it's warmed up, although not 100% perfect.

I agree with Richard - sounds like an ignition problem (arking was suggested before, but you seem to have excluded that?). I suspect it is running on 4 rather than 5 cylinders, and that one bank is compromised when the fault presents itself.

So starting and warm up are OK, then it goes wrong. Does switching off and restarting solve the problem, or do you need to wait for the engine to cool down again?

 

[jefft]

Thanks guys...

Found out today that it's not exclusively a warmed-up fault, it does run rather lumpy from cold though not consistently so.

Had a thorough look at the HT leads today (all out), and cleaned the caps again for good measure. One of the coil-to-cap leads has some insulation damage - seems to have been melted slightly. I've taped that liberally until I can get a replacement, but seems not to affect the running greatly so far, still idling rough.

While I had things apart, also noticed something I'd not spotted before - the rubber hoses from the cold start injector to the main injectors are ageing and some have small cracks in them, especially those closest to the injectors. I'll get some replacements for those ordered and see what happens - I'm sure a sudden input of extra air into the system through a split hose wouldn't be helpful and that could be something that worsens with heat.

I've half an eye on Richard's other suggestion - fuelling. The die/surge behaviour could be either that or ignition, I agree. Not sure what to check there - fuel filter was replaced about 5,000 miles ago and testing injectors and fuel pressure regulators is beyond my toolkit. Maybe that's the next stage, one to get the Indy to do, if the car will run well enough to cover the 15 miles to him!

I'll get those hoses and new coil HT leads and report back...

Thanks for the ideas so far!

Jeff

 

[jefft]

Oh, and... anyone know how to test a coil (or who can test one properly - auto electrician, perhaps?) ?

I've checked the impedance of the primary side, which seems ok on both, but no idea how to test the thing's real job of producing a strong spark. My ignition/spark test lamp shows sparks are there, but can't tell me how strong they are.

 

[puter]

Have a good look in the dis cap for evidence of tracking. Sometimes it is difficult to see, more like oil on water effect. Clean well using a pan scourer, not too abrasive, and apply silicon MS4 compound and wipe carefully with a lint free cloth. Good luck.

 

[jefft]

Well, quick update... got most of the hoses and stuff on Thursday, fitted today. Unfortunately one of the injector collars which the air hoses connect onto had become brittle and decided to break, so now I'm waiting on yet more small parts!

Update later...

 

[jefft]

Progress, I think!

Well, so far a productive weekend.

Hoses and such arrived on Friday, fitted them on Saturday and a short test drive was ok. Somewhat unsteady idle for a short while after cold start, but settled reasonably well and the warm idle seems smooth - if not exactly perfect silky V8 stuff.

Took it to a wedding on Sunday, just returned this morning. No problems on either leg, about 45 miles each way and mostly at M-Way speeds with towns on either end. Haven't yet seen the temp gauge hit the magic 90degC, so I'll have to reserve judgement on whether this problem is really sorted or just waiting to pounce when I next find a traffic jam...

Fingers crossed!

So far and thanks for all the suggestions guys

 

[jefft]

The saga continues... R129 idling probs

Well, it's behaving fairly well when running along, but the bad habits at idle are still there. Drove about 15 miles yesterday and car behaved fine. Then took it out again later and within 5 miles it surged and stalled while waiting at a light. Carried on playing up intermittently on the way back home. The difference between the two journeys was the temperature - first journey, I don't recall seeing much over 80degC on the gauge. Second journey, some slow traffic and it hit 90degC.

So, definitely something in this about reaching 90degC. I can't think of much that would kick in at that point, though - the auxiliary fans were off, they don't come in until 100deg. My other line of thought is that something like the idle air control valve doesn't like getting hot and sticks, but I'd have thought that would be more to do with time than oil temp as it's relatively isolated from the engine block. Will hook up a meter and take it for a drive, see if the control current gives me any clues just in case.

Guys, any other ideas what would change the way the 129 idles when the temperature's around 90 ? The temperature sensor itself seems ok - took it out and checked the impedance in hot water and it's close enough to spec.

Richard, you mentioned fuel pumps - any thoughts on how to test them or to see how they're behaving while the car's running?

Any ideas welcome!

Jeff <puzzled>

 

[jefft]

Ah ha, so...

Found an entry on the Mercedesshop forum about checking out the EHA (fuel supply actuator) and the idle air valve. So, hook up two test meters and away we go for a drive. Eventually, after stopping, parking and setting off again, the fault shows itself.

Two things noteworthy:
(1) The EHA current is almost always -ve 0 to 2mA, which suggests the car's still running a little on the rich side even after my Merc guy adjusted it last visit.
(2) When the engine stumbles, the current to the idle air valve drops from its normal 950mA idling to about 600mA. Mercedesshop article says normal is 600~700mA and they saw a similar fault drop it to 300mA, but the numbers may just be from a different car.

Apparently the most common cause of that valve's current dropping off is a dodgy OVP relay - that old favourite.

So, three steps now:
(1) Adjust the mixture to run a smidge leaner - done, seems fine, initial feel of the high (650rpm) idle being a touch smoother;
(2) Replace the OVP relay - it tested ok on the bench, but they're not that expensive and replacing it rules out any intermittent fault with it;
(3) If that doesn't do the trick, then the idle valve will have to be next... a little more painful at near 200 quid.

Watch this space...

 

[television]

I admire the way you are going about it,,current checks tell you so much more as to what is going on.

Most of these things are pulse controlled making voltage checks difficult.

The idle valve is a common fault and hardly worth buying second hand as they all seem to go.

checking the voltage going into the main ECU should tell you the condition of the OVP relay

I look forward to the update

 

[jefft]

Improved, but not entirely fixed

Well, the saga continues!

Replaced the valve. Also the OVP - no symptoms previously from it, but interestingly, as I started the car to go to Mercedes and get the valve, the ABS and ASR lights came on and refused to go out. Classic OVP symptom, so replaced that too.

The idle still surges occasionally (about 100~200rpm increase briefly) when the temperature is over 90degC, but there's no longer the sudden crash in revs and stalling risk that there was previously. Suspect that's down to the valve.

I'm guessing now that the surging may be a fuel regulation issue, but it's not a major worry at the moment so it might wait until the beast next needs servicing and then the guys at Keys can look at it. The weather's turned distinctly to the "don't want to be crawling around on the driveway" side now!

Still, that's some kind of progress...

Jeff

ps - Malcolm, just noticed your footer - sorry to hear about the valve caps!

 

[rayhennig]

Stalling, surging etc

Hello jefft,

How did your problem resolve?

I now have an issue that the 300ce-24 (after it's new CPS!!!) still has its idle surging but now, when it's warm after about 15 minutes, it starts to lose power.

It would keep going if I kept the revs up but when I was forced to stop for a T junction, it died!!

I had to wait about 45 minutes for it to cool - refused to start during that period apart from once and it was groggy.

Once started, I kept the revs up and managed to get home.

I feel I'm on the verge of finding out what's been making this car stall over the 11 years I've had it.

Many things have been replaced, including:

Dist cap, rotor, leads, plugs (Bosch copper), ICV, CPS, CamPS.

Basically, I cannot now go far afield although last time, when I returned, it did not die but surged a bit before settling into a smooth idle.

Shall I torch the thing?

RayH

 

[jefft]

Sounds like you're a man with a lot of patience, Ray... 11 years of stalling?!

My SL was sorted in the end by a combination of things:
(1) that idle air supply valve (350 quid, ouch!) - definitely involved as it's a key control over the mixture at idle
(2) the OVP relay - electrical tests and other symptoms suggested that was on its way out and I guess, failing intermittently
(3) the mixture, which was set way too rich - and not noticed by two Merc specialists - until Paul at Keys checked it.

It ran nicely after that lot was resolved, give or take the occasional (typical 119 engine) wobble if there's condensation in the distribs or the contacts need cleaning.

My theory is that the idle air valve was not opening/closing smoothly or fully (I bench tested it best I could) and so causing the sudden drop-off in revs, probably helped by the OVP relay, which ultimately supplies the power for that and the fuel supply actuator (EHA). The air supply problem would make the revs drop, which coupled with a rotten mixture, would flood the inlets and stall the beast.

Whether any of that is relevant to a 300CE, which is a rather different engine, I don't know, but good luck!

J