1996 Mercedes SL 600

[Hereward]

I have tidied up the front brake calipers with a coat of rust treatment. The front pads and discs are in great shape, having being renewed by the previous owner just before I purchased the car. I also removed the front wheel arch liners and cleaned out the drain hoses that handle water coming off the windscreen. One of them had a plug of crud so good to sort out. The front ADS spring actuators ("spheres") and damper valves are also housed here so they were inspected and coated with corrosion protection. The wheel arch liners do their job well with minimal corrosion in the area, except on a few tired looking bolt threads.

I also squirted corrosion inhibitor in to all four jacking slots. Two had corrosion, the other two were in great shape.

I had noticed recently the car's idle was a little erratic so I have started the process of fixing it. The two simplest items to attend to first are to plug a tiny crack on the vacuum distributor block and renew the fuel pressure regulator:

Position of Vacuum distributor block, mounted on passenger-side (UK) engine bulkhead. Engine out of shot to the left, windscreen out of shot to the right. It's the unit with red and yellow hoses attached:


Close up you can see a small crack next to the "ball bearing" that can introduce unmetered air in to the system. This needs to be covered with epoxy resin to be made air tight. I think these are common across all R129's so if you're an owner have a look.


Next job was to replace the fuel pressure regulator. This is easy to access being mounted on the top of the engine (see the red arrow in the pic):


The old unit is easy to prise out after removal of the retaining circlip and the new one pushes straight in easily. I lubed the two o-rings with dielectric grease to ensure no tearing. New one on the left. Slightly different design so hopefully no operational issues:


I will go for a drive tomorrow if dry and see if these have made any difference to the idle stability. I have also cleaned the MAF's. If not then there's a rats nest of vacuum hoses to test and inspect...

[muchacho]

Strike me roan, you must be a proper vacuum leak sleuth to identify that leak. I was only changing the injectors on my 190 2.5-16 last weekend and thinking that I could be staring at a whole forest of vacuum leaks. Nice car Hereward - have always admired the R129.

[Hereward]

It’s been a stressful few weeks. After changing the fuel pressure regulator, sealing a potential vacuum leak and spray-cleaning the MAFs I went for a test drive. All went well until I used kickdown which immediately put the car in to Limp Mode and illuminated the BAS/ESP orange warning light. I crawled back home, put the car away and sulked. After a few hours I started the car and all was well – turning the engine off cleared the problem.

The following Saturday I took the car to my independent specialist. I drove sedately to avoid using kickdown and all was fine. They plugged in the car and read and cleared some codes then I went for a test drive. Same problem again – use of kickdown instantly puts the car in Limp Mode and activates the BAS/ESP light. Limp Mode also sends the idle crazy – I assume it uses a fixed set of fuel/air mixture values to run the engine, bypassing the MAF/O2 sensor data?

The codes were read again and it was suggested the first thing to do was replace the O2 sensors. I re-started the car (clearing the problem) and drove home carefully. The garage were brilliant, the owner refused to charge me for his time and use of the code reader.

I ordered a pair of O2 sensors and also purchased the Carsoft diagnostic software so I could read codes myself at home.

Last night I replaced the O2 sensors and went for a drive. After the engine was warm I used kickdown and Boom, same problem again. Bugger. The car is absolutely fine accelerating hard UP through the gears or pottering around, it's only kickdown conditions that trigger the issue.

I read the codes and have the following:
P0150 – O2 sensor (despite them being brand new! Maybe this in an old code from the drive home from the garage with the old sensors in place)
P0819 – Autobox Shift switch function
P0102 – MAF
P041C – EGR temperature sensor
P0068 – MAF / MAP / Throttle relationship
P4F01 – No idea what this is…

It looks like I am going to admit defeat and send her back to the garage for them to sort out. I cleaned the MAFs but maybe they are simply past it. The automatic transmission error is potentially interesting. I have read that replacing the “Conductor Plate” on the 722.6 gearbox could be something worth investigating.

Annoyingly I binned the old fuel pressure regulator valve. I would love to refit that and see if that made any difference, since the aperture on the new one is so much smaller.

On the plus side I did find a large crack in the air intake hose between the MAF and throttle body. I have temporarily sealed it with electrical tape so hopefully that will assist the idle by banishing some unmetered air.

Car parked with engine running in Limp Mode with the BAS/ESP light illuminated. Look at the idle speed!:



Diagnostic results:


The hefty crack in the air intake hose just before the throttle body. The crack is at the tip of the screwdriver. My headtorch is inside the hose, illuminating the crack:

Edited by Hereward on Monday 10th April 21:29

[Mikeeb]

I would suspect that fuel pressure regulator can't flow sufficient fuel volume due to the much smaller hole.

This could quite easily cause it to run lean and trigger the O2 sensor/MAF codes.

[Krikkit]

I'd agree - if the FPR has a noticeably smaller outlet you might run into lean running at WOT (especially high RPM) when the injectors are hitting high duty cycles.

Lovely machine, keep up the good work!

[r129sl]

Hereward said:

It looks like I am going to admit defeat and send her back to the garage for them to sort out.

I can't help, but you have no idea how much this cheered me up. I'm glad it's not just me.

[Hereward]

Damn, the problem is not the fuel pressure regulator. I found the original big-bore one late last night and immediately legged it to the garage, fitted it and went out for a triumphant drive. Once the engine was warm I booted a kickdown and immediately triggered Limp Mode and the BAS/ESP orange light.

I need to investigate the P0819 code further and identify the relevant hardware on my car in case there is a gearbox sensor/electrical issue. The internet states:
https://www.yourmechanic.com/article/p0819-obd-ii-...

I know that this doesn't necessarily mean it is a gearbox issue, though, and the other codes do hint of a fuel/air issue. The gearbox/engine behaves impeccably in all other situations other than kickdown. I drove 30 miles today with no issue, the gearbox shifts up and down perfectly when driven conservatively, it is solely a kickdown command that triggers this. I will clear the codes again, go for a drive and spend some more time playing with the diagnostic software.

I have also been messing around with the adjustable ride height. As you can see in the pic I need to drop the front of the car a little more. This is the car at standard height. It can be raised higher manually via a dash button and will also automatically lower itself a little further when driving at speed.

Edited by Hereward on Friday 7th April 23:03

[Stegel]

Without going over the thread to check, have you changed the pilot bush (the electrical connector on the RHS of the gearbox)? Although my car didn't show any problems, it flagged up a code when I was checking something else - I can't remember the code, but could well have been the same generic reading. I changed it at the same time I changed the fluid and filter, took all of 10 minutes.

[Hereward]

Thanks for that. I gave that connection a visual inspection and it was bone dry, very clean and corrosion-free. I have also checked the ATF level and that is fine. I will get in touch with a transmission specialist.

[Stegel]

Hereward said:

Thanks for that. I gave that connection a visual inspection and it was bone dry, very clean and corrosion-free. I have also checked the ATF level and that is fine. I will get in touch with a transmission specialist.

There was no sign of a leak externally with my car - it is an internal seal that fails, allowing fluid into the electrical connection (the replacement part is simply a tubular body with seals - I cleaned the electrical connection with brake cleaner, reassembled it and no problems getting on for a year later).

[Stegel]

Hereward said:

Thanks for that. I gave that connection a visual inspection and it was bone dry, very clean and corrosion-free. I have also checked the ATF level and that is fine. I will get in touch with a transmission specialist.

There was no sign of a leak externally with my car - it is an internal seal that fails, allowing fluid into the electrical connection (the replacement part is simply a tubular body with seals - I cleaned the electrical connection with brake cleaner, reassembled it and no problems getting on for a year later).

[Hereward]

Stegel said:

Hereward said:

Thanks for that. I gave that connection a visual inspection and it was bone dry, very clean and corrosion-free. I have also checked the ATF level and that is fine. I will get in touch with a transmission specialist.

There was no sign of a leak externally with my car - it is an internal seal that fails, allowing fluid into the electrical connection (the replacement part is simply a tubular body with seals - I cleaned the electrical connection with brake cleaner, reassembled it and no problems getting on for a year later).

Cheers,

Sorry, my post wasn't clear - I inspected the inside of the pilot bushing and it was perfect.

I have bitten the bullet and ordered a new gearbox conductor plate:
https://www.onestopgearboxshop.com/products/m5auto...

Relatively cheap and simple to replace and it controls the gearbox electrics. I have re-watched my Apollo space programme missions for technical inspiration and I would love to be able to resolve this issue without going to a specialist.

I've loosened off all the ATF sump retaining bolts in preparation, so that's a good start. They can seize up, which requires them to be drilled out, so that's a relief.

Since sealing the split in the pre-throttle body inlet hose the MAF error codes have all vanished. I have ordered a brand new hose section from MB, it's ready for collection at the end of the week.

[Stegel]

Ah, I see!

That plate is far cheaper than I expected - good luck with the task, and I hope it works. I've seen it recommended to clean the bolts and apply a ceramic grease to prevent the threads seizing on the sump bolts.

[lukeharding]

Awesome car! would love one of these and they're a pretty great investment too judging by the prices at the moment.

[Hairymonster]

Write 50 lines:

"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes"
"I must not go on Autotrader and start looking for a V12 Mercedes".......

http://www.autotrader.co.uk/classified/advert/2017...

Urban MPG 11.5
Extra-urban 22.6
Combined 16.7
CO2 400g/km

Edited by Hairymonster on Friday 14th April 16:07

[skylarking808]

Just fit an LPG kitand enjoy V12 motoring

There are loads of big mercs that make more man maths sense when you nearly double you fuel economy.

[Schermerhorn]

Amazing motor and a dream car from my childhood.

[Hoofy]

The MPG wouldn't bother me. It's the unexpected costs.

[ZesPak]

Why are topics like this, about cars I'll probably never own, so goddamn interesting?

I've learned about an R129's indicators' serial numbers ffs.

[el romeral]

Looks absolutely fantastic. The lack of wear and age defying nature of the materials used is so impressive. It reeks quality. I remember when this car was introduced, it proved very popular with the super wealthy in Puerto Banus. On a stroll around the harbour frontage it was not uncommon to see several examples.