'73 450 SL electronic fuel injection

[steve--o]

yes ive got mi hart set on a 70,s sl convertable,if i was in uk,i would buy the slc as both models are now very cheap to buy for projects.
im in sunny Almeria spain so i might as well spend the extra 1k and get the convertable.
we can work here outside,welding & spaying as the weathers normally 15 to 45 deg.
yer carnt earn mutch brass here but the weathers great for outside projects like cars & boats, & brit bikes plus yer dont need a big workshop and yer dont have to wait for 3 hours to rub yer filler primer down.
cheers
Steve

ps if anybodys got an mot,able SL project for sale please send pm,the motor / box must be good tho,dont mind welding but dont like wonki motors !

 

[osc]

On second thoughts, fixing it is not a realistic option. OSC, could you possibly read the Bosch number off the side of yours please if you get a chance?

Thanks, Dylan

will do when i am next out there...actually they are interchangeable but there is a VERY VERY VERY delicate adjustment.......

 

[Isdyldan]

Thanks. Actually since I wrote last I have been told by others too that they are interchangeable. It is possible that my engine is running very rich, and any extra richness brought on by vacuum to the MPS is just flooding it. It will not run at all if I disconnect the electrical connection to the MPS, but also will not run if I CONNECT the vacuum line. Strange that the ECU behaved similarly in your car - it still could be an ECU fault and the fact it currently runs is simply luck.
I am currently concentrating on connecting the gear shift linkage (fiddly), and removing the fuel tank to unblock the fuel return (petrolly).

 

[Isdyldan]

Hello All,
The car is now starting and moving forwards and backwards. However, it is still running only when the vacuum pipe is disconnected from the MPS, and after 5 mins or so (ie when it starts to warm up) it begins to misfire badly. I was reading back on previous posts and AP said that if the vacuum pipe is not connected to the MPS the engine will run very rich. I was under the impression that applying greater vacuum to the MPS would richen the mixture - is this therefore wrong? I can't work out if the whole thing is too rich or too lean, but I know that after 5 minutes of running there is not enough power to get back up the slope into the garage!
Regards, Dylan

 

[osc]

Dylan: its not as simple as that. I would investigate by cleaning up a plug (or ideally all of them) till spotless and then run her until she looses power then check the condition of the plugs. If sooty and wet then too rich. If dry then too lean.

If your car starts OK after cooling down then I would go with too lean because once the plugs start to get sooty they generally continue to get worse until you fix the problem. Mine would go too lean due to poor fuel flow rate. At idle the MPS will see maximum pressure differential and try and richen the mixture. I think the same think happens when you put your foot on the gas. But on part-load (when accelerating) it will lean out to be more efficient and not dump unburnt fuel out the back (misfiring) although I love it when mine goes pop bang when you ease off after flooring it! The MPS is a very sensitive beast.

One of my coil input wires had started to fray and got so bad that it was affecting the strength of the spark...which meant poor combustion and rich running to the point that the plugs became so sooty and wet she would not start. After fixing the coil wire I had to remove and clean all plugs to get her started again.

The D-jet is sensitive with its bits 'n' bobs!

 

[Isdyldan]

Thanks OSC, and happy new year!

I tried this today. As it warmed up, I didn't get quite the same pattern as before - it didn't really do the missfiring thing, and had plenty of power going back up the slope of the garage. Maybe last time I had left it idling too long and the plugs had got fouled - who knows. However, as soon as I connect the vacuum pipe to the MPS it runs rough and miss fires, also popping out of the air inlet (which this time did not have the air filter on - maybe that made a difference?). This bad running happens if I put either MPS on - the original which leaks or the new (replacement) unit. The electrical connection needs to be made to the MPS though, or the engine will not start at all. It does point to a bad MPS, but why both of them?
It does sound great without the MPS though! Nothing like the 380SL I had - much more 'brutal' like yours!

 

[Isdyldan]

I forgot to say, after running up to temperature the plugs looked ok really. Not wet, and not overly sooty. I can't understand what could cause this problem. Does anyone know what happens if they pull off the vacuum pipe to their MPS please? My engine is obviously running ok without the MPS and then rough (rich I think) with it connected. So does this mean something is causing rich running, and if so what could this be please? The knob on the ECU seems to make little difference, and this goes for both the ECUs I have. Could there be an air or vacuum leak somewhere causing rich running?

 

[Alex Crow]

hi dylan. what fuel pressure do you see while it is running today? with the mps pipe disconnected it ought to be VERY rich - not the other way round.

 

[Isdyldan]

Ah, that's interesting thanks AP. The fuel pressure was 2.2 when I last checked, but that was with the MPS connected and running rough. Will test again tomorrow with pipe disconnected and see if there is a difference.
What you say then must mean that the whole system must be running very lean when properly all connected up, so that it is happy with the MPS vacuum pipe disconnected. If I see low fuel pressure tomorrow I will be very happy!!

 

[Alex Crow]

that is indeed my thinking dylan. it is so unusual for these to be lean, going rich and fouling plugs is the usual trouble. as osc says, if it is rich the plugs will foul VERY quickly

 

[television]

Am I missing the point here,,if a vacuum pipe is off then the engine will idle to fast, and this will not affect the mixture.

The Co2 level is done by adjusting the screw in the fuel distributor, and hard to adjust to get lean enough and still idle OK

 

[Alex Crow]

yes malc, you are .

this is the L-jet sequential that preceded the k-jet cfi. the manifold pressure sensor is super critical on this system, and it is the vacuum pipe that communicates manifold pressure to it that we discuss here. the mapping on later hfm systems uses the air mass value as the major fuelling influence, the late(ish!) pms systems did use a mps, but took greater influence from the tps for fuelling. obviously all 3x systems also take rpm and other factors into account to some degree too. so with this archaic proto sfi system, the mps signalling atmospheric pressure calls for full throttle fuelling. yes, there is a sensor on the throttle valve too, but it is used for signalling idle/full throttle positions, and data for speed of throttle opening.

i personally detest these systems, as i detest all merc carburettors and k-jetronic. ke-jet i can bare, and hfm/pms/me is ok i guess

 

[television]

Thank you and I will pass and go back to sleep


Are you up and running

 

[Isdyldan]

Sorry, too cold to work on cars in an unheated garage today! I did have a quick poke about to see why the power steering wasn't working - no fluid. Probably important I imagine. :Oops:

 

[Alex Crow]

Are you up and running

oh so very nearly! i will soon (within 2 weeks i hope) be bombarding the forum with photos of the workshop, and updating my profile and signature with some graphics and logo. just not quite there yet, but have most stuff on board now, thanks for asking .

 

[Isdyldan]

Hi - fuel pressure when running with the MPS vacuum pipe disconnected is about 2.1 bar. For some reason the pressure is not maintained when the engine is switched off now - it falls rapidly to zero. I can't work out what is different to before though. I have unblocked the fuel return and reconnected it but for the last few months I have simply had the fuel return via a tube back to the fuel filler, so can't see why this would be any different. It seems to indicate that fuel is either going back through the pump when it is switched off, or that the regulator(s) are leaking through, but they maintain 2.1 bar when the engine is running. Any ideas please?
Regards,
Dylan

 

[osc]

i think your pump could be knackered, mine was too and you do have the original style pump which would be 20ish years old. the fuel system MUST hold good pressure for at least an hour when you switch off the engine. replace with a bosch aftermarket - the porsche pumps will work a treat (DON'T use a copy or pierburg from GSF because they won't last.)

and i remember that fuel seeps out of your regulator when you touch it, so just change it

 

[Isdyldan]

Will do - thanks OSC. It can't do any harm to get a new fuel pump as long as it's not too expensive. Odd though that the pressure sticks at a constant 2.1 Bar with the engine running - you'd have thought that this is the major requirement.

 

[osc]

mine would also get to 2.1 bar and hold when running but there was no fuel in the return line, something fishy here and you have something similar going on. i replaced both pressure reg and fuel pump and fixed it solved all my problems!

 

[Isdyldan]

Hi all, just in case anyone was wondering, I haven't given up - just temporarily run out of cash. I will continue, but it will be a few months before I can resume due to my main car packing up & needing replacing!