'73 450 SL electronic fuel injection

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Isdyldan

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Hi - I'm not sure if anyone can remember the problems I've been having, but I'm now digging the old girl out of the garage again with a fresh idea in mind.

The engine starts and runs fine but very rich when the vacuum pipe to the MPS is disconnected and the hose plugged. As the engine warms up, it becomes too rich to run. All the ancillaries are from the original 350 engine, but the installed recon engine is a 450 (as deduced from the engine number). When the MPS is connected, the engine misfires badly and stalls.
Could this be that the 450 engine produces a different vacuum and hence the MPS simply needs recalibrating to suit?

Any ideas greatly appreciated please, and thanks for bearing with me on this one!
 

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the vacuum will be the same give or take a gnats whisker. possibly though the mps gives an inverse signal to the correct one, hard to know without testing the right one as well. if you have the 450 engine it will be a 117 code, as opposed to 116 for the 350.
 
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Hi, Yes, it's definitely a 450 engine thanks. So I need to find out if the MPS is different for each engine. The problem is finding another to swap out - they are incredibly rare it seems. I have seen a web site which describes how to recalibrate it - it involves drilling out the resin, screwing out the 'cap screw' and adjusting the small screw inside. I'm reluctant to do this just yet though in case I knacker it. Has anyone else has experience of swapping these units over please?
 

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Hi Dylan, how are you? Good to see you back but shame that the sl is not working as she should yet. My observations are below:

1) How do you know she is running rich (and not lean) on warm-up? Starting is not the usually the problem on d-jet, warm/hot idle is the interesting challenge!

2) The 450 and 350 ECUs are different in that the pulse width to the injectors will be different (longer in the 450 ECU) so that you can get a bit more fuel into the larger displacement engine. When you adjust the idle screw, one of the things you are doing is adjusting the pulse width. Also the 350 should have yellow injectors but you can get away with installing the blue ones if you have the correct ECU.

3) The MPS in all the D-jet cars is the same APART from the CALIBRATION. (D-jet porsche, volvo and jag systems use the same thing too - it was the Bosch system). On the SL I know of at least of at least 4 variants: 350 Euro; 450 USA; 350 Euro; 450 Euro. American systems had a tighter emission control and EGR values. If I put any of the other MPSs on my car it runs like a dog. The link to the site I sent you explains everything you need to know about performing a recalibration but I think you'll need some very sensitive inductance meters to do this. If I remember there are 2 screw and you need to get them both 100% for the power curve to be correct - I think you will spoil a good MPS and I don't know of anyone that has re-calibrated. Also if it doesn't hold a vacuum then forgot it because its knacked.

3) I think i may have a 350 ECU + MPS that I'd be willing to swap with your 450 unit (as long as yours are working - we can test it in my car).

4) Have you sorted out the problem with the fuel pressure / pump / return line because if this is not right then the car will not run right (trust me I learned the long way on this point)
 

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the vacuum will be the same give or take a gnats whisker. possibly though the mps gives an inverse signal to the correct one, hard to know without testing the right one as well. if you have the 450 engine it will be a 117 code, as opposed to 116 for the 350.

Can't remember exactly but there may not be an MB part number, but they ran on the bosch numbers.

What i do know is the the yellow sticker denoted the 350 calibration and the blue was for 450. The E or A on the same sticker was the Euro or American.

Problem is that are 40 years the stickers sometimes come off!
 
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Isdyldan

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Hi OSC - good to hear from you - you seem to becoming an expert on these systems! The ECU and the MPS which I bought 'recently' came from 350SE, and the MPS has a yellow sticker with an E on it. I have quite a few injectors now - about 7 good blue, and 8 good yellow. There are currently blue in one bank and yellow in the other as it didn't seem to make much difference.

I think it's rich because there's sooty smoke from the exhaust and a strong smell of fuel which gets worse as it warms up.

I'm convinced now I have the wrong MPS/ECU, but am not sure whether just changing the MPS will do the trick. Thanks for the offer of the 350 bits, but it's 450 I need.

I haven't changed the pressure regulators, although I know I probably should give it a try. The engine does run with good constant pressure, and the return line is cleaned out and connected properly now.
 
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Today I drilled out the resin and the aluminium screw cap of the MPS (couldn't unscrew it) so that I could get a thin screwdriver in to adjust it. I decided to try first on the knackered MPS which didn't hold vacuum. I fired her up and offered the vacuum pipe up to the MPS - the engine leaned out and threatened to stall as the pipe came close to connecting. By winding the MPS screw out (anticlockwise) I was able to keep the engine running with the vacuum pipe connected. The engine running improved further until the screw could be turned no further, but it was still a bit rough. I couldn't tell really if it was too rich or too lean. I am now debating whether to try the same with the 'good' MPS - knowing they are hard to come by if I screw it up!
 

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Apparently it is possible to do a repair to a leaking MPS by epoxy/braising the diaphragm. You need to have a decent tool shop to take it apart, repair and reassemble. If you don't think you can then I can have a crack at it for you. If you can get your hands on a very accurate induction meter then I could use mine to calibrate your 350 to 450 euro spec. Otherwise getting those two screws set up will be like p***ing in the wind!

There is a link about a guy that reckons he cracked the calibration on either volvo or saab d-jet but I havn't bookmarked it.

Found it: http://www.swedishbricks.net/faq/fidjet.html

And this - be careful not to cook it: http://forum.w116.org/mechanicals/manifold-pressure-sensor-d-jetronic-repair-attempt/
 
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Thanks OSC.

So the two routes I will now take are to drill out the rivets and attempt to repair the leaking diaphragm on this broken MPS, and also to make up a list if I can of old cars with the same d-jet system and MPS with a view to getting a few spares to experiment with.
I think I should keep my good 350 MPS in one piece and unmolested in case I am yet again barking up the wrong tree!
 

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Sounds good. I have only skimmed those links but I would read them, then study them and read again before you start. An MPS from any car will cost a small fortune now so leave your good one in tact until you know exactly what you are doing.
 
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I've been reading those links thanks and they make good reading. One guy wrote the following:

Yes, it could be the manifold pressure sensor, and yes, contrary to what Bosch would like you to believe, it is adjustable. Herm, in fact the purpose of our 4/24 gathering is to watch me and Perry put a scope on the injector leads and watch the pulse width change as we adjust the pressure sensor). Grind off the epoxy seal covering the coverplate screw. Unscrew the coverplate screw (big slot). Inside you'll see a much smaller screw slot. This is the adjustment. If memory serves, clockwise is leaner, CCW is richer. And it's very sensitive - an 1/8 of a turn can make a noticeable difference.

From this it make sense that I am currently running lean, but adjustment of the MPS to the maximum isn't sufficient. I have checked and double checked the water and air temperature sensors, so am wondering what else could be causing lean running?
 

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I've been reading those links thanks and they make good reading. One guy wrote the following:

Yes, it could be the manifold pressure sensor, and yes, contrary to what Bosch would like you to believe, it is adjustable. Herm, in fact the purpose of our 4/24 gathering is to watch me and Perry put a scope on the injector leads and watch the pulse width change as we adjust the pressure sensor). Grind off the epoxy seal covering the coverplate screw. Unscrew the coverplate screw (big slot). Inside you'll see a much smaller screw slot. This is the adjustment. If memory serves, clockwise is leaner, CCW is richer. And it's very sensitive - an 1/8 of a turn can make a noticeable difference.

From this it make sense that I am currently running lean, but adjustment of the MPS to the maximum isn't sufficient. I have checked and double checked the water and air temperature sensors, so am wondering what else could be causing lean running?

i still think its a combination of your fuel pressure and flow rate. both have to be spot on at idle and at load else you will run lean/rich
 
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Isdyldan

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Hi there to anyone still interested - I haven't given up!
I've picked up a very rotten 1974 450SL which was apparently running ok and have transfered the ECU and the MPS over to mine. Although it now starts and runs, it still miss-fires and runs lumpy, so I will need to go back over the ignition system to see if the plugs have become fouled etc. I will also try swapping over bits and bobs in turn to see if anything makes the difference. I am reluctant to swap out the whole engine at this stage, but may have to. Will report back in a few days.
 

osc

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hi mate. i was wondering the other day if you had your car running yet. good to see you are back on the case.

it's worth noting that the 350 and 450 ecu and mps are not interchangeable. it will start but will be lumpy.

does the connector on your throttle body have 4 or 5 pins?
 
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Isdyldan

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Hi OSC, good to hear from you.

I've checked and the throttle body on my car has 4 pins, but the one on the rusty wreck has 5. Presumably then I need to swapt over the throttle body then, and also the cable back to the ecu. I'll need to look at the wiring diagram again - do you know if all the wires go back to the ecu please? Maybe I could just run an extra single core wire off to where ever it needs to go.
 

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good to catch up. since you have a working donor i would transfer the entire inlet manifold and throttle body. there are extra vac pipes you need for the 450 advance/retard that the 350 does not have. transfer the injectors (450 are blue), the distributer , MPS and ECU. you can pull on extra wire from the throttle body to the ECU - i will check on my car and let you know which pin to connect this to.
 
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Isdyldan

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Thanks - that's just the sort of info I was hoping for. It may take me a while to do all this, but I'll post again to let you know the outcome. I'll be interested to see if the injectors make any difference. There are varying opinions, so I'll swap these out last and let you know what happens. I've got loads of these now - yellow, green and blue!
 
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Isdyldan

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Hi - I've just swapped over the throttle bodies first - partly because it was really quick and easy, and partly because I really want to find out what is causing the problem. The plug on the 350 wiring loom for the throttle switch has 4 wires:

Red/Green - ECU pin 9
Reg/White - ECU pin 20
Red/Black - ECU pin 12
Red/Yellow - ECU pin 17

On the 450 loom it had an extra all Red wire which goes to ECU pin 2.

I don't know what this extra wire does, but I'll need to run this extra wire back to the ECU and somehow try and connect it. The extra vacuum tube from the 450 throttle body will be plugged at this stage to see if it makes any difference as the original 350 distributer is currently fitted which only has the single vacuum advance.
 
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Isdyldan

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Well, here's a funny thing. I was just messing around trying to remove the wiring loom from the new 'wreck', and noticed that the 'upper' vacuum pipe from the distributer went to the 'front' of the throttle body. On my car, whoever had put it together had the only (rear) output of the throttle body connected to the distributer upper connection. I swapped the pipes over and shoved the 4 wire connector into the 5 pin connection on the throttle body to see if affected anything. Fired up fine and sounded great. Tickover was quite high and seemed to be running a bit rich, but a vast improvement.
In the next day or so I will run up to temperature, but could do with finding out the purpose of the extra (red) wire to the throttle body if anyone knows please?
Another thing, on the 'wreck' there is a funny connection coming from half way along spark-plug lead 1 which goes to a connection block and then on to the bottom of the distributer. Mine doesn't have this - any ideas please? I'm sure it's to do with triggering the injection, but not sure how.
 

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sorry i missed your post asking for info about the red wire. you are right, i've chked haynes book and it goes to pin 2 on the ecu. not sure how i know this but i think they modified the wiper configuration on the 450 throttle body and i reckon it is something to do with changing from part to zero throttle to tell the ecu to leans the mixture. there are lots of stops and screws to adjust on the throttle body but i have not touched them and i suggest you don't play with these!

is that plug lead a mod or does it look like OEM? i've never seen anything from the HT lead. there is a [green?] wire in the top of the dissy for the rpms and the injector sender in the bottom of the dissy.
 

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