I just used Chris Foley's excellent rebuild kit on my leaky MPS. After removing the spider webs and varnish it all went together nicely. Chris gave me some great tips on getting an initial set. I think it is there but I don't have any experience with how a well tuned djet 2.0 914 should run. I suspect there is probably 150k or so on this engine although it could be lower, I doubt higher unless it was cared for very very well. Impossible to know with the dead odometer. Compression is great on all cylinders with no variability.

I took a little video of a drive, start up, corners, and two pulls to 60 (of various quality). After the first 40 seconds I opened my sliding rear window so you can hear the engine sounds better.

https://youtu.be/Fi4vFCWhlCw

As they would say in Minnesota, uffda, that's a little lean. Turned the inner screw a full rotation to the left. Idle fell to 500, brought it up a little with the idle air screw and then turned the inner screw a hair to the right till it idled nicely again. Really don't want to drill my exhaust for the O2 sensor... Will give it a good drive as soon as I can and check them again.Click to view attachment

Did you get the tuning kit as well? Does it come with instructions? I guess that's a question for tangerine. Thought I'd I ask about your experience with the kit.

(Since I can't seem to follow directions, i.e "Tech Tips 700", "don't mess with the mps!" @dr914 @autoatlanta.com....I did....so, now I'm ready for instructions on how fix the thing I shouldn't have messed with).

Good luck, sounds/looks like your close to getting it dialed in.

QM

Yes, I got the tuning kit, and well worth it. Chris has given me very good instructions to get it in a good place without the O2 sensor. I was pretty scared at first but I think tuning it is within the range of mere mortals. It really has taken many of the rough spots out. Just need to get it perfect, but at least I feel it is possible with the tuning kit from tangerine.

The tuning kit does not come with instructions on tuning, but Chris is outstanding about answering questions and supporting his products.

The most detailed documented instructions (that I'm aware of) for tuning are here on Anders' site: https://members.rennlist.com/pbanders/manif...sure_sensor.htm

You really need an O2 sensor to get it dialed in.

I have a good one but I just don't want to drill the exhaust... For 75 I can take it to a Dyno nearby. I think I will try to get it as close as I can by plugs and ear, then hit the Dyno. That will really get me the definitive wot anyway.

And pbanders is where I started, great info.

PB Anders did a great job of characterizing the d-jet system for us but the detailed MPS information tends to intimidate people rather than empower them.
The MPS is a simple mechanical device attached to a robust electronic sensor. Only the mechanical portion is serviceable so I ignore the electronic portion as soon as I know it's operational. I skip the bench tune and go straight to the operational tune.
I've come across some simple "seat of the pants" techniques that work well and don't require in-depth knowledge or specialized electronic instruments. The only requirement is that all the components are functioning properly. So far I haven't managed to organize my techniques into a set of tuning notes but it's only a matter of time.
For modified engines I always recommend an O2 sensor just after the collector, especially to be sure the engine doesn't go lean at WOT but I don't think it's essential on a stock engine.
I don't think spark plug appearance is a very reliable indicator of mixture except at the extremes.
A dyno run can verify WOT mixture is safe but the tailpipe sniffers used at most dyno shops aren't accurate for idle tuning. The exhaust gases get contaminated with backwash at very low flow rates.
Hot start issues are a good indicator of a lean tune. Properly tuned d-jet starts quickly, hot or cold. Imo most people chasing "vapor lock" issues with their stock injected engines are merely suffering with a lean mixture. I use this knowledge as a tuning tool: Take a drive to get the engine good and hot and then park it for 10 minutes. If it starts right up, the MPS adjustment is pretty good. However if you think it might be a little rich, screw in the small center screw by an eighth turn and try again, starting with a cold engine.

One very important thing to keep in mind though: the MPS should never be adjusted to compensate for other issues. Reliable tuning with the MPS can only be done on a properly functioning system.

Not really sure about what you adjusted, there are two inner screws.

OP, I hope this doesn't amount to hijacking your thread but it seems we're all in the same boat with this topic. I, too have just finished rebuilding my MPS using Chris' great parts and tuning kit. (When I got the car the diaphragm was toast and it ran rich.)

I made the mistake of buying a very cheap LCR meter on Amazon and had some scary results that I HOPE are the fault of the meter. In all of the write ups the inductance varies from around 1.5 down to .7 ish. On mine, the highest reading never got above .95. Even with nothing touching the operating rod (case apart) Coil resistance is great and inductance changes smoothly across the range, but it just reads too low compared to published data.

Chris, then I read your seat of pants input here - set the outer screw to bottom out at around 5"Hg and put everything back together. And lo and behold the drivability is better than it ever was! Now I'm fine tuning and both the inner screw and the ECU knob make noticeable changes. But I'm not sure how to get the right balance. Idle is a bit too high even with the idle screw closed and the whiff test points to rich conditions. I've already ruled out other vacuum leaks and am still somewhat concerned about the low impedance measurements. As it seems that lowering the entire curve would add fuel across the range. But then it seems to drive very well and if I had a bad coil I'm not sure that would be the case.

Any thoughts or advice would be appreciated. And if anyone knows of a reference inductor component I could use to spot check the meter it might help.

Thanks,

Ron

I had the exact same inductance readings as well. Both on my original (before the leak developed) and the rebuild with parts from Chris (tangerineracing.com). A range from .6 ish up to a max of .92 no matter what I did. I probably bought the same cheap meter on Amazon. It actually seems like a pretty well made meter though. Would be interesting to check its accuracy, just no clue how to do that.

I adjusted the MPS on the bench as described by Chris (range of movement on the diaphragm running 0 to 7 hg). That is all adjustment of the outer screw. Chris likes the stop screw just starting to engage the diaphragm and I have tried to get that as close as I can since it is in the car now.

Just so people know what I am referring to..
I call the big one that plugs the back of the MPS the stop screw and it would normally be covered by epoxy. Chris sells a kit that has this plug screw hollowed out (so you can adjust the inner and outer screws without removing it) with a big 8mm alan for adjustment (and a nice rubber cap to seal it up when not adjusting it).

When I say inner screw I mean the tiny innermost screw.
When I say outer screw I mean the large screw the inner screw fits into.

Once it is on the car I have been adjusting only the inner screw. Chris has a very handy tool for holding the outer screw steady while you rotate the inner screw. (or reverse).

Yes, that terminology and process is exactly what I'm working with using Chris's tuning kit.

And thanks for the input on your impedance readings - reassuring to know I'm not alone. From what I read coil failures are rare and probably very rare while the resistance is still good. So what are the chances that both of us have exactly the same failure mode? Slim I hope.

Maybe an LCR guru can chime in but in the very sketchy instructions that came with my meter it says this:

.....when measuring the inductance or capacitance containing a component of resistance it may get the wrong readings.

And even more curiously:

.. This instrument can not be used to measure the quality of the ICP, the measuring inductance is varied when there is different impedance, you may get the wrong value if you are measuring inductance of the resistance???

Maybe there's a reason why pbanders specified a professional grade LHR?

Good luck with the continued tuning. Would be interested to hear if you need to make any big changes after the dyno test.

Ron

I would be surprised if the mixture is rich. Too high idle indicates too much air, ie. not enough fuel. Proper idle rpm should require some added air from the bypass.

I'm assuming he's using the definitions from Anders's site.
Click to view attachment

People have reported different inductance readings from different meters. After getting the exact same model that Anders used, while I learned some things in using it, it really only got me into the ballpark. Once the car is running, beyond judging by how the car idles and drives, the only real way to dial it in is by accurately reading the AFR for the various conditions. Money would be better spent on that then an inductance meter.

You really need an O2 meter to get it right, but if you just want to get close, I have a Wavetec and would be happy to adjust anyone's MPS to the specs published by Anders, or any values you specify. I did 3 at Okteenerfest with good results. Just pay the shipping.

I am almost there... still a little lean on the plug tonight. Gave the inner screw another loosening turn, idle dropped again, brought it up with the idle screw. ECU knob has a very large effect as well but I am keeping it at the factory mark for the time being.

And as I said before, I have an AFR, I just don't want to drill my exhaust.

I was chasing my tail on inductance readings with my cheap meter. I finally got ahold of an unmolested 043 MPS and used that as my baseline by which to measure the other MPS's I had to adjust. You need to take Anders' absolute numbers with a grain of salt. The slope across the range is probably more relevant. You also need to use several inputs as clues about how you're running as applicable and available: O2 sensor (if you have one), dyno, butt dyno, plugs, exhaust smell, head temps, tailpipe, etc. etc.

If one does use an O2 sensor, I recommend running a "tee" vacuum hose off the MPS hose and attach it to a vacuum gauge in the cabin. That way you can see what vacuum you're actually drawing under what driving conditions. Many people are confused about what "part load" may mean (I think I was). Anyway, it's a good learning experience if you don't already know.

Yeah, as soon as I get it in a good spot I am going to record the inductance readings for future reference.

I reread everything pbanders had on operation of the MPS and all of the info Chris has given me and pulled it again this morning.

1) This time I started by loosening the inner screw all the way till it bottomed out on the outer screw.
2) I then adjusted the outer screw until I had the diaphragm range of motion that Chris suggests with his kit (movement stops a little before 7#).
3) I then adjusted the stop screw downwards until it just begins to move the diaphragm at 0#.
Rinse and repeat a little on both 2 and 3.
4) I left the inner screw bottomed out and will tighten that alone at this point to taste (making it leaner if needed).

Drove it to work. Runs great, feels a little richer. No bucking or strange partial throttle transitions.

Will get it on the highway and check the plugs later.

As another point of reference:
I just unscrewed an inner screw of a diaphragm which I believe was unchanged from factory. It went a little less than 3 full turns to the left before it locked against the outer screw.
I normally recommend enrichment of 1 1/2 turns from stock.

Arggh... No change on the plug. I am going to double check my fuel pressure, I have a pretty cheap gauge so I will try a friend's.

Click to view attachment

Brought the fuel pressure up from 28.5ish to 32psi. Significant change in smell (not bad, just different) and idle speed dropped again. Sadly another darn northeaster is hitting us tonight and tomorrow so I won't be able to test it out till tomorrow evening.

Just an update on this. No matter how I fiddled with the rebuilt MPS or the fuel I couldn't richen the part throttle mix. So... Started thinking about trigger points going bad. My actual points were not looking too hot either. Figured now was the time to go for the 123injection distributor. Bought one off of tangerine racing and popped that in. After dealing with a busted oil hose (just plugged it for the moment), I started adjusting the MPS again. Sure enough, the MPS makes a big difference in afr now. I put the fuel pressure back at 29 before I started.


After a couple days of experimenting I have gone from this
Click to view attachment

To this

Click to view attachment

I thought I would close this thread up with my results. So after the Dyno tune (posted elsewhere) I put my own afr on it and did some further tweaking. Everything looks good at partial, WOT, and idle. Runs better now than I could have ever hoped. I took my inductance values just in case anyone might find them useful. 850 feet above sea level. 0inhg is 0.87. 4inhg is 83. And 15inhg is 0.61. these were taken with a szbj bm4070 lcr. The cheap one on Amazon. All tuning was done using the Dyno and my afr. Obviously these values look nothing like what pbanders has up.

It is running between 13 and 14 (more towards 13) at partial loads. 11 to 13 at wot (more towards 12). And 12 at idle. Idle smells fine and it is idling at 850 rpm. I still have room to lean it out on the ECU but I'm going to leave it for the time being.

Engine oil is running very cool even after spirited driving.

Getting a spark plug head temp sensor next week to verify temps.

Wow. You're not kidding. If you ever get your hands on a stock, unmolested MPS, take some readings to compare.

I am sure thats differneces with the meter.

Be interesting to put an LCR55 on it to see comparative values.

I did check my original before I started working on it. Those readings were much like these (but it wouldn't hold vacuum).

First, Ander's reference in is inches of mercury, not mm, but the zero reading is off as well so there is more going on.

I have an LCR55 and have used Ander's values numerous times with good results. I've heard others that have seen questionable readings using cheap meters.

Also note that you can get Altitude conversion Here.

Sorry, I put mm but meant inches. I will fix that.

I am glad someone can use inductance. I found it to be of little value. Certainly my lcr is cheap but I do feel better about tuning it by Dyno and afr.

Would be awesome to have a way to check your lcr. Also, that table doesn't seem to do any adjustment for alt. Might just be my phone's browser.

Yeah, just checked it on my laptop. Just didn't like my phone. The readings for alt are only around .01 different for me.

Considering the MPS rebuild is around 200 dollars, buying a 250 dollar LCR could be better spent on buying a good WB02 instead.

We honestly don't know if Ander's results are accurate, but we do know that he used a LCR55 so it's best to use the same tool. Others have reported significant differences between it and other meters.

The MPS site should have a field into which you can enter your altitude and then press the submit button. The bottom half of the table should then be adjusted based on the altitude setting.

Also, if the site detects that your device has GPS capabilities you should be prompted to allow access to location services. If you allow it it should display the current altitude in the upper right with a button to use the current altitude. If you press that buttion the current altitude will be copied into the field on the left and then you need to press submit to adjust the table using that value.

I agree something else may be going on. I used a cheap LCR meter, and my numbers were off, but maybe by 5 - 10%. Not like these. That's why I found a stock unit to baseline so helpful.


That's pretty slick little page, thanks. Don't think I didn't check to see what values should be driving around the summit of Mt. Everest.

Because I did

A few of us were around when Brad developed his MPS calibration values, he wporked very methodically. He collected several NOS and otherwise excellet condition orignal, known reliable, MPSs of each part number and characterized each with the LCR55, to generate the values depicted. The values are averages of several MPS values of the same part number, and are as accurate as are available.

With that comes the knowledge we must recognize that different MPS factory original NOS MPS units are not identical calibrations, but are fairly close in range. I can also tell you that an MPS calibration based on 0, 4 and 15 is a good summary and the best informaiont known at the time, but values for 12 and 18 are also valuable and the calibrations for those load conditions are important when using a dyno or WB02 to understand the engine performance over the full range of operating conditions.

Engines and their fueling system must be balanced. You can take a perfectly correct fueling/induction system from one engine and bolt it on another engine and it may not be 100% correct for that engine. The engine itself brings with it variables in wear, compresion/vacuum, and a myriad of other things - so that an MPS calibrated perfectly for one set-up may not be 100% correct for another. It was much easier to do on the production line when everything was new and predicatable, not so much now with the variables in our engines 50 years later.

Just took it for a little spin with my Dakota digital cht installed. 220 to 240 driving around town. 305 climbing a big hill. 310 - 315 cruising at 70 in 5th.

When I pulled plug 3 for the sender it looked a tad rich.

35F degrees out

Thanks to all for the great write-up!

I'm about to attempt my own MPS repair with the Tangerine kit. Any tips for de-potting the adjustment screws and drilling out the rivets in the case?

The rivets are easy. Getting the plug out is the harder part. Be careful not to strip it. The inner and out screws will be 'free' below it. Only the outer stop is potted.

Speaking of MPS's, here's a great 3D rendering of a D-Jet MPS from a wiz on benzworld.org and his D-Jet website

https://www.djetronic.org/manifold-pressure...structed-part-3

https://www.djetronic.org/

That's awesome.

A heat gun is your friend to remove the epoxy seal. Do not force it and it will not strip. Counterintuitively, screw it in to remove, not out.

Holy crap! There is some amazing stuff on that site. Including a link to this video for tuning an MPS on the bench using the MPS 'Tune-o-Matic' and using other MPSs as a tuning template.


https://www.youtube.com/watch?v=4MQ2pD9wv3g


Looks like they've made plans available to build your own Tune-o-Matic. What a time saver it would be in the rebuild process to have this on hand.
MPS Tune-O-Matic

I don't know how I feel about that.... The outer stop is important about not nearly as important as the inner two. If someone told me they had messed with only the stop it would take me no time at all to set it back to where it should be. It's the relationship between the inner and outer and then the trickle down effects on the stop that are a beast. Particularly when you have changed the engine from stock.

That said, the device looks great, so does the software.

Heat gun.

put some serious heat on on the epoxy and it scrapes out like butter

Setting the inner screw position (a critical first step when tuning an MPS) can be set using only a vacuum pump and measuring point the diaphragm is against the part load stop. Seems like one could then use the app to set both the outer screw as well as the full load stop screw.

I contacted Andy Ayre, the inventor / developer of the Tune-O Matic. It looks like he's making built to order Tune-O- Matics available. If anybody is interested, I can provide his contact information.

@914rrr Do you know how much he's charging?

I checked with him and he'd rather quote price himself. Understandably, he's a bit reluctant to build these one at a time due to the extensive time required to set up, build and test them.

Wow - glad to see others from Mercedes community working on solving the same problems we have.

Very interesting - need to spend some time on that site