Slight miss on idle, new to forum

I am new to the forum. To introduce myself:
I have a 1972 280SE 4.5 that has an exceptional history...
I got it from a mechanic in CA in 1998. The original car was involved in an accident in California in 1975. The mechanic ordered a brand new chassis from Stuttgart, put the engine, drive train, and suspension into the new chassis (clean title, no salvage). He put a bad '70's Dupont paint job on it that crackled like a Celadon vase the first time he rolled it into the sun; he got so mad that he wheeled it back into his warehouse and never drove it a single mile. So when I bought the car it had 8,000 original miles on the engine and drive train, and ZERO miles on the chassis! In 1998 I stripped it down to the perfectly clean steel and put a hand-rubbed 3-layer paint job on it in my garage. I literally cleaned every screw with a toothbrush! I wish I had known then about re-plating techniques. I did this restoration in CA; finished it in Dec. 2001, just prior to moving myself and the car to Switzerland, my adopted home.
It has about 15,000 miles on the engine (500 miles on the top-end), and 7,000 on the chassis. I rebuilt the top end last year to resolve a long-standing over-temp problem. Turns out the mechanic never drained the block, so all of that old crystallized coolant was blocking the channels in the block. A miracle I never warped a head or worse!
Now it is really solid, and not your typical 1972 280SE!

The idle was kind of rough, and it has been sluggish and tired for quite a while. I studied Dr. D-Jet’s manual for the D-Jetronic. I checked the dwell, timing, etc. The idle was really low at 550rpm. The dwell is off a bit @ 18 degrees, the timing was right on at 5-10deg BTDC at cold start and going to 0-2 deg ATDC at warm run, above 1500rpm it will go towards 25-30 deg at 3000 rpm.

I thought it was running too rich, but it tested at 21-22 on an AFR meter. I adjusted the idle speed screw to get the revs up to 750 for a warm idle and, Lo and Behold, my AFR settled into a perfect 14.7-15 AFR! Holy Moley! After that simple adjustment, it's like a new car! Like I just lifted a 500lb stone from the trunk!
Cold and hot starts are better, has a LOT more pep, literally feels like a lighter car.

My question(s):
I still have a slight miss at idle and steady rpm - would this be the injector trigger points in the distributor? If defective, do I have to pull the distributor to replace the trigger points?
Do I need a special meter to set the dwell?

FYI, upon arrival in Switzerland when the car was tuned for the first time after reassembly, a veteran mechanic "tweaked" my MAP sensor slightly (before I knew what that meant) to smooth the engine running .

Hi kitwalker,
a warm Welcome here in the forum at jetronic.org.
We like to call one another by our first name, would you tell yours to us?

I waited some time for or Mercedes-guys as I don't own a <classic> Mercedes.
I'll try to answer your questions as far as I can.

- No, your 'problems' around the idle bahavior don't have anything to to with the trigger-points
- Yes, you will have to pull the distributor to check the trigger points
- <Yes, you have to do this ASAP, because the fibre blocks there need a regreasement after such a long time, else you could get heavy wear>

You told us the dwell is at 18°? This is not a little bit, but totally out of limits.
In which way did you measure the dwell?
You ask, if a special meter is needed for adjusting it.
Yes, you need a dwellmeter, but which way did you read it out?
A dwellmeter used for reading should be useable for adjusting the dwell also.

Another problem is your MAP sensor the old 'specialist' has mistuned.
Yes, no 'standard' engine should need any adjustment of that unit.
But you know this after having read the compendium...

You should tell us more about the components of your car. There are very great differences between the US- and the European engines. The engines are different and the D-Jetronic-components, too.
We need to know which engine type (M117/??) you have installed.
You made a rebuild of the 'top'. Are US- or EU- camshafts installed?

D-Jet-components like the ECU and the MPS should have a sticker with an 'A' on them for America-engine.

Could you please read this out and tell us about you components?

Regards
Norbert

- You car should be a W116-type. The second forum of Dr-DJet has a section for this car model, too. Very few posts in English language up to this time there, but you will be very Welcome: sternzeit-107.de/en/forum/w116-forum

Hi Norbert,
Thank you for your welcome to the forum and advice.
My name is Thomas (Tom); I live in central Switzerland.
My ’72 280SE 4.5L was built for the American market, so it is an M117-type (low compression) motor. I had the cylinder heads rebuilt last year to US specifications by a local professional machine shop. It is using the stock camshaft for the US motor.
The MAP Sensor is correct for my car: Bosch Nr. 0280100100 / OEM Nr. A 002 542 28 17
The plastic cap covering the screw has been removed(?). Red mark on adjustment screw sits at 2:00 O’Clock.
I cannot pull the ECU at this time.
A mechanic friend and I put the top end back together. The timing chain was in excellent condition. We installed new chain guides.
We replaced the short fuel hoses on the injector harness upon reassembly.
My car has the transistor ignition. I used an Innovative timing light/dwell meter to measure the dwell and timing. According to the Blue Book, dwell angle should be 30 degrees? I measured 18 degrees dwell under normal warm idle operation with vacuum. Timing response under vacuum was in line with the firing point specs.
I recently replaced the fuel filter, distributor, and rotor. I have a set of points that I will install and set dwell angle when I pull the distributor to check, clean, and lubricate the injector points.
I don’t currently own a fuel pressure gauge, but I’m looking for one locally.
I typically use Shell V-Power, sometimes local 98 octane lead-free with a lead additive periodically.

It is encouraging to find this D-Jetronic forum. The system has always been a mystery to me, but since I’ve read through the whole Dr D-Jet compendium, I feel that I understand its operation pretty well. However, I don’t have a lot of diagnostic or tuning experience.

kitwalker66 wrote: ...
I used an Innovative timing light/dwell meter to measure the dwell and timing. According to the Blue Book, dwell angle should be 30 degrees? I measured 18 degrees dwell under normal warm idle operation with vacuum.

Hi Tom,
I thought about your dwell readings...
Maybe you forgot to set the dwell meter to the correct number of cylinders?
The dwell-meter would show wrong readings then and the timing-light in advance, too.

Regards
Norbert

Hi Norbert,
Thanks. I double-checked, it was set properly for 8 cylinders. My mechanic friend had no test tools when we did the engine, so all was "done by ear" at that time. I have since bought the timing/dwell meter and AFR analyzer, so I'm a little better prepared for a proper tune-up.
Travelling this week; I hope that I can get back to the Distributor next weekend to double-check the readings. I can easily change the points and set the dwell, I don't have sufficient time available to do the whole process of pulling the distributor, etc.
I was going to say that the timing was spot-on per the specs, but upon review, the M117 spec for the American 4.5 indicates more advance, so I need to adjust that as well.

Best,
Tom

Hi Tom,

a cordial weilcome from Dr-DJet on jetronic.org. Sorry that I answer late but I was on vacation. But Norbert has already helped you.

Not sure if I get all the things you wrote at once. But 18° dwell on ignition is definitely too low. Ignition coil needs to charge properly and that is determined by dwell.

If someone has opened your MAP sensor and detuned it than that is normal US desastrous behaviour. Tuning is extremely sensitive and I definitely recommend a retuning to factory values. Also the black cap should be added to prevent spray water from entering your MAP sensor. I demonstrate such things in our free workshops. The next one is at Erlangen in 2 weeks. We had visitors from Switzerland before as well. During workshop we connect some of the cars to engine and ignition testers.