W108 question - PeachParts Mercedes-Benz Forum

R72benz · #1 05-27-2016, 10:20 PM

I am confused. I have installed the Pertronix upgrade on my 4.5 M117, and adjusted the timing as recommended on this forum. Inspecting the plugs after 500 miles indicate it is still running rich......but much better than before timing adjust. I want to check the spark plug gap but can't find any info in my Owner's manual or on any of the tags on the car. I have the MB shop manual for the engine, put nothing on plug gap. I have searched the forum and read to gap the plugs anywhere from .028" to .045". My Bosch plugs came gapped at .031". What should it be?

Ron..aka confused.

BWhitmore · #2 05-28-2016, 12:35 AM

Contact Pertronix for their recommendation concerning the plug gap.

berfinroy · #3 05-28-2016, 10:21 AM

Make sure your fuel pressure regulator is properly adjusted. Too much pressure will result in overly rich mixture.

Tomguy · #4 05-28-2016, 12:08 PM

The ignition will not typically affect your mixture. The #1 cause of rich running on a DJet is a vacuum leak. It makes the MAP think the system is under more throttle/load than it actually is. Since it's an "Open" system - IE no feedback from an O2 sensor to tell iy "Hey, you're rich!" the mixture will never compensate for leaks.

The door locks, if leaking, will make you run richer. The brake booster and transmission module also run on engine vacuum. The MAP has a hose from the manifold to it. The intake plenum has rubber seals as do the injectors that can go bad over time. Any or all of these can leak, causing rich running. Before you go off and adjust the MAP for this, spray carb cleaner (NOT WD40, it will kill the seals totally) around these areas on a cold engine to check for leaks. Have an extinguisher handy. Better yet, if you have it, use some canned air or CO2. Any change in the tone of the idle would indicate a leak (CO2 would make it lower, carb cleaner would likely raise it; either can also cause stumbling). Also, unplug the air temp sensor on the intake horn. If your idle doesn't change, the wiring or sensor may be shot, and it'll assume it's about -40° outside, and add more fuel.

As for gap, factory is 0.028" I believe, but I would assume that the Pertronix - especially if you got the better coil - can handle a much larger gap due to the better spark. Check the manual / doc they give. 0.031" should be fine "For now" with your Pertronix, with a stock coil.

Do a search on the forums for more info. Another thing that can cause rich running is a stretched timing chain. The cams will be retarded, and that'll mess up the intake/exhaust cycles, obviously. With an engine this old, if the timing chain wasn't done in the last 5 years, I'd suggest doing it to make sure that's ruled out before touching the MAP as well.

Frank Reiner · #5 05-28-2016, 02:31 PM

Quote:

Originally Posted by R72benz

I am confused. I have installed the Pertronix upgrade on my 4.5 M117, and adjusted the timing as recommended on this forum. Inspecting the plugs after 500 miles indicate it is still running rich......but much better than before timing adjust. I want to check the spark plug gap but can't find any info in my Owner's manual or on any of the tags on the car. I have the MB shop manual for the engine, put nothing on plug gap. I have searched the forum and read to gap the plugs anywhere from .028" to .045". My Bosch plugs came gapped at .031". What should it be?

Ron..aka confused.

"4.5 M117" can be either d-jet or k-jet. Tell us what year engine and model you are working on.

R72benz · #6 05-28-2016, 05:46 PM

Thank you all for the suggestions. I will begin checking all of the above. Frank, the car is a 1972 280SE with a 117.984 engine. It has the electronic fuel injection system. So assumed it was a k-jet. Is that correct?

Berfinroy, you got me thinking about the pressure. When I got the car the electric fuel pump was leaking. The local MB dealer sold me a Porshe fuel pump since the OEM model is NLA. I probably should have checked the pressure then.

Tomguy, my door locks are leaking and will address that next and try your other suggestions. I should have mentioned that the car is running great.....much better than when I first got it. I just want to make it as efficient as I can.

Again, thanks all.
Ron

Frank Reiner · #7 05-28-2016, 07:09 PM

Quote:

Originally Posted by R72benz

Thank you all for the suggestions. I will begin checking all of the above. Frank, the car is a 1972 280SE with a 117.984 engine. It has the electronic fuel injection system. So assumed it was a k-jet. Is that correct?

Berfinroy, you got me thinking about the pressure. When I got the car the electric fuel pump was leaking. The local MB dealer sold me a Porshe fuel pump since the OEM model is NLA. I probably should have checked the pressure then.

Tomguy, my door locks are leaking and will address that next and try your other suggestions. I should have mentioned that the car is running great.....much better than when I first got it. I just want to make it as efficient as I can.

Again, thanks all.
Ron

From your thread heading it would be thought that a 108 would have D-jetronic, but there have been conversions. Yours, by virtue of being electronic, is in fact D-jet; K-jet is a mechanical system, even though it superseded D-jet.

If the Porsche pump is from a D-jet 914 it should be OK. Other pumps may be satisfactory, as the pressure is controlled by a regulator on the return end of the fuel injector loop line.

R72benz · #8 05-28-2016, 10:32 PM

Thank you Frank, after researching further it is indeed a D-jet. I will be testing the pressure and adjust as necessary.

Still working on the vacuum issues as well.

Ron

97 SL320 · #9 05-29-2016, 12:23 AM

A D jet being used on an engine with low intake manifold vacuum will result in rich mixtures also.

Any fuel pump that can attain at least the maximum fuel pressure required by D jet is fine. Except for unregulated pumps designed for carburetors, the fuel pressure regulator controls max pressure. You could put a pump capable of 300 PSI into your system and the injector pressure will be no higher than a standard D jet pump. ( As long as you don't put a pump with such a vastly high volume that the regulator becomes a restriction. )

Graham · #10 05-29-2016, 09:43 PM

Quote:

Originally Posted by R72benz

I am confused. I have installed the Pertronix upgrade on my 4.5 M117, and adjusted the timing as recommended on this forum. Inspecting the plugs after 500 miles indicate it is still running rich......but much better than before timing adjust. I want to check the spark plug gap but can't find any info in my Owner's manual or on any of the tags on the car. I have the MB shop manual for the engine, put nothing on plug gap. I have searched the forum and read to gap the plugs anywhere from .028" to .045". My Bosch plugs came gapped at .031". What should it be?

Ron..aka confused.

After reading the thread, I suspect you may be even more confused

And even more so after reading this and checking chart below?

Spark plug gap as as set by manufacturers like NGK and Bosch is usually correct. But it needs to be checked. You are right that MB seems to keep the gap a secret. It is probably in a Bosch manual somewhere. I have always used the pregap of 0.031" for the M117 in my 350SL 4.5. But perhaps it should be less. There are some variations by model in the chart below . I don't know where I found this or what it says at beginning in German. For some reason, same engine in a sedan has a smaller gap than in a roadster. (chart is metric - divide by 25.4 for inches)

As other said - Pertronix and even timing won't affect your mixture much. Many other things do, not only vacuum. And mixture can only really be determined by using an exhaust gas sensor both at idle and under load. What did your plugs look like? Any pictures? You could hook up a vacuum gauge to your intake manifold (tee into line at back of motor or line to MPS) It should be about 15-16" HG at idle. This is affected by timing. If timing is too retarded you won't get that vacuum. You could check if car runs any different with vacuum line to locking system plugged.

Check your T1&2 temperature sensors to determine if they meet specs. Check sensors first, then check again from ECU plug so that any bad connections will be included. Bad T2 is a known cause of overly rich mixture. Someone messing with MPS and ECU mixture screws is another!

R72benz · #11 05-31-2016, 11:23 PM

Thanks Graham, I did pull temp sensor wire and idle did change. Also checked for under hood vacuum leaks with starter fluid with no change in idle. I will be measuring vacuum later in the week and will try plugging the door actuator line. One step at a time.
Ron

97 SL320 · #12 05-31-2016, 11:47 PM

Plug gap is more or less dictated by available ignition system secondary voltage. In the days of points, max voltage was pretty low so tight gaps ( 0.025" ish ) were needed to prevent miss fire at high load.

A basic electronic ignition system will have more voltage available so gaps ( 0.030- 0.035 ) can be slightly larger than points. A modern high energy ignition system even more. ( 0.045 to 0.060" )

Modern cars are pretty much immune to large gaps however, the larger gap places greater voltage stresses on the entire system. If the gaps are too large, insulation will break down leading to an intermittent miss fire under load.

kapi123 · #13 06-02-2016, 06:32 PM

Quote:

Originally Posted by berfinroy

Make sure your fuel pressure regulator is properly adjusted. Too much pressure will result in overly rich mixture.

Well, i am agreed with berfinroy as you must check out the pressure adjustment first.

R72benz · #14 06-20-2016, 11:39 PM

Finally got around to measuring vacuum and found a very constant 17" HG at idle. Also disconnected the door lock vacuum hose and the idle changed noticeably. When the connection was plugged idle returned to same as when the door locks were connected. I am confident the door lock vacuum leak is small and not impacting fuel mixture. Thanks for those suggestions Graham. Now it is on to testing fuel pressure as Berfinroy suggested.
BTW, I feel the plug gap at .031" is not an issue.

Graham · #15 06-21-2016, 06:35 PM

Quote:

Originally Posted by R72benz

Finally got around to measuring vacuum and found a very constant 17" HG at idle. Also disconnected the door lock vacuum hose and the idle changed noticeably. When the connection was plugged idle returned to same as when the door locks were connected. I am confident the door lock vacuum leak is small and not impacting fuel mixture. Thanks for those suggestions Graham. Now it is on to testing fuel pressure as Berfinroy suggested.
BTW, I feel the plug gap at .031" is not an issue.

17" Hg is good. Plug gap of 0.31" also good.

Fuel pressure check is good idea - it should be in 29-30 psig for standard mixture settings.

Don't forget to check T2 (coolant temperature sensor) and it's associated wiring. It has a big affect on mixture.