75 450 SL - Any ideas? - PeachParts Mercedes-Benz Forum

TrailerWorks · #1 06-30-2010, 05:29 PM

Hey Folks,

Having a time with my new (to me) 1975 450 SL 4.5. 66K original miles, so very little wear. Bought it a few weeks ago and it had been driven very little (2000 miles) in the last 5 years or so. Started right up and ran just fine at first couple of weeks. Last week it developed a very pronounced missfire (500 RPM drop) that came and went. I assumed fuel, although all it made it through two tanks of Premium before this issue started.

I ran a pint of Lucas Injector cleaner through it, but the problem has gotten worse. As of today it will start right up and idle at about 700 - 800. Skips at times and drops to 500 but recovers on its own. Load it up to 2000 rpm and it really misses - drops to 1500, but comes back, then goes out, etc. My head says fuel issue, but my gut thinks ignition. It really breaks down under load.

I replaced the distributor cap and rotor. Lots of arcs, but not too bad. Cleaned/inspected the points, checked the condenser wire, switchgear wire, etc. Pulled the distributor and cleaned the trigger points (they were amazingly clean). Pulled all of the plugs - nice and even tan. Inspected all wires. Swapped out the Bosch Blue coil (cracked top) for a good Bosch Silver coil I had from my '73 280 SE. Changed the cracked, blue bracket ballast resistor.

It really feels like ignition, but as the problem came on steadily over a week or so, maybe it is fuel. My long screwdriver "stethescope" tells me all of the injectors are firing.

Before I go pulling injectors, I thought I'd look for answers here. Also, I converted my 280 SE to Pertronix and loved it, but they do not make an electronic ignition for the '75 450. Anyone had any luck finding a system?

Sorry for the long post, but now you've all got the facts. Thoughts?

E.

RadioTek · #2 06-30-2010, 06:09 PM

Have you adjusted the dwell angle of the points, yet? This is done by adjusting the gap of the points themselves. You have to loosen the screw a bit, change the gap, tighten, and measure the dwell angle. This is done with a dwell meter at the (-) terminal of the coil. I chased a few weird troubles before fixing them by setting my dwell.

Better check to make sure on the terminal, I'm not near my reference materials.

TrailerWorks · #3 06-30-2010, 07:12 PM

No, have not tried that. The car ran great two weeks ago - in fact it runs great in between the missfiring. Seems more like a component failure. Slow, progressive breakdown of some component? Started with a miss or two, now a steady miss under load. Fuel starvation? The pump seems ok, filter is good.

Graham · #4 06-30-2010, 09:04 PM

I would first check the points. Set them to 0.014" and then check dwell - it should be about right.

While cap is off, check for wear in rotor shaft - If it is worn, it could be affecting the points. Pertronix fixes this, but as you say, there is a problem fitting them on the '75 model. Maybe the Crane optical system is available again and maybe it will fit?

- Check that cap is properly seated - Don't ask why I say this

- Check type of plugs and wires. If wires are original, best to use non-resistor plugs (The wires already have resistors in them)
- check wires are not shorting to ground - I once had one short to the front rubber fuel rail hose and burn a hole almost all the way through it!
- Set the timing to about 27-30deg BTDC at 3000rpm with vac disconnected and plugged. Make sure that it does actually advance centrifugally and vacuum retard at idle to about TDC. (your car probably has vacuum retard and advance ( I only have vac retard) Maybe check that is all working properly.
- Check vacuum at idle - should be about 16" Hg. Watch what gauge does as car misses - maybe you have a vacuum leak?
- Check injector and other grounds - especially one on top of engine near back where injectors are grounded.

Not sure about the silver replacement coil - make sure it is the right one for the car.

Good luck. Basically I learned that you just have to methodically go through each system using the test procedure in the shop manual. If you don't have a shop manual, you need one! There is a W116 one on line that is almost the same.

Be aware that I am no mechanic - just an owner who has lived with an early D-jet for 21 years

TrailerWorks · #5 06-30-2010, 09:36 PM

Hey Graham,

Thanks for the quick reply. I'm off to check gap, dwell (assuming I can find a meter) and timing. I'm just thrown by the fact that it went from a perfect runner to major misfiring in just under a week. Seems odd that the point gap/dwell would change radically over a short time period - it seems more logical that an issue like that would be noted gradually.

Too bad about the Pertronix - I spoke to them today and they have no intention of making a new kit for the '75 model. Grrr...

I've got non-resistor plugs and original MB wires in great shape. I've been all over them and can't find so much as a nick. Coil wire seems good, too. The problem was present with the old coil, so the swap is not the wildcard.

All vac lines look good - found one off today but it was actually plugged with debris. No change once cleaned and reconnected.

Frustrating. I'll have another go at it tomorrow. Thanks for the thoughts.

Graham · #6 06-30-2010, 10:22 PM

Quote:

Originally Posted by TrailerWorks

All vac lines look good - found one off today but it was actually plugged with debris. No change once cleaned and reconnected.

Debris in vacuum lines doesn't sound good. Might be worth while blowing back through the two lines that go to the throttle body and make sure the tiny holes above and below the throttle plate are not blocked.

GaryF · #7 07-01-2010, 04:18 AM

You should be using the blue coil. The dwell doesnt have to be spot on as there's an electronic ignition box that cleans up the signal just as long as the gap is actually being reliably created. Talking of the ignition box, it could be starting to fail. You could try bypassing it temporarily and drive the coil directly from the points if you're confident with electrics or see if you can get a known working unit to try.

TrailerWorks · #8 07-01-2010, 07:28 AM

Hey Gary,

Thanks for the help from across the pond! The world is truly getting smaller. I was thinking the switchgear is likely the culprit, but had not bypassed it - will try that this morning. I really wish Pertronix made the kit for the '75. I'm going to order one for a '74 and have a look at modifying it. Might even consider a '74 distributor.

Thanks again for the help!

meltedpanda · #9 07-01-2010, 09:37 AM

DJET correct ? Have you checked and cleaned TPS, side of throttle body? Aux air valve working??

blk914 · #10 07-01-2010, 02:36 PM

Sounds like fuel starvation from a clogged filter screen that's in bottom of tank. My 72 sl had similar symptoms, starts easy, runs perfect at times but would miss and really cut out bad when you try to give it gas. Tried to remove fitting on bottom of tank while still in car but found impossible to do because of limited access thru hole in bottom of trunk. Ended up pulling tank since car had been in storage for about a year and I was pretty certain I had a rust problem. Maybe running a couple of tanks of fresh gas in your car loosened up some deposits (varnish and such?) that clogged screen that even worsened when you put fuel injection cleaner in tank... just a theory if all else fails. I've learned to never overlook the obvious sometimes...

good luck

blk914 · #11 07-01-2010, 02:42 PM

Forgot to mention, this screen is made of very, very fine mesh and once you see it you'll see how it can get easily obstructed.

TrailerWorks · #12 07-01-2010, 06:16 PM

Hey folks - thanks for all the great advice. I started out the day cleaning and regapping points, checking the switchgear, etc. No luck. Moved back to fuel as a possible cause. Dropped the fuel pump, inspected, tested, etc - worked just fine. Installed a new fuel filter. Checked the fuel flow out of the bottom of the tank (just by gravity) - poured out clean. No varnish smell. I did all of this in an airconditioned shop. Started well cold, but quickly ran very rough as soon as it went outside into the 88 degree SC heat and humidity - cool day here

So, what to do next. RPM bouncing around 500 and stalling now and then. Back under the hood to look at fuel pressure/injectors. Pulled off the entire air cleaner assembly and pulled the intake air sensor plug - as soon as I did, the engine settled and ran smooth. Took it around the block with no air cleaner and no sensor - ran great, no missing. Air cleaner back on, no problem. Plug in the temp sensor and it misses and almost quits within seconds. Victory?

I've ordered a new sensor ($40) and have my fingers and toes crossed that this is the issue and not just a contributor. If I put in a new sensor and the problem returns, at least I know which direction to head in.

I drove it a bit. Without the sensor it can't seem to find an idle point that it likes, but all other RPMs are smooth up to at least 4000. I'm guessing whatever computer gizmo this sensor supplies info to defaults to cold (rich) ???

For now, I'm up and running. Will post again Tuesday when the part arrives. Thanks again for all the help - what a great resource!

RadioTek · #13 07-01-2010, 09:38 PM

The Crane company was bought out by S&S Cycle company last year, and are selling the ignition retrofit kit. I have one on my '75; one of the first ones under the new company. It works great, as it replaces both the points and the switchgear. I have what I suspect is some shaft wobble that was wearing the folower pad on the points excessively. I was having to adjust the points about every 2000 miles.

The installation is fairly easy, you just have to be careful not to drop a tiny screw or two. The most difficult part was fabricating a bracket to mount the new electronics package to the fender using the existing holes from the switchgear.

I had to do a little 'trial and error' with some resistors to get the tach working. I still don't have it quite right, but it's 110F outside, and I've had a bunch of things going on for the last few months. It ma get put on the back burner until things cool off a bit. Literally.

The cost of the S&S kit was approx $110.00 and the switchgear was more than double that.

There are some automotive type multimeters available at prts stores that have the ability to test dwell, in case you need a pointer on where to look for one.

Good luck,
Scott

meltedpanda · #14 07-01-2010, 10:13 PM

you can test the temp sender - cold it should 260-340ohms. Disconnecting simply tells the ECU to enrich the mixture throughout the operating range, which often smooths thinkgs out. An old trick but not the fix. A djet requires step by step diagnosis which includes OHMing out all sensors. If this one is bad - good for you but if not , work still lies ahead. As Graham mentioned , ECU can be the issue, once again the TPS could be the issue. Keep us posted

TrailerWorks · #15 07-02-2010, 08:54 AM

Hey MeltedPanda,

Thanks for the info. Cold (75 degrees) the sensor reads about 220 ohms. If I warm it with my hand it drops a few ohms, and cooling it a bit raises the resistance to about 235. Out of your suggested range, but apparently somewhat funtional. I have a new one on the way so we'll see soon enough.

Tell me about the TPS - I found some info online that suggested a check for the Bosch TPS, but it was for a later car so likely does not apply. Key on but not running, open the throttle full and the injectors should fire 20 times. (?) My injectors do not fire unless the engine is running, but I expected this as the trigger points need to operate to fire the injectors, correct? Time to buy a manual...

I'll have a look at the TPS today. I'll give it a dose of electronic contact cleaner, but is there an ohm range to check for?

Running quite well today without the temp probe installed...

PS - I picked up a new blue bracket ballast resistor at the local MB dealer. Yes, I said at the dealer. On Hilton Head Island. He had it next day and cheaper than any I found online when you added even the cheapest shipping. Go figure. I thought sure they'd be much higher. The world is a changing place...