'72 350SL Surge Mystery
 

Hey All,

I've read about all I can on this forum and elsewhere on D-Jet trying to tune in my M116.982 and now I need opinions. (Sorry for the novel).

This is where I stand: All new vacuum lines (well, to the vac adv & MAP, as there are no other vac lines on my euro) and I'm pulling just under 16in @ the vac adv line @ idle. All new hoses btw cold start/idle valve & throttle body. Inspected & cleaned trigger points w/ index card & they look shiny & not pitted. Vac adv seems to work: +/-6degATDC with vac, 7degBTDC without, & 30ish deg at +/-3250RPM. Point gap (per dwell) @ .016. Plugs Bosch W8DC gapped to .8mm. New cap/rotor/wires. Fuel Pressure Reg & MAP both look new (very shiny). Idle speed always smooth @ 750-800RPM* (note below). She was sitting for awhile with the tank almost empty, so she got a big dose of fuel stabalizer & almost 18gal of fresh Premium. The oil smelled gas-ie when I got her so I changed it after all of the above. I cleaned the connector for the trigger points where it comes out of the dist as best I could (connector is right on the dist instead of up above the therm housing). Cleaned the contacts on both temp sensors above the thermostat housing & the contacts on the sensor in the air intake snorkle. Idle screw moves freely & adjusts the idle as it should.

Symptoms: Idles well & maintains speed fine, but bucks & surges BAD under any load. We're talking neck-breaking. If you're cruising along in 4th (after much lurching and embarassment to get there :rolleyes: ) happily maintain speed she's fine. But push the throttle in at all and she'll buck or cease to accelerate at all (like near-total fuel shut off, kinda like you put it in neutral) and you have to feather the throttle to get fuel (?) back. One or two of my accelerations from stop were almost tolerable, but they were still rough and definitely in the minority.

*Odd side note: After a long (and very buck-ie) ride, I started her up again to check the intake mani vac and upon startup, she started to surge @ idle (+/-2500RPM to 500 & back every few secs). I pulled the vac adv off the distributor and the surging sped up to VERY quickly revving up & down. Intake mani vac was still around 15in (though oscillating wildly), and as soon as I reattached the vac adv line the idle smoothed right out again to a steady 900RPMs or so before settling quietly back to 750 after a 30sec or so. Not sure if that helps any diagnosis or just confuses it...:dizzy2:

Finally, I cleaned as much of the Throttle Position Sensor with an eraser as I could reach w/o taking it off (didn't want to mess up the adjustment until I was sure it needed one). There appeared to be a bit of a line where the 'finger' goes across the 20-some contacts on the circuit board, but I could hear it clicking when I pulled the linkage. I also noticed that it was "adjusted" all the way to one end of its spectrum; turned all the way back counter-clockwise toward the firewall. Are there any tests for the TPS? It seems very much like the car is completely shutting off fuel when you depress the accelerator (or, cutting it just to idle speed, as it never stalls & the engine just starts to coast down to idle speed...hence the smooth idle not under load).

Any suggestions? Other tests that can be done to the TPS, MAP, or points? She runs great otherwise and I'm dying to see how the euro engine matted with the 4spd rates for grin-factor ;) .

-M-

Update:

Just found a thread on testing the TPS. Ignition on, pulled throttle through it's cycle. 20 distinct clicks. Should be good, right?

:confused:

Any ideas?

How do your spark plug elecrodes look after a bucking drive? Black, white or light brown? Black: way too much fuel (flooding the cylinder), white: way too little fuel, light brown: perfect combustion.

Bert

sounds like a lean condition. have you checked for vac leaks?

sorry if i missed it and you did this already.

tom w

I actually didn't pull a plug after the ride...:rolleyes: I'll do that first thing...

As far having a vac leak, I thought pulling almost 16in at the vac adv would preclude a big enough leak to cause such a bad lean mix. Of course I could be wrong. I'm getting pretty damn good at that, actually...;)

I'll check the vac at the MAP in the AM too...

Any other ideas?

A weak secondary ignition can cause the "flame" to go out when the engine is put under load. In most cases, the engine will idle fine becase the Kv needed to idle the engine is low, but when faced with additional fuel and load, the mixture isn't lit by the plugs. These D-jet cars tend to run a bit fat(rich mixture), so a 100% right ignition is mandatory. I learned the hard way on my 1971 300SEL 3.5(same engine).

If it was my car, I would make sure the distributer cap and rotor are ok first, with no carbon tracks or corrosion inside the cap. I would replace the rotor just because, imho, your symtoms sound like the classic signs of a bad dist. rotor. The cap should have brass contacts, and they should be shiny clean. I have seen rotors short to ground(the distributer shaft) before, the electricity takes the path of least resistance. A bad coil can cause this as well, it happened on my 1959 220SE, and the symptoms were identical. I forget the specs, but you can ohm out the primary and secondary sides of the coil to see if it is shorted, though sometimes they do this when they get hot after the car has run a while.

Some people hook up a timing light, and tape the trigger on while also taping it to the windshield. Then you can see if you lose spark when accelerating.

My guess would be a combination of bad plug wires, bad rotor, or a bad cap.

The plug wires can be ohmed out, to see if they are any good.

I'm sorry, I forgot to read where you said it has new cap, rotor, and wires.

I still think something is wrong with the secondary ignition, a bad coil or a bad coil resistor(there are two, and they can crack internally and reduce voltage to the coil, causing the same effects you have).

My next guess would be a coolant temp sensor that is making the mixture too lean. It has a two pronged plug with a blue/green top, and is screwed into the intake manifold. You can also ohn that part out to see what readings you get.

The coil is one of the only things that looks like hell (like 294k KM). I've already replaced the wires on both sides of the coil, as one was broken almost in two.

Could bad resistors actually cause good idle/bad load?

Could a faulty ICM cause these faults?

Thanks!

The resistors could be dropping the pos. voltage to the coil, thus reducing the secondary output, but usually they cut off all voltage to the coil and the car won't run at all. I would test/replace the coil, after making sure the coil is getting the proper voltage. The coil can weak over time, and have a low output.

The icm could be weak, but all it does is turn the coil on and off, you could try checking the dwell angle and see if it changes when the car starts to run bad, that would point a finger at the icm. Frankly, I got tired of mucking about with the points in my 300SEL and used a Crane XR-700 ignition, which bypassed the icm and used the points to trigger the ignition. It cost less than $100.00, though this was many years ago.

One other thought just crossed my mind, if the distributer shaft has any perceptible side play, this could affect both the fuel injection and the igntion.

I'm curious, can you rev the engine ok in park or nuetral?

TPS adjustment is tested by seeing if it senses 0 throttle (idle/rest), not counting clicks.

I remember you having similar issues a while ago. How long ago did you clean those trigger points? If your distributor's bushing is a bit too worn you may have to do it every 3-6 months (just like an oil change). Other than that, it really does sound like a bad cap or rotor. Just check the carbon brush on the cap - I've ruined a brand new cap before by putting it on slightly crooked and cracking that brush.The bucking/surging issue does sound more ignition than fuel though - replace the ballast resistors, see if that helps. If not, some people here surely have a spare coil. I think I do somewhere.

Thanks for the input guys.

Tomguy: Car is new to me. PO probably posted the thread that you're thinking of. I have rechecked/redone all of his adjustments/cleaning in the last two days, as he was a self-proclaimed "Chevy-guy".

I just pulled the plugs after my 'drive' yesterday and they are all a perfect light brown. Am I wrong in thinking that low spark condition would foul the plugs??

I checked my vac again. Still getting 15+in at the vac adv. I pulled off the big vac line (which is new w/ clamps) at the MAP and got some weird results: If you put your finger over the end of the vac line idle drops to almost nil. So putting a vac gauge on pulls the idle down a bit, but I'm still only getting 5in or so. Does this sound normal? If not, what the heck would cause that? What is good vac at the MAP?

As far as the coil/resistors go, I think I'm just going to go ahead and replace the whole she-bang with the Petronix/Flamethrower combo as soon as funds allow. In the mean-time, I may swap out the coil and see where that gets me. Testing the resistance on the one in there seems pointless, as I do have a good idle.

Edit: I just went out and checked the cap. Definite line across the contacts, bigger on the trailing edge of the rotor rotation (assuming clock-wise rotor rotation), but it doesn't look bad. See pic below. The PO replaced the dist, is there any chance it's the wrong one?

Keep in mind this is not a miss or even a failure of 4cyls. It's not a stall either: when you press the accelorator gently in 4th gear the RPMs would slowly start to drop to idle and you would start to coast. A blip of the throttle from this point and she would buck you hard. :confused:

Oops. Here's the dist pic:

http://i24.photobucket.com/albums/c7/mvenn/DSC_0025.jpg

How does one test to see if the TPS is finding "0"?

Back at it:

I have now re-replaced what the PO may have replaced. So now I have new plugs/wires/cap/rotor. Also, after several days banging my head on the fenders, I have installed a new Pertronix/Flamethrower coil combo and re-timed the car. (I must say, just at idle & rev-ing a bit, she already sounds better than before the upgrade).

I also checked the entire intake mani for leaks with an (unlit) propane torch and everything checked out. Couldn't get the engine to move an RPM.

She idles perfect and revs up in neutral great, warm or cold. But put it in gear and pedal down and she'll go a bit and then quit (returning to idle, NOT stalling) then rev up hard, then let off returning to idle speed all while you keep your foot at the same throttle level. All the while you're (almost) going down the road looking like you can't work a clutch (or just finished your second case of Warsteiner)...

I know now that my ECU was probably tampered with when it was federalized in the '70's, but I'm assuming that it ran fine like that until recently or it would have been junked long ago.

Any ideas?

just want to make sure I have this right, your plugs show a light tan colour so you are getting the right mix and its burning right, otherwise you would have sooty plugs if either too much fuel or too cold a burn and white plugs for the reverse. If you have light tan plugs then its likely that your problem is caused by the ecu not increasing gas to the engine when you accelerate. either the MAP is broken, have you tested it? or the vacuum is not 15 inches but much lower at the MAP or you do not have the full 28lbs of fuel pressure at the rail either because of a poorly operating pump or broken regulators. THe only other thing might be a really streatched chain and everything looks good at idel but when you pressure it the timing is running out?
just some thoughts, hope they help
cheers
Barri

I appreciate the comment.

I actually pulled the timing cover off (a little late perhaps) and found some serious chain stretch. I can't confirm that it's what is causing my bucking, but it has to be changed out before I can continue the search.

I will most definitly post if I can confirm if this is at fault or not.