My 1974 Mercedes-Benz is a mystery machine to me. Working under the hood of my 1965 Ford F100 I learned a lot. But I didn’t learn enough to prepare me for the Benz. It has a much smaller engine, less barrels of carburetor, and yet it’s far more complex. My problem today, however, isn’t a complex one but it’s one I never would’ve figured out on my own.
The weather in California has been dreadful this summer. Unusually high temperatures are the norm now so as my Benz has been running hotter than usual, I chalked it up to the outside air. Until the other day, that is.
Driving up to Redondo Beach to record the Hooniverse Podcast, my car began running hot. Not overheating but far hotter than I’d like. As the drive continued, the car also started running rough. There was stuttering and a general sense that my journey was ready to cut itself short. I reached my destination, said some swears, and went into record the show. After it was over, the car had cooled off and the drive home started off normally enough.
On the highway, the temp again began to rise. This was a delightfully cool evening and I was simply driving down the road with little traffic in front of me. Yet I had to crank the heat to keep the temp in check on the higher side of the gauge. Once I got off the freeway, the rough engine issues returned. I made it home and parked the car for the night.
Today I went out to start my car (after not driving it for one full day) to run to the store and then shoot a bit of video. The Benz wouldn’t start. It would get nearly there but never quite catch. So in my head, I assumed the thermostat is bad and that’s causing the heating issue. That in turn messed with the carburetor and/or the timing, which is why I now can’t start my car.
So I had it towed to a nearby shop. My car’s been there before and the mechanic said to come over as soon as was convenient. When I arrived, the mechanic picked his head up from out of the engine bay of a lovely BMW 1600. We walked over to my Benz, where Kevin lifted the hood and mentioned this sounded similar to a problem he had with an old Audi 100 back in the day.
He fiddled with the points, which looked to be in bad shape, and adjusted a screw in the distributor. The Benz fired to life and idled smoothly. Because of course it did. We both laughed, and he went in to order fresh points.
We’re going to keep an eye on the heating issue and see if its related to the junky old points. If not, it may be time to go through the cooling system but for now I’m happy to see that this is something simple. At least simple for the mechanic.
I never would’ve thought to look at the points.
How A Car Works – A topic that continues to not make sense to me…
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27 responses to “How A Car Works – A topic that continues to not make sense to me…”
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Always suspect the radiator cap before the thermostat unless the system is full to the tippy-tippy top. If you replace the cap and it’s the cap, you don’t go to the effort or expense of a thermostat replacement, and if it’s the thermostat, it was super cheap and it’s the shiniest thing under the hood now, so why not?
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Only cars with an overflow tank should be full to the tippy top. If there is no overflow tank the proper cold level is 1/2″-1″ below the neck.
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This article uses weak points as a foundation.
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Yes, but it sparked my interest, and his timing although bad at the beginning of the essay it got better when he brought in the mechanic so he made a good adjustment to the points he mentioned. I know he didn’t say anything specific, let alone brand specific but I kind of expected a plug or two.
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It was fine, except for the gap in narrative.
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Jeff is advanced, perhaps too much for his own good. You guys are retarded.
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Try not to dwell on it.
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To do so would be retarded.
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I’m currently struggling with a “thermo-time switch”, so I’m right there with ya. I’m sorely wishing I HAD a carburetor at this point. I’m a lost ball in high weeds.
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I have a vague idea what points do, and I understand they shift the ignition timing through mechanical magic. Can they throw off the timing that much that it causes overheating by, well, slow burn or something?
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The points are a piece of plastic bearing on a polished steel cam, like a rocker on a camshaft. The bit that rubs on the steel wears away and the points don’t open or close at the same time as they did when they were new, nor do they open as far. The points don’t actively influence timing, unless they’ve worn out of adjustment. They exist on a plate that can rotate in relationship to the distributor, which is fixed, therefore changing the timing. That plate is moved by weights (advance weights) that are flung out under spring tension to varying degrees as engine speed changes, and vacuum advance, which is a diaphragm connected to the intake manifold to compensate for how hard the engine is working. If either fails or gets stuck, the timing will not advance when it should, or to the degree that it should and cause driveability issues.
Many Jaguar V12’s have met their fate at the hands of stuck advance weights because the lube at the pivot point would solidify with age and heat. They’d overheat and cook the engine, or become so unreliable that they’d get replaced with something else because a mechanic would overlook them.-
Thank you for taking the time to explain it to me!
I also asked around, so I think I do understand why a late spark can lead to thermal problems: the mixture is in full flames with the piston way down, so the burning cloud is pushed out overheating the outlet valve and exhaust, thus rising the coolant temperature.
Early spark leads to knocking, I’m not sure yet how that will increase coolant temp though. -
The plate the points are connected to are only for the vacuum advance, the mechanical advance changes the relationship of the upper shaft/cam to the lower shaft/drive.
The rubbing block is only one of the wear areas on a set of points. The actual metal contacts also fail, When the points open that flowing current will arc over the gap as it forms and will take a few molecules of the metal. Eventually that will result in a pit on one point and a mound on the other (unless they are the ventilated points that have no center on the contact where the mound would form). This wear will accelerate once it reaches a certain level as it will create a longer period of the points being close enough together to keep the arc going.
The rubbing block wear will progressively retard the timing and reduce the dwell time reducing spark strength.-
I have NEVER seen a set of points where the rubbing block failed before the point contacts themselves failed. The car would become undriveable long before the rubbing block wore down. Engine overheating as a result of a worn rubbing block on a set of points has got to be the remotest possible cause, if it is possible at all.
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I have seen rubbing block wear failure many times, granted that is only with extremely low quality points or a case where there was zero lube. When the rubbing block wears it does retard the timing because it delays the point where the points open. Retarded timing can put extra heat into the cooling system and if is marginal it could be the tipping point, though I agree it is highly unlikely that is the root cause of running hot. It does however seem to be a problem that needs to be taken care of.
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I hear ya! 🙂
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Uh – one word: Freakin China
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This is indeed a weather issue. Because of humidity these old distributors tend to misfire or even to fall back to current – and not fire at all. In Europe many owners of classic cars do replace the original distributor with a CDI-Ignition. It does look like the original distributor, but it is an electronic component. The factory is in Netherlands. It is named “123ignition.NL”, because originally they only made CDIs for the Mercedes-Benz W123, but nowadays they make CDIs for nearly every classic car.
http://www.123ignition.nl/product.phtml?id=209
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One of the first things I did on my Thunderbird was replace the points with a Petronix (sp?) electronic module. That was 18 years ago, haven’t thought much about it since.
No offense, but I never quite understand the mental process wherein a car clearly has a problem (one that could ruin the engine and/or cost a lot of money to remedy), and the driver just goes out and starts it up for subsequent drives assuming whatever it was will have fixed itself. Gnomes? Fairies? Leprechauns? Someone needs a car buddy he can call when problems arise. Failing that, he’d better have very deep pockets.
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They’re gnomes. They’re rare these days and it should never be expected that one has come and fixed your car while you weren’t looking.
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Are they the same ones who keep stealing my underwear?
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Similarly, why do men keep marrying beautiful but bitchy women expecting a different result?
As I mentioned in another reply the points wearing will retard the timing. If the timing is significantly retarded it will put more heat into the cooling system (and exhaust). However that will only cause the car to run hot if that extra heat input exceeds the cooling system’s ability to shed heat.
That said a fresh set of points and condensor and setting them with a dwell meter is definitely indicated as the first step, which should be followed up with a proper power timing. This consists of setting the timing to the factory specification and driving the car up a hill to load it and listen for pinging. If no pinging is heard then advance the timing 2 or 3 degrees and repeat the drive until you find the point where it pings. Once you hit the point where it pings under a load return it to the last setting where it didn’t ping.
If that doesn’t eliminate the problem then it is time to diagnose the cooling system, which starts with a stone cold engine. Set the heater to full hot temp and low fan then start the engine. Monitor the gauge and the temp of the heater hoses and the radiator side of the upper radiator hose. The heater hoses should get to hot to touch before the upper radiator hose gets warm. If they warm up at similar rates the thermostat is definitely not working properly, though passing this test doesn’t prove it is working properly.
Once you’ve passed the warm up T-stat test it is time to let it idle for an extended period and observe the gauge. If the temp stays right, then go for a low speed drive with speeds of 25 or less.
Most older cars transition from fan based cooling to ram based cooling around 25-30 mph. So if the temps climb in low speed driving the problem is flow, either the fan isn’t moving enough air or in rare cases the water pump isn’t moving enough coolant.
Then take it for a drive at speeds of 40 or so and if it doesn’t get too hot try 60 or so. If it heats up under those conditions the radiator isn’t able to shed enough heat, that is likely a clogged radiator or fins that are rotting away, or in some cases an issue with the water pump.
redondo beach! that’s where i live!!
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also: shit like this is why EFI and distributorless ignition is a godsend. nothing breaks, and when it does the car just tells you what’s wrong and you replace it.
i know we all say we like to work on cars blah blah blah, but nobody likes diagnosing problems.
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