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Spark plugs

6K views 27 replies 11 participants last post by  MBL87560SEC 
#1 ·
Just a note on spark plugs and the advise of a fellow member in the archives Jono. While conducting a search for a hard starting issue I ran across a posting from him regarding the spark plug gap on our V8 engines and, he advises the plug gap to be set to .042".

When I replaced my timing chain and upper guides, I purchased some NGK BP5ES and installed them to my 91 560SEC and thought I'd leave the gap to .032" and I started to experience hard start problems. Then I did the search and followed the advise from Jono on his posting and regapped the plugs to the suggested .042" and no more hard starting.

Thought I should share that with you all, as I've found that sharing information is the only thing that advances objectives of this great forum. Such wealth of good info. Look and you shall find.

Best Regards,
 
#3 ·
Mine behaves exactly like yours. Engine starts first time but almost feels like it's slow starting. I don't mean long cranking time but seems like when it starts it's from low idle. I think I should have bought the BP6ES plugs. No real starting problems though. I thought maybe from not cleaning the plugs when I regapped them but, I think I'll live with it since your engine is behaving the same.

Let me know if anything changes. I originally bought Bosch WR9DC+ plugs for my car as those were installed when I bought the car last year. Then I was on here after that and also bought the NGK plugs but got the heat range wrong. To be honest I think I may just use the Bosch as the car started a whole lot better with those plugs.

I did call the dealer and they informed me the correct plugs for 91 560SEC are W9DC0. So really the NGK non resistor plugs should be fine but I'm not finding that. The hard starting was only rectified by adjusting the gap, so if I were you I'd remove the plugs and gap them to .042 as I did, you may see an improvement in starting behavior.

Good luck and keep me informed of the results by PM if you don't mind. If you don't see improvement then I'll probably switch to the Bosch plugs which I think will be better anyway.

Regards,
Dave.
 
#4 ·
What a coincidence! I've been reading about spark plugs on this forum extensively for the last week.

About a year and a half ago, I changed out my old spark plugs, which were old and no longer working correctly, with some platinum or iridium resistor spark plugs (a big no-no for these cars). I didn't know any better at the time as these plugs came up as the "recommended" plugs for my car (I forgot at which parts store). Anyway, after installing these plugs, my car developed a hard starting issue that's only gotten worse over time. I thought it was something else this entire time until I came across a thread recently about the correct plugs to use for these cars.


So I bought a full set of NGK BP6ES spark plugs (7333) and I plan on installing them this weekend. I've been debating on whether or not to gap them at 0.042 as that seemed large and I read it puts a strain on the ignition system (though as one member said, the ignition systems in these cars are more than up to the task), but I think I'm just going to go with the larger gap just to be safe.

Thanks for the thread!
 
#5 ·
I've run both gaps, but of late I shoot for .040. I can't say I've noticed a huge difference going to a bigger gap, but it'll generally help, especially at idle. The upside of a bigger gap is that you have more air/fuel mixture exposed to the spark and therefore a more consistent flame, especially at lower throttle where there's considerably less mixture to ignite. The downside is mostly high throttle, high RPM where there's greater demand on the ignition (more RPM = more sparks per second the system has to generate) and the greater density of air acts as an insulator, either degrading the spark or causing no spark. I don't think it creates a problem in terms of wear on the rest of the system, certainly not as much as resistor plugs do. The only thing I'd look out for with a wider gap is misfire and/or a loss of power during high demand; that would be the signal that it's too wide for the system to reliably fire (which could point to other weak links).
 
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#6 ·
.042 is the bomb
 
#12 ·
There comes a point of diminishing returns. Besides, it's a lot harder to fire the plugs at 5000 RPM than at cranking speeds. Aircraft engines start on magnetos (lower voltage at cranking speeds) with gaps of .016", 30's vintage carbs, lousy mixture distribution and pistons the size of coffee cans. I can't see how the difference between .032 and .042 could have a measurable effect on starting.

I'm not saying it's not real, just that I can't see a reasonable explanation for it. But, I admit, with only 55 years of experience wrenching, I haven't seen it all.
 
#15 ·
Thanks for that Green. Sorry John, if I sounded cheeky. You do have a point John as I think back to the seventies back in England I do remember that four and six cylinder ford engines designed in Cologne Germany used .025" as gap for plugs and points. When I came here in the early eighties most cars had a larger gap. I think that was because most cars had big V8 engines back then. I do take your point though.

I've read your contributions often and appreciate the fact that you are knowledgable about such things as CSI and generally most things related to cars. I do have a question for you:- Being that we have resistors in the mercedes high tension wire ends, is it really prudent to not use resistor spark plugs? The reason I ask this is because my car when I bought it had bosch resistor plugs in it and ran better than when I installed the NGK's that everyone is so thrilled about. Personally I think I might just switch back and see how it goes. As I stated according to the dealer the correct plug for my 91 560sec is WDC0 or something like that.

Your thoughts.

Dave .
 
#16 · (Edited)
Long winded, rambling post...

The move to wider plug gaps correlates with the move to electronic ignition. The ability to provide higher secondary voltages under all conditions permits reliable firing of the wider gaps under starting, idle and high speed conditions. Back in the days of points that was not true. Had a TR3 back in the day, and if the point gap was bigger than .014 (IIRC) the dwell was too short and the coil didn't have enough time to charge fully for anything over about 4000 RPM. The engine wild miss fire so badly it might as well have nad a governor on it. With electronic ignition the coil is saturated in much less time than that available at RPMs way above redline. (Not exactly correct technically, but I don't know how to better explain it.) Today we have motorcycle engines that routinely fire reliably at 12,000 RPM.

As far as resistance goes, it serves to limit the duration of the spark. The main functional reason is to reduce plug wear. (A side benefit is reduced RFI.) Excess resistance means more reduction of the spark duration. Eventually it will cause performance issues, but, apparently, combining the "recommended" resistor plugs with the standard resistor wires, at least if everything else is up to snuff, doesn't cause significant problems. On the other hand, there is no significant benefit, either.

In a nutshell, what happens is this: current flows through the coil primary winding. A magnetic field created by the current surrounds the coil. It takes a bit of time for the current, and the resulting magnetic field, to reach their maximum value, at which point the coil is "saturated." When time comes to fire the plug, the current in the primary winding is suddenly interrupted (points open in the old world). The magnetic field surrounding the primary coil collapses rapidly and this changing magnetic field induces a voltage in the secondary windings of the coil. This voltage is several thousand times higher than the voltage applied to the primary windings and could easily be as high as 50,000 volts were there no spark plugs. The voltage is conducted to the spark plug electrodes without loss as there is no current flowing at this point (Voltage loss = current times resistance; current is zero and so is the drop). The electrostatic field created in the spark plug gap by the increasing voltage difference between the two electrodes eventually causes the air in the plug gap to ionize. Ionized air is a much better conductor so current flows (and the voltage stops rising). This causes a voltage drop across the resistor wires and any resistor in the plug. That drop is proportional to the current times the total resistance. The drop quickly reaches a high enough value that there is insufficient voltage to sustain the spark and the plug stops firing. The time this takes depends also on the resistance provided by the plug gap and the ionized air. The bigger the plug gap the faster the drop will reach the point where the plug stops firing, so, at least performance wise, increasing the gap has the same effect as increasing the resistance, assuming you don't increase it to the point where the plug can't fire at all.

Side thoughts: Denser air is a better insulator, so increasing compression or supercharging both require higher voltages or smaller plug gaps to fire reliably. Again, though, modern electronic ignitions make this a moot point.

Increasing the plug gap (assuming nothing else changes) requires a higher voltage before the spark event occurs, and so has the effect of retarding the timing slightly. This could facilitate starting, but I doubt the effect would be large enough to cause a noticeable difference. In fact, I'd be surprised if it was even measurable (in degrees of crankshaft rotation).
 
#17 · (Edited)
In my opinion John is right in many respects. In addition to what was already said, a smaller gap generates a more powerful spark, more stable and for longer time than a greater gap. Basically you can see that the smaller gap has a 'wealthier' spark.

I don't know how it starts with a greater gap. Maybe it starts better, I will not argue that, but once started the engine sure doesn't run healthier at speeds above idle with a greater gap. As the electrode wears out, the gap will increase and that will not help the combustion at the right timing.

EDIT: I just checked the new plugs Bosch W9DC that I have. They come adjusted from the factory at 0.035" (0.9mm). If it would be me and I would put those in my engine, I would not change the factory gap. I would leave them as they are.
 
#21 ·
I put my spark plugs (BP6ES #7333) in the other day and left them at the factory gap (I think 0.031") since I figured widening the gap was more of a band-aid for another issue rather than a real fix.

I didn't notice any ill effects, though my startup issue is unchanged (probably a fuel pump or fuel check valve issue considering those are the only things that aren't new on that car). Other than that, my idle feels a bit better at stoplights, but my car has always had a smooth idle.

As GreenT said, doing a vacuum test might help sort out the problem. I did a poor man's vacuum test with carb cleaner last year and nothing came up, but I might try it again with smoke tomorrow.
 
#18 ·
Tight gap non resistor copper plugs were (are?) used in racing for decades because of the power they delivered. Current resistor plugs running ~5K ohm plus the cable at ~1K ohm starves the engine of combustion power.

Don't screw with your K-Jet system. The Germans knew what worked with it, so much so that the Italians ran some of their most iconic supercars with it. Plus Porsche, Rolls Royce, blah, blah...
 
#19 ·
Thanks to all of you for taking the time to educate the average joe with very useful information. I just can't understand why it made a difference initially but today the flipping thing is acting up again.

I had a hard starting problem from cold, it would start up for a split second and quit within 0.5 second. I tested the cold start injector tested good, fuel accumulator, ok, tested the FPR, ok, checked fuel pressure at cold start port 92psi, ok, so then I decided to replace the injectors and it started up ok for a while. Then one day i replaced my chain guides and had to remove spark plugs so I replaced those with the NGK BP5ES. I think I should have used the BP6ES instead, that is when I began to having starting problems. The timing chain went on perfectly no running issues everything was hunky dory. So I took the plugs out again and thought I should replace with the Bosch but forgot to bring them with me so decided to increase the gap to .042 and the damn thing started first time. So now it's acting up again and I'm beginning to think that I'm losing my logical mind.

My throttle sensor plate wasn't free enough so I removed the maf housing complete with F/distributor and cleaned the shit out of it, made no difference at all. So I had a used housing complete with F/distributor i removed that FD and used the good used housing with my old FD and it ran ok but starting was terrible. So once again I removed the whole thing again and swapped the old housing in again but using the used FD this time. Runs ok and all but the starting issue I still there.

Now I'm thinking of removing the whole thing again and swapping just the housing and trying the used one again.
I'm not fed up yet but sort of running out of ideas.

You know since it doesn't seem to have a pattern to what's going on, I wonder if fuel related at all and is actually ignition.

Best regards,
Dave
 
#22 ·
Good advise anytime. The econo gauge is begged to the left and indicates no leaks, but I think that can't be as accurate as a smoke test. So I will have smoke test performed and accurately identify the origin of any and all leaks. What's baffling to me is there seems to be no pattern in the hard starting. I could have a low residual pressure developed again by some strange reason. I have a spare EZL I could try, just seems a blasted crap shoot at the moment.

John what do you think. I know it's hard to diagnose like this but an idea would suffice.

Best regards,
Dave.
 
#23 ·
Update

So I thought you guys might want to know, the hard starting was remedied by replacing the EHA. No more hard starting. The plug gap did make a slight difference but the remedy was the EHA as stated above. The damn thing did have me chasing my tail for a while but it was fun to tinker with the car.

Thanks to all for the contributions to the thread.

Regards,
Dave.
 
#25 ·
The damn thing did have me chasing my tail for a while but it was fun to tinker with the car.

Regards,
Dave.
Another plus to the tinkering is other parts were checked so now you know that X system is good to go and Y system needs this or that but is still good. Or at least that's one way I look at it, knowing the car better and it's overall condition
 
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#27 ·
Hey Viz, my advice is don't spray a hot flammable liquid on to a 300plus degree surface and not expect to one day have a bad experience. I can't afford a smoke machine, but I can afford a handheld blow torch. You know the Benzomatics. Then take some rubber hose a few feet long with a width of "just so it fits" over the igniter. Open the valve until you hear a slight hiss. Don't ignite it. Instead, wave the end of the hose around areas of vacuum. If you have a leak the propane will increase the idle. The chances are much less of ignition of the gas than the flammable liquid.

I saw you mentioned EZL's. Do yourselves a favor, a little PM and take your EZL off and put some new heat sink paste on the back of it. I used Arctic Grey. The factory heat paste lasts 10 years maybe. Plus the technology of heat pastes has evolved since the 90s. Remove old paste with GooGone and clean surface with alcohol. Then apply paste and some saran wrap. It is just like a computer chip you need to keep cool. I learned the hard way on my 500SL about EZL burn out. Much pricier EZL's.

Lastly on the NGKs as the number goes up the cooler the plug. So in hot as F... Texas I would run the BP7ES. The 5's are running as too hot a plug for my driving style, distance, climate and gapping. I am running the blue NGK wires with proper boot resistance, Bosch cap, and rotor and an original non-resistor copper plug. Never pull the wire or boot when removing. Always grab the metal housing with "grabbers" or you just bought a new set of wires.
 
#28 ·
Hey Viz, my advice is don't spray a hot flammable liquid on to a 300plus degree surface and not expect to one day have a bad experience. I can't afford a smoke machine, but I can afford a handheld blow torch. You know the Benzomatics. Then take some rubber hose a few feet long with a width of "just so it fits" over the igniter. Open the valve until you hear a slight hiss. Don't ignite it. Instead, wave the end of the hose around areas of vacuum. If you have a leak the propane will increase the idle. The chances are much less of ignition of the gas than the flammable liquid.
That's a great tip, and with a smile to boot! :)

I saw you mentioned EZL's. Do yourselves a favor, a little PM and take your EZL off and put some new heat sink paste on the back of it. I used Arctic Grey. The factory heat paste lasts 10 years maybe. Plus the technology of heat pastes has evolved since the 90s. Remove old paste with GooGone and clean surface with alcohol. Then apply paste and some saran wrap. It is just like a computer chip you need to keep cool. I learned the hard way on my 500SL about EZL burn out. Much pricier EZL's.
This looks to be an issue we might all benefit from.. Might you have some photos of this operation? Even with your explanation, I am unsure as to exactly what you are saying.. re: the food wrap part. It would make a great writeup for our BW site. Thanks

Lastly on the NGKs as the number goes up the cooler the plug. So in hot as F... Texas I would run the BP7ES. The 5's are running as too hot a plug for my driving style, distance, climate and gapping. I am running the blue NGK wires with proper boot resistance, Bosch cap, and rotor and an original non-resistor copper plug. Never pull the wire or boot when removing. Always grab the metal housing with "grabbers" or you just bought a new set of wires.
Agreed, even with the robust design, one can damage the wireset if not using a deliberate approach.

MBL
 
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