Do spark plug wires overheat?? - PeachParts Mercedes-Benz Forum

EricSilver · #1 06-26-2003, 11:47 AM

Background:
1989 260E, 183,000 miles
Weather: has been hot – 95+ with engine coolant temperature at 80-85 during all types of driving.

In the past two days, my car failed to immediately start after sitting for about 15 – 20 minutes after normal city driving. It would crank but not start. This only happened during the daytime, when the outside temperature was highest.

In the first incident, I was coming back from the grocery store after about 20 minutes. After three or four attempts the car would crank strongly, but not start. I raised the hood and saw that the long plastic case that contains the plug wires had detached from the block and was dangling over the side. The wires had come mostly out, but the case was still snapped together at the end, towards the back of the block. I laid the whole thing on the block, and tried starting the car. It fired right up, and behaved normally thereafter. When I got home, I reseated the wires and reattached the plastic case to the block.

The second incident occurred this morning. Again, it was about 90 degrees outside and as I was rolling into a parking space, the engine quit. (Actually, the timing was perfect; it stopped at the very moment I rolled into the space.) On two restart attempts, the engine would fire weakly – in the 200-300 rpm range and then stop. After looking under the hood, the plug wires were in their proper place. All I did was tap the wire case to be sure it was not loose. Tried starting the car and it fired up with a little struggle, and I pulled into an indoor parking garage to cool the car off, where the engine conveniently quit, again, just as I rolled into a space.

I checked under the hood again, and on a whim, pulled the distributor wire (where it connects to the ignition coil) and then reseated it. Restarted the car and it fired up very strongly, and idled steadily.

Based on that, I don’t think anything is wrong with the plug wires themselves (after all, how does a wire fail?). Is it possible for old wires (mine appear to be original) to overheat and stop conducting??

I don't think there is a problem with the ignition coil, OVP, or pump relay, which are all still fairly new. The alternator is fine. I don’t think it is vapor lock either. The only thing that seems to make sense is a heat + electric problem involving the connections between the plug wires and the ignition coil and/or distributor and/or spark plugs. I assume there is dirt, oil, corrosion or some other impedance where they connect to the spark plug wires. Perhaps there is similar corrosion in the distributor itself.

Because I am at work right now there is not a great deal I can do. I am really paranoid because when I drive home the outside temperature will be over 100 degrees. If I make it there without incident, my plan is to clean every electrical contact between the ignition coil and spark plugs. If anyone can offer some additional advice and insight, I’d really appreciate it.

EricSilver · #2 06-26-2003, 11:59 AM

I should add two additional things:

1.
Is it possible I may also be running a bit lean?

2.
In the past month or two I have noticed the engine stumbling after the first startup of the day. It was as if it was starved of fuel; you floor the pedal and nothing it would rev right away; only after a second or three would it rev normally. Once it got warm, or if I revved it at 2000 rpm for 30 to 60 seconds before driving, the stumbling would not happen. This morning was the first time that did not happen, which seems to suggest an electrical issue.

Also, duruing the past couple of months, the plug wire case had been very loose, but generally remained on top of the engine.

Is it reasonable to assume that after weeks of driving with dangling plug wires that one or more of them came loose at the distributor and, as we have seen, the ignition coil?

Arthur Dalton · #3 06-26-2003, 12:11 PM

My first check would be fuel filter ...

Lean along w/vapor lock ... it comes out of vapor when you open the hood ..

EricSilver · #4 06-26-2003, 12:54 PM

Arthur thanks!

Each time I opened the hood, it restarted, which is why I am unsure about my electrical problem theory.

What concerns me is the stalling that occurred this morning, when I was parking. Would that be related?

Arthur Dalton · #5 06-26-2003, 01:46 PM

Could be lots of trhings ,
I simply think I would start with a new fuel filter and/or a fuel pressure test..
Opening the hood relates to letting the heat out of the engine compartment , not changing the conductivity of the plug wires,,

EricSilver · #6 06-26-2003, 02:21 PM

Understood.

Movement of the wires still seemed to be a factor, particulalrly disconnecting and reconnecting the ignition coil wire.

I still plan to clean all of my electrical contacts between the battery and spark plugs. This type of heat issue arose once before in an ignition wire. The basic dynamics of both problems match. I'll know for sure after I clean the wires and distributor parts tonight. If the problem persists I'll shift more of my focus to fuel/air mixture.

Incidently, Is that area of the valve cover that the wires sit on function as a type of heat sink?

Arthur Dalton · #7 06-26-2003, 02:31 PM

You may well have an ignition problem,..
It is usually one or the other ...fuel/spark
I do the pressure/flow test first b/c it is so simple to do and that gets that part of the equation out of the way..
Ignition faults/connection testings can be a little more involved..
You may want to just try another coil wire first ... and I would also , when the condition exist, give the OVP a tap or two...

EricSilver · #8 06-26-2003, 04:55 PM

I will definitely check the OVP. And while I am back there, will check the fuel relay, though I really don't know what to look for other than dirty electrical contacts.

The ignition coil was replaced exactly three years ago and I assume their lifespan is longer than that. The battery is new. OVP and fuel pump relays are about 3 years old also.

Fuel pump(s) are in good shape. Fuel filter has not been repalced in a while. Accumulator, as far as I can tell, is fine -- never had any pressure problems.

Spark plugs are due for replacement but are probably not part of this problem.

Distributor cap and rotor and spark plug wires are the only components that have not been serviced, replaced or even given much attention in at least five years - and may indeed be original equipment -- and thus seem to be the most likely suspects.

#9 06-26-2003, 05:23 PM

''clean all of my electrical contacts between the battery and spark plugs''. That would be a h*** of a job.
You said the spark plugs/distributor cap/rotors are due for replacement. Before spending any other $$, I would do it.
I would also forget about the OVP. Those poor OVP's seemed to be blamed here for all sins of the world.
Get your ignition in top shape first (you said it needed it). If the problem is not resolved, get to the fuel system (filter, accumulator, pump).
I don't think what you described about your plug wires is the cause of your problem.
JackD

EricSilver · #10 06-26-2003, 05:52 PM

Hi Jack,

Well, not literally every electrical connection, but definitely the battery leads, distributor and ignition coil.

I focused on the plug wires because they seemed to be the one component that when touched or otherwise manipulated would cause the car to immediately start.

The logic I am applying stems from childhood: when my electric trains were running crappy, or not at all, I cleaned the metal tracks and locomotive wheels of all dirt and oxidation. After that, they were good as new. Because I cannot remember the last time I cleaned my distributor and spark plug wire contact points, their potential condition or cleanliness causes me to focus so hard on them. That plus the fact that the wires were draped over the side of the engine in such an unnatural manner, and after restoring them to their proper place, all was (almost) right again.

I am also surprised at how easily the ignition coil wire came off. The end on the distributor requires considerable effort to remove. The end on the coil slid right off, indicating a rather loose connection.

Also, I went through a period of months when my engine bay was being drenched (literally) by leaking power steering fluid. It is reasonable to assume that some oil and dirt seeped into key electrical contact areas. I'll know for sure when I remove them later on.

EricSilver · #11 06-27-2003, 12:06 AM

Okaayy,

Based on what I am seeing and have more recently experienced, the distibutor is the culprit. Corroded, green gunk and carbonixation were on the contact points. I have cleaned and polished them and am preparing to reinstall, but this is clearly a replacement item since the erosion is so evident.

Also, I noticed that one of the six points (?) is more eroded than the others, causing a wider gap between it and the rotor. Perhaps that is the cause of persistent stumble at idle?

In any event, it died a few times on the way home, jerking and shuddering when I was able to restart it, and then conk out again. It did not behave normally until after the sun went down and it cooled off outside.

The ignition coil was nowhere near hot, and produced a strong spark (through the wire's insulation and into my hand

) when I tested it. The spark plug wires are old, but clean and otherwise in good shape.

My only dilemma now is whether to risk driving the 25 miles to the mechanics tomorrow to get a new distributor, or have the car flat bedded.

Thanks everyone for your good feedback.

manny · #12 06-27-2003, 08:39 AM

Next time you hold a coil wire while testing for spark.........hang onto a friends hand.

P.S.
I would take a chance & drive the car to your mechanic. If it acts up, you know exactly what it is.
The cost of flat-bedding it will pay for your ignition parts.

EricSilver · #13 06-27-2003, 09:04 AM

Hi Manny,

I cleaned everything as planned, and the car idles with the usual stumbling, but because of the temperature outside right now (HOT!) I am too paranoid to risk stalling on the Washington DC beltway during rush hour. I'll take a leisurely drive up the flatbed ramp and hopefully be done in time to go back to work this afternoon. Plus, the towing is free for me under my roadside insurance policy.

And thanks for the coil suggestion!

EricSilver · #14 06-28-2003, 12:08 AM

And the culprint was the ....

Position Sender Unit

This is some sort of sensor wire connected to the ignition control unit, and which evidently controls ignition timing (??) .

The insulation was gone and the metal braided wire core was exposed, shorting out due to moisture and heat.

Still plan to replace distributor, plugs and wires for good measure.