Engine Spark plugs problem

Hi Everybody writing from Italy, Sorry for my english.
I have a problem on my Daimler 4.0 90’.
Engine run perfectly when cold, but when arived to the normal running temperature ( 80/90 c) the Spark plugs Number One and six stop working
I have chandged distributor, the interior rotor, and the engine Spark plugs, but still dont work. Has anybody any ideas?

Ciao Michael …

The fact that your engine runs perfectly when cold seems to eliminates any mechanical problems. And since only two of your spark plugs are affected (how did you determine that?) eliminates a fuel delivery problem, so …

Since you’ve already replace the distributor cap and rotor I would recommend also changing the spark plugs and wires and see if that solves your problem.

hi, thanks for the reply. it was the mechanic with a special tool that determined that only spark plugs No. 2,3,4,5 work. he also checked the correct functionality of the cables and everything is working. is it likely that the problem is that the control unit receives incorrect information? maybe the crankshaft sensor? or the lambda probe?

Michael …

Let’s try this bit of troubleshooting …

  1. Start your car when it’s cold and running smoothly. Now one at a time pull a spark plug wire off the plug. The engine should now run a little rough but run smoothly again when you replace the wire on the plug. Do this one at a time for all 6 plugs. Did this past the test?

  2. Now with the car warmed up and running roughly do the same test. If plugs 1 and 6 are not firing there should be no change in the way the engine is running when you pull those wires. Actually if those two plugs are not firing you should be able to pull both of them at the same time with no change in the way the engine is running. This will tell you if your mechanics diagnosis is correct.

Screen Shot 2021-04-23 at 11.30.32 AM

thank you very much for the explanation.
we noticed with my mechanic that the spark plugs 1 and 6 stop working only once they have exceeded 1500 rpm.
for the sake of scruple we have replaced the lambda probe with an identical one used and this time the spark plugs 1 and 6 are able to work up to 2500rpm.
Maybe if I buy a new probe I solve the problem?

I can’t see the connection where the o2 sensor can affect only two cylinders. The o2 talks to the ECU to adjust the fuel delivery on a basically global arena not individual cylinders.
I had the earlier 2.9 and when it was cold it worked on 5 1/2 cylinders, I tracked that down to corrosion between the spark plug lead and the nipple on the spark plug. Once I had cleaned those areas it was back onto 6
I would be inclined to do as Groove suggests and change the plug leads.

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What plugs are you using if I recall,they should be champion, NOT NGK because of heat range…
Give it a try.

Michael …

I have to agree with Robin (and I’d be foolish not to :grinning:) that your O2 sensor isn’t the problem. All it does is tell the ECU what your fuel trim is by measuring the amount of unburned O2 in the exhaust. Based on that information the ECU either increases or decreases the duration that the fuel injectors are open. As Robin mentioned it sends the same signal to all the injectors, not individually. Are you getting a check engine warning ?

Good advice to check that all the connections from the plugs to the distributor cap are corrosion free.
Did you try my tip on pulling the plug wires??

If your car ran smoothly at all temperatures but only ran roughly at higher RPM’s I suspect the ignition coil was breaking down but what you discribe doesn’t support that.

“Check Engine” when the engine start to work a 4.
yesterday with the mechanic we did this test:
with the stroboscopic sensor (thanks to this it has identified the spark plugs involved), we exchanged the cables 1 with 2 and vice versa, and 5 with 6 and vice versa and also with the cables of numbers 2 and 5 mounted on 1 and 6 at 2500rpm stop working, and 2 and 5 work perfectly …today i try to do your test.

Michael …

A check engine warning with a code “4” … hmm. The cars up to 1989 have codes 1-8. 1990 and later models have 24 codes starting at “11” and going up (so no code 4). Are you sure it’s not a code “44” ?

Check Engine fuel Fail 44

Michael …

OK, a code 44 is basically telling you that the O2 sensor is either …

  1. working properly and detecting a too rich condition in the exhaust

  2. not working properly (bad O2 sensor)

If one (or more) of the spark plugs isn’t firing then that fuel is going right out the exhaust manifold. When the O2 sensor sees this it tells the ECU. The ECU then tries to lean out the fuel until it get to a preprogrammed point and then it just gives up and switches to a stored program ignoring all the other sensor inputs … and it turns on the check engine warning code 44.

thanks for the reply and the useful information, now, knowing that I have this problem with ignitions at pistons 1 and 6, which in the meantime can now work up to over 3500rpm, (so I cannot use the sport button), and knowing that I have the “check Engine” light on, what other connection could there be? Is it possible that too much fuel cannot be ignited by the spark?
i have ordered a new lambda probe in the meantime.

Michael …

I hate to say it but you seem to be all over the place explaining your problem.

  1. Are you now saying that your problem doesn’t have anything to do with engine temperature but is RPM related ? In other words the engine runs just fine until you reach 3500 RPM and then plugs 1 and 6 stop firing ? The only possible cause for this would be your ignition coil breaking down at high RPM but that would affect all the spark plugs not just 1 and 6, so that doesn’t make sense.

  2. Did you ever do the simple test of pulling each spark plug wire one at a time and see if it changed how your engine runs at idle ? Not that I don’t trust your stroboscope test but …

  3. A bad O2 sensor is NOT going to shut down any of your spark plugs. As I mentioned in my previous post all it does is advise the ECU and in turn the ECU then changes the pulse time of the fuel coming out of ALL the fuel injectors (it doesn’t trim each injector differently) Replacing it may cure your check engine warning but it’s not going to be the cause or fix dead cylinders.

  4. A bad O2 sensor won’t flood properly working cylinders with too much fuel. Once the ECU gets to a certain point and fails to obtain the desired oxygen content in the exhaust it gives up and goes to a preset fuel trim which is slightly rich. In other words it doesn’t keep going richer and richer.

  5. The fuel rail delivers the same amount of fuel at the same pressure to all the injector so if all the injectors are working properly you’ve determined that a dead cylinder isn’t fuel related.

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sorry i probably can’t express myself well in another language.
the situation is this:
the cold machine works perfectly.
once the temperature is reached, once they have exceeded 3500rpm *, the spark plugs 1 and 6 stop working.

  • before I changed the lambda probe with a good used one it only reached 1500rpm.
    I ran all possible tests for the spark plug cables, stroboscopic, your test, with water spray, and also swapping cables 1 and 6 with those that arrived ie 2 and 5.

Hi Michael …

Absolutely no need to apologize, your English is much better than my Italian (which is non existent).

Here’s my recommendations …

Install NEW spark plugs and wires. If you’ve already done this then swap plugs 1 and 6 and their wires for any other two. Make absolutely sure that you have a NEW distributor cap. Even if you’ve done all this, or been told it’s been done, … DO IT AGAIN. If you do this I can say with certainty there is no way that plugs 1 and 6 (and only 1 and 6) can not be firing.

Here’s a simple tester to visually check if your plug is getting power

Screen Shot 2021-04-26 at 11.43.17 AM

Only about $10 or 16,000 ITL. Don’t trust someone else and test each plug yourself

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