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Different spark intensity

Tiny
WISH30
  • MEMBER
  • 1999 MERCURY COUGAR
  • 2.0L
  • 4 CYL
  • 2WD
  • MANUAL
  • 250,000 MILES
Should the spark be with same intensity on all cylinders? Mine is weak on 1 and 2, and very strong on 3 and 4. I have replaced the spark plugs, wires, and ignition coil pack, but result is the same. What can be the problem? Car listed above has had engine replaced with Zetec 1.8.
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Wednesday, September 30th, 2020 AT 4:04 PM

41 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Let's start with what model and year vehicle is the new engine out of?

Also, a weak spark is caused by the secondary side of the coil not building high enough voltage. This is either due to a short to the primary side or an issue in the coil. The fact that the coil is replaced we need to dig deeper.

Is this causing an issue or did you just notice this? Can you get a video of the spark on a weak cylinder and on a strong cylinder so we can see the difference?
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Wednesday, September 30th, 2020 AT 4:14 PM
Tiny
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I will add video tomorrow, and it has problem that makes stalling while driving at constant speed without accelerating. Some mechanics told me the spark is not a problem, it is maybe IAC valve that cause the problem, they said. Also I know only that engine is Zetec 1.8.
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Wednesday, September 30th, 2020 AT 4:31 PM
Tiny
KASEKENNY
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Okay. The videos will be great. So if that is the issue the weak spark would cause a misfire or rough running. It could cause a stall if bad enough but it would run very poorly.
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Wednesday, September 30th, 2020 AT 6:15 PM
Tiny
WISH30
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Yes, rough I think is the right word in English. The engine is not working evenly when driving, it is most noticeable on lower speed, like 2 or 3 shift 30-40-50 kmph. Also often is noticeable on higher speeds. Also rpm on idle is not stable and usually very high, like 1200-1300-1500.
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Wednesday, September 30th, 2020 AT 7:32 PM
Tiny
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I'm going to make short video with the sparks in about 2 hours when will be back at home because I'm night shift at work at the moment.
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Wednesday, September 30th, 2020 AT 7:39 PM
Tiny
WISH30
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Here it is. First and second are the same, third and fourth have strong spark.
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Wednesday, September 30th, 2020 AT 10:13 PM
Tiny
KASEKENNY
  • MECHANIC
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Okay. That helps. I don't think you have weak spark. Just because it is not arcing or sparking out of the boot when you remove the wire doesn't mean it is weak. I agree that the other cylinder sounds stronger but if you have a rough running condition then we need confirm some other things first before we go to a weak spark.

As I was saying earlier, the strength of the spark is determined by the primary side of the coil supplying the low voltage and then the igniter in the coil causes the high voltage for the secondary side to deliver to the spark plug.

I would suggest checking for a vacuum leak and compression of the engine before we go much further.

https://www.2carpros.com/articles/how-to-test-engine-compression

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

The other reason I don't think you have weak spark is because when you pull the wire from that cylinder you can hear the engine change as it is now misfiring and it is about the same as the other cylinder did.
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Thursday, October 1st, 2020 AT 6:45 PM
Tiny
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I went to a local mechanic and he says that compression and vacuum is good. After that I removed and cleaned the IAC valve, measured its resistance it was 11.1 ohms and tested with 12v adapter. After placing, the driving is more smooth, stalling is almost unnoticeable but still persist, and idle rpm is more stable. But, rpm is still high and not as normal as it should be. When stay in one point and in idle, rpm is about 1200, when driving in some shift it variates depending of the speed of moving, but, when moving in idle it stays on 1500 until the engine completely stop and then it rise to about 2000 for a second and drops to 1200. What I noticed is that one of the cracks of the pipe that comes from exhaust is connected by hose to EGR valve, and other is not connected to anything. Also the upper right corner opening on the head cover is just a hose plugged and not connected to anything.
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Thursday, October 8th, 2020 AT 3:06 AM
Tiny
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Okay, that is progress.

Let's start with clarifying this so that we are on the same page. On your drawing did I number the locations of open hoses correctly?

If so, number one should be connected to the engine/airtake. This is where the exhaust gases enter back into the engine so that they can be used to reduce emissions.

Number 2 appears to be the PCV system that takes engine crankcase gases/blow by gases and puts them back into the intake to be reused. Again to reduce emissions.

Number 3 I do not know what that would go too so I am hoping that is not an open hose.
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Thursday, October 8th, 2020 AT 6:39 PM
Tiny
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Sorry I have a mistake on my drawing. There should be one more main pipe that goes directly to EGR valve, and those two fork pipes should be connected to DPMF sensor (I saw it on diagram on Google). One of the sensor hoses was unplugged and I plugged it on the pipe but nothing got better or worse. Then I tried my last option, unplugged the vacuum hose from EGR and blocked it to prevent vacuum leak. After that driving got too much better and very smooth, but idle got worst. When stay in place it is still high, about 1250, but when drive in idle, it fluctuates between 1000 and 2000 (I made a short video). When driving in some shift, and want to stay and change in lower shift, in that moment while stopping and pressing foot brake and clutch, rpm rises to almost 3000 until you put the gear in shift and rpm normalizes while unpressing the clutch slowly to get drive. Also what is suspicious (maybe not a factor) is that digital speed meter show some value when staying in place for example 20-30.
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Thursday, October 8th, 2020 AT 8:02 PM
Tiny
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Okay. Glad to hear it is running smooth. However I am a little confused on the RPM issue. You said it will range around 2,000-3,000 while sitting still? If that is the case, the video shows the RPM going up and down while you are driving around 60 Km/h. Is that what it is doing with no accelerator input?
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Saturday, October 10th, 2020 AT 9:36 AM
Tiny
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Last night I did another step, tried to turn off the DPMF sensor and to turn off EGR vacuum switch, then removed the battery for 10 minutes and start again and wait 5 minutes in idle and then drive. Now idle got stable but too high. When car stays in one place, idle rpm is 1,250, and when car drives in iddle, its rpm is 1,500.
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Saturday, October 10th, 2020 AT 9:47 AM
Tiny
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Okay. How long have you driven the vehicle since you unhooked the battery? You are in effect wiping the keep alive memory which supports the fuel adaptives and the idle. Take a look at the information where it explains what happens when you reset the PCM. Clearly the best way to reset this is with a scan tool.

Are you touching the cables when you unhook the battery and then leaving them off for 10 minutes? This is a capacitive discharge which will drain the PCM's keep alive memory. If you unhook the battery for 10 minutes the keep alive memory may be wiped but there is a chance it will remain as this was designed to try and hold this small amount of power so the vehicle doesn't have to relearn all this if the battery is changed.
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Saturday, October 10th, 2020 AT 3:11 PM
Tiny
WISH30
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Yes, I connected both cables with a wire while the battery was disconnected.
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Saturday, October 10th, 2020 AT 6:01 PM
Tiny
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Okay. Take a look at the last attachment where it runs through the learn process for the idle. Then it will adjust as you drive the vehicle. If it is running stable but just a high idle or RPM then I would recommend putting some miles on it and see if it adjusts. If not we can address the idle control which is part of the throttle body.

I would say to let it idle for 15 minutes and then just put about 50 -100 miles of normal driving on it and see if it brings it down.
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Saturday, October 10th, 2020 AT 6:06 PM
Tiny
WISH30
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Can it be caused by cam cover ventilation hose (that one attached on the upper right corner of engine cover) because it is not connected on intake system?
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Saturday, October 10th, 2020 AT 6:46 PM
Tiny
KASEKENNY
  • MECHANIC
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You mean the positive crankcase ventilation (PCV) hose? Here is the diagram of it. Yes. If this is not hooked up then more air will enter the engine and can raise the RPM. I guess I made the assumption that everything was hooked up correctly.

Maybe get a picture of it just to be sure. Also, is there anything else not hooked up?
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Saturday, October 10th, 2020 AT 6:53 PM
Tiny
WISH30
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I mean for this hose, this is not my car, it is from Google, but on mine is the same.
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Saturday, October 10th, 2020 AT 7:06 PM
Tiny
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Yeah. That looks like the PCV. They call it the crankcase ventilation hose. See the attached.
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Saturday, October 10th, 2020 AT 7:19 PM
Tiny
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It is not attached to intake.
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Saturday, October 10th, 2020 AT 7:21 PM

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