Wierd miss
Wierd miss First if this should be placed in the external column, move it there. Second, in all the years I have worked on cars, I have never had such a problem. No. one cylinder is not firing. It has spark, fuel, and compression so it should fire. But, it is cool.
I checked the spark by placing a spark checker on the plug wire, and it lite up like a Christmas tree. I also took the plug out and grounded and started the motor with another plug in the head. It also sparked.
Next I checked the injector plug for power with a condinuity checker and it lite up. I then attached the checker to the positive on the battery and probed the plug for ground, and it lite up. Then I installed a Noids light on the plug, started the motor and it flashed, indicating the drivers are OK.
So yah, I figured it must be the injector. I replaced it with a new one I had around, although 42 instead of 36 lbs, and it made no difference. After each of the above tests the motor was started and the cylinder tested with a temp sensor and showed no more than 150 degrees. So, I checked the compression thinking valves or piston, and it was 140.
So, now I am really stumped. The valves were done last year. The rotating assembly this year. The motor had a bad piston slap last year, but doesn't now because it was rebuilt. It runs real well for 7 cylinders, but has an exhaust pop from the unburned gas. The rebuilder mentioned that he thought it was possible that no. 1 was wiped out by the tune (Gas).
I need a sense of direction. Any and all help is appreciated.
I checked the spark by placing a spark checker on the plug wire, and it lite up like a Christmas tree. I also took the plug out and grounded and started the motor with another plug in the head. It also sparked.
Next I checked the injector plug for power with a condinuity checker and it lite up. I then attached the checker to the positive on the battery and probed the plug for ground, and it lite up. Then I installed a Noids light on the plug, started the motor and it flashed, indicating the drivers are OK.
So yah, I figured it must be the injector. I replaced it with a new one I had around, although 42 instead of 36 lbs, and it made no difference. After each of the above tests the motor was started and the cylinder tested with a temp sensor and showed no more than 150 degrees. So, I checked the compression thinking valves or piston, and it was 140.
So, now I am really stumped. The valves were done last year. The rotating assembly this year. The motor had a bad piston slap last year, but doesn't now because it was rebuilt. It runs real well for 7 cylinders, but has an exhaust pop from the unburned gas. The rebuilder mentioned that he thought it was possible that no. 1 was wiped out by the tune (Gas).
I need a sense of direction. Any and all help is appreciated.
No not today, but before and they were ok. I agree its low, but I think it is because of the gas wash down. When I checked compression before starting the motor it was at 180, and now after running for 15 or 20 minutes, its lower.
TECH Fanatic
Joined: Jan 2018
Posts: 1,250
Likes: 239
From: Dade City, Fl.
If that is the case. I would get the engine up to operating temp and then do a cylinder leak down check on the suspect cylinder. That should give you a pretty good idea of what is going on
You might want to swap the coil/plug wire and spark plug from another cylinder to that one. Just because it has good spark at atmospheric pressure doesn't mean it will with the air/fuel mixture compressed. 140psi does sound on the lower side but it's more than enough to fire and make some heat in that cylinder.
Yes I swapped the coils two times. I know its not a mechanical issue because it is intermittent. Whats bothering me is that I have spark and fuel and more than enough compression, and the cylinder won't fire. Yet occasionally it will fire. When it won't fire I ran the coil tests, the noids tests etc over and again and it still has spark and fuel but won't fire. I am staring to think it must be in the crank or cam sensors, although it doesn't show a trouble code for those sensors.
I think that because it has gas and spark, but appears to be at the wrong time. I have worked on it on and off all week. Help!
I think that because it has gas and spark, but appears to be at the wrong time. I have worked on it on and off all week. Help!
Well it turned out to be the ECM was the final culprit. The missing and running rich was solved when the O2 plugs wires were fixed. The motor would start and would run smooth when warm. But on the next day, when it was cold, it would not start. This pattern went on for weeks. The motor ran long enough finally for a code to set. It set a 340 cam sensor code. Changing it allowed the motor to run smooth.
A used ECM was installed and it seems to be fixed. But I am taking a cautious approach until I am certain its fixed before I go for a long drive. I've never had anything like this before. Will keep you posted.
A used ECM was installed and it seems to be fixed. But I am taking a cautious approach until I am certain its fixed before I go for a long drive. I've never had anything like this before. Will keep you posted.
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For those of you that have read this thread I would like to add some final thoughts. The problem turned out to be the computer, the last place to look. This problem started a month ago and turned into an agony. Using the "its probably something simple theory," I went through weeks of on and off checking and rechecking of wires, sensors, etc. Endless ohms and voltage tests, piecing wires, noids tests and much more finally lead to the computer.
I am firm believer in getting the right diagnosis and not just throwing parts at the motor. But, this time is different. It would have been so much easier and less frustrating to replace the crank and cam sensors when I first suspected them. But, it turned out to be the least likely component to fail, the ECM, and the last to check after looking at a laundry list of items first. I knew there was something crazy going on, but never suspected the ECM. Lesson learned.
I am firm believer in getting the right diagnosis and not just throwing parts at the motor. But, this time is different. It would have been so much easier and less frustrating to replace the crank and cam sensors when I first suspected them. But, it turned out to be the least likely component to fail, the ECM, and the last to check after looking at a laundry list of items first. I knew there was something crazy going on, but never suspected the ECM. Lesson learned.
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Brett Foote
Joined: Jun 2009
Posts: 723
Likes: 25
From: go get your shine box tommy
Interesting how the ecu caused a misfire in just one cylinder. I'm very curious about this because I just put a hole in #5 piston due to misfire. I was driving on the road course and got a check engine light for #5 misfire and a slight loss of power at high RPM. I put in a new plug and it ran a few sessions no problem then the check engine light came back and then poof. The fuel injectors tested good which is the only thing that could cause a melted piston so I'm at a loss. I'm NA and only bolt on and tuned but tune or setup hasn't changed in years.
Mike, originally I had these conditions, no firing on No. 1 (cold exhaust header), motor running rich, and intermittent start ups. Fixed the bad O2 plug in wires and it leaned out. 2) as I continued testing number one, the miss started to circulate and affected all cylinders at different times. After fixing the O2s, the motor finally ran long enough to set a code. It was the CPS. Repaired that and got it to run smooth, but intermittently, meaning it would not always start.
This takes use to 3). The motor would start and run fine while warm. The next day it would not start. This happened even if the car was taken for a test drive and fully warmed up. Somehow the ECM went bad. It is believed to have a short in it (took it to a shop with the right equipment) which caused the CPS to fail and the motor to hard start when cold.
Good luck to you. It sounds like a fuel problem caused the melted piston.
This takes use to 3). The motor would start and run fine while warm. The next day it would not start. This happened even if the car was taken for a test drive and fully warmed up. Somehow the ECM went bad. It is believed to have a short in it (took it to a shop with the right equipment) which caused the CPS to fail and the motor to hard start when cold.
Good luck to you. It sounds like a fuel problem caused the melted piston.