3800sc random multi cylinder misfire (Page 1/2)
Madess FEB 01, 04:58 PM
I have a 3800sc with f23. I have been fighting a misfire for a while. I have swapped the coils, wires and plugs. I have looked for vacuum leaks, and have not found any. I have cleaned the MAF. I have not tested fuel pressure yet. I am checking the fuel pressure next.
OntarioKev FEB 01, 06:27 PM
When you monitor the misfires live what cylinder(s) are they primarily in?
Reallybig FEB 01, 07:10 PM
The coils are mounted to a platform that has a plug going into it. The ignition module? I think...
In the stock car it is bolted to the cast iron accessory tensioner bracket. The mating surface on both need to be clean so there is decent conductivity from the module to ground. Corrosion on the aluminum side or both can cause problems. Make sure on yours it is clean with good conductivity. If I'm in that area for maintenance or repairs, I like to clean off any corrosion and apply a light film of dielectric grease to both sides before bolting it back on. Worth a shot
Reallybig FEB 01, 07:13 PM
I have also had misfires that turn out to be improperly gapped spark plugs or a spark plug where the end was bent during install and shorted to the tip to ground
Spoon FEB 01, 09:05 PM

quote
Originally posted by Madess:

I have a 3800sc with f23. I have been fighting a misfire for a while. I have swapped the coils, wires and plugs. I have looked for vacuum leaks, and have not found any. I have cleaned the MAF. I have not tested fuel pressure yet. I am checking the fuel pressure next.



So I'm not the only one chasing a 3800SC misfire. During my swap I replaced all sensors, ICM, plugs, etc that every body will tell you to do. Figured I would start off all new. The list is extensive. And I endd up with a cylinder #1 misfire code. I even removed the metal shield from the ignition wire boot on #1 figuring it was arcing out. The odd thing is the "ChkEngLigt" only comes on when the tach hits 3K rpm (standing or moving) and you still couldn't tell there was a misfire.

The above was during warm weather. Now its winter and I can actually feel the misfire at 3K rpm. I can also hear an ocassional misfire at idle but not with every firing of the cylinder in question. I connected a vacuum gauge and it holds steady, no load, from 0-3500 rpm with no noticeable needle shaking. Compression check all cylinders were within spec, no highs / no lows.

Bought a new scanner Actron 9680 that shows graphs. Did some recordings but haven't had a chance to decipher the readings yet. May post it on here for the experts. This scanner gives the most likely problem based on the readings and what actual mechanics have done with the same #1 misfire code on same engine.
At the top of the list was "REPLACE #1 EXHAUST VALVE SPRING".

With that info I'm thinking a weak spring could float valve at 3K rpm causing the misfire if it can't maintain contact with the camshaft lobe. Sorry for being long winded.
Madess, get back to me pm or otherwise if you get this figured out and I'll do the same to you should I.

ps: #1 happens to be in one of the most congested areas to work in. ie: cradle drop...

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

Madess FEB 02, 10:47 AM
I checked fuel pressure and it is a little below 40psi, I am guessing that is too low. When I rev the motor it does not go up much. I am assuming I need a better fuel pump. I do have a F20000169 sitting on the shelf. I am going to install it, along with some sound deadening, because I hear it has a terrible whine. I will let you know how it goes.
Dennis LaGrua FEB 02, 06:26 PM

quote
Originally posted by Madess:

I checked fuel pressure and it is a little below 40psi, I am guessing that is too low. When I rev the motor it does not go up much. I am assuming I need a better fuel pump. I do have a F20000169 sitting on the shelf. I am going to install it, along with some sound deadening, because I hear it has a terrible whine. I will let you know how it goes.



I do not believe that a defective fuel pump would show a random misfire on #1 cylinder as Spoon is reporting. I would lean towards a partially clogged #1 injector or
erratic power to it caused by bad wiring.

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"THE COLUSSUS"
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Spoon FEB 02, 06:49 PM

quote
Originally posted by Dennis LaGrua:


I do not believe that a defective fuel pump would show a random misfire on #1 cylinder as Spoon is reporting. I would lean towards a partially clogged #1 injector or
erratic power to it caused by bad wiring.




Mades / Dennis, I did swap #1 injector with #6 opposite bank with no change. Still got chk eng light at 3K rpm. Put injector back where it was and checked wiring with Noids and all wiring checked good. Did test light back probe on injector while running and results were positive. One lead was constant hot and the ECM was flashing away on the other lead. Keep the ideas coming.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

OntarioKev FEB 02, 09:41 PM
When moving coils/injectors to track down a misfire you need to be live monitoring all the cylinder misfires to watch for any changes. Just waiting for codes to pop is not sufficient to track this down.
Madess FEB 03, 08:32 AM

quote
Originally posted by Dennis LaGrua:


I do not believe that a defective fuel pump would show a random misfire on #1 cylinder as Spoon is reporting. I would lean towards a partially clogged #1 injector or
erratic power to it caused by bad wiring.




I agree. If it is only cylinder 1, can't remember if you have done one, but I would do a compression check on all the cylinders and see if #1 is a significantly lower than the rest.