missfire cylinder 3 and 4

[arizzle]

i searched this and still nothing has come up.

motor is a freshly built 7mgte, eagle rods/ JE pistons no more then 3 months old.
its running a maft-pro w/ speed density, 550 injectors , ngk ird7's,

this just suddenly happened while driving doing speed limit car starts to struggle to maintain speed, get home checked spark it was good, warrented the plugs just incase they are fouled out from running really rich. replaced my jz COP for stock thought it was a bad coil/ lead still no change.

checked fuel next, consis 12v in the inj plugs. injectors have resistance. so i tried pulling the plugs while the cars running 1 2 5 6 car ran rougher then it was but when i pulled 3 -4 there was no change in idle.

im going to be doing a compression test but its a fresh motor i dont see why it would have blown the rings or the HG, not losing coolent and it seems to be burning only on morning start up meaning valve stem seals. besides that nothing out of the ordinary.

please help, what am i missing if its miss firing only in cylinder 3-4 ??
broken valve?
bad rings?
ecu?

 

[whitemike]

The fact that It's two cyls next to each other leads me to the head gasket.. what kind of HG did you use? I think a compression check should be done right after you check for error codes.

 

[mkiii222]

Sounds like it's not firing at all on 3-4. Since you've already changed the coils (COP to stock) that should eliminate coils as the source.

Check the wiring from the igniter to the coil packs. 3/4 are on the same lead so I would start looking at a wiring issue. Maybe something melted or came loose.

http://cygnusx1.net/Supra/Library/TEWD/MK3/manual.aspx?S=Main&P=40

 

[whitemike]

Could check for spark on 3-4 ..

 

[Nick M]

3 and 4 are companion on the ignition. A failure to seal between mechanical companion is also possible.

 

[arizzle]

all the awnsers,
im running a 1.2MM hks with ARP studs, block and head were machined.

there are no codes.

as for spark thats the first thing i had checked and there is spark in all 6 cylinders.

 

[Nick M]

Those cylinders don't share injection. If what you are saying is true, you have two broken wires, two bad injectors, or two bad transistors in the ECU. Those are the only choices.

And since the cylinders are side by side, and share waste spark, that is a much more likely choice. And if you have good spark, that narrows it down even more.

 

[arizzle]

^^^ i think im leaning towards a bad transistors in the ECU, i checked all the injectors and all of them have the same resistance.

but i guess the only way to test all the option is a new ECU and injectors. im getting voltage to the plugs so that will take the broken wires away.

 

[CajunKenny]

How did you confirm spark...just by pulling the plug wires?

 

[arizzle]

removing the spark plugs.

 

[CajunKenny]

Harbor Freight sells a spark/coil tester for around seven bucks. That's a safer way of doing it for you and your coils. You also get to see the intensity of the spark.

 

[metaphysico]

Also just cause an injectors resistance is ok does not mean the injector is working, use a stethoscope or something to listen to the injectors to make sure they click. If they are clicking that pretty much rules out the ecu and the injectors as being bad. My money would be on a hg fail, if you can do a leak down test that will be the easiest to tell. Compression test helps too and its easier to get hold of.

 

[arizzle]

^^ yah i used a stethoscope and there was clicking in the injector it self,

wel looks like everytime i drive seem like im losing a bit of coolent. but yeah looks like a HG to me also.

well thanks guys for the help!

 

[colin39]

blown hg between ports, soz budd :3d_frown:

 

[arizzle]

sucks indeed!