Engine Spluttering Or Misfiring On Level Throttle?

[h00dstarr]

Hey guys, have never posted on here but have been reading on here alot in the last year or so, however have got no idea whats possibly wrong with the car. I have an 89 gtr only has exhaust, filters and brass boost restricter removed.Over the past 2 months has been progressivly getting worse. The problem is when engine is cold even after 10mins driving it will splutter or misfire/cut out when throttle is left on level ie sticking at 60kms an hour. Hard to explain, ive been in cars misfiring or cutting out under boost and it felt alot harsher than what my car is doing, it feels like blown headgasket but that is not the problem. If anyone has any Ideas as to what it is that would be awesome. Could be a few things, fuel pump? AFMs? coilpacks? Any ideas?

[ashneel]

coil packs are the first things you should check as they are very common with misfires

[h00dstarr]

coil packs are the first things you should check as they are very common with misfires

thanks, is there a way I can test them or is it a case of buying new ones and hoping that fixes it. Another thing, the car is fine under load even before boost when cold surely Im not the only one to have had this problem even if it seems uncommon. Cheers

[h00dstarr]

Common guys, any ideas? I dont understand why the problem is non existant when the car is hot. Only when cold, as with coil packs hot temperatures shouldnt make them better. If anyones heard of this let me know or point me in the right direction. Thanks

[FAT_3L]

doubt itl be coils if the cars fine when its warm.

[YCOPY]

Clean the AFM's

Check your o2 sensors, see if any wires to them are damaged.

Check the vac line off your FPR, see if its cracked or super loose.

Failing that...are you loosing water? any water oil mixing happing? ie milky oil/water. or bubbles in the radiator when you take the cap off?

[DannyzGTR]

yea my 89 r32 gtr does the same. its got new spark plugs, splitfire coils. under heavy load such as 3rd about 4500rpm and full thorttle it misses once and then again and then heaps. its not losing water.

car specs:

pretty much standard 12psi with 3.5 exhaust, m's air intake & mines comp.

my thoughts:

incorrect spark plug gap?

oil travelling through the intercooler pipes from blow by or turbos?

not enough fuel pressure?

plugs fouling up to much fuel due to oxygen sensors not reading right?

one of my oxygen not workin 100% but still, wat about ppl with 800cc+ injectors when they foul up?

ps. sorry invading your thread but i thought to myself were in the same situation.

[__PhaseShiftDown]

could be AFM. Mine was doing the same, replaced the AFM and it went away.

[Beer Baron]

it is most likely your 02 sensors.

does the problem only occur when the throttle is held steady at a small opening? say 10% throttle, cruising along at 60km/h? does the car run fine through the revs if you floor it?

at small throttle openings and low load/low RPM the cars ecu runs closed loop mixture control. so it's reading the values from the 02 sensor and adjusting fuelling to reach it's target value. if your 02 sensor fails then obviously it can't do this properly. the reason the car runs fine when you are acclerating is at those times the ecu is disregarding the 02 sensor input and is just running off the AFM, RPM and throttle position to give it a location on the fuel map.

it's very easy to test the 02 sensors. if you have stock ecu you can use 'consult' or one of the products out there that interface with the diagnostic port and that will give you the 02 sensor readings. or if you have a power FC you can just look it up in the sensor check screen.

I'd be checking the 02 sensors first as it's very easy to diagnose.