3400 fiero missfire under load, help please

[LZeppelin513]

This is a 3400 engine running on a 7730 computer in my fiero. It runs great except that it misfires under load. The engine has it's oem plugs, wires, and coils on it. its an '04 and has about 23k miles on it.

When doing a hard run from 1st at WOT it will miss in second or higher if i shift early like around 4000 or less rpms and stay at wot. The engine will rev to about 3000 rpms in 2nd or higher and then start missing and bucking. It will do this through about 1000 rps or more if in a higher gear or stop if i let off the throttle. If i shift at 5500 rpms while running up the gears it wont miss. It also never has missed in first no matter how it is driven. If i am going up a hill and dont down shift it will start missing, especially if i open up the throttle a lot. If i am driving around town i can induce the missing by shifting in to fourth and open up the throttle, it would miss at any rpm until it either revs up high enough or until i let off the throttle. It never is random, I can almost always tell when it will do it because it follows these patterns so well.

Does any one have any thoughts, thanks a lot for listening.

[tractorman]

In my experience, if an engine is *hard* missing under a load, then the fault is almost always in the ignition system- seen this more times than I can recall on more types of vehicles, tractors, forklifts, etc. than I can recall- including a 3100 I had. Turned out to be plug wires in that case.

If I wuz in your shoes, I'd start by closely looking over the coils and plug wire terminals, then ohm the plug wires. I don't know exactly what they should ohm- it's so much per foot. I generally just check them all, and if they're all within the same order of magnitude, then I call 'em good (as far as resistance goes). I've seen lots of these 3100's and 3400's corrode the plug wire terminals where they snap onto the coils- even with relatively few miles. The only way I know of checking coils is to make sure that the resistance on the high voltage sides and the low voltage sides are pretty consistent among the three- and check for cracks/leaks. Somebody here might have a more precise method of checking them.

If none of this turns up anything obvious, then throwing parts at it isn't neccesarily a bad strategery. Plugs and wires are pretty cheap and easy to change.

[merlot566jka]

look at your plugs, are they white? sounds like your going lean!
what size injectors are you running?
hows the coolant level doing?
got any idle issues that may entail a vacuum leak?
who did your ecm? if your running the 91-92 camaro code then its probably got too much advance, the alum heads are more effiecent than the iron heads, thus needing less spark advance.
go ahead and replace the wires..its fairly cheap, and on the fiero...it gets hot back there and wires will fail more often than normal.
check the grounds from the block to the deck lid, batt to body, and the one on the back of the trans...ive seen some funny things happen because of bad grounds!
and last, data log it and watch whats going on at wot, cound be kr, could be all kinds of things. more data paints a clearer picture!

[LZeppelin513]

Ok thanks for the responses! I have data logged and if i am reading the log correctly i don't go lean, but then again i am new to this. Could a vacuum leak cause this? Right now i am suspecting that i have a small vac leak somewhere between my new brake booster/check valve and my plenum because my booster isnt holding a vacuum and when the car is on it has to build a vacuum moments after the brakes are engaged for assist to kick in. <-(Hope that makes sense) I am going to try to ohm out the wires and coils to see if anything looks weird because i am poor and in college and would hate to just throw money at the engine.

On a side note- when the engine is missing is that hard on it....am i going to break stuff when the engine starts missing?

THANKS!

edit: Coolant is good, stock multec 2 injectors from the 04 3400- i believe they are like 22 psi, ryan hess did my chip and i trust that he has a pretty good tune on it because he has tuned this same setup before.

[merlot566jka]

i belive they are 19lb injectors..if your pressure is at 22psi then there is your problem!

if your miss is knock, then yes it will damage things...and very fast!

a vac leak can cause this!