Guys Please Help (Misfire)
 

Guys, I have a problem with my Z that me and several of my mechanic friends simply can not get to the bottom of. Some of you might remember a post from me regarding igition breakdown, Everyone who drove the car told me the same thing... coil packs. Yesterday I put in 6 working coil packs and brand new spark plugs in my car... after getting excited when I drive it I find out that in fact the break down is still there. The breakdown is alittle hard to explain, basically the engine start misfiring when under a decent "load" so for example I might not experience this problem in 3rd gear till maybe 5grand rpm's... but in 5th gear I can get it as early as 3600 rpm's. Out of the mechanics that have drifven it two of them both feel that it isn't the computer and it isn't the throttle position sensor, one of them suggested checking the grounds to my car and I have already done that... everything checks out. The only thing one of my friends could think of is if this car indeed had an igition module it could possibly be bad, im not sure if it does.... Can anyone please try to help me figure this out, it's really getting on my nerves, and is really making me sad that my Z won't run right. Thanks Guys, I have confidence in you!!!
Chris

First, how many miles are on the engine? Original or rebuilt? When was your last tune-up / service? Have you done a compression check?

It could be anything from the copper inside your electrical wires breaking down to your valves/seats being so worn out that you don't have any compression.

An under load, higher rpm miss/stumble is usually caused by a lack of fuel or a vaccum leak from some kind of diaphragm that actuates at those rpm's.

Just questions and ideas for discussiond purposes........

I'm still saying MAF. I had a MAF go bad in my old Z31 and it did exactly what you're talking about.

Have you checked your timing? Did you recently change a timing belt?

car is right around 100k miles, original miles... I havent compression tested it, but by the way i feels thats proabably not it, vacume lines... I'm not sure why only high rpms would cause that to be a problem but then agian i am no master mechanic can u explain?

i wrote this in my thread about the same problem...i dont know if you ever read it but i will write it again.
try cleaning all of your connections going to the coil packs and to the injectors with some electrical spray, i think my dad used wd-40, and let them dry for about 10 minutes and then clean them off if they are still wet. then hook them back up. it worked for me so i guess you could try it
do you know if it is a certain cylinder that is not firing correctly?

Vaccum diaphragm........ Something like a EGR valve maybe. 1) Bad diaphragm = vaccum leak. 2) EGR doesn't open at the prescribed 2500-3000 rpm, or so, the engine might see a seperate lean or rich condition, causing a misfire. Have you checked you fuel pump pressure? Pressure regulator? Any codes off the ECU?

One thing is for sure, if it's not running right and you hook it up to an engine analyzer that reads your exhaust emmisions, you will get a better idea as to what is wrong. The emmisions test will tell you if the misfire is caused by a lean or rich a/f mixture. Generally, if it's a lean misfire, you have a fuel/vaccum problem. If it's a rich misfire, it's ignition/electrical.

One of my old sayings: "There's only two things that I can't do on my cars, balance the tires and do a smog check".

I have a similar issue with my z32 92' NA, I change MAF, Fuel Injectors, coil packs, and when I do a cold start it works fine and very smooth, but when it reaches the working temperature it starts to misfire, so you guys can help me to find a solution.