problem from newby

[xsive]

hi all new to this forum hope im in the right place to ask some questions?
firstly the vehicle is a 1990 300zx z32 auto non turbo .
this is what we have done to try and rectify a minor miss at idle (some times) a pop pop in the exhaust
idle speed around 900rpm which some times alternates to around 1500rpm
as you lift the revs there is a stutter untill around 1200rpm .the miss is no longer felt or audible
but as you can imagine if this continues to persist the car will not be driven for fear it stops in an inersection or ?
now for the nitty gritty as to what has been tried /replaced i might add that i am an older mechanic out of the game for many years.
manifold has been off twice to check gaskets etc for air leaks
injectors have been rebuilt and flow tested
6 coils new have been instaled
2 sets of plugs the first set nippon the set in now ngk
throttle bodies have been set with cigarette papers and are definatly closing
modified the left side linkage by taping a 6ml addjustment screw for finite adjust
timing has been checked at a starting point to 15 deg btdc
the ignition transistor has been replaced with a new unit
every terminal block has been cleaned and sprayed with contact cleaner
all injectors test again using multimetre and test between 12.7 and 13.5
fuel filter changed
tps is set to 0.46 volts at closed throttle and hard idle circuit tested for continuity at this seting OK
there is no missing ohm readings when the throttle is opened from the closed to full open position
there is no fault codes showing on the computer
compression test shows no bad cylinders
the idle valve can be heard to open and close when i remove and replace the plug although i have not removed to clean or check it
i have pinched off the vacume pipe to the valve and the vehicle drops revs to around 500rpm but the miss is still evident
i have road tested the car to over 160kph with no missing above the initial miss and goes like a scalded cat pulls right through
i have physicaly frozen the ecu unit to no avail
fuel pressure was checked .im am at wits end can any body HELP ?have i missed something

[xsive]

sorry guys did not have any room left on the first post to mention that the MAF has also been replaced

[Trimble Epic]

Have you looked at this?

http://home.swipnet.se/e-solutions/IdleTech.html

[Rager]

I didnt see anything mentioning the IACV idle air control valve , that may need cleaning/ replaced/adjusted . also look through all the connectors and make sure they are clean and not green with corrosion which is very common with theses cars.

[xsive]

thanks guys i did mention that i had not removed the iacv and now have read the link you posted and will pull it down tommorow and clean it up .heres hopeing it may work as im all out of love with it at the moment and glad to recieve any more sujestions before she goesin to the to hard corner

[Fearspect]

What type of air intake are you running? Is it in good condition?

[xsive]

thanks air intake is std new filter no cracks and tight

[xsive]

back again removed the maf and blocked te intake with my coffee cup ,the only thing that was lying around that was 80 ml pulled the brass plug in the manifold fitted an air fitting and pumped her up to 5psi and found a split hose under the opposite side the hose that is 16ml one end 8ml the other so i soldered a ruducer together with copper pipe and used two bits of oil tubing to join her up thinking it could not be this simple ?i was right it was'nt she still idles with o pop pop clean then pop pop ect ect at around 750rpm but as soon as i move the throttle from idle to lift revs stutter stutter then pow revs its freckle off any thing above 1500rpm no hesitation no poping just clean revs i have removed the iacv today checked inner workings faily clean and free moving power up the solinoids and the both activate freely i will take the other bit i forget what it is called with the diphram in it off tommorow and check its guts out aswell but i still have this feeling that this atempt will still not cure the gremlin that exists. any sugestions will be welcome and a box off matches is getting closer by the day

[zswap0429]

Im having a similar problem with mine. Some mystery vacuum leak. Or at least thats what z1 said when I explained it to them. So im looking foward to seeing the end result on this

[H1tman]

have you isolated the misfiring cylinder? fuel or spark problem?
have you checked your harness? My injector connectors were corroded from the inside when I soldered my new style connectors, you should also make sure the ECU-injector wire pins are making good contact on the ECU connector. Have you checked if there is a volt reading on the injector connectors? what condition are the PTU connectors? coil pack connectors?

random misfire or single cyl? pull spark plugs after running it and check for fuel smell. If its spark related you will get a heavy gas smell from the misfiring cylinder. If its fuel related, other plugs will have a slight scent of gas and one will just smell burnt.

[Ace2cool]

I'm gonna move this to the tech forum where it should be.

[xsive]

Thanks for all your advise guys found the problem it was the ptu or ignition module if you like not the unit it self but a former cowbow had replaced the conecting harness and not stagered the joints two wires were momentarily touching creating a crossing of coil trigger but not all the time which made identifying the missing cylinder very dificult as it was alternating from one cylinder to another so the matches and bon fire will have to be put off indefinatly .....I hope ...once again thanks for sujestions /help cheers

[Ace2cool]

Doesn't explain the revving up to 1500, but that's good that your sorted the PTU. I'm assuming it was the old style?

[xsive]

thanks ace but no it was one off the newer type pcu's but as all my prior posts regarding finding this gremlin nothing seemed to explain the miss to 1500 rpm even though i think i have read and watched nearly every thing i could find and checked cleaned replaced in relation to the miss /stutter from and including idle until i found the wiring problem i must admit that these vehicles are a breed on there own and appears that only a wish not to give up and be beaten is a must even if you think it should be easy to identify although i have been building std and modified custom cars /hot rods/and motor cycles since before my apprenticeship in 1973 this one nearly had me beat and i hope that someone else finds some answers /inspiration from this site to help with there problems as i have ..your never to old to learn something..i might finaly add that when reading a lot of posts on this site it seems it is rare for some posters to add there solution/ answers to the problem that they had asked for help with when it had been fixed .Cheers all steve Australia.

[Ace2cool]

Yeah, we ask for resolutions but it's up to the OP to provide. Sometimes I'll follow up and ask, but it's just hard. Thanks for posting your solution, Steve! Cheers to you as well!

[xsive]

all cool ace .i see your into classic bikes as well some of mine can be seen on choppers australia site in forums under your chopper pics heading of xsives xs if your interested cheers again

[t.mcginley.jr]

Wait so what is the issue now? Or was it resolved?? I got confused about halfway through lol

[Ace2cool]

Seems resolved. He said it was the PTU. Also said it hasn't revved up in a bit, so that seems to have cleared as well.

[t.mcginley.jr]

Ahhh ok sounds good to me!