Misfire problem tracked down.

[ben.smithjr]

Hey there Nicoclub, new member and first post. To start off I recently bought a 1990 300zx 2+2 that had a misfire. After doing research and looking at website after website and post after post to try to track the problem I finally said screw it and just went at with a multimeter and an LED I had laying around. This is just a little info for guys that might have my same problems. I know there are a hundred posts with information but I was experiencing really weird problems.
To start off my symptoms were that:
1. If I started my car early in the morning when it was cooler it would miss, but if I started it in the afternoon when it was warm it would run fine. Now this is where my problems started getting weird. If I started it in the morning it would miss and it would continue to miss even as it got warm. If I turned my car off after driving it for about 30 minutes and turned it back on it would continue to miss as if it was the early morning. It would do this pretty much however long I drove it for if I started it in the morning but would run fine if I started it in the afternoon no matter how many times I turned it off and back on. First weird problem. 2. After doing a little research I came across a topic about setting the voltage on the TPS(throttle position sensor). After checking the voltage on it it was way off, I mean way off like 1.4 volts. I didn't even think it would go that high. I adjusted it to the proper voltage but in the process it was acting strange. When I would turn the TPS to lower the voltage it would jump from .4 volts all the way to 1.9 volts. I didn't know if this was normal so I just managed to get through and get the correct voltage. I tightened the screws back up hoping this would solve the misfire. I started the car back up and it still missed. Sad face. 3. After the let down of the TPS I came across another post for the throttle bodies. It explained how to set the throttle bodies and synchronize them to open together. After reading this article I looked at my throttle bodies and noticed one was opening a little bit sooner then the other one. The right up said that they didn't have to be removed in order to adjust them but I went ahead and took them off so that I could clean them. I removed them from the plenum and they were super dirty with built up carbon. One of them was so bad that the build up was keeping it from closing all the way which was what was making the other one move before it was. Well I cleaned them, adjusted them, reinstalled them and hooked up the hoses to them. Thinking 100 percent that it was a problem fixed I started the car back up. It wasn't missing anymore, but them I realized it was the middle of the day and was pretty warm outside. So after feeling that it was resolved I called it a day and came out the next morning and fired it up to go to work. To my surprise it was still missing. Epic sad face. I was a little fed up at this point. 4.I thought that my timing must have been off due to it just acting completely weird. I bought a new timing belt and when I took the cover off I lined one of the marks on the gears up with the mark on the back of the belt cover and noticed that the marks on the belt for it were off and that each gear was off by one tooth either to the right or to the left. I was thinking whoever did this must have just not given a crap about it. I put the new belt on and lined everything up like it is supposed to be and got the cover back on and crank pulley and it was at night so I new if it was still going to miss it would do it then. Well again I fired it up and still it misfired. This is when I went redneck on it and figured it was either a fuel or spark problem. 5. I employed an old trick I knew by unplugging coil pack after coil pack until I noticed it not make a difference in the idle. Well I narrowed it down to the number 5 cylinder, of course it had to be one of the hardest ones to get to. Next I had to determine if it was missing spark or fuel. I took out the coil pack and found an older spark plug laying around and shoved it in there and held it close to something metal and had a buddy start it up for me. Spark was fine. Then I took a screw driver to the fuel injector and held it to my ear. It wasn't clicking. I felt a little relieved after I found this out but it didn't last long when I started looking into what would cause this. 6. I saw another right on checking injectors and I took my multimeter and checked the ohms on the injector. It had 12.9 ohms, which meant good injector. Then I checked the voltage at the connector and I didn't get any voltage what so ever. I started thinking, "crap bad ecu". While i was making this check I reached over to grab my light and I bumped the ptu, when I did this the voltage started reading 3.1 volts. I still don't know why it did that but I just know it had me stumped. I was pretty sure that the ptu didn't have anything to do with the injectors but I again went looking for how to check the ptu and looked to see if it could effect the injectors. I found a test for it that was a little confusing but I ended up figuring it out and it was fine. I was still trying to find out why it was getting only 3 volts instead of the necessary 9 volts. I have only heard of it either getting all the volts it needs or none what so ever, not a third of the volts. I then looked into how the injectors worked and discovered that they have continuous voltage and they get ground from the ecu. Again, "crap ecu". 7. My last hope was that maybe the wiring harness was cut or corroded and was not letting the ground get to the injector. I still had the LED hooked up to the connector and I dug the ECU out of its little hiding area. I unplugged the connector (unbolted to be more exact) and then I ran a wire from the number 5 injector pin to a bolt to ground it out and my led lit up. So I knew the wires were good. Now this is when dumb luck helped out, I was just about to call it a day and order a new ECU but I wanted to hook it back up and get any codes off of that it might be throwing. While I was hooking it back up I noticed that my led flickered a little bit. I thought I was tripping, so I tried to get it to light up again and sat there for about 5 minutes holding it upside down and hitting it and just whatever dumb stuff to get it to work. Still couldn't get it light back up. I started to unbolt it again and i grabbed the connector and the LED lit up. I was ecstatic, if I pushed down on the connector it would light up and I would let go and it would shut off. So I took the connector off and cleaned the connector and pin and kinda bent the pin on the ECU to make a better connection. I fired the vehicle and "holy crap" it runs great. This has taken me a week to try a diagnose. Sorry if this was long and drug out but I didn't want to miss any detail in case some one else has this problem. I hope this helps someone out one day.

[Ziggy1621]

Welcome to Nico! And congrats on figuring it out! Funny what 22 years can do to connections

[Shoey2]

im currently having a very similar issue, i looked up the ptu price and its approximatley $140 employee cost at the nissan dealer im at. so its not a real big deal if it dosnt cure it, mines a 1990 and still has the pre recall ECU and i already tried the NGK PFR6G-11B. The guy before had put in the non turbo(N/A) spark plugs. i think ill order that tomorrow and make sure to clean the ground as OP mentioned while im in there...

[ben.smithjr]

Well first find out if your not getting spark or fuel. The ptu has nothing to do with the injectors. Just track down what cylinder it is then take a screw driver and see if the injectors firing. When you do this your going to hear either a very distinct sound from that injector or a faint clicking from its neighbors. First listen to a known working injector to get the idea.