1990 300zx down 6th cylinder runs really rough

[bucrucox08]

1990 300zx just picked it up 2 days ago. 6th cylinder doens't seem to be firing. I did plugs and swapped the coils- still bad. the connector's terminals that connect to the coil read out 10.6ish vots while the rest read over 12. Any typical failures in wiring in these cars? Defective harnesses that attach to the coils? Any help would be great since my experience with 300zx's dates back only 2 days...any suggestions would be awesome. thanks

[izmyttz]

being that it is a 90 model like mine..i'm sure the harness is junk if it's the original one. I replaced mine for 325.00 from concept z. they sell them on ebay. Or you could just get the coil pack plug replacements for around 50 bucks and cut and splice the new ones on. Either way mine did the exact same thing. I took it to NISSAN and they said new harness...blah..i could have told them that.

[Zwicked]

The 1990 I have has 250,000 km on it and the harness works as good as new. It's not to say without doubt your harness is bad, but it's not real likely unless someone has had the engine out many times and not been careful with the harness.These cars are prone to injector failure though and corroded terminals. If the injector plugs are destroyed, you can buy new ones and splice them in.

My suggestions:

1. Remove #6 coil pack (all for that matter) and then the injector plugs. Using a digital ohm meter, check if the injectors themselves are close to between 10 and 14 ohms. If they are much off that, they need to be changed. They seem to go randomly and not over any trackable period of time. It's pretty clear now that Ethanol fuels shorten their life.2. When testing the injectors, check the connector ont he injector and the harness. If there is any of that greenish corrosion, it must be scraped/wire brushed off, sprayed with a GOOD contact cleaner, and the dielectric grease applied before installation. Any of the connectors with that wire clip usually end up getting corroded as they aren't really weather proof, plus you may noice some of the rubber boots cracked, that's a sure sign.***The harness can be tested as well if all test good and #3 below is done and no change.3. Remove the connectors for the: CAS, PTU, MAF, Cooltant Temp Sensor, Fuel Temp Sensor, !AA (both solenoids), and Auxiliary Air Valve.All of these are controlled by/provide feedback to the ECU and if any are compromised, the engine doesn't run right.

This is a starting point, but I'd also recommend going to http://www.twinturbo.net and seach 6 P's and do all of those to ensure your car is right to start off with

[izmyttz]

wow that harness must be the NISMO one to last that long.........

[Zwicked]

Well, no it is a stock harness. I think it all depends on the life they have lived. Anything dies early if treat poorly.

[bucrucox08]

Thanks guys....I had a hunch of that....then found out today that it's a fairly typical issue...the injectors are a 178 at cost at my local dealership and the harness was over 300 bucks....Sooo....instead of ordering the parts and going back and doing a little body work on the car...i went and bought a second one and 3rd engine...engine was junk...but it was 50 bucks...6 injectors and an ac pump...the other car only has 55k on it.... the guy couldn't get it to run b/c he installed his fuel lines wrong. The only issue is the rocker pannels are rusted pretty badly...more than cosmetically...so how much structural support or integrity do the rocker pannels give the car...can a do some welding and bondo?

[jdstealth66]

It woul not show anything bad with the voltage if the harness is bad. Resistance is key! If there is to much corrosion from the ptu to the coil it will cause the coil to missfire or my not even work. My suggestion is check the ptu harness and see if there is any corrosion i.e. green crap. Then check the coil connector for the same thing. If you have a manual and a ohm meter check for resistance if i remember it shouldnt be more than .5 ohms but check to verify. hope this info helps. I had the same problem but now i got it fixed but now gave in to a new tt harness because im doing a swap.