1990 300ZX Compression Test

[dallasee]

I purchased this car as an every day driver. Changed the oil every 3-4k miles. has a few mods and a chip?? nt sure what kind but it ran great for several years (8). noticed the gas mileage dropping so I decided to put new fuel injectors in and replace the injector wiring while they had it apart. Ran pretty good and had a dyno before and after to ensure injectors were working properly. I have driven it for about 5k miles since that work. Recently it started running rough. I checked all the coil packs and came up even across the board with about .9 to 1.0 ohm. Also checked the injectors and all were within range. Compression test revealed about 170-180 all except holes 5 and 6 which I hear usually have problems. all plugs look great except #6. Little bit dark. For the past 3k miles I have heard a bogging noise like a exhaust leak from that side (near 5-6). #5 was pretty low (like 100-115) put a bit of oil in and jumped to 150 or a bit better. I likely will have to pull the motor but reluctant to because the miss is not that bad when over 1500 rpm. Low ide the miss is pretty obvious. I have followed several threads here and decided to join (newbe) but I guess the question is: is it worth pulling the heads or just get a low mile engine with good specs and swap it out? I think the rings are likely not that great and the valves give up pressure pretty fast but that may be my 30$ compression tester.

[twinturbozxtacy]

really up to you, a used engine is a crap shoot, and a rebuild would cost about the same, maybe even cheaper if you are handy... either way you have to pull the motor. More than likely it needs a rebuild if it was a DD for 8+ years. rebuild(kit with overbore gaskets, pistons and rings - 4-500ish) plus labor to pull and install motor, plus motor machining, plus motor assembly, plus new clutch(assuming its a manual, otherwise new trans filter and oil) now a diy'er could get the rebuild, pull the motor, spend 40 bucks for the valve tool and 5 for lapping compound and do the valves himself along with all the other mechanics. might need a hone then a hone brush like a flexhone (200 for both) and some beers and a lot of hours and its done for around 8-1k, if you have a shop do it fugetaboudit!

[dallasee]

Your right. Kind of knew the answer but wanted someone to weigh in. Ive rebuilt small and big blocks ford and chev but this motor and all the hises are out of my realm. Not that I could figure it out but as anal as I am it would take months. I drive a 350z now and this ine just sits. I have kids that lay claim to it but have no idea what is takes to maintain theae type of vehicles. I wowuld likely buy a block and rebuild it top to bottom. Then do thr swap. Either way its not about the money I just love my z cars. I wont deive it anymore but will likely take you advise and pull the motor. Right thing to do as I will not sell it without full disclosure. Would rather gift it. Thanks for the help and it does run oretty good for only 5 performing. Cant say that about my chev or ford. Again thanks for the input as it seconds what I feared

[dallasee]

Sorry about spelling. Darn cell phones

[twinturbozxtacy]

sounds like a good project for you and kids! if they want it let em work on it with ya

[dallasee]

Hey. Thanks again. I had the injectors put in by a new shop in town. The more I look at it the wires to the injectors were never replaced. Like they charged me for....... they look cracked and old. I can hear the injectors TWAKing though. Not sure that means much but they are responding. I don't have a leak down tester but could probably make one. Anyway for now I put it all back together with fresh plugs. Still a miss. Retested that cyl. (5) today and got 150. The miss is definitely coming from the drivers bank. Do you think an O2 sensor on that side could be causing the issue? I have the 300 page manual for this car but how do you check an O2 sensor? The previous owner put a different ECU in the car and everyone I have brought it to in the past for tune ups and such say they cannot communicate (check DTCs). no check engine lights. Wonder if it's worth checking? Any thoughts? I know 150 is not good but wouldn't think it would run that rough. Maybe I'm wrong.

[twinturbozxtacy]

did you do a powerbalance test? if not do it now!