First 300zx - misfiring?

[gibygreen]

Hey guys, new 300zx owner here, looking for some help on the car I bought recently. It's a 1990 (Built 05/89) NA 2+0 Manual.
When I first got the car, it had a high idle. Which i suspected was either a vacuum leak or bad IAC. Turns out pinching the hose that supplies air to the IAC was enough to stall the car from 1200 immediately. For now I've just got that hose plugged off and the idle raised manually, So that's solved the idling problem.

Along with this, the number 4 cylinder was not firing. Changed the plug and swapped coil packs, still missing. Turns out the injector wire had been corroded badly and was not making contact. Cleaned it up and ran much better.
But after cleaning nearly every connection on the engine, changing the plugs, resetting the timing, cleaning the maf, tuning the throttle bodies, and replacing the air filters. The car still has a random misfire. It hits on all 6 just fine, but under 2500 <50% throttle the car noticeable shakes/stutters. Above 50% throttle and 3000 rpm it runs smooth and pulls great.

What am i missing here?

[DCaff300ZX]

Hey guys, new 300zx owner here, looking for some help on the car I bought recently. It's a 1990 (Built 05/89) NA 2+0 Manual.
When I first got the car, it had a high idle. Which i suspected was either a vacuum leak or bad IAC. Turns out pinching the hose that supplies air to the IAC was enough to stall the car from 1200 immediately. For now I've just got that hose plugged off and the idle raised manually, So that's solved the idling problem.

Along with this, the number 4 cylinder was not firing. Changed the plug and swapped coil packs, still missing. Turns out the injector wire had been corroded badly and was not making contact. Cleaned it up and ran much better.
But after cleaning nearly every connection on the engine, changing the plugs, resetting the timing, cleaning the maf, tuning the throttle bodies, and replacing the air filters. The car still has a random misfire. It hits on all 6 just fine, but under 2500 <50% throttle the car noticeable shakes/stutters. Above 50% throttle and 3000 rpm it runs smooth and pulls great.

What am i missing here?

First I'll say that shake/stutter and misfire have quite a few possible causes, so in this case the best way to go is to use the FSM and it's tests for this symptom to try and narrow down the options. You've done some good things and some not-so-good, and the FSM tests will help there.
One area however to look hard at is the IACV that you just pinched the hose shut. The IACV is a problem unit for several reasons and the FSM covers testing it, if bad (VERY common, especially on early pre-94 Z's) replace with new ONLY and be sure to also get the upgraded harness. Another area is the PTU and it's harness with large connectors that corrode badly. Clean harness connectors and replace it if necessary (I have), and a relocate to somewhere less hot and out of the way of water soaking is also a great idea...the front area "side rails" above the airbox/filter can work for a custom mount position, or on the frame rail down lower and on the driver's side where the PTU mount can be bolted right into two open nuts on the frame rail, PTU harness pulled through top area near rad opening.
The FSM can be downloaded from our "everything you need to know...Z32" sticky, or googled...Good Luck!

[gibygreen]

Well, as advised I fooled around a bit more with all of the Idle controls, and i've got everything unplugged and blocked off and the idle set manually. As far as i can tell this isn't causing any vacuum leaks or idle issues (apart from the ever present misfire). But even with the miss, the car still starts perfectly fine when cold.

I also cleaned all the plugs for the PTU and CAS though all were in great shape. And I did a bit more poking around with the injectors and coil packs, specifically the num. 2 cylinder (which i mistakenly called num. 4) and I found that the resistance on the injector was way higher than spec, around 150-170 ohms.

Long-story-short I replaced the injector with a new one and it runs substantially better now!
But it's still missing, turns out the num. 4 injector's resistance is around 40 ohms, causing it to only intermittently work. So, I've ordered another which i'll replace tomorrow morning.
Hopefully this should eliminate the miss, as it already runs very smooth as a 5 cylinder.

Now I just have to put new brakes, wheels, tires, paint, and exhaust on it!

As a side note though, do these cars normally ride extremely rough? As far as I can tell the suspension is all stock on the one I've got and it rides absolutely terrible! Even the smallest bumps in the road make the whole interior shake and doors rattle. It also likes to.. Wheel hop? around corners. I'm not really sure how to describe it other than that, a tight corner makes the rear shake and wheel hop to the outside.

Thanks! - Levi

[DCaff300ZX]

Well, as advised I fooled around a bit more with all of the Idle controls, and i've got everything unplugged and blocked off and the idle set manually. As far as i can tell this isn't causing any vacuum leaks or idle issues (apart from the ever present misfire). But even with the miss, the car still starts perfectly fine when cold.

I also cleaned all the plugs for the PTU and CAS though all were in great shape. And I did a bit more poking around with the injectors and coil packs, specifically the num. 2 cylinder (which i mistakenly called num. 4) and I found that the resistance on the injector was way higher than spec, around 150-170 ohms.

Long-story-short I replaced the injector with a new one and it runs substantially better now!
But it's still missing, turns out the num. 4 injector's resistance is around 40 ohms, causing it to only intermittently work. So, I've ordered another which i'll replace tomorrow morning.
Hopefully this should eliminate the miss, as it already runs very smooth as a 5 cylinder.

Now I just have to put new brakes, wheels, tires, paint, and exhaust on it!

As a side note though, do these cars normally ride extremely rough? As far as I can tell the suspension is all stock on the one I've got and it rides absolutely terrible! Even the smallest bumps in the road make the whole interior shake and doors rattle. It also likes to.. Wheel hop? around corners. I'm not really sure how to describe it other than that, a tight corner makes the rear shake and wheel hop to the outside.

Thanks! - Levi

Glad to hear you have the injectors figured out and got some maintenance done that has paid off. As an early model the injectors will continue to fail so keep an eye on them, and as you also mention the 27 year old suspension is bad and needs replacing. The front upper control arm is notorious for failure, and sounds as if your rears are also not good and more most likely...google the wheel hop problem for help there.
Good Luck!

[amc49]

Compression test, one being too low can miss regardless of what you do to the car.........it may well show in a plug that is darker than others.