Z31 poor running

T

timo944

New member
Well I'm off to the ARRC. I got the 300ZX good and warm last night and it doesn't seem to run too well. I think it has a condition that may be getting worse.

Basically, it backfires quite a bit. It has done this in the past, for example during gear changes, etc, it seems to have trouble running well between idle and about 3500 rpm, and sometimes it has backfired a little through the intake while I have been tinkering with it.

The motor has none of the emissions stuff - all disconnected and plugged. There are no vacuum leaks I can find. I have checked cam timing, ignition timing (15 degs at idle), condition of rotor, cap etc. The only codes showing on the compter are the fuel temp sensor (the tab is busted off) and once it showed the crank sensor position.
I have tried to play with the screw in the AFM but it seems to make little difference to the way it runs. I am guessing that turning the screw in (clockwise) is going lean? I have also tried different timing but that doesn't seem to help either. I have also tried different ECU's.

I was wondering if anybody would have any ideas - things to try. Without having an air/fuel mixture reading it's hard to say what is really going on. Maybe a treip to the dyno is in order.

Some basic questions:
- does turning the mixture crew on the AFM clockwise go lean?
- Any way to verify operation ot the cyl head temp sensor?
- Any guess on an optimal timing setting for a race car?

Thanks, timo
 
Timo,
I have quite a bit of experience with the VG from my Maxima days, and the Z31 is very similar, although they don't have a fuel temp sensor.

the service manual should have resistance readings for all of the temp sensors at cold and hot temps. you should be able to check the temp sensors with a multimeter.

when you say the fuel temp sensor tab is broken, do you mean the sensor's tab, or part of the wiring harness going to the ECU?

My thought is that the fuel temp sensor isn't necessarily important (not every car has one and they all run, right?), so I'd grab a resistor from the parts bin that corresponds to the fuel temp at reasonable temp (anywhere from 50 to 100F), and plug that into the harness to fool the ECU into thinking the fuel temp sensor is connected.

If the coolant/head temp sensor is bad, it will cause all kinds of ugly things- usually that causes the engine to run really rich, to the point of flooding the engine. they often only run at full throttle or high RPM because of it.

You say all emissions stuff is removed? do you have an oxygen sensor? If not, can you plug one back in? Nissan ECUs really like to have Oxygen sensors on them. if not, they go into a failsafe mode and dial back timing and run full rich to make sure the engine stays cool.

also, the engine is looking for an O2 sensor signal while it's in closed loop mode, which isn't a huge issue for the car on track- you're usually either at full throttle (open loop mode) or off the gas (don't care what mode it's in long as the engine doesn't die).
But an Oxygen sensor will cause the ECU to do stupid things. they're cheap enough to reinstall or replace, so you might try it.

IIRC, factory timing on a Maxima was around 15deg BTDC. On 93 octane you could run about 18* or so and get better throttle response and more power, but watch your engine temps under track conditions--- I don't think it's an issue in the fall but when it's 100deg outside, that'll cook an engine in a hurry.


A backfire through the intake sounds like a timing issue to me.. these old distributors have been known to "wear out" internally where the gears and such get just enough play in them to cause erratic readings to the ECU. If that's the case, the ignition timing could be off and cause the backfire through the intake. If you have a spare distributor, it probably wouldn't hurt to try it. (you can probably get them cheap from a junkyard if you can still find one. I'm not confident a Maxima dizzy will work though- I think they're slightly different.)


So based on the stuff you've mentioned, it could be any of those thigns.. fuel/engine temp sensor, oxygen sensor, crank position sensor..

the airflow meter could also be an issue if you've knocked it too car out of calibration. I don't know what turning the screws in the meter do since mine always worked and I left it alone. I believe there's a setting in the manual that it should be like 0V with engine off (key on), 0.5V at idle, and 3.5 or 4V at full air flow. the problem is that's pretty hard to check at WOT without having the car on a dyno...

hope that helps some..
 
Matt

Very helpful - thank you. I ended up discovering that the cam timing was off a tooth on the left cam so I reset the timing. In the process I discovered a home made woodruff key on the crank shaft that was VERY sloppy so I went to NAPA and bought an imperial one and filed to fit. After this and setting timing to 15 degrees it ran a little better. This of course trumped my practice day at the track!

Anyhow, the motor ran great above 4000 rpm, and seemed to get better as the weekend wore on. Still pops quite a bit but backfiring not near as bad as before. The timing was set to about 5-10 degs - maybe that was part of the problem.

I still think it runs rich, so I'll check the head temp sensor and fool the fuel temp like you said and then head to the dyno shop to see whats going on, or maybe get an O2 sensor.

Unfotunately I had problems with ignition in both races. Not sure exactly what yet but it could be the computer or the coil overheating. It happened after 1/2 hour in the first race and after 20 mins in the 2nd race. So it coudl takle a while to be sure its resolved.

Thanks again

Tim
 
I wondered about cam timing, but just *assumed* it would be right. A compression check would have ruled that out pretty quick.

As for the rich/lean issues, if you're not running an O2 sensor, you're most likely running rich, unless you adjusted the MAF too far one way or the other.

I haven't seen Nissan computers overheat like that unless there's another problem. I would first suspect the coil, and that's easy enough to check with an infrared thermometer on both parts.
If it winds up being the coil, you might try to find some way to put a heat sink on it, but I would think that the coil overheating is indicative of other issues- they shouldn't do that even on a >120 degree day.
 
Well, as usual there is more to the story. On Friday AM in the very first session I blew a fuse - the one that powers the ECU and the coil too. I replaced it with a 20A fuse (was 15A).

The coil is an MSD of some kind. A "high energy" spark I imagine. It is pretty old and couild be bad. I figured I'd go ahead and replace it with a standard off the shelf coil, something reliable, and fuse it separately.

The distrib looks fine - nothing amiss there.

As for the AFM - somebody in the past has played with it - the plug for the adjustment is missing!

Timo
 
Sorry to be late to the game, but the rich running has to do w/ the head temp sensor behind the LH cam sprocket. It (should) run in a warm up mode until that reads 120deg F. Mine always sounds raspy and studders badly until it switches over to normal mode.

Need more advance than that, mine always ran in the 20-25 deg range (at idle).

AFM adjuster is a placebo I think. I messed w/ one at the dyno once and couldn't see a lick of difference on the wide band. (so we cranked fuel pressure up & down to tune it).

OE coils seemed to work well for me. Never blew the fuse you describe except when the O2 lead got loose, melted on the exhaust & shorted out.

Matt
 
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