Cylinder Misfire, Wont Pass Smog

[Droldaerd]

Hi, I have a 1991 300zx

I took it to get smog and the mechanic said it was running High on NO2 and HFCs and that it might be due to the catalytic converter needing to be replaced or that one or more of the cylinders are not getting a spark. It did not pass smog.

This was after replacing all spark plugs and spark boots. I noticed it has a bit of a misfire on the engine.

If I remove the coil connector for cylinder 1 the engine doesn't seem to change, but pulling 2, 3, and 4 does. I haven't tested 5 or 6 yet.

I switched ignition coils with 1 and 2 and still had the problem on 1. I pulled ignition coil 1 out, put in a spark plug and it is indeed receiving a decent spark. I pulled the spark plug from cylinder 1 and put it in and it is also receiving spark.

Can anyone tell me what else it may be or what I can do to diagnose it and repair it?

[ThisIsSparTTa]

Please fill out your profile including if its a TT or NA. The fuel injectors go out on these very commonly, especially if you have the originals they will fail due to ethanol. You can test an injector by listening for a click, and measuring the resistance. It should be between 10-14 ohms: http://z32.wikispaces.com/Fuel+Injector .

Also, I would recommend testing your PTU: dead-cylinder-can-t-figure-out-why-read ... 64452.html

[Droldaerd]

Sorry, its an NA

Am I able to test the fuel injectors with them still plugged into fuel rail? Would that cause it to fail emissions test?

Ill run the PTU test tomorrow and see if that is the issue

[ThisIsSparTTa]

Yes, just pull the connectors off and put a DMM with resistance mode across the 2 leads. With the old injectors, this is kind of tough because they have the pin that wraps around. You'll need a pointy tool to help get it off.

Yes missing a cylinder would cause you to fail emissions because you aren't properly combusting on that cylinder.

[Droldaerd]

Sorry total Newb I know, but out of curiosity, I'm was under the impression if the fuel injector fails, no fuel to cylinder, and thus lower emission anyways. Could you explain how it still raises emissions, I'd like to know!

[t.mcginley.jr]

Because usually when a fuel injector fails, it doesn't get stuck closed. Instead, it lets in way more fuel then it should and produces a lot of unburnt fuel in the exhaust, raising emissions. Instead of switching on and off likes it should, it just sprays all the time. After a long enough time it can actually drop the compression in a cylinder from excess wear

[ThisIsSparTTa]

And it throws the engine off balance from a power standpoint. And the ECU is trying to compensate because its assuming all cylinders are firing, and its getting weird readings from the O2s, etc.

[Droldaerd]

Assuming I have lost some compression to the chamber, but have now replaced the fuel injector, how does one go about fixing the compression? Thanks for the replies as well

[t.mcginley.jr]

How long was it running with a bad injector? If it was only a short amount of time I wouldn't worry about it unless you have high mileage already

[Droldaerd]

only 350 mi, but car has 192k and is supposedly "rebuilt" recently

[t.mcginley.jr]

Do you know what was "rebuilt"? 192k is fairly high, but depending on how it was taken care of it should still be in pretty good shape.

Most auto parts stores rent compression testers so you can actually see what each cylinder looks like. The compression (in psi) tells you the "health" of the cylinder. For our cars, all cylinders should be:

NA: 186psi is 'perfect', minimum of 136psi.
TT: 174psi is 'perfect', minimum of 121psi

You can also do a leakdown test (which they should also rent at Advance/Auto Zone/O'Riellys) is easier IMO and also more accurate. You just need an air compressor to do it. It tells the leak percentage of each cylinder, 10% or less is great, 15% is ok, 20% or higher is bad.

[Droldaerd]

Not sure. The guy gave me this story that he put royal purple into it and that it wasn't meant for early NA 300s and that it screwed his engine and his dad had to rebuild it in order to clean it out. Not sure what much of its true or if that was the reason, but there's visible silicons in some regions of the engine, valve cover leak, oil pan leak, and possible leak from behind timing chain.

[Droldaerd]

Didn't have a chance to get the DMM and test it; finals week at my school right now. Ill get that tester and the DMM and reply the information back soon. Also, Do I need to have the balance tube attached to run the compression tests?

[z32loverboy]

Compression tester screws into the spark plug holes. Just twist it in with the hose that's attached to the fitting till it's tight and let the engine "bump" five time by cranking the engine with the key. Just be sure to pull the fuel pump relay beside the battery before you start.

[t.mcginley.jr]

Not sure. The guy gave me this story that he put royal purple into it and that it wasn't meant for early NA 300s and that it screwed his engine and his dad had to rebuild it in order to clean it out. Not sure what much of its true or if that was the reason, but there's visible silicons in some regions of the engine, valve cover leak, oil pan leak, and possible leak from behind timing chain.

Sounds like he put synthetic oil in a high mileage engine that was used to conventional (dino) oil. This will cause old seals to leak, so that's probably what happened. Post up the results of the compression test when you get around to doing it, it will tell how healthy the internals are.

[t.mcginley.jr]

You shouldn't need the balance tube attached since you're only testing one cylinder at a time, but I don't think you need to remove it. Here's a how-to on doing the test:

http://www.twinturbo.net/nissan/300zx/f ... ow-To.html

[Droldaerd]

well plug 5 and 6 are under the balance tube if im not mistaken

[z32loverboy]

Yeah you can take it off. you're not running the car with the compression tester on there so it'll be fine. Just remember to pull the fuel pump relay.

[Droldaerd]

Thanks for the info everyone!

Update: I haven't run the test yet, but after double checking to make sure I know which cylinder gives me the problem. I also noticed that moving the any part of the harness near the coil packs or the connectors that are near the ptu (not the ones attached to it, just the ones that have a connector near it), will cause the engine to nearly die out. Am I due for a new harness? Giving the pricing on them, even used (with risk), I'm starting to wonder If i should ditch this Z and start with a different one.

[t.mcginley.jr]

Thanks for the info everyone!

Update: I haven't run the test yet, but after double checking to make sure I know which cylinder gives me the problem. I also noticed that moving the any part of the harness near the coil packs or the connectors that are near the ptu (not the ones attached to it, just the ones that have a connector near it), will cause the engine to nearly die out. Am I due for a new harness? Giving the pricing on them, even used (with risk), I'm starting to wonder If i should ditch this Z and start with a different one.

Question, are all the clips on the coil pack connectors broken? I'm betting they are (90% of peoples are) and don't hold tight any more. Z1 sells a set of brand new connectors for $30, you just have to de-pin the old ones and push them into the new ones. It's a fairly simple process.

http://www.z1motorsports.com/product_in ... ts_id=5001

[z32loverboy]

Good question^
I replaced my harness this spring. I'm in the process of rebuilding my Z engine, you have to decide at some point if you're going to shuck out the cash and get rid of the problem causers or go get someone else's problem child. I don't know of anyone who'll sell a perfectly running Z that has absolutely no problems and won't give anyone problems. You have to take that into consideration.

[ThisIsSparTTa]

You have a 91, which means you originally had the old style PTU. After the recall, the new style used a sub-harness to connect to the existing harness. This subharness creates all kinds of problems. I could kill my engine by moving it, as you described. Best solution is to cut the wires 1 by 1 and directly solder them together. You don't actually need the subharness for anything, and can still disconnect from the PTU without it.

[Droldaerd]

You have a 91, which means you originally had the old style PTU. After the recall, the new style used a sub-harness to connect to the existing harness. This subharness creates all kinds of problems. I could kill my engine by moving it, as you described. Best solution is to cut the wires 1 by 1 and directly solder them together. You don't actually need the subharness for anything, and can still disconnect from the PTU without it.

I will do that, thanks for the info

[Droldaerd]

well i tried the continuity test but i think i have a s*** DMM because it did not give me a pass any any combination of the PTU. It works when touching the prongs together but nothing on PTU terminal combinations. I hate throwing money at something that may not be wrong, but I think im gonna buy one off ebay just to see.

[agent4573]

The subharness was causing me to lose at least one cylinder. I cut out the sunharness and soldered everything together, ran great today. You can also check the injectors the same way you tested the coil packs. Start the car and unplug one injector at a time. The idle should change noticably. If you can affect the idle by wiggling wires though, definitely start there. Replace all the connectors and then try again.

[t.mcginley.jr]

You have a 91, which means you originally had the old style PTU. After the recall, the new style used a sub-harness to connect to the existing harness. This subharness creates all kinds of problems. I could kill my engine by moving it, as you described. Best solution is to cut the wires 1 by 1 and directly solder them together. You don't actually need the subharness for anything, and can still disconnect from the PTU without it.

On that note, here's the how-to:

http://www.aus300zx.com/forum/showthrea ... hlight=ptu

And IIRC I saw new style PTU's on ebay for $24 with pigtails attached.