Cyl 6 has spark, fuel and compression but wont fire?

[cm1wrx]

So I have been beating my head against a wall for months now....

91 300zx 2+2 N/A

As the title states... I have looked at EVERYTHING and still cant make cylinder 6 run! Here's what I have done so far...

Coil is firing as verified by ohm test on coil pack, all wiring to coil verified continuity and signal (tested with a power probe). With coil removed and a known good plug inserted into it (along with one in the engine) I can verify the spark plug is firing and looks strong! That plug was then installed in cylinder 6. Compared it to another cylinder/coil and it was identical. I have swapped coils numerous times with the problem always staying with cylinder 6. I have also replaced all the coil pack connectors with new ones as well as all the other connectors on the engine. (should have just bought a harness but too late for that).

Fuel injector- The car has never run correctly since I purchased it (resto project) so I have no way of confirming the condition before but it did have original (first version) injectors in it when I bought it but I replaced them with a set of cheap ones I bought off amazon. With these injectors (second version) the car ran but I had other issues (like MAF wiring) that prevented me from even noticing the situation. Once I sorted everything else out, I was able to diagnose the miss fire by performing a cylinder contribution test. Only cylinder 6 was not working correctly. I tested the wiring back to the ecu to verify good continuity, which it has. I put a noid light in the injector harness (new connector) and it lit up correctly. When I pull the plugs after running the engine for a bit, cyl6 plug is wet with fuel and does not appear to have fired at all. I ohmed the injector found it around 12 ohms. Not knowing what to do, I thought it might still be a bad injector so I replaced the entire set with 6 OEM, reconditioned injectors and installed them. Same cylinder 6 issue. Ohmed out that injector just now and got 11.1 ohms and the plug still comes up wet when I pull it. Tested the coil and plug again and they are perfectly fine.

compression test. So from there, I thought it might be an issue with the cylinder itself but when I perform the compression test I get 170psi on that cylinder!

Also, I've even tried swaping out the CAS (yes, I know it only controls spark but still..... )


So I am totally stumped as to what to do next. I have put a TON of work into this car and I still haven't driven it!

Other new parts...

PTU and sub harness are new.
A bunch of new plugs of various NGK designs (iridium, laser, standard)
Injectors
MAF
All connectors (ALL OF THEM)
The timing belt (tensioner, pulleys, etc.) have been replaced and verified correct
I pulled all the emissions related components off of the engine (because it just simplifies everything and the car is too old for emissions in my state so it doesn't matter) including the avcs
All vacuum lines were replaced. Gaskets and o-rings are all new, so I don't think the issue is vacuum leak related.
Both coolant sensors, O2's, TPS, MAF
Fuel filter twice
Fuel pump
Fuel pump controller mod

I have removed and cleaned both throttle bodies.
I have gone through the entire fuel system including blowing out the lines from the engine to the tank, put in an aeromotive fuel pump (the big one), did the fuel pump controller mod to put it in hi output only, all the injectors more than once, all the orings and what not for the fuel injectors/rail and the fuel lines and filter. So in short, I think the fuel system is good to go.

I also have ECUtalk, which only shows code 55 (all clear) and the gage readouts all look good (o2's cycling, MAF voltage correct, TPS, etc.).

Anyone ever experience anything like this? Any ideas?

[cm1wrx]

So another update. I just joined the forum recently and I didn't see the sticky at the top that seems to cover exactly what I am experiencing! The problem is, I just ran through the test and it appears that my PTU is fine. I don't see anything out of the ordinary and power seems to be flowing as intended (no open circuits, ohm readings seem to be even an consistent). Oh well.... thought I was finally at the end of this, never-ending test of my patience.....

Any ideas welcome at this point.....

[VStar650CL]

If 6 is wet all the time, are you sure the injector isn't stuck open or leaking from the lower seal? One thing all Nissan techs know from rude experience is that nothing Nissan will fire when flooded.

[cm1wrx]

I'm not 100% but at least 95% as I have put two different injectors in the same hole and got the same result for each (cyl 6 no fire). I also replaced all the seals and 0-rings on the first install and the remans I put in had new o-rings as well. All were lubed with vasoline before installing and I don't see any fuel leaking from anywhere so I think they are ok? The issue that I keep seeing is a wet plug that doesn't look like it even tried to fire once (its only wet with fuel, no carbon build up at all) but when I have it out and hooked up to the coil with a ground lead running from the body of the spark plug to the intake manifold, it has a solid, consistent spark. I also looked in the cylinder and it doesn't appear to be excessively wet with fuel. Basically, everything I am seeing says it should fire but doesn't. I just ran through the PTU check again this afternoon and I don't see anything out of the ordinary with it at all. Passes all the tests..... frustrating!

[VStar650CL]

If it sparks outside the cylinder, then unless it's something wonky like a wire that's broken inside the insulation and losing contact when repositioned, the problem can't be electrical. Whether the plug is wet because it isn't firing or isn't firing because it's wet is a chicken and egg problem you probably can't answer, but I'd be pretty sure you're looking at a mechanical issue. There aren't too many things that can allow good compression but not let the cylinder breathe and fire. Besides a bum injector, the only things that come to mind are a blocked exhaust port or a broken cam lobe.

[VStar650CL]

Hey, wait a minute, I just got a flash from the Wayback Machine. Have you checked your fuel pressure regulator to make sure the diaphragm isn't blown? IIRC, even though the plenums are different, the ZX FPR's tapped vacuum from more or less the same place as the VG Pathies, near the #6 tube in the vacuum gallery. If the diaphragm in the regulator is blown, the vacuum hose will be full of fuel and dumping it selectively into the #6 cylinder. Pull the FPR hose after running the engine at idle for a bit, if fuel comes out then you have a bad FPR.

[cm1wrx]

The FPR is new and it held 43psi on prime with no leak down last time I tested it. Might have to take another look though. I really don't think its mechanical because the compression is solid and I don't hear any strange noises or anything when it runs (actually runs pretty smooth on 5 cylinders but its noticeable without adding throttle and more so with (delayed reaction to throttle input)). Think it might be time to hook it back up to the fuel pressure tester and check it again. I will look into it and report back.

[VStar650CL]

No, you're not getting this. If the FPR diaphragm is blown, it may or may not affect the fuel pressure. Usually it doesn't, they work fine and will often pass a leakdown test on the fuel side, only leaking when the valve is cracked open by vacuum. What you're looking for is an internal leak from the fuel side of the FPR into the vacuum side, so the symptom will be fuel inside the vacuum line after idling, not abnormal fuel pressure.

[itsa300zx]

Hey, wait a minute, I just got a flash from the Wayback Machine. Have you checked your fuel pressure regulator to make sure the diaphragm isn't blown? IIRC, even though the plenums are different, the ZX FPR's tapped vacuum from more or less the same place as the VG Pathies, near the #6 tube in the vacuum gallery. If the diaphragm in the regulator is blown, the vacuum hose will be full of fuel and dumping it selectively into the #6 cylinder. Pull the FPR hose after running the engine at idle for a bit, if fuel comes out then you have a bad FPR.

You might be onto something here. This is the general area where the FPR connects to vac source, near #6

[NolimitZ32]

Do you have phase 1 or phase 2 injectors? IIRC there's a plug back there that goes to the IACV I think that is identical to the injector plug and can be confused. You say you've tested the wire back to the source and confirmed the signal so little chance of this being the issue but it's something to check.

[cm1wrx]

Sorry I haven't posted in a while. Had to take a break from the car for work stuff. Thanks for all the suggestions as to what to look for as well! Regarding the FPR, I mentioned it was new but I should have said its new and its working correctly. I found no fuel in any of the vacuum lines and it holds a vacuum when applied so its good. However, I did find an actual issue that I think will solve it once I put everything back together. The injectors that I ordered (phase 2) had too large of an o-ring on the base adapter and I think it was leaking into the cylinder (which is why I couldn't see a leak) because I pulled the injectors and when I inspected the o-rings, 4 of them were rolled and had a small cut in them. I changed them all for smaller (diameter) ones and 6 started working again but 1, 4 and 5 stopped working. That's when I realized I went down two sizes of o-ring instead of 1, which is what I meant to do in the first place (I have a large kit of metric o-rings and it had three different diameter o-rings per size, I thought there were only two). I reinstalled them again with the correct size and pressure tested the fuel rail with no leaks. Hopefully, once I reinstall everything tomorrow it should be working correctly. Will keep you all posted. Again, thanks for all the help! I knew it wasn't electrical, I just couldn't figure out what else could be causing it! These cars are waaayyyy more sensitive to fuel issues than I first thought. Live and learn I guess.....

[cm1wrx]

Well, cylinder 4 is still not operating. I have replaced all the injector o rings with new ones that I measured to be within spec (had some issues with the o-ring diameter being either too small or too big for the 2.5mm thickness lower seal). Verified rails and injectors aren't leaking. Checked every single hose connection for seepage or just to tighten a little extra to make sure. Ohm'ed out #4 injector (11.3 ohms). No issues there. Pulled the fuel pump to inspect. Found all connections tight and the pump is cycling normally so we are good there as well. Put a noid light on the injector harness (its firing correctly). Pulled the coil pack and hooked up a plug to verify spark (looks strong and consistent. appears the same as the one next to it that is working correctly so I am pretty sure its good). Only issue I can see is the fuel pressure dropping after priming the pump. It will completely bleed out in less than 10 minutes but usually drops down to 20psi... holds for a minute or two... and then bottoms out. The only thing I can think is that my new FPR is bad so I ordered another. Since pressure is simply resistance to flow, I assume its the only other component in the system that could cause a pressure loss if its hanging open or damaged. FPR should be here tomorrow so I will throw it in and report back. If I am missing anything, please let me know.... Damn these things are frustrating....

[cm1wrx]

FYI all the injectors were re-manufactured recently and all of the cylinders that have stopped working have started again with the exception of 4 (which was working before) so I don't think the injectors have any issues.

[cm1wrx]

And the fun continues.... just put in the new FPR and exactly the same condition. Pressure bleeds off about the same as before. That's every part...... I don't know what else to look for? It just makes no sense....

[cm1wrx]

Once I shut the car off the pressure starts dropping out to zero within 10 seconds. Power balance test only shows front drivers cylinder (#4) I assume, is not firing. Noid light and visual spark inspection confirmed. Injector at 11.3 ohms. No leaks from the rail (internally or externally... had the rails off and pressurized from the pump. Not leaking at all. The thing I don't understand is that with the car running, I have excellent fuel pressure. More than enough to run the fuel injectors so, why does #4 not work if there are no leaks. I am going to pull the coil and plug to see if the plug is wet with fuel.

[cm1wrx]

Plug was wet but didn't look like it had fired. I just replaced it with new. Same result.....