1990 Q45 misfiring

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Ryantzer
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Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

1990 Q45 misfiring

Postby Ryantzer » Mon Jun 28, 2021 12:00 pm

I've been working on my Q45 for the last year or so when time allowed - new timing chain guides & tensioners, new lifters, new NGK Laser Platinum plugs, reman 93-96 style injectors from AUS Injection, later-style fuel rails, new fuel filter, new OEM air and coolant hoses under the plenum, new OEM knock sensors, new OEM radiator hoses and heater hoses, new OEM water pump, new OEM thermostat & radiator caps, and new gaskets and seals for everything that was removed. I finally got it all reassembled on Saturday and was rewarded with a misfire - feels like 1 cylinder. I tested all the coils while it was apart and they were all within the factory spec for resistance. There is no check engine light on.

At this point I'm guessing it's either a bad coil or a COP boot that's causing the misfire, but the only way I can think of to test them is pull each one out individually, stick a plug in the end, and visually see if there's a spark. Does anyone have any suggestions on a better way to diagnose which cylinder is misfiring and what component is causing the issue?
Last edited by Ryantzer on Mon Jun 28, 2021 12:34 pm, edited 1 time in total.


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VStar650CL
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Re: 1990 Q45 misfiring

Postby VStar650CL » Mon Jun 28, 2021 12:30 pm

When it's a boot arcing out the side, a gap tester won't find it. Usually you'll see a visible carbon track or dot someplace on the boot or the inside of the tube, but not always. If you don't find any, use a grounded test lead and sweep it over the coil boot while letting it fire with a plug that's deliberately gapped wide. If the boot is leaking you'll get sparks jumping to the test lead instead of the plug.

3Q Jay
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Re: 1990 Q45 misfiring

Postby 3Q Jay » Mon Jun 28, 2021 2:58 pm

It's alive!
What makes you suspect #1?
Can you do a (consult-type) power balance?
Not 100%, but another way I've been able to narrow an offending cylinder fairly quickly is to use an IR thermogun. get close to the exhaust mani runner. look for an out of family reading.

Ryantzer
Posts: 89
Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

Re: 1990 Q45 misfiring

Postby Ryantzer » Mon Jun 28, 2021 3:44 pm

3Q Jay wrote:
Mon Jun 28, 2021 2:58 pm
What makes you suspect #1?
Sorry, poor wording - it feels like it's one cylinder, not necessarily cylinder #1.

I don't have a Consult, that would have been my first choice if I did.

I like the idea of comparing exhaust temps - don't know why I didn't think of that myself.

Ryantzer
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Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

Re: 1990 Q45 misfiring

Postby Ryantzer » Mon Jun 28, 2021 4:05 pm

Ok, the exhaust manifold runner for cylinder #7 is a little bit cooler than the others, but it's not cold like the cylinder isn't firing at all. Looks like it's not simply a bad ignition component.

3Q Jay
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Re: 1990 Q45 misfiring

Postby 3Q Jay » Mon Jun 28, 2021 5:13 pm

I maybe should have added that the IR test works best starting from a cold(ish) engine. easier to pick up then as the heat tends to equalise more over time.
Misfires can certainly be intermittent, and the cyl may be firing some of the time and producing some heat.
what was the condition of your subharness between the ignitor and the CoP pax? My guess it would be pretty brittle after 30 years. the plug boots could also be dried out and leaking as you earlier surmised. I like to use a little dielectric grease between the porcelain and the very bottom of the boot. True dielectric grease you do not want to get on the terminal of the plug body....just the porcelain. Ignitor could have partial failure (transistor for #7), though I've not seen many fail. They usually either work or they don't. And....what do your injectors ohm--both stone cold, and after full warmup? again, look for an out of family.

Ryantzer
Posts: 89
Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

Re: 1990 Q45 misfiring

Postby Ryantzer » Mon Jun 28, 2021 6:10 pm

I started the engine dead cold and took several rounds of measurements as it warmed up - #7 was consistently 10-15% lower temp than the rest.

The ignition subharness connectors are intact but the wiring is pretty stiff. If a new one was available I would have replaced it just to eliminate it as a possible future problem. I did use dielectric grease on the bottom inside of the boots, as well as a very light smear of anti-seize on the plug threads. The injectors were replaced with the 93-96 style - a reman set from AUS Injection that all ohm'd in spec before I installed them.

I'm leaning towards getting the smart coil conversion from VH Performance as it would eliminate the coils, ignitors, and harnesses as being possible issues. I think I also need to do a smoke test just to verify that there are no vacuum leaks, even though every single hose, gasket, and seal on the top of the motor is a new OEM part.

3Q Jay
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Re: 1990 Q45 misfiring

Postby 3Q Jay » Mon Jun 28, 2021 6:29 pm

hmmmmm...smart coil conversion, you say?

vh45de-smart-coil-conversion-t623673.html

Ryantzer
Posts: 89
Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
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Re: 1990 Q45 misfiring

Postby Ryantzer » Mon Jun 28, 2021 6:54 pm

Lol, I just read and bookmarked your post a few hours ago while doing research on the smart coil conversion.

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Q451990
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Re: 1990 Q45 misfiring

Postby Q451990 » Mon Jun 28, 2021 9:05 pm

The coil packs tend to be fairly bulletproof, but injectors aren't. It's also easy to pinch a lower o-ring and cause a fuel leak that spills in to the corresponding cylinder. I didn't know to lube them the first time I did an injector job, and pinched one causing a hard start when hot issue. At this age, anything is possible, but just because they're new, I wouldn't totally rule out an injector issue.

Ryantzer
Posts: 89
Joined: Sun Aug 11, 2013 5:37 pm
Car: 1990 Infiniti Q45
Location: Phoenix, AZ

Re: 1990 Q45 misfiring

Postby Ryantzer » Tue Jun 29, 2021 6:39 am

Before installing the fuel rails I pressurized them to make sure there were no injector leaks.

3Q Jay
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Re: 1990 Q45 misfiring

Postby 3Q Jay » Tue Jun 29, 2021 7:24 am

Ryantzer wrote:
Tue Jun 29, 2021 6:39 am
Before installing the fuel rails I pressurized them to make sure there were no injector leaks.
smart call
Consider that a static ohm of the injectors is a single data point (even if taken multiple iterations). Very difficult to discern dynamic response without an inductive current probe, scope capture of waveform. I don't have a cheap PC interface o-scope, although I keep telling myself to invest in one. Without that, then my earlier suggestion stands to at least look at the static resistance of the injectors (particularly #7 in your case) over temperature. Good news is #7 can likely be R&R if necessary without disturbing the octoplenum.
PS- my smart coils now have 1250 miles on them (car doesn't get a ton of miles since I have 3, and limited my trips last year due to COVID) no issues, but I believe fuel consumption is up slightly.
Again, back to my oscope comment....with that I could compare coil waveforms between the old nissan ones on the '94, and the VAG shorties on the '95....

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Q451990
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Re: 1990 Q45 misfiring

Postby Q451990 » Tue Jun 29, 2021 10:21 pm

Ryantzer wrote:
Tue Jun 29, 2021 6:39 am
Before installing the fuel rails I pressurized them to make sure there were no injector leaks.
Smart! I'd say move the coils around if you think the IR temperature test is enough to identify the miss. While you're at it, you might as well take a look at the spark plugs to see if one shows fouling.


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