Misfire in 1991 Q45

[ti286]

Rough idle. Mechanic diagnoses it as a misfire.

I'll change all 8 spark plugs. If that doesn't solve it, I'll buy 2 working coils and by process of elimination, hopefully find the failed coils.

Is this a good gameplan? Or is there something I'm missing?

[DAEDALUS]

You're assuming it's a coil and not an injector? You might want to ohm the injectors, takes 5-10 minutes. If they're really out though, they'd light the CEL. A stethoscope can at least confirm the clicking, but they could be clogged. I like changing the plugs because they may indicate which cylinder is misfiring and possibly why. A timing light and plug wire can help isolate it if it is indeed the coil.

[Q451990]

I would say injector failure is more likely than coil failure... and mine never tripped the CEL. Maybe a Federal vs. CA emissions difference.

In my case the plug on the bad cylinder was black, to the point that I still had a miss with the new injector until I pulled the plug and cleaned it - so it was pretty easy to tell which one was the problem. A less invasive way to check is to get a non-contact digital thermometer and check the temperature of each leg of the exhaust manifold. If it's not firing that one will be much cooler.

Of course ohming the injectors is a good idea too. You may find that you have another one or two on the way out that you want to change at the same time. Trust me, once you have the plenum off you won't want to repeat the operation frequently.

Heath

[ti286]

How do I "ohm" the injectors? I read the article on q45.com (http://www.q45.org/ohminjectors.html) but it leaves out many critical details.

My plan now is to check exhaust temps for the cylinder producing the coldest exhaust and change the injector to that cylinder.

Any good sources for a good, used injector?

[DAEDALUS]

Put an ohmmeter across the indicated points. I still think you should determine what's wrong before you replace it. If the injector is under the plenum you want to be darn certain it's the problem before you remove the plenum.

[maxnix]

Any good sources for a good, used injector?

There is no such thing. Injectors are wear items. Get a new one from http://www.everythinginfiniti.com . If you have to remove the plenum, move suspect injectors to the three locations where you don't have to remove the plenum to replace them.

[ti286]

You guys have been very helpful. Thanks.

Results of the OHM resistance test:

Cylinder OHM1. 132. 123. 144. 135. 136. 147. 518. 12

How accessible is the seventh cylinder, and about how much time/$ would it take to replace?

[Q451990]

Well, it certainly looks like you found your problem! From looking at mine I think you'll need to pull the plenum... there's a metal line that looks like it would be in the way. Not sure if anyone has had any luck getting to #7 w/o pulling the plenum or not.

The injector with o-rings should cost about $150. T-3 plenum labor is about $500 if I recall correctly - but if you're pulling the plenum there are a lot of other things you might want to address, like all of the hoses - runner o-rings, knock sensors/harness, etc. The cost will vary a lot depending on what all you decide to tackle while you're under there.

Heath

[CastorTroy777]

I been having same problem....

I did my injectors and got some scary results....

#1/ 73.2#2/ 43.2#3/ 13.5#4/ 13.5#5/ 13.5#6/ 13.9#7/ 13.5#8/ 13.5

The service center said i had a misfire on #8?But according to this my #1 & 2 injectors are bad?

[DAEDALUS]

I've seen an injector measure in at 48 ohms--bit of a stumble but no constant misfire. Ohming doesn't tell you if the injector's clogged.

[CastorTroy777]

U ever see one at 73.2???

[DAEDALUS]

I would replace both #1 and #2, after confirming that the injectors are bad and not the harness, since you're measuring through the harness. But the point is that you still don't know with 100% certainty either one of those or both are missing.