I have decided to leave my life of lurking behind to petition the collective wisdom of this forum.
I have been having engine trouble with my 91 Infiniti M30 convertible, and due to its rarity, it is difficult for me to find any ideas for troubleshooting.

Here’s the quick version:
100,000 miles, relative regular maintenance. Timing belt replace at 65,000 miles.
The engine does the “silent stall” at low RPM like when coming to a stop and stutters intermittently at any speed
Stalls and skips/stutters seem to be cylinder specific, although I am not sure which one
The smell of un-burnt fuel accompanies the stuttering

Other possibly related symptoms:
Tach and temperature gauge don’t work

What I’ve tried and replaced:
Dealership tuned throttle
Refurbed MAF (car wouldn’t even starts put old one back in)
Aftermarket distributor (rejected by car)
New distributor and rotor (seemed to help a lot)
Seafoam fuel injector gas treatment (used to help a lot, but now is ineffective)
Rebuilt alternator
New spark plugs
New wires
New battery
Rebuilt alternator #2 (1st rebuilt failed)

I need ideas on what to fix next and how to cost effectively troubleshoot possible culprits.
Any M30/VG30E gurus out there?

Here is the long and detailed story of my M30:

History:
I purchased this vehicle a little over a year ago with 95,000 miles on it. Everything worked well, other than a broken trunk release switch and the tach would intermittently flat line or peg for no apparent reason. I have the majority of the service records, most notably my favorite Infiniti dealership replacing the timing belt at around 62,000 miles. I drove the car as a toy on sunny days until my lease ended then I put about 5000 problem free miles on it over the course of the summer and early fall with a few long road trips and daily driving. I took it to the local Infiniti dealership for all its routine maintenance, mainly oil changes at this point. I also replaced the trunk switch release, only to find out the actuator wasn’t responding, but that’s another problem for another day.
Problems start:
Fast forward to fall and some cooler rainy weather after just crossing the 100,000 milestone. Driving home one night the car stutters when starting from a stop at a light, almost feeling like it was trying to start from 2nd gear (car is an automatic for those who don’t already know). Only does this once or twice in the 10 mile drive, and drive fine for a few days. That weekend I started the car, and while I was putting the top down for a sunny Sunday drive home, the car sputtered, dropping in RPM for a few seconds, then returning to normal, which repeated a few cycles, then died. It started right up without an issue and drove home, stuttering a bit at stops, but stayed running. I took it to the Infiniti dealership the next day and they set me up in a loaner. They could only get it to die on them once, but couldn’t find a cause. They checked for codes (nothing) and the master 20 years of Nissan service mechanic said he dialed the throttle and such back into factory specs. They handed me the keys back in exchange for a wad of cash and I was off again. Later that week all the symptoms returned.
The past symptoms:
At low speeds, usually when decelerating the engine does the silent stall, which would be hard to notice if it wasn’t for the abundant idiot lights coming on. While running it would stutter and if not given extra gas would stall, it would stall at traffic lights and in general became hard to drive in any kind of traffic. It almost seems like its switching off when it shuts down, and whenever it sputters it smells of unburnt gas. Still no check engine light or codes, and still intermittently enough to not be predictable. At this point I am of the opinion that the dealership doesn’t know the M30 very well, and being fall and rainy, I park it for the winter and buy a 1997 I30 to drive. While it was parked for the winter I swapped out the distributor cap and rotor, which were as old as the car and ran some Seafoam injector cleaner through it. At this point the car could idle for hours without the slightest problem. I also realized just how corroded and rusty some parts are under the hood.
Spring 2015:
Car still occasionally stutters and stalls, but not unmanageably for a trip to the grocery store or to church on Sunday. Being suspicious that other parts of the car are as old as the distributor, I decided to do an electrical system tune up. I replaced the decrepit spark plugs with the manufacture recommended double platinum. I put in a new set of Ngk wires, new battery which wasn’t holding a full charge and led to me replacing the alternator (AC Delco rebuilt), which failed the bench test, specifically with the voltage regulator. I also disconnected and cleaned all the grounds I could easily find. At this point the temperature gauge next to the tach flat lined and doesn’t respond.
Summer 2015:
I continue to run Seafoam with every tank of gas as that seemed to reduce the frequency of stalls. The new alternator failed both regulator and bearing, leaving me stranded for the first time with a dead battery in the middle of the freeway. I put in another one under warranty. I took the car out for a sunny drive for a few weeks ago, because usually at freeway speeds the engine used to run fine. This time even at 70 mph the engine would stutter and felt like it was missing on a cylinder or two. By the time I got home I was two-footing the pedals to keep the engine running when I stopped at lights, or it would stall and keep stalling right after restarting.
This brings us to today. Sorry for the book, but usually when I read these forums people sometimes don’t give enough details.

I need ideas on where to go next-- and ways to cost effectively troubleshoot. I am suspicious that at least two fuel injectors are failing, but I read somewhere that if the transistor ignition unit can cause similar symptoms.

For those out there having similar issues, when I do finally fix this, I will be sure to post my conclusion.

does this engine have ignition coil packs that attach to the spark plugs like the J30? Have you replaced any of those?

Thanks for the reply. The M30 has a single ignition coil, not individual. It also has a single transistor ignition unit right under the coil. Neither has been replaced yet, as the issue seems to be cylinder specific. Can a single coil fail in a rythmatic pattern?

Everything you say is wrong with the car sounds like a bad MAF....very common ailment on these cars!
I know you tried a remanufactured one and the car balked at it...but I would try to see if you can find a good one to test on your car.....
you might want to test the grounding of the transistor ignition unit...in the J30 it grounds within the ECU...just a thought....

The MAF in there is a salvage from before my ownership (I can tell by the markings on it). I conveniently have another spare salvaged MAF from what I was told was a totaled car, but have never tried using it because I was told that when I MAF goes bad the car can't reach high RPM like 2200+... which I guess could be false in my case? On occasions when my tach is working I can red-line the engine, and even with it stuttering on the freeway, I could accelerate.
I'm kind of like a hypochondriac on mebMD with this car. I read into all the symptoms and assume worst case scenario; my injectors are all bad, the ECU is bad, the TIU is bad, etc.
Speaking of the transistor ignition unit, I understand on the M30 it is a simple two-poster. Is there a way to test it and the ground? There is a part of me that wonders if the alternator and voltage regulator dying didn't slowly cook something important.
Thanks for the advice. I guess since I have a spare MAF laying around it won't cost me to try. I'll swap it out this weekend and go for a spin.

I'm back! And I have an update.

I started the car before doing anything. It ran, a bit rough, but didn't stall in the 2 minutes or so I let it idle. I then swapped out the MAF for the spare I had laying around and put everything back together. It took me 40 minutes to swap it out and 1 hour to chat with the neighbor. Started the engine and it ran quite rough, possibly more rough than before. Didn't outright stall, but there was a lot of unburnt fuel in the air, both from the tail pipe and I could smell it in the engine.

It is interesting to note how unpredictable the stuttering is. It does seem to be cylinder specific (I can listen to the engine and count as the cylinders fire: "123 - 5 -123 - 5 -"), but on that last time I was driving it, there seemed to be a cycle to when it ran like nothing was wrong for a few minutes, then stutter for a few, then repeats.

I can't swear to the quality of the MAF I installed, but every time I swap the MAF it seems to correlate with the same or worse problems. Buying OEM factory new MAF is ~$600 from the dealership and I would rather avoid this unless I can prove it is indeed the issue. Used they are around $80-$100 (I think I paid $90 for the spare I had). Not really sure what to try next.

without expert advice and some computer diagnostic help, all you can do by yourself is replace part after part, hoping to find the right one.
You can try taking off one spark plug wire at a time while idling to determine which cylinder might be misfiring.
Once you might determine that, I would switch spark plug wires to see if changes, otherwise, suspect a leaking fuel injector....
I think you need to find a competent mechanic with a computer scope to diagnose your issue.

I'm not opposed to paying someone to fix it, the problem is finding someone who knows the car, and actually can find what's wrong. I can "Johnny Cash" (One Piece at a time) the engine together without paying a mechanic by the hour. I have no issue with paying someone good money to fix this for once and for all. I've paid a certified Nissan/Infiniti mechanic to troubleshoot on day one, but they didn't have a clue. They were going to just start replacing stuff, which I can do myself for less.

Any recommendations for the best mechanic in the Seattle area?

I'll check it out. Thanks for all your comments and ideas.

Case isn't closed yet... I'll be back with updates.

I have to agree with what fourdrinie said, it sounds a lot like the MAF issue i had with my wifes j30. you also said it sounds like its just affecting certain cylinders, the best way of i know to check that is to pull plug wires one at a time while the car is running. If it get worse that cylinder was fine, if there is no change you have one that is not firing. I have located a few bad plug wires this way.

That's two votes on the MAF, and to be clear I lack the experience to challenge that by any means. Without doing the one spark plug at a time method, I can't say it is the same cylinder every time, but definitely rhythmic.

As I pondered my results today, it occurred to me that each time I swap out the MAF the symptoms do change, although in both cases they worsened.
FIrst MAF swap (October 2014) was for a NAPA re-manufactured part. I couldn't even keep the car running with the part, and returned to the old one.
Second time (last night) was for a salvaged part, and the engine ran rougher than it did 2 hours before I swapped the part.

Is it possible there is a step I am missing? Both times I just unbolted the air cleaner, loosened the clamp on the intake hose and undo the big MAF plug. Then I just bolt in the new one and hook everything back up. Do I need to pull the battery or reset the computer or anything?

I would try unplugging the battery, i know on my newer vehicles that is usually recommended after changing or cleaning any electrical part.

Ok I'll try that tonight. I say it's worth reprogramming my radio presets and clock if I get my smooth running convertible back.

another thing to keep in mind, reman parts especially electrical are not always 100%. if you know anyone that has the same car and a working MAF try a swap to see if that does anything.

The most bittersweet part of being a M30 owner is no one else has one. I've seen 3 on the road in person (since being old enough to know what they are) not counting mine. Two white verts and one was a white coupe. Flip side is every part is a special order, no matter how simple. Absolutely zero in scrap yards locally. Last winter when someone tried stealing the car I had to call salvage yards across the U.S. to find replacement trim parts.
I understand it shares a MAF with the early 90's Maxima and a very close match (screw holes are different but same primary part number) with the 300ZX. I have a friend with a 89 Z.

Do I risk damaging one electronically if I borrow it?

Back again folks. To fill in the gaps, after swapping MAF to the one I had laying around, the car wouldn't stay running without extra gas, and was coughing and sputtering like a chain smoker with emphysema. Based on the strong correlation between doing anything with the MAF and the car somewhat running/not at all running I ordered another (salvaged) MAF, Nissan part # 22680 16V00 for the curious. Call me crazy for trying used again, but this crazy guy doesn't want to blow $600 on a non-returnable trouble-shooting part. Used M30 MAFs sell for around $90 as mentioned before, but the part is exactly the same as the 1991 Maxima, which I found in abundance for $30-$40.
The new-to-me-and-my-M30 MAF arrived today and this time I wasn't leaving anything to chance. I blasted it thoroughly with MAF cleaner (yes, they make such a thing) and sanded the electrical contacts until they were shinier then a new penny. After I installed it, I put the battery terminal back in place and she started and stayed running on the first try!
The exhaust smelled a bit funny at first, but I figured that might be the last bit of the cleaner running through. I am going to go about my Friday night activities with the car, and will keep everyone posted on how everything goes.

Special thanks to gsf1200s and fourdrinie for the helpful advice!

Good to hear, glad i could be of some help

how did it run this weekend?

It ran solidly all weekend, thanks for asking. I put about 40 miles on it, without the slightest stutter or hesitation. I discovered there is power under that hood! I can actually spin the tires at a traffic light, which I have never been able to do before. The 0-40 acceleration used to put me to sleep. Now its every bit as fast as my I30 off the line. It also seems to be getting much better fuel economy.
Last time I was under the hood I found the loose thermostat gauge wire, which fixed that issue, but I also discovered a very slight coolant leak, which I why I didn't push for a longer "victory" drive this weekend.
Next adventure is to hunt down the leak...

Ok...now that we got a used MAF that works....I would suggest the following:
1. Never know how long it will last...if you see a good deal on another one, buy it!!!!!
2. If you're handy with electronics and soldering, try taking the old MAFs and cutting out the top of the black box...inside are two circuit boards, look for bad solder joints, broken wires, etc....or just apply a good touch of solder to all solder joints....reseal the top with silicone caulk, whatever.....try the MAF and see if it works now....if it does, keep it as a good spare.

As far as the water leak, check the top neck of the radiator for cracks, check the bottom of the water pump as it would indicate a ready to fail condition (good time to replace along with timing belt) and of course water bypass hoses on side of engine.

Questions?

Thanks for the advice fourdrinie. I am very comfortable with soldering and if it's that simple to fix a MAF then I shouldn't ever need to buy another one. However, if say I buy new from the dealership I should get 15+ years without problems, right? In a perfect world!
I saw the drips of coolant on the bottom engine pulley so it's either a hose or the water pump.
My question to you would be: since the timing belt is still under mileage (but not years?) by 20k or so, should I replace it with the pump? Dealership recommended to replace the belt every 65k-72k miles. It's on a "new" belt, replaced around 65k miles at the dealership, and currently only has 101,700 miles on the engine total. Thoughts?

I believe to bet to and replace the water pump, you have to replace the timing belt, it at least remove the timing belt cover and everything needed to change it...
When you replaced the timing belt previously did you change all the idler springs and pulleys that go with it?
And the question for you is why didn't you change the water pump when you replaced the timing belt previously?
The drip from the crankshaft pulley is "usually" a water pump bearing beginning to fail..
Any new part for these "old" infinitis are almost 20 years old sitting on a shelf somewhere...even though you re solder all the joint connections, the electrical components are still twenty years old and subject to failure due to cold and heat climate variations.

I didn't own the car when the timing belt was done, but they had it serviced at "my" dealership and they had the record of it being done on schedule with recommended maintenance. I have most of the paperwork for the service records and will double check if the pump was replaced, however since I assume it is leaking, that point is probably moot.
I could be mistaken, but I think the water pump is external on this vehicle (more like the 300ZX than the J30), although the pulleys and belts will have to come off, and if I've gone that far, might as well replace the timing belt.
I'm fairly mechanically inclined when it comes to backyard mechanic stuff, however timing belts make me nervous. I had a 4 cylinder Toyota pickup with a failed timing chain and it was a pain to work on and never recovered. Hugh waste of time and money and in the end I should have just paid someone to do it. Are Nissan timing belts considered "easier"? (PS I have the factory service manual for the car, which I assume will walk me through it. If I run into trouble I'll start a new thread)
Back to the MAF: if cost wasn't an issue, would you suggest forking over the $600 to have factory "new" or stick to repairing old ones or salvage units? Will there be equitable improvements from spending 20x as much, or just buy 20 spares instead?

I would get one or two good spare MAFs, all used or reconditioned...I have had good luck buying a knock-off as well.

On your car, the timing belt does NOT have to be removed to replace the water pump.
I would also check the part number on the timing belt that is currently on the car, I believe Infiniti switched it to a Kevlar belt that should last 105K

Any idea what the part number is for the Kevlar belt? I called the dealership yesterday and had them pull my car by VIN. Timing belt was replaced at 59,000 miles in 1997. At or around the same time the water pump was replaced because it was leaking, according to their records. I'm going to see if I can find the bill and the timing belt part number this evening. Thanks for checking on that.

That belt is almost twenty years old.
If my water pump was leaking, I would replace that old hard t-belt and every spring and tensioner under there.
Technically, that belt might be originally manufactured in the 1980's and sat in a warehouse until 1997, making it closer to thirty years old.

Yeah, that's my thought when I heard it was in 97... I assume it is recommended to use OEM parts-- if (and that's a big IF) I can find them?