Fouled plugs -- stumped

Because I helped my brother-in-law get ready for the Runoffs in FF, I am just now trying to diagnose a problem in my ITS 1986 300ZX...

At the regional race at Texas Motor Speedway in September, the car ran ok during qualifying -- it would seem to miss occasionally, like it was flooded then it would clean out and run ok. But on the pace lap for the race the car would literally die completely, then pick up and run, then die completely again, then pick up and run only to die again. I was able to keep it running -- angering everyone behind me because I couldn't keep up with the pace car -- until they threw the green flag. The car then ran but would not pull beyond 3500 rpm no matter what I did. It never died again, but just would not pull beyond 3500rpm in any gear.

I needed to finish so I kept on motoring, staying out of everyone's way. Twice coming out of a sharp left-hand turn on to the back straight it would take off like a rocket in 2nd gear, running all the way up to 6000rpm, only to stop revving past 3500 when I shifted into 3rd, 4th, 5th, etc. Running great in 2nd gear out of that one particular corner only happened twice during the race. The rest of the time it would pull up to 3500, then just not rev anymore no matter what I did with the throttle, etc.

I just pulled all the plugs and found that all 3 of the plugs on the passenger side (1 - 3 - 5) are black, wet, and fouled with gas. The 3 on the driver's side (2 - 4 - 6) are fine. Obviously that is my problem. I checked the spark plug wires 1st, checking each one with a diagnostic tool and they are all firing perfectly. No difference in the voltage for 1 - 3 - 5 compared to 2 - 4 - 6. Both sides are getting spark.

Not electrics, must be too much fuel.

Obviously the passenger side is flooding the 1 - 3 - 5 plugs with fuel which leads me to wonder if all 3 injectors are stuck open (unlikely in my mind) or that the sensor at the front of the fuel rail is somehow faulty causing too much fuel to be sent to the passenger side.

Does this sound like a possibility? Any other ideas?

As always, thanks for your help,

Bob Marcho

I would check your computer for codes (blinking lights) first but it sounds like an air flow meter problem, Could be fuel pressure regulator, cylinder head temp sensor or maybe a wierd inition problem to those cylinders like half the distributor isn't picking up? I bet its the AFM though, I've seen them fail before like that with similar symptoms. Anyone else? Good Luck

Check all your harness grounds.

Would the AFM affect just one side of the motor though? Seems like you'd see the same issue in all cylinders. I've only worked on the 90+ Zs, but is there maybe a separate coil for 1-3-5? Or maybe those injectors all use the same ground strap like Joe said. Seems like it'd be something common to those 3 cylinders, but not the other 3. Just throwing stuff out.

David

Dave and Joe are probably right now that I think about it a little more. Also, how is your rev limiter setup? The stock limiter cuts fuel to evey other cylinder if I remember correctly, cold be a problem in the ecu. In any case, check the service manual first....good luck and please post what the issue was.

Thanks for all the advice, obviously I have a lot to look at...

My struggle is that I will not be able to test out any of the suggestions for some time. The local drag strip -- where I do all my testing after working on the engine -- is closed for the winter and there are no races in the area (Texas) until March. Needless to say, my neighbors are not particularly fond of me starting up the race car and revving it up to see if I solved the problem. I may try to "engineer" some kind of additional muffler so I can try out your suggestions. In any case, I will share anything I find out.

Since the first post I have:

* Checked all of the harness plugs and they were all plugged in and tight.

* I think I am going to rule out the A/FM because my problem is only on 1 - 3 - 5 plugs.

* To further explain the pulling to 3500rpm and then not pulling anymore, I do not mean it pulled strongly to 3500 and then quit, it was running very weakly/slowly to 3500 then would not pull anymore. Thus I doubt that it is the rev limiter; whatever the problem it appears to be related only to 1 - 3 - 5. (By the way, one learns to be smooth and not loose speed int he corners when the engine will not pull at all -- my lap times were only about 6 seconds off my qualifying after I learned to keep up the speed and not use the brakes).

* I cannot check the computer codes, I have so many sensors disconnected to by-pass various emissions controls that the "check engine" light is always on. No, I did not by-pass anything before the last race that had not been by-passed for previous races. It ran great at a race in July and again in August, and I did nothing other than change the oil before September when I had my problem.

What next? In the following order I will attempt the fix and test run as best I can after each step -- not try to do them all and never know which one worked/didn't:

* I have a couple of spare distributors so I will probably swap it out just in case it is an intermittant short in the distributor. But using my timing light on each wire shows that each plug wire is firing.

* I will pull the injectors and see if they appear to be stuck open. I seems to me that the injectors can still click even if they are stuck open, then again, maybe not.

* I have 2 spare parts cars and I will pull the computers out of them and replace the current computer.

* I will hook up another muffler and run it up to see if the problem has already solved itself -- which says to me an intermittant electrical problem.

Thanks again,

Bob Marcho

I have no idea why I'm reading this thread, but what the Hell.... :D

Isn't the 300zx FI system a batch-fire (or bank-fire) arrangement, where the ECU triggers one bank of 3 injectors at once? If so, it might be worthwhile to investigate the triggering mechanism, whatever that may be. If one bank is firing more often than it's supposed to (or possibly in a constant-on state), you would end up slobbering rich on that side of the engine. My ancient ITB car ('71 Volvo) uses a double set of FI-specific points in the distributor to batch fire 2 injectors per crankshaft revolution, but I would suspect your '86 would have something a little more sophisticated?

Hmmm, a Volvo guy who adds an interesting thought...

You are correct, it is a batch fire system, the injectors on each side dumping fuel to all the cylinders on one side at the same time. Sounds like I need to find out where the trigger for the batch fire is done. Perhaps in the ECU? Maybe in the distributor like your Volvo?

Sounds logical that there might be a problem that is causing the dumping of fuel at all times to one side. It seems to me that the ignition system might be able to handle that for awhile and then after awhile the plugs would be so overwhlemed they would foul out.

Thinking out loud...

BTW, my wife drives a 2002 S60 and loves it.

Thanks,

Bob

Quote:

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BTW, my wife drives a 2002 S60 and loves it.

Bob [/b]

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Amazingly small world, ain't it... So does mine. :023:

Awww crap. I thought of this when you first asked, didn't think to mention it since it didn't come to light. Gary L got me thinking.

The book is out in the shop, but....

Yes, it batch fires in two modes. One is w/ 3 injectors at a time (but I don't remember which three). Then above a certain RPM (again, the book is out in the shop) it switches to all 6 at once. I bet the # is 3500.

It also does this as part of its warm up cycle. Mine will idle, then after about a minute it runs like crap, sounds like an old tractor, then once it hits a particular temp, it goes back to 3 then 3 and runs nicely.

Perhaps fool the ECU into thinking its cold, then seeing how it runs. Have to look in the factory manual to see where it is pulling its information on what temperature it changes over at. I think its the plug at the front of the LH head. If it can't properly fire all 6 at once, it should stumble all over itself again at low RPM if you can get it into this warm up mode.

Matt

Wrong answer on my part. It groups them 123 and 456. The cut over to firing all 6 at once happens at 3000 not 3500.

Matt

Concerning my 300ZX ITS that had clean plugs on 2 - 4 - 6 and fouled plugs on 1 - 3 - 5...

I looked at a ton of stuff to see what I could see and for some reason decided to open up the distributor to look at it. I found that the etched mylar piece no longer had a flat side on the center, it was totally round; the keyed piece that holds the mylar piece in place was also round in the middle; in fact everything inside the distributor was wallowed out and no longer keyed.

I don't have a clue how it happened.

Put "new" (it has at least 100k miles on it) guts in the distributor from a spare parts car -- including a "new" trigger; new spark plugs to replace the fouled out ones, and it runs great. I let it run until the thermostat opened and then for another 20 minutes after that. Ran it up to 5k rpm many times and no miss at all. In fact my sweet wife said she hadn't heard it run that good in a year or more. It idled smooth and I could not detect a single miss throughout the rpm range. I also pulled a spare ECU off the parts car and threw it in the race-spares box just in case.

I hope this solves the problem. I will run it at the local 1/8 mile drag strip as soon as it opens in February just to make sure.

Thanks for all your advice,

Bob

Glad to hear you got it fixed! Thanks for posting the results also. Bob, I don't know what the cost of the local drag strip is but for the same time investment loading/unloading etc.. you should hit a local chassis dyno to check things out. Even if it's a couple of hundred bucke later, you'll find it is money VERY well spent versus the 1/4 mile crap shoot or a shot weekend at a regional race.

Thanks, Chris, but out here in West Texas the nearest chassis dyno is Dallas/Ft Worth, 175 miles away. But I would dearly love to get on a dyno and find out what my horsepower is and play around with a couple of things to see if they make a difference.

The "Test and Tune" night at the drags is only $10.00, although everytime we go my wife says "I wish there was someplace where you could drive it more than a 1/8 mile so you can see how it does for more than 10 seconds."

Me too.

Bob