I know that I should probably post this in the 4th gen forum, but I think we all know the kind of responses I will get there, yall are obviously more knowledgeable and will say more than 'search', because I have.

For a while now, my max has been occassionally missing at idle (as in it doesn't always happen, but when it does, it'll sit there and miss-miss-miss-pause-missmissmissmiss, etc). No symptoms under partial throttle with no load, but under WOT or part throttle with load (i.e. up a hill), it'll misfire very badly, to the point it's unsafe to merge on to the highway. This as well is an intermittent condition - when it shows up, it happens constantly, but when it doesn't happen, she pulls hard (not as hard as when I first got her, but that's another thread altogether).

I normally would immediately say 'bad coils', especially that they have something like 110k on them, but I can't help but feel that this is related:
Under and only under WOT, she'll backfire out the intake with seemingly every other misfire (when the misfires occur).

It's a stock VQ30, stock '95 fed spec ECU.

Common sense would tell me that intake backfire is one of three things -

1. Incorrect cam/crank timing
2. Incorrect ignition timing
3. Worn exhaust cam lobes (worn to the point where the valves don't open, when the intake valves open on intake stroke the exhaust gases escape out the intake valve)

Since it's intermittent, the 'worn cam lobes' idea is eliminated. That leaves me with physical timing, and ignition timing. If it were off a tooth from cam(s) to crank, it would not be an intermittent condition, but constant, correct?

If I've been correct so far, what would cause ignition timing to be jumping around under some conditions (so far, all intake backfiring and heavy missing have been in approximately the same relative conditions) to the point that an intake backfire would occur?

I have no codes stored, and oddly I don't get the flashing CEL under any misfiring (my ECU doesn't care about the safety of my converter, not that there's anything left in it ).

Any help is appreciated, right now I'm heading out to pull both CKPS POS and REF, and the CPS, clean them up, and possibly install a heavy cable from negative battery terminal directly to the block (I believe one of the all motor guys had a problem like that a long long time ago, according to search - reading from the CKPS REF would jump around with a shoddy ground between bellhousing and block). At some point I'm going to try to borrow a scanner and watch my ignition timing when it happens, but it's so hit-or-miss that god knows when I'll get the chance.

 

Sounds like you are on the right track, and don't really need much advice. Don't forget about the ground straps at the upper intake though

 

If you have access to a timing light you should check the base timing with it.

 

Check the injectors. I had a misfire and it took forever to throw a CEL. Turned out it was a bad connection on one of my injectors.

 

I'll definitely look into that, seems like a good place to start.

That's the thing, this has been happening around the same time that I replaced my water pump, which turned into a whole big thing. No CEL, and it's been a fair bit of time. I understand the injectors will definitely cause the miss issue, but what about the intake backfiring?

I can't help but think this is related to that water pump job, because I never noticed it missing at idle except after that. I did have to remove the timing chain and reset the timing (long story, it's posted around here somewhere), but if I got it wrong, would it not be a constant thing?

Okay, R&Red the CKPS POS and REF, and the CPS. POS has some ferrous shavings on it, cleaned them off, REF has some minor dirt build up, and of course the CPS was covered in oil so it was clean. Haven't taken it for a test drive yet to see if any difference, but it's so intermittent I may not know for a couple of days.

So do yall believe this is indeed an ignition timing issue?

 

Mine did that for 2 weeks intermittently before it finally threw a MAF code. But the IGN timing seems like a likely culprit as well, ESPECIALLY since you just did the H2O pump.

I also had a CPS code, but I bought a new sensor, and it went away for about a week and half, then shortly returned, along w/ a MAF code.

 

Another thing that may or not be related, I have overheated the **** out of this motor a few times. I tend to not pay attention to the temp gauge out of the pure assumption after replacing the WP that the cooling system would be fine, and it ends up overheating to the point that the motor shuts off, which is when I see the smoke coming from the hood.

Somehow, it's loosing coolant. I haven't had an issue since the water pump replacement, but on 81N moving back up to MD from TN, it overheated (when I was pacing a Max, damnit). Radiator was bone dry, reservoir was still full (I think my rad cap is bad). Anyway, it's not leaking in any amount that I can see, and I don't see smoke or any evidence that coolant is burning, no antifreeze smell in the exhaust gases either. Is it possible (though not very likely) that one of the heads or the deck slightly warped from running so hot, just barely enough to let a small amount of coolant run from a water jacket into the combustion chamber? Even then, I would imagine that would be a constant thing as well.

Hell, I don't know. This damn car.

 

MAF would adversely affect ignition timing to the point of intake backfiring, right?

That's another thing I forgot to mention, I've been having MAF issues intermittently as well as of late. I had to reground it to make it even drivable. Occassionally, it will buck and stall when taking off in gear or slowing down in gear (only with load, in P/N it's no issue). I assumed since the reground worked for a while, it was a harness issue, do you think the contacts within the MAF are loose/partially broken? Come to think of it, it almost seems like a cold solder joint, since whenever it starts doing that, I unplug it and plug it back in, move around my ground wire and the harness a bit, and it goes away.

 

Update:

Well, after cleaning off the CKPS POS/REF and the CPS yesterday, I did a few high speed runs on the highway, got it nice and heatsoaked, and nothing. Never once did it hiccup. It's still slow as ever, but at least it's not backfiring through the intake for now. Usually at least once during a highway drive it'd act up, but I went up and down 100 for about 40 minutes (should've been like 15, but traffic...damnit), most of it WOT from different speeds to downshift to either 1st or 2nd.

It did miss once at idle a few minutes ago on the way home from the gas station, while stopped at a red light, but no biggie. Hopefully it keeps this up, and this issue won't come back. Thinking about opening up my MAF and resoldering the contacts in there anyway.

 

Glad to hear there was a simple and effective resolution to this. Hope it doesn't come back.

 

Well occasionally get a hiccup at idle now, and still some missing at WOT, but no other symptoms (such as intake backfire) have resurfaced yet. I'm going to pull the three sensors again and see if they've managed to get another coating of ferrous particles.