User 52121

This is just NOT my season.

So my car has a misfire over 5000rpm. Here's the symptoms:

When the car is cold (or mostly cold), start it, drive it a mile (get a little heat in it), pull out onto a 50mph road, hit it in 2nd gear and she SPRINTS to 6k+. Runs all the way up, almost frantic.

Once the car gets some heat in it (usually about 2 miles down the road when I catch the next light), it no longer pulls cleanly to 6k. It goes but it's no longer smooth, it has a minor stumble that shows up around 5500. Subsequent pulls, it gets worse. Stumble starts at 5000, and by 5500 it's really breaking up, might not even rev past 58-5900, it's just sputtering.

Here's what I know:
AFRs according to the logger are steady, mid-11s.
Turn boost down from ~14 to 10, makes 0 difference.
Plugs come out looking nice.

Relevant car mods:
T04E (~14psi)
M-Tune w/logger and tuner
MSD Blaster coil

I've started doing the "crappy mechanic" thing and have just been throwing parts at it:
New Blaster coil
New Beru wires
New NGK BPR7ES plugs gapped to .025
New cap
New rotor
New TPS (I had one...)

None of these items have made ANY difference whatsoever, and I've done them one at a time. The symptoms are repeatable - runs great when cold, warms up and progressively gets worse. The parts above were replaced after doing many searches here and finding similar symptoms (and trying their fixes).

The motor is new (to me, ~5k now since rebuild). The under hood wiring harness is also new (to me). Last fall, before the new motor, and new wiring harness, the misfire was not present.

The guy who installed the motor set the reference sensor depth. Or, I assume he did.

Thoughts on cause:
Is it possible that the reference sensors are set such that it runs fine under 5000, but causes a misfire at high rpm when warm?
DME starting to flake out?
Something I should look for in the harness? It's in WAY better shape than the old one I removed from the car...

Could use some ideas. Car is parked... costing me an event tomorrow, going to share a car with a friend instead: a 996. My wife already commented that she prefers the 996 to my car.

EDIT: SOLVED!

Turned out to be electrical interference with the new battery cables. Issue went away when I separated all the battery and sensor wiring and kept them at least 1" (or more) apart.

Items I replaced that didn't fix it (in addition to the items listed above): KLR, DME, coil wiring, Speed/Reference sensor wiring, alternator.

gpr8er

this is a valuable upgrade to the weak stock ignition

of our cars. Credit to Tom McGuinn for turning me onto this.

black944 turbo

Mine started this yesterday as well. Runs great until fool boost in second and then it hits a bunch of flat spots, like it is pulling a ton of timing and then going back. I have yet to investigate mine. Keep us updated.

User 52121

When it gets really bad, it's almost like a broken valve spring. But I'd think if that were it, then the car would sputter ALL the time.

I did notice something today (drove it to work as it runs great so long as I'm not running it hard) - it was super foggy out this morning so I drove with the headlights on. And the misfire was worse. Now I had pockets of traffic, so I couldn't do tests immediately back-to-back, but it sure seemed like the misfire was MUCH worse with lights on than off (I tried a few times over the course of 20-25 minutes with lights on and off).

I'll have to experiment more tonight when I get home. If it's repeatable that way then clearly the issue is electrical.

bw993

Start with the basic stuff. Check to see if your car system voltage is above 12.3V when your car is warm. Low voltage will have an effect your ignition and fuel injection systems. Check your voltage regulator, main grounds and battery condition.

Paulyy

if it's a broken valve spring i think you'd know about it

Try gapping the S&R sensors.

reno808

Does your AFR change? maybe a vac line is collapsing as things get hot.... That happened to me once. check all of the wiring where you have high temps.

User 52121

AFR's stay flat - mid 11s.

That said, I've never had it do the "bad, won't rev past 4500" misfire on the logger (Note: I have a WBO2, but no gauge - it's just wired to the logger). But with the headlight oddity I found this morning, I'll run the logs and do a bunch of back-to-back runs later this week and see if I can capture something different and get it to misfire the "bad" way. But for the smaller stumbles/misfires/gets ugly after 5500, AFR's stay flat.

Tom M'Guinn

How is your battery voltage, and when did you last clean the bell housing grounds? Are you adjusting coil dwell time and/or any chance the ignition driver in the DME is overtaxed or going bad? Timing runs through the klr, so don't forget to check it too for cracked solder joints...

refresh951

Jim - Are you running the stock dwell map in the tuner?

User 52121

Sorry - just saw this. Yes, still running the stock dwell map.

User 52121

Ok - so I'll post what I have so far. I tried to borrow a DME (thanks Andy!) but it dawned on me - my DME was modified for the Rogue Tuner. I was a beta tester for Joshua and as such, never actually received an "off the shelf" 28-pin DME chip for the M-tune. The 40-pin processor on my DME was modded (by Joshua, for the tuner) and I plug the Ostrich directly into the 28-pin socket. I can swap in a standard 28-pin chip in my DME, flip some DIP switches, and run a normal chipset... but not the other way around (can't swap in another DME without a 28-pin M-Tune chip!) Damn! I'll have to ping him to see if I can get a base M-Tune chip to try.

That said, I'm not 100% sure it's a DME anyways.

Headlights on DEFINITELY affects it. Did a bunch of back-to-back, 2nd gear runs, and the misfire is MUCH MUCH worse with the headlights on. Lights off, car will rev to 6,000+, just not very smoothly and it will misfire. Lights ON, car will not rev past 5500.

Also noticed: boost is 1psi higher when the lights are on. I didn't touch the MBC. Hits ~14.0 with the lights off, and ~15.0 with the lights on. AFRs get richer too - though that might be a function of the tune itself.

DME voltage drops under heavy load, but not like it falls out to 12v or anything. Looking at an averaging trend line (thanks Excel!) it might drop from an average of 13.5v at cruise to ~13.3v at WOT.

All the battery and ground wires are new. (The "Robby" setup). Nissan alternator with the case grounded to the driver's side headlight ground. (It had this misfiring issue before I had it grounded there...) Extra ground wire running from the body ground at the battery to one of the fuel rail bolts. Extra, auxiliary (+) wire running to the fuse panel from the battery. All other grounds were cleaned with a wire wheel.

Log files are here: http://omniglh.net/951/logs.zip

4 WOT passes. I alternated - lights off-on-off-on. I added a column to the .xls file showing lights on (=1000) and off (=0).

Would love any insight. Beyond frustrated at this point.

ramius665

Jim,
I've been having a misfire as my 951 comes on boost and on occasion will not//not rev past 5k. The symptoms are very similar to what you describe. My car has a SciVision MAF and Zeitronix suite. After reading everything you've posted above, I noticed that my 951 has lower than normal voltage, and the running condition seems to get worse as the voltage drops. I've got a Quest alternator and a set of battery cables from JohnKoaWood. I'm going to clean all of my grounds this weekend and I'll let you know if it makes a noticeable difference. I know this probably doesn't help your running issues but it has helped me out quite a bit.

User 52121

Cool good luck!

I have another DME coming from a fellow RL'er that's got the Rogue Tuner in it, so I'm gonna swap it out and see if that fixes it.

LUCKY DAVE

This really sounds like low voltage to the coil. The headlights are the clue.....