User 52121

My old cables looked like that

Tom M'Guinn

I had a short at the starter once and the car still started and ran fine, but when I finally changed the cable at the battery, the first 3 or 4 inches inside the insulation looked like a long cigar ash. There was still enough copper left in the middle to crank the car, but it really drove home to me that you can't judge a wire by its insulation. This was back before the iPhone, when the Mavica was the next big thing -- so no pictures of it unfortunately.

I also had a brand new, just out of the box, Magnecor wire set that created a high load miss.

What stark plug gap are you guys running. I've had bosch and ngk plugs gapped to .32 out of the box -- and always close them to .26 unless running low boost levels. At 18+ psi, big gaps can cause missing, even with a strong ignition system.

User 52121

Plugs are gapped to .025. NGK BPR-7ES. Behaved the same with 6ES's too.

Tom M'Guinn

That should make it easier to track down. If by chance you have a scope you could watch the trigger wire on the coil and see if the problem is in the ignition signal (suggesting something on the engine management side) or on the engine side (suggesting coil, gaps, plugs, wires, etc.).

If you can get it to happen at light load but high rpm, you might be temped to try bypassing the klr... I've never done it though so can't say for sure it's safe...

dmcampbell

Reduce spark plug gap to .023 and report back. Mike

User 52121

I don't have a scope but I'm pretty sure a friend of mine does. I'll give him a buzz tomorrow.

And it's not the KLR. Borrowed the one out of my buddy's car... did the same thing. Headlights on = pops and farts and bangs and won't go over ~5400. Headlights off = it'll get up there but not smoothly once warm. Headlights off and a cold car = thing friggin RIPS.

Mike - regarding the gap: do you really think that could be an issue? Guys run much bigger gap than I do, at higher boost levels. I'm only running ~14psi, .025 gap, with a Blaster coil, and it misfires the same whether I have the boost set to 14 or 10, Blaster coil or stock coil. I mean, I'm game to try it if you have a hunch that could still be it... (and appreciate the thought and suggestion!)

Considering it happens in neutral in the driveway, and while on the road - I'm wondering if, perhaps, it IS something to do with the reference sensors? Another mechanic friend of mine (not super familiar with 944's), when hearing my symptoms, said (without hesitation): crank position sensor.

Tom M'Guinn

If your logs show a nice clean rpm/ignition signal, with no weird blips, then that would suggest the speed/ref sensors are ok -- no guarantee, but not sure how it would have a clean rpm/ignition signal without clean speed/ref sensor signals. If it happens in the driveway and at the same rpm as on the road, then I'd agree that doesn't sound like a spark plug gap issue.

Any chance you have a soft rev limit set by mistake in the m-tune? What brand wires did you put in? Is the cap seated firmly? Is the rotor spinning on the cam? Try rev'ing it in the dark and look for arcing when it misses. I'd be inclined to go back to a stock chip and ignition for testing...

docwyte

WTF refresh?! My post said NOTHING about V8's. At all. I was just pointing out that the problem is most likely electrical in nature and coming from the engine wiring harness.

It's up to Omni now to spend the time and trace it.

That has NOTHING to do with V8's.

User 52121

Good guess - but no, not a rev-limit. I even went and downloaded the base .bin from Joshua's site and loaded it into the car the other day, just to rule anything like that out. No difference.

And just to point out/recap from page 1 - the misfire, and "no revving past 5400"... is with the headlights on. Headlights off, it'll rev past 5400. Not smoothly, but it will do it.

Basically - first run of the day (1.5 miles from the house, pull out onto a 50mph road), car RIPS to redline. It's AWESOME. Drive 2 more miles down the road, merge on the expressway and nail it, and it'll pull normally until ~4500, then start to feel like it gets choked up, and it's no longer smooth to redline. It'll pull but it'll have little stumbles along the way up - and the hotter it is outside, the more pronounced the sputtering is. Flip the headlights on and it all goes to hell - 4500 it's breaking up BAD and stops revving completely by 5400.

Wires are Beru. I ordered them direct from a local import parts place here. Beru branded. Cap is snug, rotor is locked on with the screw (which is blue loctite'd). And it did all this with the previous cap and rotor... I replaced them hoping to resolve this issue.

Now that I know it'll do it in the driveway, I suppose I could run it in the garage with the lights off to look for arcs-n-sparks. Maybe I'll give that a shot this week after work. Or maybe next Saturday to spare my poor neighbors from listening to me run what sounds like a Pro Rally car with anti-lag...

I suppose I can also try yanking the M-Tune out, stick my old A-Tune chip back in along with the stock injectors and AFM... I don't have a J-boot that fits my T-O4E though (for the AFM)...

TurboTommy

This sounds so much like weak ignition, it's not even funny.
I know you said you have a new coil in there, but maybe check your source and trace it where they got it from. I know it sounds crazy but there are so many counterfeit automotive aftermarket products out there.
I'm just sayin'; check it out!

Droops83

Hi Jim,

A bit late to this one, but I agree this totally sounds like a voltage drop and/or charging issue.

Before changing out any more parts, you need to check the entire primary ignition circuit and charging system.

Run the car till it gets hot and starts acting up, and check voltage everywhere: at the back of the alternator, at the battery, at the coil, fuse box, ignition switch, etc. There is likely a voltage drop somewhere.

Switch your voltmeter to the AC setting and check for AC voltage at the alternator----if there is any (more than a few 10ths of a volt) you have found your problem.

You'll have to go through the wiring diagram carefully and check the voltage at every leg of the primary ignition circuit.

Good luck.

odurandina

I think it's the one and only [v8 lover emeritus] you really want.
http://www.thetruthaboutcars.com/2006/07/v8s-rule/

I've made the analogy dozens of times.... we're not running our '90s laptop anymore.

but, these harnesses/computers/alarms/dme's were already going bad by the '90s.....

exposed to the elements; road salt, dirt, corrosives, and endless heat cycling, and finally time.

and we got the 2,700 odd no start/can't start/will never start threads to prove it....

the op is a solid wrench and all around stand up guy - and he's stumped.

ain't the first time this has happened with an extremely knowledgeable enthusiast.

these electrical systems are fried alright.... in the final analysis, we're lucky we got this far.

is it possible to run a 968 harness??

blown 944

I havn't really read through this whole thing, but I had n experience that sounds similar where my alternator cable was causing interference with the sr sensors

Tom M'Guinn

What did you do to solve it? I wonder if that is one of the reasons these cars will kick back when cranking to start -- despite the shielded sensor bracket, etc.?

User 52121

Bought it from Summit.

Thanks man. This combined with Lee's comments about AC voltage (WTF? Where the hell is THAT coming from?!?), I guess that's the next thing I'll look at. Looking for AC is easy enough, and I'll take a shot at looking at vdrop from the alternator back to the battery, etc. Though I can see ~13.5v at the DME according to my logger, and with the wiring straight to the coil from the battery, so I would think that the 2 key components needed for proper ignition (DME and coil) had all the volts they need?

What would cause a new Nissan alternator with probably 10k miles (or less) on it to suddenly start spitting out AC?

Sid: Ditto to Tom. How did you decipher that this interference was the issue, and how did you resolve it?

My car does the kick back too - done it ever since I notched the bellhousing 2 winters ago. Though all last year, bellhousing was notched, misfire didn't exist... but with new, fresh battery wiring now, I'm willing to look at it again. I made sure at least the sensor side of the reference sensor cables are nowhere near the power wire - though the harness/DME side, the power wires are zip-tied at the firewall next to the harness.

Thanks Od for the props and respect - I'm a little surprised (and appreciative) you didn't post your usual "put an LS1 in it" post in my thread. Clearly the pro-LS crowd gets under the skin of some folks (and it IS a little tiring)... but I generally just ignore them.

I agree with you that we're sorta lucky we've gotten this far. Electronics don't age well in this environment... especially in my personal car, that has seen a lot of track time and autocross (sitting in a hot parking lot, on a hot day, idling and running hard for 60 seconds at a time) over the years.

I feel bad for the guys, 25 years from now, that buy a 996+ and have to troubleshoot electronics issues.... at least with the 951, the harness is relatively simple and can be rebuilt.

I guess I'll start ordering up automotive grade wiring too, and will start in on my "build a new harness from scratch" project that I put on hold last winter.