1992 240sx : Heavy mis-fires under no load before warmed up

[slipnfall]

Hi Folks, This is quite a long-standing problem with my car... Basically this problem seems to surface every fall/winter. This is the problem:

If I don't let me car idle for a good 10mins, I get heavy mis-fires when taking off. It also mis-fires when I slowly raise the idle before warmed up. This all occurs between 1-3000rpm, untill I have gone about a mile down the road. Everything is good after that. If I blip the throttle, it revs OK. If I hold the idle, this is where I have the troubles.

This car has an OEM O2 sensor, about a year old. OEM spark plug wires, about 4-5months oldOEM cap/rotor, roughly 9-10months oldNew fuel pump, and aftermarket regulator. I have 'disabled' the EGR by pluggin the control line coming from the BPT to EGR. The problem still exists.

Any thoughts? It can't be doing my car good. When this problem occured last time, I concentrated on my EGR and O2 sensor(when I bought the OEM O2). Why would I have gone through an O2 sensor so fast? I don't burn any oil, but have a little blowby above 4.5-5000rpm under load.

Thanks for reading!Jamie

[slipnfall]

Forgot to add: NGK standard plugs (BK5E11 from memory??). Nice mashmallow-brown on the plugs, no signs of detonation or excessive oil consumtion.

Also: I was under the impression that the ECU *ignores* the O2 sensor signal untill it has reached optimum temperature (~185?).

I always run '93 octane.

Never been any codes store in the ECU(I check often).

[slipnfall]

bump for some help.

[Bwana]

Have you checked for a vacuum leak?

[slipnfall]

To the best of my abilities, yes: vac at idle is roughly 21hg(?)

I've sprayed carb cleaner almost everywhere and couldn't find anything. I'm consider taking it to a shop to have it hooked to a smoke machine.

Any other ideas? A vac leak should be pretty consistent: meaning it shouldn't go away once warmed up.

Thanks,Jamie

[boriquaS13]

Does the missfire seem like only one cyl. is the problem or does it feel like it may be random cylinders. Could you possibly do a cyl. balance test when its occuring? How does your car idle when warm? Does it sound like its backfireing through the intake when its happening (maybe leaning out cold)?

[slipnfall]

Does the missfire seem like only one cyl. is the problem or does it feel like it may be random cylinders. Could you possibly do a cyl. balance test when its occuring? How does your car idle when warm? Does it sound like its backfireing through the intake when its happening (maybe leaning out cold)?

It feels like random cylinders. I could try a ballance test: I have a CONSULT interface that I can do that through (though it's only really usefull for turning off individual cylinders, not a *true* factory ballance test). May I can narrow it down to one cylinder.

When warmed up, the idle is OK, although about ever 20-30seconds I hear a slight miss (I hear it because of the 2.5" exhaust). If I'm watching the engine closesly at idle I can see it 'twitch' (for lack of better terms) when it does this.

It's certainly not a backfire, and I haven't honestly listened to it while in the engine bay.

Thanks for your help!

Oh, worth mentioning: I did a datalog of the ECU while trying to hold 2000rpm on a cold start. Nothing really jumped out, the RPM obviously was bumpy, along with MAF and injector times as well(to compensate I presumed).

[boriquaS13]

Since you have a consult handy what does the coolant temp read cold is it pretty close to ambient? And you said you tried disconnecting the egr, if I recall correctly it should have an egr transducer inline to the egr valve I have seen those to allow vacuum to the egr valve but its weird that no rough idle warm though. Try to remove the egr valve and block the vacuum side of the port (cold engine obviosly) and start to see if the missfire is still there. Does it have good spark output at least 30kv? This problem actually intrigues me and I hope to help you fix it keep me posted.

[slipnfall]

Yes coolant temp is on par with ambient: anywhere from 32-40*F.

What you described is the way I have 'disabled' the EGR: I have removed the control line from the BPT going to the EGR, and capped the BPT end.

I don't know about secondary output: I don't have any means of testing kV right now. From what I understand it is the primary current that matters more.

[boriquaS13]

That is true but if your coil cannot put out at least 30kv its concidered weak spark. Just a thought have you had to add coolant at all? Could it be possible you have a blown head gasket leaking coolant into the cylinder and causing miss in intial start up until the coolant burns out. Just had a subi in the shop today with a miss on start up ended up failing a block test (blown head gasket enough to leak in but not enough to cause overheating). Any how keep me posted.

[slipnfall]

I actually just tested compression about a month ago: ~185psi across the board. I have a leakdown tester but haven't had the chance to use it yet ( I was planning on using that to test for intake leaks). Do you think this would be an effective means of testing for such a problem?

Again I'm looking around for a shop with a smoke machine. While it's there maybe I can get them to test the secondary.

I have another spare coil, I'll toss that on first.

[boriquaS13]

The block test that I speak of is the actual checking of the co and hc in the cooling system (combustion gases). Its a type of catalyst that changes color when it comes incontact with combustion gases in the cooling system. Anyway good luck and keep posting and hope to help.