missing/rough running

[Z06scentair]

Originally Posted by 8cam

A little more clarity and I think I'm homing in on it. Engine not throwing any codes at all, and vacuum holds 20" for several minutes, pump is cycling like it should. Cruise also verified working perfectly, even though that plunger still doesn't affect the idle.

So I disconnected the IAC and started pulling plug wires. When I got to #4 there was no change in the idle and it's the only one that didn't arc/spark as I pulled it. So I pulled that plug and I swear it wasn't tight. The plug looked very good, no residue, and when I reinstalled it the idle was immediately better. I actually thought I had fixed it, but a quick test drive and it's still missing badly. I ran out of daylight, but tomorrow I'll pull all the plugs and just make sure they are all seated/torqued properly. Then it looks like I have something to figure out with #4.

Make sure the spark is not tracking on the plugs. That's what it sounds like to me. Add Di-electric grease to each plug boot.

Have the injectors been changed in this car? Someone above mentioned that but you never state if they have been changed or not.

If not that is something you should plan for sooner than later. While your under the plenum go ahead and replace the coil packs. Total cost of parts should be around 1k for SS new injectors, coils, and gaskets.

[8cam]

Injectors and coil packs are about 18 months and 5000 miles old. If I wasn't working so fricken much, I'd be able to get into the garage and knock this out. I'm going to try and ohm out the coil packs this afternoon and see if I have a bad one, which is my chief suspect right now.

[We Gone]

Did you replace the plug wires also?

[8cam]

I just pulled the invoice for when all that work was done, and 16 months ago it was injectors, fuel pump, plug wires, plugs and a battery among other things.

Just checked the coils across the wires with my crappy analog meter (got to get a better one) and they look like this...all measures are approximate.

1/6 - 32k
2/3 - 32k
4/7 - 28k
5/8 - 24k

So nothing definitive other than they are all suspect?

[Z06scentair]

Originally Posted by 8cam

I just pulled the invoice for when all that work was done, and 16 months ago it was injectors, fuel pump, plug wires, plugs and a battery among other things.

Just checked the coils across the wires with my crappy analog meter (got to get a better one) and they look like this...all measures are approximate.

1/6 - 32k
2/3 - 32k
4/7 - 28k
5/8 - 24k

So nothing definitive other than they are all suspect?

Im betting on spark tracking now. Fill those plug boots with grease and see if the misfire is gone away.

[8cam]

Originally Posted by Z06scentair

Im betting on spark tracking now. Fill those plug boots with grease and see if the misfire is gone away.

Definitely the most inexpensive option! I'll be giving this a try tomorrow, thanks!

[Schrade]

Mid-routed wires emit EMI; same as a nuke, and will jam up Solid State electronics, and light up false SES lights.

[Paul Workman]

Believe me when I say, checking the invoice ain't the same thing as checking resistance!

When I bought my Z, the dealer handed me an invoice to show the injectors had been replaced. But, as it happened, they'd been replaced with NOS injectors, and after a couple seasons, alcohol got two of 'em. BTW, the symptoms were very similar to yours, from the "sound" of it; especially when you said the plug looked more or less normal when you pulled it. (Typical of a miss when the injector is not pulsing correctly.)

No codes either. So, let the engine warm up to operating temp and then measure the resistance across each injector. (Refer to the FSM. Resistance can be checked easily at the relay connectors for both primary and secondary and secondary injectors.) Depending on injector type, the resistance will vary between types: some read 12Ω and another model will read closer to say 14Ω, for example. (This is important to know, as an injector or two that are only a couple ohms low, compared to the rest, are "suspect".) If, after the engine is at operating temp, any injector(s) are reading 2Ω lower than average, they're suspect. But, if they're reading 4Ω lower than average or more, you have an issue without doubt: a LOT more reliable test method than looking at the invoice (I can say with a red fact!)

In any case, I think you've narrowed it down to injectors, or plugs (not likely), or coil. Methodical testing and verification/comparison is the key. And, also, failure when hot is a classic case of coil winding failure.

Beit windings in an injector or the spark coil winding; neither will throw a code. I'd die to have an "O'scope", but the spark coils resistance too can be measured and compared. It too is best done with them being hot, and they should all read relatively close. (although 2 of my 4 read "open" across the sparkplug terminals, it ran OK. But, zero ohms make me think there may have been some internal arching due to an internal open somewhere. So, just in case, I replaced them while I had the plenum off (with new Accel coils). (5 years and 18k miles later and no problems.)

Tho I don't think fuel pressure is the problem at this time, do you know what it is?? Middle 40s is normal at idle. 52(±) psi is in the normal range at WOT.



Checking dynamic fuel pressure across the spectrum of throttle openings by taping pressure gauge to the windshield and going for a spirited drive!

Hope this helps...

Paul.

[8cam]

Thanks Paul, really appreciate it. I'll be picking up a better meter this morning and will check the injectors too. Believe me I know checking the invoice isn't always right, but in this case it's at least step one. I just went back and reread it, and apparently I was wrong. It says coil packs "checked" but not replaced. So there's a possibility too. Going to try and find an hour or two this afternoon to spend with the car and hopefully I'll be able to home in on exactly what the problem is.

[8cam]

I'm under a Monday deadline and will probably be working all weekend, so garage time is almost nil. But I was able to test the wires with a better digital meter this afternoon and got some better numbers but still a little high.

1/6 - 27.9
2/3 - 27.7
4/7 - 24.7
5/8 - 22.9

I also pulled the IAC and started pulling wires again. Very little rpm drop when pulling 1-4, noticable chugging when pulling 5-8. Also very intermittent/weak spark on #4 and not much better on 1 & 2. 5-8 look like an arc welder in comparison. Pretty sure at this point I'm pulling the plenum and looking at bad coils.

BTW dielectric grease made no difference, but given the weak spark I'm seeing I wouldn't expect it to.