Just recently I started picking up what either can be described as a rough idle or mis-firing plug.

History: I have Taylor Spiro Pros 8mm, NGK TR55-ix plugs (both have about 30-35K on them) removed cleaned throttle body and new IAC about a year ago, new ignition coil, new EGR, new vacuum harness plumbing, new vacuum check valves, and new O2 sensors. All these parts are GM OEM except the plugs and wires.

Symptoms: On first start of morning, w/AC off the moment I touch the start position it fires off with a perfect smooth idle (if I turn the AC on then I get a rolling idle with the cycling of the AC compressor) engine temp begins about 85F and at around 103-105 O2's kick in and all seems fine. If I put it in OD or D and hold the brake you can hear/sense a miss in the idle (similar to a mis-firing plug). Occassionally, while idling in D, like muddling through a parking lot, it feels like it wants to stumble but doesn't. I've taken it out on the interstate, and ran it right up to peak RPM getting up to the triples and holding it there it runs perfect.

So do you think this is indicative of a fouling plug? I have all the gaskets and cleaners to R&R the throttle body and IAC but I don't want to do it if that is not the issue. (No need to waste my gasket sets prematurely.) It seems very noticable when its in the low idle range with a load.

Comments Opinions?

 

Gee... how many miles are on the car in general?

The only way you are going to confirm a fouling plug is to remove and clean each one. And determine WHY it's fouling, if it actually is.

This could also be an injector problem. Is the car running a bit rich? Any sign of black out the tailpipes?

 

121,000

No not a whole lot of black on the corsa tail pipe. It gets just a tad bit black there on the inside pipe. I usually wipe them clean when I wash it.

It just seems to occur at that low load rpm something like 600-700 rpm with it in D/OD and holding the brake stopped.

I know it could be any number of things. I wonder too, what the useful life is of these irridium plugs?

 

What is the history of the opti?

 

Ya know I knew I was forgetting stuff when I originated the post.

Around 80K the opti-spark and water pump were replaced. So it has around 40K on the combo. The water pump started weeping as I was new to this vette and I was aware that if you replace one you might as well do the other. So I just told Chevy to replace both.

I was going to do some reading this evening to see if it covers this symptom in the charts. First I need to wash it, having just detailed it this past weekend I parked near a oak tree today and have tiny drops of sap on it.

 

hm.... anytime there is weakness at low RPMs like that, I want to blame something in spark delivery.

 

You mean wires, plugs, coil and coil module?

Its a new coil and yes I coated it with heat sink compound. The 8mm taylors and ngk's are probably 3yrs old. I suppose I could ohm the wires, and of course pull some plugs.

I'm just wondering about the NGKs. With that tiny irridium electrode and being a non-gaping plug what its useful life is.

Hmmm came across this tidbit:

Service life of some OEM Iridium spark plugs can be as much as 120000 miles with very little degrading of performance in that lifespan. NGK Iridium IX types have a centre electrode tipped with Iridium alloy with a 0.6mm diameter. NGK believe this gives the optimum balance between performance and service life. Service life estimates can be up to 60000 miles for normally aspirated car engines and up to 20000 miles for motorcycles - this is extremely subjective though, a high revving motorcycle or extremely modified car engine will most likely wear plugs out in a few thousand miles.

 

Try checking the voltage on your throttle position sensor. My sensor acted the way you described and created a slight miss only with the A/C on. I back-probed the TPS wires with sewing needles and checked it with a digital voltmeter. My TPS voltage was 1.8 volts at idle. I moved it back to .6 greatly improving my off idle miss.
History piece, I slotted my TPS to adjust for my idle stop adjustment after my hot cam install.Apparently too much and once you jack with the idle stop screw, you inadvertently, change your TPS setting. Etc.In effect, I created my own miss problem during my learning curve with the tune... Hope you solve the problem.

 

Thanks I'll go read about the TPS and see what gives. Yeah its weird this just started a couple weeks ago. Since it was rolling so much during its warm up idle I've since been turning the AC off first then start let it come up to 103 and then engage the AC. While its idling with AC off its a perfect idle. I haven't driven it with AC off, too damn hot/humid so that makes me wonder if its load related to the AC? I need to learn about the TPS.

 

Just heard back from NGK on useful life. They estimate between 40K and 50K. Since I'm right there I guess I need to pickup another set of plugs. And look at the TPS.

 

Check the plugs. Those TR55ix iridiums are more prone to fouling than other plugs.

 

I called NAPA earlier and have a new set arriving tomorrow, but I'll pick them up Friday. Looks like I know what I'll be doing on Saturday morning!!

 

Hint: Replace your #8 plug first while you are cool, calm and collected. Then replacing 1-7 is a breeze!!! For whatever reason, I was able to get my #8 threaded and torqued in less than 5 minutes using this method....don't save it for last when you are tired and cranked out...

Have fun....

 

Follow-up... still this issue remains. Has me perplexed but I did pickup a new set of NGKs this morning. I'll install them in the morning on a cold engine. But interesting, it is just an obvious miss firing when at the low load rpm that is idle, in drive, AC on, stopped. Last nite I took it out the interstate to run it hard to see if I could clean them up, as I entered the on ramp its a 270 degree ramp and I accelerated hard just after I came on to the interstate. Just as I peaked RPM I got a 2 second SES lamp then it went out. It runs fine at constant cruise or hard acceleration, but back at the next stop light same thing. Hopefully it will be a plug. Just thought I'd mention this in case anyone has other ideas. I was also thinking of pulling the throttle body and IAC too, I have the complete gasket set. And checking the TPS after the plugs. Going to be a fun filled Saturday.... naught.

 

Well the saga continues, little over 4 hrs today installed new plugs. The old one's still looked perfect. And after installation it did not resolve the issue. Still misses at the low loaded idle. But it is a little smoother and peppier with the new plugs. Now I just have to figure this thing out who or what is causing this problem???

Had enough fun for one day, tomorrow I'll be looking at the TPS and other items to test.

 

any update on this, i am having a similar problem under load a mis that causes the car to bog down alitle only under load- snap the throttle in nuetral and it sounds strong... i have done plugs wires and still have the problem, 92 six speed 190000mi, the previous owner says the opti was replaced 15000mi ago,,, i only noticed this problem after a coolent fan problem and the card got up to 245reg,, i guess its deff related but where do i start, no water in the oil or exhaust-
thanks,, any imput would be great- oh yeah and no codes, i was going to go with the deltec but i could bettere spend the $650 if its not needed- thanks again

 

My '93 has a very similar problem. Runs great at WOT, but when you come to a stop, the idle becomes very erratic (like it wants to die) and the exhaust fumes become unbearable. This happens about half the time. It's in the shop now, and they discovered that sometime in the past, a battery had leaked down in the fusebox and onto the fusible link that goes to the ECM. Everything was really corroded. They believe (and I am hoping) that replacing the link, the fusebox and the battery cable will solve the problem. Should know in a day or two.

 

man, what is it with 90-120 mile LT1's having this problem? I got the same thing...

Same symptoms, same engines... You would think that we could all come up with the definative answer to what's causing this!

 

Had same problem at 80k in 93 LT1. Turned out to be leaking injectors. Replaced with Ford SVO's and idle greatly improved. I agree with bogus that low RPM misses are generally spark related. By the same thinking having too much fuel makes it difficult for the plug to light off. Someone mentioned that exhaust fumes become unbearable, sounds like its running rich to me.

 

Actually it got carried over to a new post.

Your welcome to entertain yourselves. Yes I finally got it solved.

Entailed replacing:

ECM - primary problem
Tach Filter - primary problem prevent ignition coil module from working
Coil
Ignition Coil Module
Ignition Coil Module Connector

Already had installed new NGK TR55ix and new set of AC Delco OEM wires. And a new set of O2's.

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