ShardayGirl

One hot day, my 94 Z28 had one incident of an alternating racing idle (e.g., 500 rpms, then 1000 rpms, then back to 500, etc. -- all in a matter of moments). The dealer I take my car to said, after both a computer analysis and a test drive, he thinks it's the optispark sensor & it needs to be replaced, which costs (parts & labor) over $1200. He then told me he can't guarantee that's what caused the problem; it could be the computer. He said if I don't get it fixed, one day, it will simply stop and won't start again.

Since I know nothing about cars, I was wondering if this sounds right and if the cost is in line with what it should be. Any advice you can give would be appreciated. I'm taking it to someone else for a 2nd opinion, but I thought this might be a good place to find out the real scoop.

Thanks.

shoebox

The only replacement for the opti is the whole thing. Separate parts are not available (except cap and rotor). Everything you quote is about right on from a dealer (because they are going to be the highest priced place). What they say about the guarantee is also probably true. Of course it could be something else entirely causing your problem. Since you seem to be at the mercy of mechanics, you should get another opinion or two.

raroz28

Opti is very easy to replace. Find a friend good with cars. 1200 is outrageous. The only part that sucks is draining the coolant - i hate draining fluids, but even that isn't hard, just messy.

That being said, I've never heard of an opti causing an idle issue like you're describing. Bad opti's usually misfire, long crank, breaking up at higher RPM's, or a no start condition.

I would find another mechanic. BTW, not sure how he did a computer analysis of the opti. The only way I've ever known to test it is with a multimeter and even then it isn't totally conclusive.

Injuneer

I have recently worked on three separate cases of helping people with erratic idle problems. In all three cases the problem turned out to be an erratic high resolution pulse signal from the Opti's optical cam position sensor. In two of the cases, it was and MSD Opti (one of them just rebuilt by MSD) and one was an AC Delco.

In all cases, you could see the erratic idle RPM readings in the data log, and the resulting attempt by the PCM to adjust the IAC counts to correct the idle, which it couldn't do because the RPM readings were incorrect. In at least two cases, the owner was able to observe a brief "twitch" of the tach needle just before the RPM started to surge.

I am the last person to say "replace the Opti", but that may be what is required. If you have the time, I would recommend that you download a free copy of Scan9495 (link to free download in a "sticky" at the top of the "Computer Diagnostics and Tuning" forum), and I will review the data log for you to see if there is a similarity with the recent problems of errratic idle surge caused by the Opti.

ShardayGirl

Injuneer,

Thanks for the offer. I've got an appointment with another mechanic Tuesday. If the news is still bad, I may take you up on your offer to review the data log from the Scan9495.

ShardayGirl

Thank goodness for 2nd opinions. The new mechanic said the hole I had in the intake duct could have caused the problem (a relatively cheap fix), which may be why he couldn't replicate the problem since the hole has been taped up. Anyway, I 'm holding off on doing anything with the OptiSpark for now; I'm just getting the intake duct replaced.

Thanks for the helpful input.

Injuneer

Where was the hole in the intake duct?

HardcoreRM125

Before or After the Mass Air Flow sensor?

ShardayGirl

Unfortunately, I don't know. By the time I saw the duct, it had been all taped up. Would that matter as far as possibly causing the problem I had?

HardcoreRM125

Sure would. If it is after the Mass Air Flow sensor (that measures how much air the engine draws in), it is unmetered air. If you have more air than the computer thinks, it will result in a lean mixture. The RPM's would probably jump up. As a result the IAC would try closing off to help bring it back down to the proper RPM range. Unmetered air can cause all sorts of odd problems with high idle, low idle, missing, stalling, not even starting in extreme cases, etc.

Injuneer

The long term fuel corrections may be able to adjust for the bad MAF sensor readings, unless the hole is so big that it creates a flow that exceeds the measured flow by more than 25%.

ShardayGirl

It appears that the hole in the duct probably was the problem since the surging idle hasn't happened again. Thank you all again for your input. It was very helpful and let me avoid a very large, and apparently unnecessary, expense. Even the service manager at the dealer admitted that the duct hole could have caused the problem, but only after I asked him about it, which I wouldn't have known to do without you.

Chimera96

shardaygirl

if you haven't replaced your duct that is taped to fix the "hole"...I suggest you do.

"tape" is not a permanent fix for anything

ShardayGirl

I've already had it replaced. Thanks.

94Z28LS1toLT1

Ive been to the dealer before to just have an engine code scanned because my ses light was on, and the lady told me they said my computer was bad,egr solenoid was bad,egr valve was bad,and that i need to replace my
WHOLE wiring harness wow!!!!! costed 150 dollars just to get it scanned, I got a 2nd opinion found a better mechanic and he scanned my cars computer and just said maf sensor was bad needed it replaced,and to check the wiring to egr solenoid i did both of those and my light came off,goes to show you when get a 2nd opinion