Who bets its the opti?(fixed)

Well I have developed a miss. It was very intermittent a week ago. Didn't seem to be related to engine load. Just as it felt. Checked battery cables; they were tight. Pulled plugs and they all looked good. It felt almost like a lean miss. Fuel pressure is good, changes just like it should. Makes a solid 18" of steady vacuum, erg valve checks out. Didn't see any plug wires worn through, they got about 25k miles on them.

I had replaced the coil a year ago with a Delphi unit. It's out of warranty, but appears to be in spec. Replaced it anyways because it's fairly cheap, no change. Replaced fuel filter,it was pretty dirty and about 25k on it also. No change. I got 7 of 8 plugs changed, but broke one. May feel better, but could be in my head. Gave up for tonight.

Opti has about 25k on it also, new unit. So, what else would cause this miss. Under full throttle, don't really notice it. When it gets into OD and I change throttle input, I seem to notice it. That's why I assumed lean miss as the car should be leaning out fuel mixture in OD.

Brand of new opti? If it's a genuine Delphi with the Mitsubishi sensor then the odds are it is still good. Have you checked the operation of the vent system? One problem that was identified by GM in a tech bulletin -- the vacuum can pull in the large surface area of the opti cap and cause problems if the air inlet becomes plugged.

It was as new opti unit I installed 25k miles ago. I haven't checked the operation of the vent system. Its all hooked up and do not see and splits. Its making solid vacuum, so I would assume no leaks. One thing I look to have left out was it doesn't seem to start missing until warmed up. Air system is deleted and tuned out by GM dealer.

I think I will know the answer tonight. Got about 3 miles down the road today and noticed something was really wrong. Turned around and went to step on the gas and it really started missing. So much, the CEL started flashing. Had to fire up the old T/A today to make it to work (poor me).

well, looking at it more tonight, I'm going with ignition control module.

so, there was no test for the actual ignition control module, just the wiring going to it. test the wiring, it was good. So, replaced the module today, and surprise, no luck.

when the car is cold and in open loop, it idles and runs fine.
when it warm up and goes into closed loop, it miss fires pretty good.
One code said miss fire on #8. Checked the wires, looked good. Cleared it.
Ran it again, then it was random misfire.
So, WTH? What else would cause it to misfire when it warms up?

Mixture. Maybe time to clean injectors? Did you get a chance to check the vent system operation yet?

I recently had a weird issue with SWMBOs Regal -- it would actually die on the highway, throwing a MAF code. The car would not restart until the codes were cleared, then it would run fine. The MAF data looked just fine, I couldn't understand why it was throwing the code -- until I started watching other, related sensors while my wife drove. I noticed the MAP sensor values were barely changing. Sure enough, when I changed the MAP sensor it stopped throwing the MAF code and has run fine ever since. As near as I can figure, the PCM was comparing the calculated MAF (derived in part from the MAP) to the measured MAF, and for some reason decided the discrepancy meant the MAF was faulty, so it reverted to speed density mode (ie: using the calculated MAF) and died.

I'm not saying your problem is identical, but I would take a similar approach. Use your scan tool to monitor the data stream while someone else drives. Look for anomalies -- readings that don't add up under current conditions. Does the 96 have a live misfire monitor? If it's only one bank that misfires, then it could be O2 sensor related. If it's only one particular cylinder, then plug, wire, injector. If it's two sequentially firing cylinders, then possibly carbon tracing in the cap or wire routing. If it's truly random, then probably something like a MAF or MAP or ICM that affects all cylinders equally.

Well I cleared codes and ran it agian. It said misfire on #8. I ohmed the plug wire and nothing. No change. I couldn't believe it so I test another wire and it ohmed good. I pulled the wire and looked it over. No damage anywhere. So, they have a lifetime warranty and I used it. They replaced the wire for me. I put it on, and still had a miss. Wtf. Plug in and now it says #7 misfire....

I go back and swap all the wires under warranty. After going out and doing some WOT, I am pleased to say no misfires

Nice! I have also fixed a few misfires on Subaru's lately btly changing wires with OEM wires. I should probably take my own medicine and do them on the Roady but i hear it is a royal PITA.

71novaguy wrote:

I ohmed the plug wire and nothing. No change. I couldn't believe it so I test another wire and it ohmed good. I pulled the wire and looked it over. No damage anywhere. So, they have a lifetime warranty and I used it. They replaced the wire for me. I put it on, and still had a miss. Wtf. Plug in and now it says #7 misfire....

First off, congratulations on tracking this down and fixing the problem.

When you say "ohmed the plug wire", do you mean you measured the resistance with a standard multi-meter? The problem with relying on such a meter is that it only applies a small battery voltage (usually no more than 9v) across the leads for testing. The secondary ignition system generates 10s of thousands of volts. So a wire might test "good" at low voltage, but not be able to carry full current at full voltage. Conversely, the conductor or insulation might show infinite resistance to the meter, yet still leak some current when coil voltage is applied.

Correct, I would have expected about 4k resistance per foot on these wires. When it read nothing, I was surprised. I've never had a wire fail. They usually rub through and short out, never just fail. I really can't believe I had two fail at the same time, and for the back two cylinders. The wires still looked brand new.

I can only guess that it didn't have a miss when at cold idle because it had enough power to jump the break in the wire. When it warmed up and resistance increased, it couldn't make the gap anymore and started missing.

It runs much better now. I just got done out running a Prius on the interstate ( I know, I know). He was riding my but so I kept it at 90-95 for about 5 miles. I think I broke his car because he dropped off all the sudden and pulled over to the side of the road with his flashers on.