Out of the blue came a mystery misfire on my 93 vert. I changed the plugs to no effect. I've only had this car for a year, so I don't know what's recently been replaced. Plugs seemed like a good idea! The wires are Taylor 8MM, which have a good reputation, but I don't know how old. I used my scan tool to see if anything was amiss, and the O2 sensors seem like they're trying to tell me something: The right bank is fairly steady around 440mv most of the time, while the left sensor wanders over a much wider range.

When i was changing the plugs I observed that the wires from the loom feeding the fuel injectors were stripped of a lot of their insulation (probably from the heat, not chafing), so I wrapped them in electrical tape, to no effect.

I'd like to know if there's a non-life-threatening way to check the coil to see if it could be on the way out.

And of course there's the Opti. When the engine was cold I reached my hand under the water pump and it felt nice and dry with no crud detectable so I don't think my Opti is bad. (I'm sure that's the definitive test for that component! )

Any suggestions (Take it to the dealer doesn't count!).

Thanks!
Paul

 

If it's a steady/constant miss fire--try pulling the connector from the fuel injectors one at a time and see if you can isolate which cylinder is missing.

 

You do have an O2 issue but just the opposite of what you think. The O2's should be constantly switching from lean to rich as the pcm dials in fueling. It will never steady out unless it is in open loop or the sensor has gone lazy. The sensor that is steady at 440mv might be telling you that your miss fire is on that bank.

 

Thanks for both replies!
I'll try the FI wiring trick -starting on the right bank! Once (if) I isolate it to a single cylinder, is it likely to be a plug wire? Or...?

Thanks much!
Paul

 

check your spark plugs for fouling due to your stated issue with an o2 sensor.

If one fouls, then it wont fire.

I know its easier said than done (to check your plugs).

If you put long tube headers on there it becomes a piece-of-cake instead of PITA to service your plugs.

good luck!

btw i have a 93, and there is normal slight popping (on everyone here with 93's) when you let off the gas. Which I enjoy the attention of if Im out on the town, etc. Your popping sounds more like a misfire, and Id look into it.

Oh! and dont run any of the fancy platinum or iridium plugs they have a smaller electrode (bc the precious metal is expensive) that is more prone to fouling.

Been there done that. NGK TR55 (the cheap stuff) is what you want.

 

Block/clamp off the air hose to the right header and see if it goes away. If so, the Check Valve is broken which means it's sucking air which is why the O2 is staying Lean. Surprised there's no Code.

 

I recently replaced a non functioning EGR with a new one. perhaps the non working EGR was masking a broken check valve? I'll try to figure a way to block it off as you suggest. I might consider putting the broken EGR valve back on to see if it has an effect.

Thanks for the idea.

DizWiz- Those long tube headers are looking better all the time.

Thanks for the other ideas. All will be explored until the problem is resolved!
Paul

 

Easier way to isolate a missing cylinder: put an infrared heat gun on each header right by the head. The one(s) that is cold is misfiring. If you don't have a temp gun, just grab it with your hand (kidding...kidding) just splash water on each one. Seriously. It will hiss and evaporate instantly on a good cylinder, it will slowly dry on a misfiring one. Trust me, it works. If you aren't sure what it's supposed to look like, pull one plug and try the water test so you see the difference.

Rule of thumb: low RPM miss means plug, hIgh RPM miss usually means plug wires. I also have a write up another forum member posted years ago with a step by step ignition system check. You'll need a multimeter for this, though.

PM me your email if you'd like it.

BTW LT1's run smoooooooth even with missing cylinders. In trying to isolate one once on my car I had unplugged coils (I have a Delteq) until only 4 cylinders running. It was like butter. Seriously. It's amazing.

 

I tried the splash test tonight. Inconclusive with the time I had to do it, so I'll try it again this weekend. My observation was that the center exhaust (serving 2 cylinders- 3/5 and 4/6) got hotter than the corners. I wasn't starting with a cold engine, though, and I think that will help.


I'll send a PM. I have a multimeter. Thanks for the offer!
You're right about the smoothness of the engine! I might be running on 4 cylinders myself!

Paul

 

I would check the path the Arborbarber suggested. I have had issues on 2 different occasions (I apperantly am a slow learner) both times it turned out to be the O2 sensor. I changed 1 then the other (I should have replaced both at the same time). Fairly easy removal and install if you have the O2 extractor socket (or whatever it is called).

 

I've changed O2 sensors on other cars. Usually not a big task. I'll do that here. I may have misunderstood Arborbarber's post/ I didnt think he was pointing at the sensor as the problem, but that the readings indicated something else causing a problem on that bank. Anyway, O2 sensors are a 'wear item' and even if they're not the culprit, they can stand to be replaced.

I hope they use the 22MM slotted socket that I've used before. Makes the job a piece of cake.
Thanks for the suggestions.
Paul

 

OK- I installed new O2 sensors. soaked them in KROIL for a few minutes and that made extraction much easier than others have experienced. Took it for a ride, and the first few minutes were great with one or two backfires. Then as it warmed up the problem came back.

The O2 sensors are now reading as they should- swinging from high to low readings depending on acceleration or deceleration.
On the principple that I'd rather spend money on parts than turn my baby over to the dealer, I'm going to replace the check valves. While I'm at the parts store I'm going to buy a spark plug test wire so I can see which plugs are giving the most trouble. That may point to one or more plug wires.

The good news is I can still drive it, it's just that acceleration sucks and makes noises (bangs!).

Stay tuned! No Pun Intended!!

 

Mines a 93 , May want to check out mt thread http://forums.corvetteforum.com/c4-t...-question.htmlif you never replaced the famous Optispark.

Dont know how many mile are on your Corvette but this is my Opti after 128K and yes its the original

This could be your problem, because what you explained is what my Corvette was doing and heres the problem .

pictures in thread will explained the problem clearly

GL

 

Thanks, Repzard!
That isn't what I wanted to hear!
However, the Opti is one of the prime suspects. Having said that, I would much prefer advice that 1.) costs less to repair; and 2) doesn't involve tearing apart so much of the front of the motor!

Like any other Opti owner, I'm trying to blame the easier to replace parts. I have one or two other avenues to explore before I buy an Opti , plug wires and a water pump and put the car on jackstands for a couple of weeks:

Plug wires- I bought a tester that is a pain to use, but should tell me if I'm getting spark to all the plugs. I tested #2 a little while ago, and it checks out OK. The remainder will be a pain, but that will help narrow it down. If a plug is not firing, then the unburnt gas in the exhaust could be the source of the backfire.


Fuel injector system- heat and age have damaged the wires to the injectors. Could there be a short that is making one or more injectors fire at the wrong time? That could explain the backfiring.

Optispark- deteriorating with age. Highly likely, just a pain as noted above. And what's the best replacement? This car had a new opti at 50,000 miles ( according to Previous owner) and now it's pushing 150K.
It's time is likely up.
GM, Chandler, others??

Thanks

 

All this and no codes? Your SES light should be on and blinking. A scan would save you money and time.

For $60 you can get an ALDL USP cable and free download a scan program and you're all set. If you have a laptop that is.

 

That's making me scratch my head, too. I have a scanner (that's what made me change my oxygen sensors), but it shows no codes.
A few weeks ago I was getting Code 34- EGR, and a vacuum test of the EGR valve showed it was inoperative, so I replaced that device.
Now I have no codes according to my scanner.
I've been trying to bum a inductive timing light from my neighbors to avoid taking the plug wires off the plugs and using my newly purchased spark plug tester. If i don't find a timing light soon, I'll be elbow deep trying to see how many plugs are getting no spark.

If I get the cable and SW, will it tell me which plugs are misfiring? My AutoXray scanner can't do that. Remember, I have OBD1, not the newer technology OBD2.

Thanks!
Paul

 

Advanced Auto Parts loaned me the O2 socket kit. Only had to put down a deposit but it was refunded in whole when I brought it back couple days later.

 

Old school wire test is mist the wires in a dark spot, make some popcorn and look for blue lights.

If it's ignition there would be codes. Fuel maybe not. Not completely sure about OBD1. I wrastle OBDII.

 

MSD no doubt...

 

Before I saw your post I tested the plugs with the engine on. First I used an inline tester to get good news on 4, 6, and 8, but varying (therefore bad) results on 2. The other side was to hard to get to with the inline tester. So i got an inductive tester that clamps on the wire. Results on 7 were good, but 5 seems to be dead. 1 and 3 were too hard to get to without taking the fender liners off, so I plan to do those later. (mostly to verify how necessary the replacement is).

The absence of codes is strange.

But... I already know that 2 out of 6 plugs (NEW Plugs) are not firing consistently, so I'm trying to draw a conclusion from that.

I believe that a ten year old opti and 10 year old wires are the prime suspects- they might both be bad.

Conventional wisdom says that I might as well do the whole kit and kaboodle- Optispark, wires and waterpump.

Not my favorite use of money, but I think it gives me a 99% shot at solving the misfire problem. Any disagreeement with that??
I don't think it could be the coil, and I don't even know what the IC[B]M does (except destroy cities thousands of miles away!).

I'll put the car on jackstands this weekend, order the parts and start disassembling the fender liners, water pump, PS pump, etc.

Thanks to all for the diagnostic help and ideas. I'll let you know how it turns out. If it turns out the Opti is in great shape, and didnt need replacing, I'll sell it (my old one) on the Forum!
Stay tuned!
Paul