Random misfire injector code 18
 

Hey guys. I'm getting a somewhat random misfire two shops can't figure out. I'd love to look at it myself but I'm a T5 paraplegic and drive with hand controls so I'm relegated to giving the shops information at best. 95 z, OBD1. Both shops scanned it and came up with the code below. Shop one said the injectors looked good and said it was the opti. Actually had a warranty on the opti at shop 2 and they scanned it, saw the code, ohmed the injectors, and they ohmed good so they agreed it was the opti. Put two different opti's on it and seemed to make it "better" but not cure it. Shop two said to take it to a performance shop that might use aftermarket tools to dig into it further because they've gone as far as they can go considering it'sl OBD1, they'd have to just start throwing parts at it.
Car seems okay when driving around, can feel slight hiccup and sounds like slight spark knock which shop two said. At idle its *** and almost dies BUT not constantly. It will idle fine and then all of a sudden drop and come back up. It's not a constant misfire. It also pops out the exhaust at idle. I'm at my wits end... I've got a call into a performance shop to see what they think and if they think they can look at it. If its got the injector code but injectors seem fine, i guess maybe wiring? Does this somewhat "intermittent" misfire seem injector related? Any guidance would be appreciated.
https://cimg0.ibsrv.net/gimg/camarof...c259962c15.jpg

Not sure how much I can help you. Drivability issues can be some of the most complicated problems. This is why drivability technicians are some of the highest paid mechanics. It is not un common for shops to charge more per hour to fix these issues. Not being at the car can cause a complicated issue exponentially harder to figure out.

I have some information. First, what does random misfire mean? As your engine runs the computer closely monitors the engine RPMs. There are codes that define a cylinder. example miss fire in cylinder 3. The computer is watching the RPM and it detects the motor slows at the cylinder 3 position. Then you get a miss fire in cylinder 3 code. With random misfires the computer detects the motor slow down but it is not at the same cylinder each time.

A good scan tool will track miss fires based on a time frame. This would allow to see in the last 2 minutes cylinder 3 missed 143 time and cylinder 8 missed 64 times. The computer could report this as random misfire or give you 2 cylinders misfire errors if the rest of the cylinders have almost no misfires. Also the codes do not set because of a single miss fire. They are looking for a number in a certain time.

There is a very low chance that a bad injector would set a random error. Fuel pressure on the other had will. In fact there are many things that will set a random code, pretty much anything that triggers spark or can effect fuel flow across the whole engine. That is why I was surprised both mechanics lasered in on injectors. They would be a common case of a cylinder miss fire but not a random one.

Examples of things that can cause random misfires. Fuel pump pressure or regulator. A failing connector that causes intermittent connection. There are several parts inside the Opti that can cause it AKA most of the ignition system. Vacuum leaks, MAF sensor or MAP sensor, coolant temp sensor Stuck egr valve and a lot more. An injector can cause a random misfire but that mean that some of injectors failed at the exact same time. Dirt in the injectors could do it but a flow test is the only way to detect that.

The best way to find out what is causing the issue is a high end scan tool and the ability to understand what everything means. You then find somethings that are not reading right and track down the cause of each anomaly. A good scan tool will have a digital oscilloscope built right in that will let you watch the spark. It would easily find an opti issue because it is reading the secondary spark in the coil from the PCM. In some cases you may need to pull out a “real” oscilloscope that actually hooks to the coil.

DO NOT set off a parts Cannon. You would be amazed how many new parts are defective. As it stands now you probably have just one issue, if you load up with a bunch of new parts you could add a second issue. This can make getting the car running right a lot harder and more costly. You know as a tech if you have a drivability issue and a customer have a list of parts he has already tried it is going to be a long day.

For you the work is finding the right mechanic with the right tools. Normally I would say the dealer is your best chance but with a car this old there is a good chance the current techs have never worked on an old school LT1.

This guy does a pretty good job. Watch some of his videos and you can get a better idea about what I am talking about.
https://www.youtube.com/channel/UCn4...3wMT6VBzQoKPUA

Appreciate the info. I've got a message into a dealer to see what they think. I'm hoping I can find someone able to look at OBD1. I'm assuming it's probably just a single issue (hopefully), but who can say. I was told they ruled out vacuum leaks. It's a misfire like I've never had before. Normally it's a spark issue and it's constant. It feels weird at a stoplight that it's fine for 5 seconds, then almost dies, then kind of back and forth.