ejb55

I bought my first Corvette 4 weeks ago tomorrow. Its a 1998 Coupe Nassau Blue with a M6. Had 24,770 original miles. It has the BBK cold air intake, BBK stainless headers with the Mid America exhaust, and MSD coils and wires. O.K. the problem is this. The first 800 miles was flawless, not a hiccup. The last 300 or so it has given me some flashing engine and it never stays on. It will clean itself up and run 100% on its own. I took it to a small shop that some friends have used and he hooked it up to a scanner. He said its a Random Misfire Code. Apparently that involves 2 or more cylinders? I did some research last night after installing a new set of Delco plugs per factory part number. Which did not solve it. It has got worse in the last week. I do smell raw fuel so some degree when it throws the code. I did research last night on the P0300 code and it talked about low fuel pressure, vacuum leak, and a possible leaky EGR valve.

Thanks in advance. I am in the St. Petersburg, Fl. Area...

enoniam

When you say "it never stays on" are you saying it won't idle?

Not knowing the answer to my question, my initial guess is that there's a vacuum leak downstream of the MAF.

ejb55

Idles fine. Just runs rough like a dropped cylinder. Today it started out rough. 15 miles. Drove a total of 100 miles and it ran flawlessly for 85 miles right up until I parked it tonight. I really don't get it.. Light does not stay on. Very Random and cleans itself up...

enoniam

I'm thinking it's something electrical - an issue with a sensor or some wiring. It would be helpful if you had some means to monitor front o2 sensors, MAF, IAT and ECT signals. I'm guessing one of them is "off" when the car is acting up. Since your shop didn't see any other codes than the misfire code I'm leaning towards IAT (intake air temp) or ECT (engine coolant temp). Have you ever looked much at your gauges, specifically the coolant temp, when the car wasn't running well?

ejb55

I have watched all the gauges. I do not see anything off. Coolant temp. has never changed once its fully warmed up. I almost took it to GM today. I had a appt. for a diagnostic. I called and cancelled this a.m. since I ran so good yesterday. I am sure it will resurface but it needs to be seen preferably when someone can see and hear the misfire.

tblu92

A random misfire code IS a misfire---The only exception is if you have an automatic with a stall converter----Stall converters create 'false misfire codes"
Being that your car is a M6 I suspect a real misfire---Plus the smell of fuel indicates a cylinder not firing----
Most common issues would be wires- or plugs---I would make sure all your plug gaps are around .040--It is common to bend a gap closed during installation of new plugs--I have seen this many many times-
Then aftermarket wires--Some of them especially MSD's the coil boot is so tight that they pop off after awhile--Make sure on the coils that you hear them "click on" when you install them---
I am assuming you can actually smell or hear a misfire----and it runs rough at idle or stumbles on acceleration----If not-let us know and we can go from there----1st thing to do is to identify if the misfire is real or false !!!

ejb55

I am certain of a misfire. I can feel it even when I do not have a flashing engine light. No question of that. I have checked my plug wires and installed a new set of pre gapped better than a double platinum per GM specs. Looked for any wiring issues. I will say the MSD wires are tough to work with as far as the boots. I did not install them as they were on the car when I purchased it. All seems good, but something still is not right. If its a Random Misfire does it involve more than one cylinder? It sounds like it would. If that's the case a bad coil or a bad connection would only involve one cylinder. I took it to GM today with a 1:30 appt. which I did not want to do but I am frustrated. When I got there they could not even get to it today and they had no loaners. I did not have anybody to chase me so I brought it home...

tblu92

Ok so if you have a real misfire--Ignition related--- It can only be a few things---Plugs---Wires---Coils----Or coil primary wiring (pigtails)
It is rare to have a Misfire that is fuel related but not impossible--
Can be from a dirty or fouled injector--A failing fuel injector--Or loss of fuel flow and/or pressure---
The only other thing that is again rare and hard to diagnose---Is a "Broken" valve spring------Which does not have a code by itself--But the vibrations from a broken spring simulates a misfire--hence the code---
Hate to tell you again---But I would take out the plugs again--and look at them closely with a light and magnifying glass to see which one looks the worst and seems to be mis-firing---
Then you can switch the plugs and wires around to see if it goes away--If NOT the try switching injector locations to see if it goes away--If again no---what ever side left or right that you find the suspicious plug ---You take the valve cover OFF and inspect the rockers and springs

enoniam

I doubt that it is anything mechanical as I don't think it would come and go as you describe.

You can get additional info through the OBDII interface on which cylinders the PCM thinks are misfiring. You will often get the PO300 general misfire code instead of a specific cylinder code even if the problem is with a single cylinder because the PCM's ability to accurately identify the correct cylinder to a misfire is far from 100%. If you can read some misfire counts on the cylinders the one with the most is probably the one causing you problems. You'll probably also see a good deal of misfire counts on the cylinders firing immediately before and after the one with the high count.

I just reread your initial post and this time "MSD coils" jumped out at me. For most applications the stock ones are just fine, and probably more reliable than the MSDs. So you may have a problem with one of those coils. On the other hand someone may have replaced the stock coils due to a perceived problem with at least one of them when in fact the problem was with the wiring connected to one of them.

tblu92

I agree---The stock coils perform perfectly---I have seen a few "bad coils come out of MSD-----The early ones were prone to failures---
A scanner or tuning software can isolate which cylinder is mis firing-Switching coil locations can determine if the coil is bad--If the misfire FOLLOWS the coil change--then it's a bad coil----( But I still think it's your wires --!!!!! )