So this is kind of a weird one. Kind of a long post as well, for backstory's sake.

I developed a misfire last night down in London as myself and a group were doing some high RPM tunnel runs, catching video, and generally having a good time.

I typically fill up on base with the available 94 octane, but have used 91 before with no issues. Last night with about a quarter tank of the fuel I always run, I filled up off base with another quarter (total of 1/2 tank now) of 91 from a podunk looking station. Car drove well until I got back down around a quarter tank, and threw a CEL code.

Slightly after I noticed the code, I started picking up misfires that got progressively worse. I figured perhaps I got bad fuel from the bottom of that station's tanks or something, and seperation had caused water to float to the top. Getting back down to the bottom of the tank, I figured if that's the case, the pickup was now getting the ****ty fuel.

Stopped at a Shell, filled completely up with 94 V-Power, trying to dillute whatever supposed fuel I had left. Pulled away from the station, car sputtering a bit, and within about 5 miles, it started running perfectly and purring like a kitty down the highway. I figured the problem was solved at this point.

But, within a few, it started hiccuping again and missing fairly bad. With the Instant MPG's displayed on the DIC, where I would typically be seeing 28-29mpg (6th gear, 70mph), I'm down to about 12-14. Car is guzzling fuel, and the tail pipes are extremely sooty. This is still after burning through over an 8th of a tank of the new fuel. So the old stuff should be gone by now, I would figure.

I thought maybe I broke a valvespring at high RPM (7k+), but here's the weird part.... the misfire comes and goes. It will be very apparent for a couple minutes, then just after upshifting or downshifting, it will smooth out perfectly for 5-10 seconds, then come back. It does it at all RPM's. Change the RPM, it pretty much goes away during acceleration, but as soon as I level off the throttle and hold an RPM, the misfire returns badly.

I tried disconnecting each coil during idle, but with a lumpy *** camshaft (240 .608 on a 112 LSA), it's hard to tell if pulling a coil changes the idle at all, since it's so erratic anyway with the cam.

What do you guys think?

Sorry for the long post, btw. I just wanted to be clear in my description.

 

Have you tried to drain the tank completley and start with some new gas.

I would try this first, being the most non evasive way to start.

 

No, I haven't yet. I'm actually a little reluctant to, because that was a $130 tank of fuel. British exchange rate. And I think you meant to type "invasive" versus "evasive", .

Besides, I had under a 16th of a tank left of the old fuel, and after refilling with the new fuel, have burned through an 8th. It makes sense to me that the old fuel would've cycled through.

 

Check for trouble codes don't think it's fuel related. Check for loose plug wire at plug or mass air flow sensor connector. My car had a skip and Mech could not find with scan tool it was missing all over the place turns out I found # 8 plug wire not pushed on when I touched the wire it fell off.z51vett

 

Thanks for the suggestion, man. I definitely went through the plug wires to make sure all were seated. That was the very first option I tried. All are seated and on tight, at both ends.

Also pulled the Vararam off, no blockages, didn't suck anything up. Filter is mostly clean with the exception of a handful of flies, but all of them look like they've been there awhile. Vararam section upstream of the filter is clean and dry, MAF screen is clean, dry, and undamaged. MAF plug was properly seated and on tight, and it's little filaments arent broken or damaged as far as I can see through the screen.

When I started the car up just now and went to back it out of the driveway, I was watching the mirrors as I was backing out, and at the fist jab of throttle I got a pretty significant puff of black smoke. I couldnt reproduce it though, and driving up the block and back, the car is still acting the same. I didn't get any black smoke after I pulled off though at any other point. This leads me to believe that there was some puddled fuel in one of the cylinders.

Anyone think it's possible I lost an injector? Maybe one is just dumping fuel?

 

You need to pull the code with a code reader...here in the states you can get one from Walmart for $69 - there, I'm not so sure.

the code will get us on the right track to solving the issue.

Did your brake booster hose come off? Sounds like a massive vacuum leak to me.

Do you have a modded FAST intake? I've had trouble with my grafts coming loose and developing huge vacuum leaks which causes the computer to dump fuel and set a check engine light.

 

I do have a Fast. Two of them. A ported 90 and a ported 92. Neither of which are on the car .

I know, I know. I'm ashamed for not having a working code reader on hand. I had a nice one that's managed to evade me for the last couple years. I'm not sure if it was lent to someone and forgotten about, or what. But it hasnt turned up. I simply don't understand why GM eliminated the code-check feature that the C5 was so well loved for.

Here's a curveball, though...

Just took the car out again after pulling and reseating all plugs and connectors. Essentially every single sensor I could get my hands on under the hood was removed, hit with compressed air, and reinstalled. I took the car up the road to pick up takeout, in the naive hope that maybe that'd change something.

Got about a few hundred feet up the road to the Fish and Chips joint, and the car was still missing the whole way. Hop back in the car to come home, and it fires up, idles right, and only misses noticeably about twice in the half-mile to my house. It sat in the driveway just now for a good 20 seconds idling like a champ.

Unfortunately, in consideration for my neighbors (I live in a cul de sac of brick homes), I can't leave it to idle too long in the driveway or else the police will be called. Lol. But yeah, it's running perfect as of this second.


W.

T.

F.

 

Stab in the dark........you dont have a code reader.......try pulling the "ECM" fuse for a while then starting it back up.........

 

Disco'd the battery for a few hours, let it sit, took it out on some errands today, and it drove perfectly, threw no codes, nada.

I'm really thinking that I just got crap fuel, and it finally cycled itself though the engine. It seems to be running as good as ever now with a full tank of Shell 99 (British 99 is equivalent to about 94.5 in the US), and that's on a Florida 93 octane tune. As of this second, I'm assuming the car has sorted itself out. I will probably change the plugs though, as I'm guessing they're going to be pretty black about now.

 

Try putting in new spark plugs. If any of yours had a hairline crack it would do the same thing.

Or if you want to be daring pull your fuel rail out of your intake manifold and put the car into the run position to prime the fuel pump. If any of them are spraying its because they are stuck open and flooding a cylinder. Although with the L92 injectors it would be ridiculously profound...

 

I would suggest you stop driving the car until you read the CEL codes.

 

There are no remaining CEL codes. Car's running perfectly.