Poppy's Vette

Hi All, any advice or ideas appreciated.

Left Thursday on a road trip in the vette. The car seemed to be running fine but I noticed I was only 100 miles from home and the fuel gauge was already getting close to half! I checked the DIC and it said I was getting 15 mpg! WTF! My car has never been one of those 30 mpg cars but I always can count on 25 or 26 at the worst. This was on a fresh tank of Chevron Supreme. The car has 34k and mods in my sig, and driving on the cruise control at 68 mph. But the point is it has NEVER sucked gas like this. I even stopped to check to see if I had a leak somewhere.

Anyway, I stopped at 160 miles and refueled with Shell. I hand calculated the mileage and it was a whopping 14.90 mpg. I also noticed my tail pipes and even the back of the car were heavily coated in black soot. I've seen that on the track but not so on the street and again, never this bad. The second tank averaged a little better at 19.80 but NOTHING like it should obviously.

All this time the car seemed to be running fine with no check engine lights. On the way home today I finally did get a check engine light and I also started noticing a mild bucking or surging and would have to down shift in situations the car used to be fine in (not real, real bad and it may have been my paranoia but it felt that way). 2 more tanks continued to average 18-19 mpg and tons of black soot.

At first I thought it might be a bad tank of gas but I have run 3 tanks through it since and no real improvement.

Any ideas or suggestions?

PS. the car is out of warranty

bearphoto

Since you did get a check engine light you can go to one of the auto parts stores and have them read the code for you. I'm not an expert on fuel injection so no advice here.

Initially I thought perhaps you might not have filled the tank all the way but that does not seem to be the issue here. Keep us posted!

Poppy's Vette

I do have access to a code reader and will do that this week but was hoping to get a head start on ideas as I know there are some really sharp folks here.

A few weeks back I got a check engine light and drove by my dealership and he read a nondescript "misfire" code and cleared it for me. I wonder if that was the beginning of my current problems? I need to look at plugs and wires but that seems to be crazy at only 34,000 miles, no?

bearphoto

34k miles is way low for that kind of issue. Your car really looks nice.

I'd agree on some kind of injector issue rather than a plug or wire but who knows.

Yes there are lots of experts on here, hopefully someone will chime in.

In the meantime you can pull the plugs, maybe identify which cylinder(s) is causing the problem.

Dave68

Wait, can't you look at codes on the digital display? I don't have a C6 but I can look at current and history codes on my C5's display. Once you have the code(s) you can look up what it means by doing a quick search.
Back in the old days, I'd suspect a loose spark plug, spark plug wire, or satuarated carburator float. I agree that you should look at the wires, first. Make sure that each is securely plugged into its spark plug.

One Nut

O2 sensor ('s) my guess. Is it sooting from one side or both evenly.

Poppy's Vette

Dave68, for whatever brilliant reason GM discontinued that function in the C6. I do have access to a code reader and will check it this week.

One Nut, it appears to be both sides evenly.

Dave68

Poppy, I guess their thought was that if you can't query the car for the fault code, you'll bring it to the dealership.....

Dave

stano

Time to upgrade

Seriously, hope you get it fixed soon Paul

Stano

stano

If it is the injectors, have RC blueprint them. I did on my '06 Z and was happy with the results. Surprised by the results. Cost was not bad.

Good luck Paul

Poppy's Vette

Thanks Stan. I may see you in a few weeks if you are going to be in the restaurant on the 24th! We'll be passing through and spending the night on our way north for some fishing. We'll be eating there in any case if your not there, but would love to say hey!

cv67

my mileage went in the toilet when the 02s got funky on me.

OP really need to pull codes or have it scanned all we can do is guess.

Varmit

do the simplest of things first, check the sensor on the MAF to see if it had somehow unplugged. My 99 did that from time to time. If not it is probably an 02 sensor. Perhaps the wire to the 02 sensor has found the tender touch of you long tubes!!!!!!!!!!!!!

stano

I'll try to make it up there if I can Paul. My daughter and grand kids are here from Japan.

What time do you think you'll be rolling into town?

Stano

Poppy's Vette

I'll definitely check those Benny. Thanks

Poppy's Vette

Well don't worry about me Stano, it will be late in the day, dinner time. That should be great to see your daughter and grand kids for sure. We're meeting ours at Convict Lake for a week!

Really, don't worry about us. I need to get into to town to get a wilderness pass and spend the night so I hope to be there early afternoon. I'll tell your staff to say hi!

Poppy's Vette

I have been away for 4 weeks and just got home so spent the day on the car.

- I started by pulling all the plugs and wires to inspect. They all look fine, no abnormal coloring or wear. The were all gapped around 0.38 (manual says 0.40 so that's fine).

- All the wires were in excellent condition and all were seated well.

- I checked the MAF and it was plugged in.

- I inspected under the car and all the O2 wires were in excellent condition and still neatly managed out of harms way.

- I scanned the ECM and did find a PO300 DTC which is "Engine Misfire Detected" (can't get much more non-descript than that!) This is the same code the dealer cleared a couple of months ago for me (although that was really only a couple hundred miles ago).

- I used the scan tool to monitor some engine functions. In closed loop the O2s seem to be working fine. I was able to graph the front O2s and watch them at the same time and they were tracing like I would expect from them and the same as each other so no smoking gun there that I can see. I looked at some other parameters but everything seemed normal or at least nothing stood out as puzzling.

I also put a fuel pressure gauge on it to verify I have the proper fuel pressure, which I do.

So I'm at a loss as to what to do next. I hate to take it to the dealer especially since it has minor mods and the PCM has been modified. I suspect they will just blame the problem on that even though it's been running fine for years with the current mods and tune. I have a full set of GM Service Manuals and have gone through the trouble shooting for poor fuel economy and

I have placed a call to CMS but haven't heard back yet and I'm not sure if they primarily work on performance upgrades rather than basic diagnosis issues. I do look forward to hearing from them to see if they can help.

Any other thoughts based on this information?

TIA

FamousRR

Clogged cats?

Poppy's Vette

Thought of that. They are high flow CATS that came with the headers and probably have 20k miles or so. Seems like a stretch and if they (or one) is clogged, what caused it?

I did check them on the scanner to see if they came up to temp and if the system went into closed loop and it did. The CATs came up to 1500 deg F.

I'm thinking I may just put the entire exhaust back to stock (maybe with the exception of the Corsa's. I've been thinking about doing that anyway. If the problem goes away then it was the CATs. If not, I'm back where I started.

Streetk14

My experience with aftermarket cats is that they aren't of OEM quality. At 20k miles, you could have a problem. What's been done to the car for mods?

Also, the cats have nothing to do with whether it goes into closed-loop or not. If you have long tube headers (sounds like you do), then your catalyst efficency codes are probably turned off with tuning software. High-flow cats don't work well enough to keep those codes active -- or if you do, you'll likely be getting check engine lights on a regular basis since the rear 02 sensors aren't seeing what they want.

I also believe the cat temp status is calculated, not an actual reading. Probably not super accurate with headers and cats that are much further downstream.


PS : Is your intake manifold a ported stock unit or a FAST?