Corvette Wild

Hello fellow forum friends, I wanted to see if I could some advice on where to go next. I have '98 M6 with no under the hood mods, and since I bought it, almost three years ago, it has had a rough idle, but never any check engine lights. After getting the car, I went through everything that I know which could cause a rough idle (plugs, wires, injectors rebuilt & balanced) and this made improvements on many levels however it still has that slight misfire that I can feel sitting at idle. When driving, I feel no issues...no misfires, no hesitation, plenty of power. I let the misfire thing go for a while while I did other work on the car but now I want to get back at diagnosing this misfire. Misfire is enough at idle that I can see the engine shake and can it inside the cabin but it is still not enough to light the CEL; also, the car went through smog check a couple of months ago and passed with flying colors. Here is what I have found and tried up to this point:
1) I took a timing light to each cylinder while idling, about half the cylinders you can see interruptions in the light pulses, the other half pulse perfectly. When I say half, it is not all one bank that fires good or bad, it is all mixed.
2) Swapped coils between one good cylinder and one misfiring cylinder, problem did not follow.
3) Swapped plug wires between one good cylinder and one misfiring cylinder, problem did not follow.
4) Checked voltages at coil connectors between one good cylinder and one misfiring cylinder, voltages are right in line with battery voltages.

Anyone have a though on what else to check? Does anyone know if the signals sent from the computer to each cylinder to fire the coil can been seen on either a Foxwell NT301 scanner or a clone Tech 2? I would like to look at the ignition control signals going to each cylinder to see if they are steady or not but don't have an oscilloscope to read right at the coils; if it comes down to it I may have to get one.

Any insight would be appreciated. Thanks.

Bill Curlee

My very first thoughts are VACUUM LEAKS! Do you have a scanner or tool that you can read your LONG TERM & SHORT TERM Fuel Trims?

First things first,

1. Disassemble the complete air filter/bridge assy. Clean the MAF, Clean the Throttle Body and reassemble being careful to make SURE that there are NO unmetered air leaks between the MAF and TB.
2. Your PCV system is SEVERLY SUSPECT!!! . It lives behind the intake manifold and it is very well known to get very brittle and CRACK. That allows (you guessed it) Unmetered air into the manifold. If it were ME, I would follow every inch of that PCV plumbing and see if there is any cracked / damaged or worn plastic/rubber PCV tubing. Try this. Remove the Oil Fill Cap. Put a 3 X 5 Card over the oil fill opening. You SHOULD get a very slight VACUUM that sucks the card down to the oil fill spout opening. If gasses are being blown out, the PCV System is NOT working correctly.
Mis fires are seen by looking at the Check Engine Light. It FLASHes during a misfire event. You should also be able to see a P-0300 DTC if you read the DTCs BEFORE you turn the ignition OFF! Once you cycle the ignition, the misfire counter resets and the DTC erases.

Corvette Wild

Hi Bill, thanks for the reply. I have had a vacuum gauge on the car when I was first looking into the rough idle issue and the vacuum is in the green on the gauge and rock steady. Also, when snapping the throttle the needle on the vacuum gauge snaps right back into the green without any hesitation whatsoever. Also, as part of the original tune up, I did replace the PCV valve and the tubing from the valve cover to the PCV valve to connection at the throttle body looked good. However, I will try the card on the oil fill tube, I am interested to see that result but I think it will be okay based on my vacuum gauge. I don't have DTCs and the check engine has never flashed however I will hook up the scanner again while the engine is idling to see if there may be a pending P0300.

I do appreciate the tips but I don't see a vacuum leak causing the firing of the coil to "skip a beat" (so to speak) on the timing light.

Bill Curlee

Examine your LTFT & STFT to see if they are in an acceptable range.

C5MSG2004Vert

You need a scanner that can read the mode 6 data. It will tell you the total misfire counts per cylinder. At least you will be able to tell if it's truly random or the same cylinders.

Bill Curlee

A good scanner will also tell you what the Short Term and Long Term fuel trims are doing. Very important data if the engine is not idling or running correctly

Misfire data is also very important data as C5MSG2004Vert stated. It will allow you to see what cylinders are not happy. .

Go to your scanned LIVE DATA and select ALL PIDS and examine all the data that it provides. You will learn a LOT about your engine operating parameters and readings. Its a little confusing at first but the more that you use it/ ask questions and learn what it can tell you, the more useful of a tool it will be.

Corvette Wild

Sounds like I will need to do some playing with my scanner when I get some time. Thanks for the suggestions.

Corvette Wild

I finally got a chance to get my scanner on it today and look at some data. Unfortunately, this scanner will not allow me to see the misfire data, not sure why because I could see it with this scanner on my 370Z. Anyway, I don't know a lot about fuel trims but from the research I have done, these fuel trims don't seem out of line. Before I looked at this data, I warmed up the car good, took it for a couple of good rides, one easy drive and one "fun" drive; after that sitting in my driveway idling, this is the data I see on the fuel trims:Bank 1 STFT: -0.8 to 1.6

Back 2 STFT: -0.8 to 1.6

Bank 1 LTFT: -1.6

Bank 2 LTFT : 0.8

Will the clone Tech 2 allow me to see the misfire data? The misfire monitor is complete, as well as all the other monitors.

Thanks.

C5MSG2004Vert

The clone tech2 or a vcx nano will tell you the misfire data. What obd2 scanner do you have? I'm supprised it reads misfire data on one vehicle, but not the corvette. Do you select the vehicle or does it automatically detect what vehicle its connected to?

Corvette Wild

The scanner I have is a Foxwell NT301. I did have to select Chevrolet for some of the on-board tests. If the misfire monitor data is there somewhere, I may be overlooking it but I don't think so. I will have to scan back through the manual for the scanner one more time to make sure I am not missing something.

Corvette Wild

My scanner is a Foxwell NT301. I was a bit surprised I could not find the misfire data either as it was really simple to see on the 370. I did have to select the type of vehicle for some of the on-boards tests. I think I will browse through the scanner's manual one more to make sure I am not overlooking something.

To add onto this, the on-board component test menu changes for a CAN system vs a non-CAN system. The 370Z is a CAN system, the Corvette I guess is not so that it why i cannot see the misfire data on this scanner on this car.

redzg

Is it possible that you could be expecting a smoother idle than a proper LS1 is capable of? I’m just wondering if the Corvette cam is just aggressive enough to feel a bit more than other applications.
If there are any car gatherings in your area I would not think it would be difficult to observe another car at idle. Heck, I know of two other C5s in my neighborhood, and have yet to find a fellow owner who was not delighted to talk about his car.

Corvette Wild

I guess this is possible, so far everything I looked at is normal other than the intermittent drop out of the pulse from the timing light on about half the cylinders. Comparing to another C5 might be a good next step.

Bill Curlee

If one bank is dropping out the timing light and you are SURE its not the timing light issue, you need to check power and ground to the bad bank. Each Cylinder BANK has its own Power and Ground to the coils. Power is the PINK WIRE and Ground is the BLACK WIRE/WHITE STRIPE. With the Ignition ON/RUN (engine off) Examine the power to each coil bank. Measure it with a meter and put a LOAD on the power wire (incandescent lamp) and see if the circuit can properly support the load. The grounds for the coils are secured to the rear of the drivers cylinder head. You also need to check the bank connectors and wiring to see if there are poor/corroded/damaged pins in the connectors.

THIS POST has ALL of the Coil power and ground schematics in it. Check it out. Measure the actual under LOAD voltages at the FUSE for each bank and insure it had FULL battery voltage under load. You can measure the two test pins on top of each fuse to chassis ground to test for proper voltage from the ignition powered B+ Supply. If you don't have full battery voltage supplying both banks, the engine will NOT run/perform properly.

Main voltage loss to coils??? - CorvetteForum - Chevrolet Corvette Forum Discussion

Disconnect the bank main plug and test the BLACK wire (ground ) with an OHM Meter. Again, the GROUND is attached to the back of the drivers side cylinder head (G-107) You should see little to no resistance if you read the connector Ground wire to chassis ground or cylinder head ground.

Bill

Corvette Wild

I have checked voltage on one cylinder that showed no misfires (via timing light) and another cylinder that did show misfire and the voltages were the same. I will plan to check voltage at each coil, with and without a load, and document that. To be clear, it is not one bank that is good and one bank misfiring, I have good and bad cylinders on both banks. What is the easiest way to see the ground on the back of the cylinder head? From underneath the car??

In the meantime I will read up on this post and get to work. Thanks.

Corvette Wild

With the wife out of town, I was finally able get some time to take a look at this. Here is the data I took:
Voltage measured directly at the battery: 12.33
Coil Voltages with no load: #1 - 12.23, #2 - 12.30, #3 - 12.22. #4 - 12.30, #5 - 12.23, #6 - 12.30, #7 - 12.22, #8 - 12.30
Coil Voltage with a load (12vdc fan): #1 - 12.16, #2 - 12.23, #3 - 12.16, #4 - 12.22, #5 - 12.16, #6 - 12.22, #7 - 12.16, #8 - 12.22
Ground wire resistance (one lead connected directly to battery negative cable): #1 - 0.4, #2 - 0.5, #3 - 0.4, #4 - 0.5, #5 - 0.4, #6 - 0.4, #7 - 0.4, #8 - 0.4
Fuse Voltages: Fuse 22 engine not running - 12.14, engine running - 14.57......Fuse 18 engine not running - 12.14, engine running - 14.56

I was trying to see the ignition ground on the back of the head but my wi-fi endoscope was not cooperating and I don't know how else to see it. As soon as I get the endoscope working again, I will take a look.

The other interesting thing is now that I have had the ignition coils harnesses completely disconnected for testing and then reconnected everything, the engine is idling smoother, although misfires are still there but when I went across all cylinders with the timing light now only three cylinders are showing a skip in the light whereas before I started, 5 cylinders were showing this skip in the light. Skips in the timing light pulses are showing on cylinders 4, 6 and 8. I am wondering if those coil harnesses are the problem.

Tron Z

^ This
I would compare the car to a similar specimen before going deeper. I purchased my Z06 new and it has exhibited a noticeable shake at idle since the day I took delivery. It is coming up on its 21st birthday in April and it has never set the MIL for a misfire, or any other engine issue

Note also that old / failing engine mounts could increase the NVH that is being transmitted into the passenger compartment.

Best of luck getting to the bottom of this dilemma!

Bill Curlee

YEP, Agree~! If you have misfire concerns, find a Scanner/code reader that is able to read/display real time engine data and read the Cylinder Misfire data. See what cylinders are showing misfire counts and how many. ONE BANK misfires are easier than random all bank misfires. BC

grinder11

OP says he bought the car used 3 years ago, and says it has had a misfire at idle since he bought it. He also says it has had no mods. My question would be; is he certain there have been no mods, like a mild cam? OP, I ask this because I have a Diablosport handheld tuner that reads engine functions in real time. On one set of parameters, it states random, multi cylinder misfire, cause unknown. I do have a cam, and no MIL lights. But it has been tuned well, and the tuner set things so no codes are thrown for certain things. It could be the PO had a mild cam installed, but now its starting to throw codes. Just asking.......

C5MSG2004Vert

Since you have apparently fixed a cylinder that was missing by messing with the harness, maybe there is a connection issue. As Grinder11 stated if you can monitor live data, that will show when a cylinder is missing and confirm what your timing light is showing. The reason that you don't have a check engine light is that unless there are enough misfires to damage the CAT or there's enough of them in a given time, the PCM will ignore them and a MIL will not be set.