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Hi all, I have noticed now that the temperature is starting to drop (New England), my C5 (2002 MN6) will run very rough until the engine coolant temp gets to around 100 degrees F.
It will register no codes in the PCM nor a check engine light. I purchased the car in June and have put about 2,500 miles on it and this has not happened until now. Was thinking of changing the plugs and wires as it is close to 100k miles but I feel like it is something else.
Sounds like the ECT sensor may be skewed at cold startup…after the car sits overnight look at the coolant temp with the key on…the coolant temp should be very close to the outside air temp…if the PCM thinks the engine is warm at startup it’s like having no choke..the engine will run rough !!
Sounds like the ECT sensor may be skewed at cold startup…after the car sits overnight look at the coolant temp with the key on…the coolant temp should be very close to the outside air temp…if the PCM thinks the engine is warm at startup it’s like having no choke..the engine will run rough !!
Thank you for the response! Will check in the morning and report back.
I let it idle at work a bit and got check engine light with P0300 in the DIC.
Since the ECT may not be working correctly your fuel injectors may be not suppling enough fuel to each cylinder so that may trigger a “lean misfire” !!
Since the ECT may not be working correctly your fuel injectors may be not suppling enough fuel to each cylinder so that may trigger a “lean misfire” !!
Started the Corvette this morning and the coolant temp reading was within a degree of the real ambient temp.
I had read that intake manifold gaskets can cause this as they can let air in if cold and once the engine expands due to heat it seals it up. Going to try and pop off the fuel rail covers after work today and maybe spray with brake parts cleaner?
Has the intake manifold been off recently ??…if you have a scan tool this can be easily diagnosed…if it is a vacuum leak your long term fuel trims will set the check engine light (P0171/0174) once they reach 23%…a P0300 is all that shows now ??
Has the intake manifold been off recently ??…if you have a scan tool this can be easily diagnosed…if it is a vacuum leak your long term fuel trims will set the check engine light (P0171/0174) once they reach 23%…a P0300 is all that shows now ??
Thank you for the quick responses! Yes, P0300 was the only thing showing on the DIC with an H.
Intake manifold has not been off during my ownership. I will hook up a scan tool to it later and see what the trim is.
Think cleaning the MAF and adding a bottle of Techron may help? (I know shooting in the dark). PO had a gravel driveway.
Edit: I have changed the air filter on the car and it was very dirty but this was about 2 months ago. Replaced with WIX
Anything upstream of the MAF sensor will not affect the fuel trims…if you had your intake bellows after the MAF sensor was removed I’d look there…if your long term fuel trims look elevated (over 10%) raise the RPM to 3000 for a few minutes and see if the long terms slowly decrease…the short terms will quickly decrease…that is a vacuum leak !!…if dirty MAF or a fuel delivery issue the fuel trims will INCREASE as the RPM is increased…you may have to check this when the engine is cold.Let us know what your fuel trims look like when you start the car.
Anything upstream of the MAF sensor will not affect the fuel trims…if you had your intake bellows after the MAF sensor was removed I’d look there…if your long term fuel trims look elevated (over 10%) raise the RPM to 3000 for a few minutes and see if the long terms slowly decrease…the short terms will quickly decrease…that is a vacuum leak !!…if dirty MAF or a fuel delivery issue the fuel trims will INCREASE as the RPM is increased…you may have to check this when the engine is cold.Let us know what your fuel trims look like when you start the car.
So I had the scanner on the car and the data screen froze while it was misfiring so I wasn’t able to get LT or ST trims but I got these photos.
Anything upstream of the MAF sensor will not affect the fuel trims…if you had your intake bellows after the MAF sensor was removed I’d look there…if your long term fuel trims look elevated (over 10%) raise the RPM to 3000 for a few minutes and see if the long terms slowly decrease…the short terms will quickly decrease…that is a vacuum leak !!…if dirty MAF or a fuel delivery issue the fuel trims will INCREASE as the RPM is increased…you may have to check this when the engine is cold.Let us know what your fuel trims look like when you start the car.
Tried to post some attachment photos but for some reason they wouldn't go through.
Got the car to misfire and throw the p0300 code again but the live data froze on the scanner. ST trim1 and ST trim 2 were at 24 at one point. Does this signify vacuum leak?
Noticed the misfire would get worse if I blipped the throttle and would almost stall the car.
Your fuel trims look good. I would look at the cylinder misfire counters and see which cylinder is misfiring. P0300 is for a random misfire which is usually ignition related rather than fuel. I doubt if you have a vacuum leak but maybe a bad coil, wire or plug.
A Snap On scanner froze ??…unheard of…LOL !!…short terms are what they are reacting to the O2 sensors at the present moment…short and long terms should not be more than plus or minus 10%…you will not see a P0171/0174 until the long terms hit 23%…raise the idle to 3K and see if those short terms quickly go towards 0…injector pulse width at idle should be around 2.5 milliseconds…you show about 3.2 so the PCM is adding more fuel…look at freeze frame data and it will show what the engine was doing when the P0300 set…misfires are a little more difficult to disgnose !!…you can try spraying that Brake Kleen around the intake runners while looking at the short terms after the car is started in the morning and see if the short terms go negative that signifies a vacuum leak.
A Snap On scanner froze ??…unheard of…LOL !!…short terms are what they are reacting to the O2 sensors at the present moment…short and long terms should not be more than plus or minus 10%…you will not see a P0171/0174 until the long terms hit 23%…raise the idle to 3K and see if those short terms quickly go towards 0…injector pulse width at idle should be around 2.5 milliseconds…you show about 3.2 so the PCM is adding more fuel…look at freeze frame data and it will show what the engine was doing when the P0300 set…misfires are a little more difficult to disgnose !!…you can try spraying that Brake Kleen around the intake runners while looking at the short terms after the car is started in the morning and see if the short terms go negative that signifies a vacuum leak.
Was able to look further into the snap on scanner after it came back to life lol.
Cyl 2 showed misfires 80 something times and no other cyl showed misfiring so I’ll be changing the plugs, wires, and cyl 2 coil as well. I’ll report back this weekend or next week on what happens after.
I have a set of AC delco plugs and GM performance wires waiting to be installed and I’ll pick up an OEM coil pack. Never done plugs on one of these before and I’ve heard #7 and #8 plug are fun…. #5 doesn’t look too fun either with the air system.
Happy thanksgiving and thank you all for the responses I really appreciate it. This forum has been great.
Looking at fuel trims can help diagnose a misfire without using specialized equipment like a scope…an injector misfire will show higher fuel trims than ignition related since you have a lot of excess oxygen in the cylinder.
Since this is your first time changing plugs in a C5, this might be helpful.
To start, you probably should checkout youtube for - < How To Change Spark Plugs And Wires On A Corvette !!!!> but view it with a grain of salt… (the video makes it look easy to remove the boots and plugs, this is not even remotely true)!!!
I changed the plugs on my C5, first time ever in a C5. It was a PITA. My advice is use a pair of offset needle nose pliers as short as possible because you ain't got a lot of room to move around (short vice grips may also be an option but I never tried them). Do the plug with the most room first, the easiest, it will give you the confidence you may need for the plugs against the firewall which are a bear. Don't try to save the wires. Try to rotate the spark plug boot 180* to break it loose from the plug. At plug #7, I covered all sharp areas with layers of duct tape after I donated about a pint of blood to my Vette. I did manage to change all the plugs and wires, took me 7 hours for the first 4 plugs and 1.5 hours for the last 4 - Alley Oop September 2021.
These are the Corvette Forum posts that saved my sorry ****. I edited some of the posts for brevity. Here are a variety of approaches…
While I do have a C5, I've not yet gotten into the spark plugs on it. But today I started to change the plugs on my daughter's 2004 Grand Prix GTP, and ran into the same issue, same type of connectors. I finally got all six off without breaking them, but it was not easy. Here was my approach, which uses some of what was discussed above:
1. Get the heat shield loose from the boot. It is held down by a spring type device. With your hand, rotate the shield clockwise as if you were tightening it (because the wound spring is like a left hand thread). This should break the shield loose from the engine and the boot.
2. Use a large pliers, set it to allow a big opening. Pull the heat shield back down toward the engine and clamp the big teeth of the pliers on the rubber boot, between the large molded hex on the boot and the metal heat shield.
3. As said before by others, wiggle and manipulate the boot with the pliers. If possible, rotate the boot 90 degrees in one direction, then 90 degrees in the other. I suspect that is what the rubber molded hex is for, but in some locations on the engine, I couldn't turn it with my hand, but could with the pliers.
4. While keeping the pliers tight on the boot, use the other hand to pull outward on the pliers hard while wiggling the boot with the pliers.
5. Here is the tough part. Keep doing #4 until the boot finally comes loose and pops off. It can take several minutes or more of putting heavy force on it. It is tiring and can cause parts of your body to ache intensely. Especially when laying across the top of a transverse V6 to get to the back row of plugs (much harder than the Vette). But eventually it will come loose.
6. There was one plug on the GTP where there was not enough room to get the second hand on the pliers. Eventually I took a 2 1/2 foot long 3/8 inch round rod, and got it behind the pliers and pried the pliers away from the engine while keeping the pliers clamped on the boot. It popped right off.
If I can get all six off the GTP using this technique on boots that had never been removed in 100,000 miles, any Vette should be a cinch with this method. But you have to be persistent and confident that it will eventually come loose. Don't give up
I just did this job on our 45K mile 2000. One thing I have not seen mentioned in this thread that may help somewhat is to pull the wires off the coil packs first. Grab the top boot and twist it both ways until it breaks loose it will then easily pull off the coil pack. Now grab the heat shield around the lower boot and turn it from side-to-side. Pull only on the heat shield or the boot, I guarantee if you pull on the wire you will break at least one of them, probably more.
Many people have tried to help you (in their own inimitable ways), so without being a "smart-aleck" I will tell you the awful truth: You would be foolish to attempt this project without knowing you want to change the plugs and wires. This is not a project you want to be doing just for practice. You will sweat, you will curse, you will bleed - make it worthwhile and change the damn plugs and wires.
Hardest part, getting the old wire boots off the plugs. After doing one side, reading about the angled needle nose - I used that idea on the other side... gripping the heat shield with the angled needle nose and along with tugging on the wire boot itself, they popped right off. No room for me to twist and turn them on the plug to free them up, so the pliers helped me a lot.
I used small, angled needle nose pliers to grab the edge of the metal shields. You could try taping the pliers if you're concerned about marking the shields. Just be patient and work at it.... the only wires I destroyed met their demise when I was really good and aggravated.
try using a light coat of dielectric grease on the plug tip and ceramic. Since I started doing this, I have never had a problem with sticking wires.
I replaced my wires and plugs on my 2002 Z06 a few months ago at 32K miles.
The wires were indeed a PITA- even with a special plug wire puller. Lot's of twisting and pulling and choice English language helped. Several of the wires pulled apart - one boot almost got shredded.
Once the wires were off removing the 8 year old spark plugs was a piece of cake. New GM red wires installed with the dielectric grease stuff to hopefully help the new wires not stick as hard
I used pliers designed to pull the boots. Grab the boot low, squeeze hard, twist, pull. Heat shields, wire/boots, were all removed undamaged. This was when they had 50k+ miles on them and been on since built, which made them 10 years old at that time.
With the metal sleeves on, I found it was really easy to pull the sleeve and the boot as one.......
A tool I used was a needle-nose vise grip. Get it in between the boot and grab the metal insulators and pull. You'll still tear up a couple wires but they will come off.
I found a pair of long-handle needle nose pliers in the $4 bin at Adnvance Auto Parts. Work perfectly for all the plug wires. Haven't broken one yet. Attach them on the boot and pull on the pliers.
Make sure you put some gloves on, grab by the boot the best you can and keep twisting and pulling ........... eventually they'll slip off ........ it's a very tight fit and actually feels like a suction releasing when they free up.
Be sure to squirt a little dielectric grease into the new boots before putting them on.
I didn't remove anything extra. I just removed the red beauty covers. Disconected the wires from the coils. Heres the hard part. At each plug grab real tight on the boot and rotate one full turn to break loose. Then continue to rotate and pull. Its tough but works.
A tool I used was a needle-nose vise grip. Get it in between the boot and grab the metal insulators and pull. You'll still tear up a couple wires but they will come off.
I can tell you from experience that you will more than likely break or mangle a plug wire or more.. I broke 3 on my Ls6 and 4 on my Ls1.. The wires get really brittle over time, from the extreme heat and literally weld themselves in place, and a few are a down right bitch to get off. A few of the wire boots did come of really easily. Just remember when you break a wire you will have to buy a whole set of wires or go to the dealership and buy 1 individual wire for $33.00 (rip off).. Just a heads up before you start.. Gene Cully has the RED GM Hot Wires for $56.00 for a set of 8.. Superb performance wire at a steal of a deal price http://www.gmpartshouse.com/chevy/chevy-parts.html
I'm doing my plugs/wires as we speak (or type/read). I have the driver's side wires ripped (and I mean ripped) out and all the plugs (AC-Delco 41-110 iridiums) changed out.
#7 is a PITA, for sure, but #1 is fun to get leverage on with a ratchet. That alternator seems to freeze you up from moving the ratchet very far.
I'm using the anti-seize compound on the threads and dielectric crap on the inside of the boot and on the white ceramic of the plug.
Moral lesson today, always PLAN on buying new wires when changing plugs on a C5 for the 1st time. My 02 has 67k on it and the wire boots are/were so frozen to the plugs, I had to pop the contact off the plug and grab the boot with channel locks and tug.
WEAR GLOVES and put something over the fender to protect it from your legs, beer belly and tools.
I changed spark plugs and wires on my 02 with 50k miles that's all original. 18 yr old plugs and wires didn't come out easily. I only changed out for peace of mind. I believe the plugs would of lasted to 100k but not the wires. Since I was changing wires anyways I pulled on the wires to get them out. The ones that ripped apart, ripped with minimal force. Maybe #3-6 get the most heat from headers? After all of this the idle completely smoothed out. Went back with AC Delco plugs and GM performance red wires (added wire boots).
So… changed all plugs, wires, and cyl 2 coil with GM performance red wires, AC Delco coil, and AC Delco 41-110 plugs. (Not incredibly hard, some of the wires and plugs even still had dielectric grease inside) about two hours worth of work with a snack break in between.
However, when I started the car, it still ran slightly rough for about 1-2 minutes but didn’t throw a check engine light. May be worth smoking the intake to check for vacuum leak? About 35 degrees out today and both oil temp and coolant temp showed that in the DIC prior to starting the car.
Pic of cyl 2 plug attached
Last edited by MAGREDC5BASE; Nov 27, 2021 at 02:43 PM.
Did you check your fuel trims at 3000 RPM vs. idle as I had mentioned to check if there is a vacuum leak…it is strange your short terms are elevated that high with your long terms still at 0…you can also look at your ECT sensor temp in OBD2 generic…in generic mode the PCM can not “substitute” a value like ECT that it can do in enhanced or factory mode.
Did you check your fuel trims at 3000 RPM vs. idle as I had mentioned to check if there is a vacuum leak…it is strange your short terms are elevated that high with your long terms still at 0…you can also look at your ECT sensor temp in OBD2 generic…in generic mode the PCM can not “substitute” a value like ECT that it can do in enhanced or factory mode.
I wasn’t able to check as the scanner froze and by the time I got it figured out the idle had smoothed out. I’ll try and catch it again on Monday and will look into the ECT even though I think it seemed normal afaik.
Got the car to misfire again this morning but did not have a scanner to check out the trims.
Sprayed water all around the intake tube and intake manifold to see if I could get it to stall but nothing.
Going to bring a scanner home tomorrow and see if I can get some shots of the trims when it misfires. Any way to get to the MAP vacuum hose without pulling the intake mani off? Just to check
Rough idle when cold - no codes
