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I did a YouTube video on doing a functional test of the coil packs on the control wire...the BEST way to check your coil packs is with a scope looking at your firing KV at idle and under load...these C5 coil packs are pretty bullet proof but like I said it may be cheaper in the long run paying for diagnostic time !!...and as I had said in the PM to you spray the secondary ignition system down with baking soda and water mix and see if you see a lightning show under the hood at night or in a dark garage...they do make adjustable spark testers that may help !!...mine is pictured be
low !!
If the power falls off suddenly.. I'd guess electronic. If it falls off smoothly I'd say a lack of fuel.
Given the clues you've mentioned.. I'd try replacing the fuel filter first. (oh... you already did that)
Second.. need to get a scanner hooked up and start watching parameters.
Last edited by wydopnthrtl; Jan 20, 2020 at 03:52 PM.
Buying a longer fuel gauge hose tomorrow so i can see what fuel pressure is doing under load. Had the gauge on earlier today, was at 61psi at idle, had my buddy blip the throttle to 4k car bogged at beginning of him hitting throttle till about 2k then came back and pressure dropped about 6 psi when he did that at 4k. Cars also stumbling at 5th and 6th at about 1500-2500 rpms at about 25% throttle, rather then just high rpm at wot in 2nd/3rd and 4th
6psi... wow.
so when the power falls is it suddenly like turning off the key or is it smooth but just a loss of power?
My guess is fuel filter or a restriction of some sort.
maybe the fuel pump... but I'd make sure there is no restriction first.
I once had a car that wouldn't start until after 4-5 try's. Then after it did it would run fine. Turned out to be the fuel filter. And it wasn't that old either.
Last edited by wydopnthrtl; Jan 22, 2020 at 11:36 AM.
I had this issue too but mine was only at WOT in 3rd and 4th gears (never went WOT in 5th or 6th gear). I noticed that when it happened I could ease of the gas for a second and then get back on it and the car ran great. I never figured out what the issue was but it only happened to me after my H/C and the tuner never had this issue when tuning it. Later the tuner rode with me on the street and he logged the street pulls. He could see the changes in vacuum pressure for the intake manifold but couldn't find an electronic reason so he assumed it was mechanical. KR, A/F, and other stuff all looked good on his computer.
The guy who did the install felt like it was electronic. So I had both guys pointing at eachother. Both are HIGHLY experienced and good guys. I know if either one of them felt it was something they could fix, they would have done so...so who knows. Once I stopped tracking my car about 5 years ago, I've not had the issue since on the street and had forgotten it even used to happen. Good luck!
I think it is normal for the pcm to keep the engine below 4k rpm under no load-throttle blip, so this may not be a symptom. I think both spark retard and fuel cutoff happen under throttle blip to 4k.
Bogging down under load may be knock sensors. They can retard timing and not throw codes. Most scan tools will data log thru bd2. So, there might be PID for spark advance you could watch as the problem happens under load. If it warms up this afternoon, I’ll hook mine up and let you know if there is a PID for spark advance.
I remembered I made a list of all PID's associated with the GM enhanced package from my AUTOENGINUITY scan tool. See the spreadsheet attached. BEWARE that this is a very large list, and applies across the GM product line, so a given PID may not be active for the C5 Corvette.
By convention, the PID's are in alphabetical order (all scan tools are this way), and later when I get my lazy A$$ motivated, I will add the PID values to the columns.
If you look at line 396, 397, you will see PIDS for fuel rail pressure. these may not work on a C5, but you could certainly rig up a pressure transducer and read fuel rail pressure easily. Next look at line 511 this PID looks like ignition timing and might be one the C5 supports.
My point is there are a lot of diagnostic trees to cut before you bring in the logging trucks.
2020 Corvette of the Year Finalist (performance mods)
C5 of Year Winner (performance mods) 2019
I think he meant pressure dropped a total of 6psi not dropped TO 6psi, the engine wouldn't run if it was that low... I agree in a perfect world it should not drop but if your tune is compensating for the pressure drop it could technically be fine but I personally wouldn't recommend doing it that way
I think he meant pressure dropped a total of 6psi not dropped TO 6psi, the engine wouldn't run if it was that low... I agree in a perfect world it should not drop but if your tune is compensating for the pressure drop it could technically be fine but I personally wouldn't recommend doing it that way
I guess I saw a “to” in there...LOL !!...still a 6 psi drop is WAY to much even under load !!
2020 Corvette of the Year Finalist (performance mods)
C5 of Year Winner (performance mods) 2019
lol, the first time I read it as a 6psi drop then saw your post and had to go back and double check to make sure I didn't misread it... it is too much though, I have seen guys let the pressure drop on forced induction applications and sometimes they get away with it if it isn't a drastic drop and they can compensate for it in the tune but in the op's case his mods shouldn't be stressing the fuel system out that much to cause such a drop
Last edited by StingrayRebel; Jan 22, 2020 at 07:34 PM.
Yes 6psi drop from 61psi to about 55 psi at a 4k throttle blip in neutral. If i ease into throttle until a little bit before WOT and not actually go fully WOT it wont get the stumble in the higher rpms in say 3rd and 4th. If i go wot it does always at 4k area in higher gear, 2nd gets the symptoms occasionally under wot in about 4k+ but rarely. Its always in the higher gear. When it does it in 2nd its only for a second then hops back into powerband fine. The fuel filter is brand new. I need to order myself a data logger to check the parameters when it happens. The car runs great other then that and for the 6th gear stumble its when im cruising cars at about 1500 rpms and i give it about 25% throttle itll stumble and shake until it gets to about 2500 rpms. The same kind of stumble it does when it does it at high rpm 3rd/4th. I checked local parts stores for a fuel gauge hose extension and no one has them. Also the vette never throws codes after it happens or during!
Last edited by MurderedLs1; Jan 22, 2020 at 10:57 PM.
I think you have a possible fuel "volume" issue. Lower gears are shorter pulls and as you get into 3rd and 4th the demand on the pump (volume output) increases allowing pressure to drop off and leaning the engine out. Fuel pump may be on it's way out or possibly a pinched fuel line. First thing I would do is check the AFR commanded vs what your getting at the times it's losing power. Also I think I read you stated you recently replaced the fuel filter. The older I get the more I seem to buy "new" parts that suck and cause or don't fix problems so I wouldn't be at all surprised to hear you say later in this thread it turned out to be a bad fuel filter.
It did it with the old filter (which looked like it was pretty new from last owner). and does it with the new. Im starting to think FuelPump but ill need to order my self a extension for my gauge so I can watch it when it happens
How long have you owned the car? How many miles have you put on it before this problem started? I agree with what c5 diag said in post #2. A tuner can put a wideband on it and datalog and easily see if its rich, lean, pulling timing, check that catalyst overtempt protection (COT) was turned back on (it gets disabled while tuning) , etc. For instance if its running lean at WOT and COT is still disabled then you can overheat your cats. Check the tune first since you have no knowledge of its integrity unless you've put a couple thousand miles on it before this happened.
... it is too much though, I have seen guys let the pressure drop on forced induction applications and sometimes they get away with it if it isn't a drastic drop and they can compensate for it in the tune but in the op's case his mods shouldn't be stressing the fuel system out that much to cause such a drop
