[C5 Z06/LS6] Troubleshooting Random Knock & Erratic Idle
06-24-2015, 04:01 PM
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[C5 Z06/LS6] Troubleshooting Random Knock & Erratic Idle Hey guys!
I'm a somewhat active member over on the corvette forums but I'm trying to diagnose a tough issue and so far I haven't been able to get any good suggestions. I'm hoping some members on here may be able to help! I'm going to copy and paste my info from my other thread. Any suggestions would be super helpful guys! Thanks
. For the record, my car is a 2003 Z06 (mods are listed below).
Originally I thought this was a tuning issue, but now I think it may be some sort of vacuum leak or other physical issue (probably not computer related). A lot of things have happened over the past month or two so I'm going to condense it into a chronological list for ease of reading:
1. Car had a cam/exhaust/intake done over a year ago (G5X3, LG Super Pro Long Tube Headers, K&N Intake). Tuned and had no issues.
2. Car is taken in a month or two ago for AFR 210 heads, FAST 102 intake manifold, electric water pump, centerforce clutch, LS2 fuel rails, 90mm throttlebody, and 42 lb injectors. Tuned and seems fine.
3. Immediately upon picking up the car I notice the idle is extremely erratic when rolling to a stop (bogs down to 200-300 rpms, revs back up to 2000 rpms -- keeps doing this back and forth, sometimes dying, until I come to a stop). Upon stopping the idle goes back to normal. AC on or off makes no difference. Car seems to drive fine though.
4. Car is returned and retuned but erratic idle persists.
5. Took the car on a road trip to California with the intention of returning the car again when I got back to address the idle issue. Notice knocking about 3/4 of the way through the trip when car is under heavy load in 6th gear (cruising up a hill usually). I was in northern Arizona and it was extremely hot so at the time I thought heat was a contributing factor.
6. Limp the car in lower gears the rest of the way and have the car retuned from scratch by a reputable tuner in California to address knocking and idle. Seems to be doing fine, idle seems much more stable and knocking is not present. Tuner mentions he did not have time to check for vacuum leaks because I was under time constraints to get the car back.
7. Begin return trip home to Texas. Seems fine initially, starts knocking in 6th again under high load at some point. Also notice that the erratic idle seems to be present at the same time the engine is knocking (not at the exact same time, just in general, the two issues seem to coincide).
8. For the middle leg of the trip both issues seem to disappear. No knocking under WOT in 6th, idle is smooth as butter.
9. Wake up to finish last leg of the trip. Engine is knocking in 6th again and I can't even roll the car to a stop without the engine dying. I'm having to give it gas while coming to a stop to keep the engine going.
10. Finish the last leg of the return trip in 5th to prevent knocking.
I'd like to point out that lower octane gas was used later in my trip, but even after putting 5 gallons of 100 octane competition fuel in the knocking was the same. When it was retuned it was tuned to 91 octane gas, and the same gas was present when it was running fine and when it was knocking so this should not be octane related knocking.
Does anyone have any ideas as to what the issue could be? Could a vacuum leak or for that matter any other type of leak cause knocking and a bad rolling idle? I'm open for any suggestions. I plan on dropping the car off at the guys that did the original work to see if they can sort it out but I'm trying to get a list of problems it could be together.
Thanks
I'm a somewhat active member over on the corvette forums but I'm trying to diagnose a tough issue and so far I haven't been able to get any good suggestions. I'm hoping some members on here may be able to help! I'm going to copy and paste my info from my other thread. Any suggestions would be super helpful guys! Thanks
. For the record, my car is a 2003 Z06 (mods are listed below).Originally I thought this was a tuning issue, but now I think it may be some sort of vacuum leak or other physical issue (probably not computer related). A lot of things have happened over the past month or two so I'm going to condense it into a chronological list for ease of reading:
1. Car had a cam/exhaust/intake done over a year ago (G5X3, LG Super Pro Long Tube Headers, K&N Intake). Tuned and had no issues.
2. Car is taken in a month or two ago for AFR 210 heads, FAST 102 intake manifold, electric water pump, centerforce clutch, LS2 fuel rails, 90mm throttlebody, and 42 lb injectors. Tuned and seems fine.
3. Immediately upon picking up the car I notice the idle is extremely erratic when rolling to a stop (bogs down to 200-300 rpms, revs back up to 2000 rpms -- keeps doing this back and forth, sometimes dying, until I come to a stop). Upon stopping the idle goes back to normal. AC on or off makes no difference. Car seems to drive fine though.
4. Car is returned and retuned but erratic idle persists.
5. Took the car on a road trip to California with the intention of returning the car again when I got back to address the idle issue. Notice knocking about 3/4 of the way through the trip when car is under heavy load in 6th gear (cruising up a hill usually). I was in northern Arizona and it was extremely hot so at the time I thought heat was a contributing factor.
6. Limp the car in lower gears the rest of the way and have the car retuned from scratch by a reputable tuner in California to address knocking and idle. Seems to be doing fine, idle seems much more stable and knocking is not present. Tuner mentions he did not have time to check for vacuum leaks because I was under time constraints to get the car back.
7. Begin return trip home to Texas. Seems fine initially, starts knocking in 6th again under high load at some point. Also notice that the erratic idle seems to be present at the same time the engine is knocking (not at the exact same time, just in general, the two issues seem to coincide).
8. For the middle leg of the trip both issues seem to disappear. No knocking under WOT in 6th, idle is smooth as butter.
9. Wake up to finish last leg of the trip. Engine is knocking in 6th again and I can't even roll the car to a stop without the engine dying. I'm having to give it gas while coming to a stop to keep the engine going.
10. Finish the last leg of the return trip in 5th to prevent knocking.
I'd like to point out that lower octane gas was used later in my trip, but even after putting 5 gallons of 100 octane competition fuel in the knocking was the same. When it was retuned it was tuned to 91 octane gas, and the same gas was present when it was running fine and when it was knocking so this should not be octane related knocking.
Does anyone have any ideas as to what the issue could be? Could a vacuum leak or for that matter any other type of leak cause knocking and a bad rolling idle? I'm open for any suggestions. I plan on dropping the car off at the guys that did the original work to see if they can sort it out but I'm trying to get a list of problems it could be together.
Thanks
