(I posted this is tech as well.. not sure which is the more appropriate place)
I've got a high speed miss I can't seem to diagnose..
1993 383 Stroker LT1 Auto (engine specs in sig).
The miss occurs between 4800-6300 @ WOT ONLY!
So far I've done the following..
-Brand New GM Opti-Spark (this was replaced because of a no start condition - had the miss before and after this)
-Brand New Wires (while I was in there.. why not - Hoped it would cure the miss)
-Check fuel pressure as outlines in the factory service manual..
-Checked Fuel pressure at WOT (No drop in pressure.. steady @ around 49psi)
-Dataloged a WOT run... the inj DC showed 110% is some lcoations.. so I leaned out the mixture a little.. (this didnt help). I'm running the factory 1993 Multi-Tech injectors..
since you data logged it, I'd look at:
knock counts (where and how much, if any)
O2 sensor volts (is it lean or rich in the miss area)
throttle position sensor (is it showing WOT or jumping when it shouldn't be)
If there are no knock counts and the O2 shows richer than normal where the miss is, then I'd suspect the ignition system (no spark = rich exhaust) bad sparkplug?.
If your getting knock counts and a lean O2 reading, then I'd start with checking the base timing (if thats possible on an LT1) and the timing in the tuning, it might also be a clogging injector.
If the throttle position sensor isn't showing around 4.0v or more then the ECM may not see it as WOT. also dead spots in the sensor can cause problems.
one last thing: how old is the gasoline? or is it cheap gas? (bad batch? bought somewhere other than normal?)
old gas will start running like crap at high load/higher rpms first, then at lower and lower rpms as it gets weaker.
cheap/bad gas will run bad at high loads/high rpm.
you would probably see knock with these.
RJ
TPS is where is should be..
02 looks rich.. 915-950mv although it did drop to the low 800mv range after the inj. DC spiked around 115-120%
it read a little knock on one pass.. and none on another (the condition existed in both) however, the spark retard was only 0.4 degrees.. this has been happening for awhile.. so I can rule out gas.. (tried different types of gas as well). I logged another drive in to the office today.. I'll peak at it later and see what new information I can post.. if anyone wants to take a peek I'll send them the files..
If you have an AFPR, turn up the pressure so that your injectors flow more. Be sure to set the injector constant accordingly in your tuning. You want about 85% DC. Retest. If it's still there....
Pull out 4* of timing in the 90-100kPa regions in the appropriate rpm ranges and retest.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
ok, new datalog shows the problem happening even when the inj DC is in the 60-75% range as well.. so i'm ruling that out.. datamaster indicated throttle postition at 100% with the TPS voltage at 3.63 volts..
knock retard when as far up as 1* in some areas... but not all areas where this condition existed... I'm thinking its got to be the plugs.. I have looked at those yet.. and I have the ACdelco platniums with the little puck in there.. maybe the puck fell off and widened the gap.. I'll pull it apart tonight
ok, new datalog shows the problem happening even when the inj DC is in the 60-75% range as well.. so i'm ruling that out.. datamaster indicated throttle postition at 100% with the TPS voltage at 3.63 volts..
knock retard when as far up as 1* in some areas... but not all areas where this condition existed... I'm thinking its got to be the plugs.. I have looked at those yet.. and I have the ACdelco platniums with the little puck in there.. maybe the puck fell off and widened the gap.. I'll pull it apart tonight
Very interesting that you're seeing 100% throttle with only 3.63v. Usually you need over 4.0v
I'd pull a few easy to check plugs as you've mentioned (1, 3, 2, 4 come to mind). If they all have their pucks, flash a new chip with 4* less timing as I suggested above.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
did you have the WOT tuning correct before this miss started happening?
I ask because IF the tune was good before, then showed rich due to the miss it could be that the opti unit is still bad (its been known to happen even on new units).
still 96GS#007 is right, getting the DC down would be a good thing.
when you pull the plugs keep track of what cylinder they come out of and look them over closely, they can tell you a good deal about whats going on inside there.
I replaced the opti because the car would barely idle much less achive WOT.. it would run fione for 20min then turn to mush.. miss.. sputter..backfire..etc..etc.. replaced the opti and bingo runs great... except for the high speed miss.. the high speed miss has been an on again off again problem for some time.. although now its a little more pronounced.. (i.e. it happens sooner..) it used to only happen sometimes in the 5300rpm and up range.. now it happens always in the 4500 and up range..
it used to only happen sometimes in the 5300rpm and up range.. now it happens always in the 4500 and up range..
Many times a miss that starts at the high end of the rpm range and over time begins happening earlier in the range is a result of fatigued valve springs.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
it could also be that you knock sensor isn't catching all the knock that it should be.
I'm not sure about LT1's but I know that L98's normally take out more than 1* of timing when they sense knock.
something funny going on there IMO
Many times a miss that starts at the high end of the rpm range and over time begins happening earlier in the range is a result of fatigued valve springs.
oh man dont tell me this..... that is not what I want to hear.. I just had a valve smack the piston on the drag car.. however, that felt different than this... the drag car would nose over at 5800rpm and fall flat on its face.. the vette revs up.. but it cuts out for a split second then picks back up climbs higher in the revs then misses for a second again then keeps climbing the revs - it feels good and strong when its not in the process of a miss.. Also, I noticed in the datalogs in one record where a miss occured the inj constant lterm value was 0 for boths sides and the sterm was 0 for both sides.. the 02 sensors leaned out about 100mv in that frame (to about 812mv) and then the next record the inj picked back up to the 128 128 lterm (which is a fixed value for WOT) the inj DC went back to 105% and the 02 sensors went back up to 900+mv.. I checked the fuel cutoff in the ECM and i have it set at 6500rpm so its not that..
oh man dont tell me this..... that is not what I want to hear.. I just had a valve smack the piston on the drag car.. however, that felt different than this... the drag car would nose over at 5800rpm and fall flat on its face.. the vette revs up.. but it cuts out for a split second then picks back up climbs higher in the revs then misses for a second again then keeps climbing the revs - it feels good and strong when its not in the process of a miss.. Also, I noticed in the datalogs in one record where a miss occured the inj constant lterm value was 0 for boths sides and the sterm was 0 for both sides.. the 02 sensors leaned out about 100mv in that frame (to about 812mv) and then the next record the inj picked back up to the 128 128 lterm (which is a fixed value for WOT) the inj DC went back to 105% and the 02 sensors went back up to 900+mv.. I checked the fuel cutoff in the ECM and i have it set at 6500rpm so its not that..
thats some interesting info,,,,
I'd take a very close look at everything in that area (and right before that area) of the datalog: rpm's, load/VE, injector pulse width, timing ect.
if the signal from the opti is dropping to 0 then there is no pulse width programed for that (bad opti or connection?).
or it could be that you've missed an area in the WOT tuning? use that area of the datalog to see whats programed for it, it could be one value mis-typed or just out of whack.
all this info is really helping! I took a second look at the wires today and ran an ohm meter on all of them.. they are all pretty close to 8000 ohms.. so they all looked ok.. however, the #6 cyl wire was 34k+ohms.. upon closer inspection i noticed it had been burned through .. it must have been touching the exhaust manifold heat shield near the plug boot.. anyways, I happen to have a few spare wires laying around (brand new) and replaced that wire and went for a ride.. it felt much stronger.. but still the miss in the 4k-6.3k rpm range.. I got to thinking maybe its because the old program was a program for 7 cyl. since this one wasnt firing.. soooo.. I did another datalog run and found that since i leaned the fuel mixture at WOT out the new 02 readings at WOT are in the 912mv range... but with the new wire I was getting quite a bit of ping (knock count) and the computer was pulling out a full degree of timing almost accross the whole board.. 85 kpa-100kpa from 2000rpm-6500rpm.. so I did as suggested by GS and took 4* of spark out of the whole range.. WOW what a difference!! no missing throughout MOST of the range.. the only place i get a miss is around 6300rpm in the 1st-2nd gear shift.. it might be the fuel cutoff.. but its too late at night to make another datalog run.. so i'll hook up the computer on the way in to work and see if its needs more timing taken out at the 5500-6500 range or I need to raise the fuel cutoff again...
This leads me to another question.. I have the stock Auto Tranny with no shift ki or nothin'.. why is it waiting so long to shift? 6300rpm in the 1-2 shift?? the tranny is strong and has about 20k miles since the last rebuild.. it seems the 383 just rev's a heck of alot faster than the stock displacment configuration does?? any ideas??
Lastly.. with a SCAT 9000 crank, ARP rod bolts, factory rods (reconditioned, relieved and polished with KB hyperutetic pistons.. whats a resonable redline to set the limiter (fuel cutoff) at???
Last edited by Z06_BluByU; 12-21-2004 at 11:53 PM.
To determine fuel cutoff, you have to account for the cylinder head configuration (weight of the valve train, spring pressures, etc) as well as your cam specs. Your bottom end should be fine for a 6300 shift point, 6500 fuel cutoff. Just gotta match it to the other stuff I mentioned. No reason to spin an engine to 6500 if the hp peak is 5900.
What's your timing map look like where the miss is occuring? More timing down low helps, but up high you want less. Remember, the idea of timing advance is to optimize the combustion event relative to piston position. Too much advance and the combustion process is fighting the upward motion of the piston.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
can't help with the auto trannie shift points (don't know squat about that). I also think it should be safe up 6400-6500rpm (at least for now).
the smart thing to do would be to get you tune good and close, then head to the dyno/with wide band O2.
that way you'll find out where the HP peak is and can have the shift points adjusted accordingly afterwards.
ok did a data log today.. and the crap is back.. the car is pulling 1* of knock retard out @ WOT conditions (STILL - EVEN AFTER I TOOK TIMING OUT LAST NIGHT). it has a huge miss around 6300rpm.. (between the 1-2 shift). WTF!!! I'm hating this!!! I went to bed last night happy because I thought I had it fixed. oh nooo.. not my luck.. think i'm gonna pull some plugs tonight.. maybe its those AC delco platniums.. I dont remember this being a problem with the old optispark.. damit!
ok did a data log today.. and the crap is back.. the car is pulling 1* of knock retard out @ WOT conditions (STILL - EVEN AFTER I TOOK TIMING OUT LAST NIGHT). it has a huge miss around 6300rpm.. (between the 1-2 shift). WTF!!! I'm hating this!!! I went to bed last night happy because I thought I had it fixed. oh nooo.. not my luck.. think i'm gonna pull some plugs tonight.. maybe its those AC delco platniums.. I dont remember this being a problem with the old optispark.. damit!
So it appears the 4800-6300 area is resolved, or is the miss still there as well? Assuming it's not, now it's time to look at the timing in the 6300 & up range. I'd expect total timing to be in the 32* - 36* range.
Check a few of the plugs. If you're running the OEM recommended plug, it may be a bit too hot. You might want a step or two colder plug.
In your first post you said you had the miss with the old Opti, but above you indicate that may not be the case. Which is it?
Has this car ever been on a dyno with a wide band o2 sensor? What was the AFR? As an experiment, you might want to add a half point to the PEvsRPM table to see if a richer mixture helps.
I think you have a lean misfire as a result of a combination of incorrect timing and fueling.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
yes to both accounts but in different ways.. with the old opti (even right after the 383 was compleated) sometimes at the top of 1st (at the 1-2 shift) it felt like it bounced off the rev limiter (fuel cutoff).. this would only happen under certian tempature conditions.. i thinkit was when it was warm outside or the engine was hot from idling long periods.. After about a year or so I developed a problem where on the way in tow rok in the morning (or the way home in in the afternoon) after driving for about 20min the engine would cut out, miss, stumble at all engine RPMS.. this is what lead to the opti-replacment.. after the new opti was in this wasnt an issue anymore.. but then this WOT miss happened.. immediatly (as in during the first test run after the install was complete).
2. is the miss gone in the 3800-6500- range... Not its back (DAMIT!) I thought for sure that cut plug wire was it... then I thought it was the programming.. but alass.. it was back this morning and this afternoon..
3. I am running the recommend ACdelco platnium plugs for the stock 1993 LT1 350 engine.. what plug should i run to achive the two step colder plug (I'm not familiar with plug numbers as they corrispond to temp range).
4. The car has never been on a dyno.. and has never been scanned by a wide band 02
tomorrow i'm going to pull all the plugs and look to see if one of the platnium pplug pucks fell off thereby increaseing the gap.. and check all the grounds.. again..
BTW: as I mentioned earlier my datalogs from this morning indicated the ECM pulled out 1* of timing (knock retard) when the car went into WOT.. even after I pulled out the timing yesturday.. I think i'm running in the 38* total timing range @ 100kpa/5500 range..
If i cant find it in the plugs, timing maps or grounds this week i think i'm gonna take it to the only dyno shop in town and see what they can do..
tomorrow i'm going to pull all the plugs and look to see if one of the platnium pplug pucks fell off thereby increaseing the gap.. and check all the grounds.. again..
...
If i cant find it in the plugs, timing maps or grounds this week i think i'm gonna take it to the only dyno shop in town and see what they can do..
I think this is the correct approach to take. Time to put it in a controlled environment where you can strap it down and check it out versus trying to do that while driving.
38* total timing is borderline IMO for your setup. I'd pull another couple degrees. Be sure this is what your datalog software is saying. On '94 & up LT1/4, the PCM mysteriously adds up to 4* of timing to what is in the map depending on load and rpm...i.e. in my case, the table says 32* but at 6000rpm and 100kPa the datalogger will report 36*. Not sure if '93s are the same. None the less, I would look for the timing to be 36* total in 5000 and up / 100kPa range as reported by your datalogging software.
You need to check with the various plug manufacturers regarding what part number plug is a step or two colder. FWIW, I run RapidFire RF12 plugs. NGK or Denso are also nice plugs to run.
__________________
Jim
'96 Grand Sport #007
A few mods here and there
Corvette Store
Dam high speed miss still...
what a difference!! no missing throughout MOST of the range.. the only place i get a miss is around 6300rpm in the 1st-2nd gear shift.. it might be the fuel cutoff.. but its too late at night to make another datalog run.. so i'll hook up the computer on the way in to work and see if its needs more timing taken out at the 5500-6500 range or I need to raise the fuel cutoff again...
I went to bed last night happy because I thought I had it fixed. oh nooo.. not my luck.. think i'm gonna pull some plugs tonight.. maybe its those AC delco platniums.. I dont remember this being a problem with the old optispark.. damit!
