LT4 Ignition Timing Question
Thread Starter
Team Owner
Joined: Feb 2006
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From: San Antonio Texas
LT4 Ignition Timing Question
For over a year now, I've been chasing a high RPM ignition problem on my LT4. The car runs just fine and gets good MPG as long as I keep it under 5000rpm. Last night I put a friends "Scan Gauge" on and checked the timing. Everything else checked out OK but the timing reads -12 to -14 at idle and reads all the way to -44 when I ran the rpm up to about 3000. How can that be? I didn't think an engine would run at all with that much retard! We hooked up the Scan Gauge to his Tahoe and the timing doesn't show the minus sign and the numbers increase with engine speed just like you'd expect. Hooked it up again to the LT4, and again got increasing negative readings
! I have an MSD opti with the adjustable timing screw on it, so we turned it first 1/4 turn one way then 1/4 turn the other way and the Scan Gauge showed No Change at all. Still -12 and increasing negative as engine speed went up. Under driving load when you get to about 5200RPM, it acts like the ignition shuts off until the rpm drops back down and then it comes back to life violently until it again hits what seems like the rev limiter around the 5k mark! I know the timing is set by the computer, but how do you know where it is supposed to be and if it is advancing or retarding correctly? Could faulty knock sensors make the computer go crazy like this?
! I have an MSD opti with the adjustable timing screw on it, so we turned it first 1/4 turn one way then 1/4 turn the other way and the Scan Gauge showed No Change at all. Still -12 and increasing negative as engine speed went up. Under driving load when you get to about 5200RPM, it acts like the ignition shuts off until the rpm drops back down and then it comes back to life violently until it again hits what seems like the rev limiter around the 5k mark! I know the timing is set by the computer, but how do you know where it is supposed to be and if it is advancing or retarding correctly? Could faulty knock sensors make the computer go crazy like this?
Race Director
Joined: Aug 2006
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From: D / FW
Race Director
Joined: Jan 2002
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From: O'Fallon Missouri
WRONG!! You need to keep whom ever gave you that advice away from your Vett.
Racer
Joined: May 2004
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From: Kissimmee FL
Are you the origional owner? If not than its possible a previous owner installed a Hypertech or similar product where you can set the rev limiter. Unless it is changed it will cut off wherever the limiter is set. I know this from an LT 1 that I worked on with the same problem, owner finally contacted the previous owner to see if he ever had the problem who told him that he did have a Hypertech installed changing the rev limiter. Reprogramming fixxed the problem immediately changing the limiter to 6200. Might not be the problem but worth checking out....G
Race Director
Joined: Jan 2002
Posts: 12,263
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From: O'Fallon Missouri
The 12* spark retard at idle makes me think you have a knock sensor problem. I datalogged a 95 LT1 that had the wrong knock sensors installed and it had a constant 12* retard at idle. Your car may be going into a default mode from a bad or bad knock sensors.
Safety Car
Joined: Dec 2007
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From: Sandhills of North Carolina
For over a year now, I've been chasing a high RPM ignition problem on my LT4. The car runs just fine and gets good MPG as long as I keep it under 5000rpm. Last night I put a friends "Scan Gauge" on and checked the timing. Everything else checked out OK but the timing reads -12 to -14 at idle and reads all the way to -44 when I ran the rpm up to about 3000. How can that be? I didn't think an engine would run at all with that much retard!
As far as the 5200 shut down.....I would see what the rev limiter is set at as mentioned above. I have mine set at 6K. I have a LT1 though.
Melting Slicks
Joined: Dec 1999
Posts: 2,816
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From: Austin, TX
Weav is right -- actually the negative (-) means ADVANCE not retard. Autotap does this too, if it reads -12, that means 12 degrees before TDC. Retard would show as +12.
I just looked at the program for a stock LT4 and 44 degrees of advance is definitely within reason at WOT.
With your MSD's adjustment, it's physically changing the timing at the rotor, not changing the program, so the computer would not know the timing has changed. Therefore the fact that you're seeing no change in the scan tool readings after adjusting the MSD is correct.
There have been several guys with high RPM miss problems on LT engines on the board (do a search on "high RPM miss") some have been due to bad Optis, others seem to have never figured it out.
I just looked at the program for a stock LT4 and 44 degrees of advance is definitely within reason at WOT.
With your MSD's adjustment, it's physically changing the timing at the rotor, not changing the program, so the computer would not know the timing has changed. Therefore the fact that you're seeing no change in the scan tool readings after adjusting the MSD is correct.
There have been several guys with high RPM miss problems on LT engines on the board (do a search on "high RPM miss") some have been due to bad Optis, others seem to have never figured it out.
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Race Director
Joined: Aug 2006
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From: D / FW
Accually it is still wired to the old computer which still controls the dash gauges and the Tranny.
Like I said I feel like he may have mods but If the engine ran allright once then unhook the offending knock sensor see if it runs better and then put in a new one.
If you are running big cam and injectors and it clears up with disconnecting the sensor then does it again with a new one seek further help. Not really enough info to venture a guess.
Race Director
Joined: Jan 2002
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From: O'Fallon Missouri
Sorry for the short post I have a heavilly modded engine and the Accel Gen. 7 computer takes care of knock by measuring the pressures according to the graphs and the knock sensor is not even wired to the computer anymore.
Accually it is still wired to the old computer which still controls the dash gauges and the Tranny.
Like I said I feel like he may have mods but If the engine ran allright once then unhook the offending knock sensor see if it runs better and then put in a new one.
If you are running big cam and injectors and it clears up with disconnecting the sensor then does it again with a new one seek further help. Not really enough info to venture a guess.
Accually it is still wired to the old computer which still controls the dash gauges and the Tranny.
Like I said I feel like he may have mods but If the engine ran allright once then unhook the offending knock sensor see if it runs better and then put in a new one.
If you are running big cam and injectors and it clears up with disconnecting the sensor then does it again with a new one seek further help. Not really enough info to venture a guess.
However, running the stock PCM. I would never recommend disconnecting the knock sensors. One bad tank of gas could take out the engine.
Safety Car
Joined: May 2002
Posts: 4,659
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From: Toronto ontario
first guess is that theres too much timing. Unless 44 degrees is at light load then it's reasonable. at wot under load you should be seeing 34-36 degrees or so at 5k rpm. Does it sound like the motor is knocking? If your knock sensors have already been disabled perhaps they are not picking up the knock. check to see if your temp guages are working ok if the pcm thinks it is cold it will add timing too.
Personally I would change the opti plugs and wires asap but that's just my way of throwing money at problems which usually works.
Keep posting any info you can get.
Personally I would change the opti plugs and wires asap but that's just my way of throwing money at problems which usually works.
Keep posting any info you can get.
Safety Car
Joined: May 2002
Posts: 4,659
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From: Toronto ontario
If you get a chance get a fuel pressure guage on your fauel rail and tape it to the windshield. watch it as you go wot. My brothers old 97 ss had this issue with the fuel pump where it died at higher rpms .
Melting Slicks
Joined: Feb 2008
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From: Palm Beach
Knock retard does not work at idle.
Race Director
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From: O'Fallon Missouri
Thread Starter
Team Owner
Joined: Feb 2006
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From: San Antonio Texas
My car is an LT4 and as far as I know it is stock. (just has resonator replaced with straight pipes) I had to change the Opti because it fried the contacts and rotor tip off. There was 160 thousandths between the contacts and the rotor. There was also an oil fog over everything in there and the sensors were failing; Opti was junk! Replaced the following:
1. MSD OPti, 2.MSD Wire Set, 3.Iridium Plugs, 4.All New O2's, 5.New Bosch III Injectors, 6.New Delco Coil, 7.New Ignition Module 8.New electric water pump with old drive shaft removed.
Before the old Opti crapped out, the car would go 6500 before it hit the limiter. I rarely went that high, but did 6k at least once a week with no breakup. Now it shuts down just past 5K! The scanner I used was borrowed from a friend and it said "Scan Gage" on it. It read + timing on his Tahoe and - timing on my LT4, so that was what had me confused. Car runs and idles just fine and gets great mileage as long as I stay under 5K. After 5k it all goes in the crapper!
1. MSD OPti, 2.MSD Wire Set, 3.Iridium Plugs, 4.All New O2's, 5.New Bosch III Injectors, 6.New Delco Coil, 7.New Ignition Module 8.New electric water pump with old drive shaft removed.
Before the old Opti crapped out, the car would go 6500 before it hit the limiter. I rarely went that high, but did 6k at least once a week with no breakup. Now it shuts down just past 5K! The scanner I used was borrowed from a friend and it said "Scan Gage" on it. It read + timing on his Tahoe and - timing on my LT4, so that was what had me confused. Car runs and idles just fine and gets great mileage as long as I stay under 5K. After 5k it all goes in the crapper!
