Ignition timing problems

[JPetroJr]

I have bought a 94 silverado with cracked heads. I built the 5.7 motor. 40 over summit vortec heads oversized comp cam 468 lift 110 lobe separation. All new parts and sensors except crank. Rebuilt TBI with new injectors IAC and TPS. Base timing at 0 computer unplugged. Plug up computer timing at 16. Idles fine but stumbles and backfire through exhaust. Base timing at 20 advanced plug up computer timing goes to 36-40 idles and cranks good no backfire but stumbles. Took thermostat out seems to run fine. I know that’s not a fix but sure need some help

 

[Schurkey]

What is the fuel pressure?

What intake manifold are you using?

Was the damper checked for indicating "0" degrees on the timing pointer, when the #1 piston is at true TDC?

Was the camshaft degreed?

Connect a scan tool, look at the data stream. Verify that all the sensors are sending "reasonable" data. Assure that the EGR is working properly--most vehicles converted to Vortec heads have butchered EGR systems.

 

[JPetroJr]

Fuel pressure is good. Using a wieand intake no egr. Crank is top dead center no cam was not degreed was told by summit put 0-0 and that was it. Got a scan tool but don’t really no what to look for . To start with it showed a 15 -33-42 code. Cleared them drove truck and still no codes coming back

 

[Schurkey]

Fuel pressure is good.

What does "good" mean? Did you test fuel pressure under load, when the vehicle is running poorly?

Using a wieand intake no egr.

I expect the computer is unhappy about that. Surprised you don't have a Code 32.

Crank is top dead center no cam was not degreed was told by summit put 0-0 and that was it.

I REFUSE to install a camshaft without putting a degree wheel and dial indicator on it; there's half-a-dozen machining operations and parts variations, any one of which could cause improper cam timing.

What is your cranking compression on at least two or three cylinders? Too low, or too high indicates the cam timing may be off.

You have VERIFIED that the damper mark and the timing indicator "0" mark line up when the #1 piston is at TRUE TDC?

Got a scan tool but don’t really no what to look for .

Look at each sensor's output. Before you start the engine, does the coolant temp sensor show the same temperature as The Great Outdoors? When the engine is fully warmed-up, does it show 190--200 or so? Does the O2 sensor flip back and forth from high (0.9V) to low (0.2V) voltage, and does it do so rapidly, once the engine has been run long enough to go into closed-loop operation? Scan tool verifies closed-loop operation once the engine has run awhile? Does the MAP sensor show about 29--30 inches of pressure before you start the engine, and about half that after the engine is idling? Knock sensor showing NO detonation at idle--but does retard the timing if you tap the intake manifold with a brass hammer? Throttle position sensor shows smoothly-increasing voltage as the throttle is slowly opened from idle to WFO. (Engine not running, of course.) Do you get a reasonable RPM signal at idle, and with the throttle opened?

To start with it showed a 15 -33-42 code. Cleared them drove truck and still no codes coming back

"Codes" are the beginning of diagnosis. They can be very helpful. However, the real answers are in the data stream--the sensor signals, the ECM outputs to the IAC, EGR, ignition timing, injectors, etc.

 

[Supercharged111]

Truck needs a tune, plain and simple.

 

[Hipster]

I REFUSE to install a camshaft without putting a degree wheel and dial indicator on it; there's half-a-dozen machining operations and parts variations, any one of which could cause improper cam timing.

^^^This

I don't imagine the ecm in stock tune is digging the 110 LSA either.

 

[JPetroJr]

Thanks for the info. Now I have something to go by. I was watching and scrolling the scanner but didn’t have a clue what I was looking at. I can reread the thread while I’m scanning. Thanks, let’s see what we come up with