98 Vortec 305 suddenly out of timing

Jeffrizzzo · Aug 6, 2022

My 98 305 with 122,000 miles is acting like the timing is off. I’ve replace the cam and crank sensor. Taken out the O2 sensors I’m front of the cats to offer back pressure relief in case they’re clogged. Plugs, wires, and cap and rotor all all about a month old with 2,000 miles on them. I lined up TDC on the balancer and the distributor rotor was pointing exactly at the mark on the distributor body (toward #1 cylinder). Pulled the distributor anyway to check the gear and it looks perfect. Lastly I’ve replaced the ECM because it’s the last thing I could think of and it’s not giving me any codes on the scanner. This thing went from perfectly running to backfiring out of the throttle body and I can’t figure it out. Perfect fuel pressure and beautiful spark. Please throw any ideas you have at me. I‘ve built engines and I’ve done an LS swap so I know a little about working on cars but, I guess I could still be missing something simple.

stutaeng · Aug 6, 2022

I know "jumping timing" is typically brought when guys experience engine problems, but I suspect the sprocket gear and chain would have a really hard timing "jumping a tooth", and if it did, you'd certainly have bigger issues.

But you verified it is timed correctly, so keep looking.

You probably have a different issue. Need to hook up a scanner that gives you live data... oxygen sensor waveform and fuel trims are your friend here. Look at those at idle and while revving your engine.

Good luck

phantom 309 · Aug 6, 2022

Jeffrizzzo said:
My 98 305 with 122,000 miles is acting like the timing is off. I’ve replace the cam and crank sensor. Taken out the O2 sensors I’m front of the cats to offer back pressure relief in case they’re clogged. Plugs, wires, and cap and rotor all all about a month old with 2,000 miles on them. I lined up TDC on the balancer and the distributor rotor was pointing exactly at the mark on the distributor body (toward #1 cylinder). Pulled the distributor anyway to check the gear and it looks perfect. Lastly I’ve replaced the ECM because it’s the last thing I could think of and it’s not giving me any codes on the scanner. This thing went from perfectly running to backfiring out of the throttle body and I can’t figure it out. Perfect fuel pressure and beautiful spark. Please throw any ideas you have at me. I‘ve built engines and I’ve done an LS swap so I know a little about working on cars but, I guess I could still be missing something simple.

Backfiring could be lean condition,. vacuum leak.?

95burban · Aug 6, 2022

Fuel pump, vortec are extremely picking about fuel pressure.

Jeffrizzzo · Aug 7, 2022

I did a compression check to further verify no jump in timing chain or distributor and I’ve also tried the battery out of my Camaro. I’ve seen batteries do weird things. After hours of reading in here I finally found someone with the same exact problems and had tried a lot of the same things as me with no results….. until he swapped the distributor cap. He already had a fairly new Accel distributor cap and it went bad giving an out of timing vibe. I have that same cap and it’s fairly new so figured it was good. Turns out the reviews on that product are not great so I’m trying that next. I have an AC Delco being delivered to my local O’reilly’s. I should have it swapped within the hour and I’ll keep ya posted.

Jeffrizzzo · Aug 7, 2022

Jeffrizzzo said:
My 98 305 with 122,000 miles is acting like the timing is off. I’ve replace the cam and crank sensor. Taken out the O2 sensors I’m front of the cats to offer back pressure relief in case they’re clogged. Plugs, wires, and cap and rotor all all about a month old with 2,000 miles on them. I lined up TDC on the balancer and the distributor rotor was pointing exactly at the mark on the distributor body (toward #1 cylinder). Pulled the distributor anyway to check the gear and it looks perfect. Lastly I’ve replaced the ECM because it’s the last thing I could think of and it’s not giving me any codes on the scanner. This thing went from perfectly running to backfiring out of the throttle body and I can’t figure it out. Perfect fuel pressure and beautiful spark. Please throw any ideas you have at me. I‘ve built engines and I’ve done an LS swap so I know a little about working on cars but, I guess I could still be missing something simple.

I should have mentioned this is a crank no start issue with all the indicators of being way out of timing. Like it sounds like it’s going to break the starter off kinda timing. I’m not getting any codes on my handheld code reader. These are the same things that happened to someone else in here. I found a thread (hours of reading) that sounds the same as mine and it was a simple distributor cap swap that fixed it. I’m hoping that’s my problem. I’ll know soon.

Jeffrizzzo · Aug 7, 2022

phantom 309 said:
Backfiring could be lean condition,. vacuum leak.?

The spark plugs look like it was running perfect prior to this issue. Fuel pressure is within spec also.

Jeffrizzzo · Aug 7, 2022

95burban said:
Fuel pump, vortec are extremely picking about fuel pressure.

Fuel pressure is great. 61 pounds while cranking.

stutaeng · Aug 7, 2022

Have you checked to see if you have spark? I would check at each plug.

Do the spark plugs smell like fuel? They should. They could also possibly be flooded. If so, take them out and cleaned dry them. Cranking with the accelerator pedal all the way to the floor stops injector pulsing and you get it to fire it that way... assuming you do have spark.

Jeffrizzzo · Aug 8, 2022

Final update. It was a faulty, one month old, Accel distributor cap. I grabbed a new AC Delco and the truck fired right up. I can’t believe I’ve worked in this thing everyday for a week and a half and it was just the cap. I did inspect it very well as I’ve had it off several times and it looks perfectly fine. Anyway I hope this thread helps someone in the future.

98 Vortec 305 suddenly out of timing

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