Shift light flashing and tachometer jumping

Tiny
SHULK40
  • MEMBER
  • 1994 CHEVROLET 1500
  • 5.7L
  • V8
  • 4WD
  • MANUAL
  • 85,000 MILES
When I start up when it is cold the shift light flashes the tachometer jumps and engine runs rough. Like the timing is off. I have set timing and checked it 3 times. When the engine warms up the shift light quit flashing and tach quits jumping. Idles good but I still don’t think it running as good as it should like the timing control doesn’t work like it should unless you are running over 2,500 RPMs. I have replaced ground wires from cab to frame to engine. And checked every other ground wire on engine. I have replaced O2 sensor. New distributor complete. Coil plugs wires. I have a ESC box on the intake but I haven’t changed it yet. Just trying to see what direction to go from here?
Saturday, February 6th, 2021 AT 6:51 AM

1 Reply

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
This is not an uncommon issue one these engines when the intake gaskets are leaking. What happens is when they are cold they will leak and pull in unmetered air and the vehicle will run poorly. Then when you warm the engine the gaskets seal a little better and the o2 sensors start to adjust for the lean condition and it smooths out.

I would suggest starting with checking for codes. Here is a guide that will help with this:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Then we need to check for vacuum leak. Please follow this guide on how to do that:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Or if you want to use a vacuum gauge hooked up to a vacuum line. I attached a diagram below on what the readings mean. If you are not sure, get a video of the gauge while it it running poorly and we can figure it out.

If you find no vacuum leak we may need to check this with a gauge anyway because you may have a sticking intake valve.

Thanks
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Sunday, February 7th, 2021 AT 7:51 AM

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