All 4 passenger cylinders not firing, but getting spark.

96GMCSIERRA

MEMBER
1996 GMC SIERRA
140,000 MILES

This is a "start-over" from my previous post: http://www.2carpros.com/questions/1996-gmc-57l-vortec-no-power-in-gear-good-idle

The other post just described symptoms, I have stumbled upon something else entirely, that I've never seen or heard before.

The entire passenger bank is completely dead. The exhaust is cool to the touch, not even trying.

I replaced plugs/wires and pulled the wires off one by one from the numbers 2,4,6,8 cylinders while the truck was running, and there was no change at all in the idle.(Yet if I take ANY of the driver side wires off, the truck nearly dies) I took the plugs out one by one and grounded them to the exhaust and saw a healthy spark from each plug and each wire. So I know it's getting fire.

The plugs had a small amount (thin layer) of gas, roughly the size around of a pencil eraser on the very end (Round part under the bend), but they were by no means soaked with unburned gas.

Any ideas where I should start checking?

(If it's worth mentioning, I do not have the breather on and the air suction into the throttle body is VERY loud.)

1 Reply

CARADIODOC

MECHANIC
33,873 POSTS

The loud sound of air flow is due to the idle speed motor opening the air passage around the throttle blade to get the idle speed up to where it should be. That same part is used on Chrysler products and it has so much control on Jeeps that a V-8 engine will reach idle speed with six cylinders disabled.

Try introducing some propane into the intake manifold, ideally through a vacuum port on the passenger side. If the idle speed picks up, that will confirm the injectors aren't firing. The first suspect would typically be the Engine Computer but start by measuring the voltage at those four injectors. There must be 12 volts on one wire and you should see voltage on the other one but that can be misleading when you're using a digital voltmeter. A test light works better in this case. It will be bright on the 12 volt feed wire and usually it will flicker and be a little dimmer on the control wire.

If the 12 volt feed is missing, suspect a corroded splice where those four injectors connect to those on the driver's side. If the 12 volts is there but it's solid and bright on the second wire too, that would point to a failure in the computer. Your engine may use what's referred to as "batch fire" where all four injectors are fired as a batch at the same time. Otherwise there would be four separate failures and that's not likely.