95 Yukon Back fireing again.

[99Vortec]

Would a compression test help to eliminate an intake valve problem?

I should do one myself just to know.


"(Multiply the highest compression reading that you recorded by .15).

So, let’s say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).

Now, the next step is to subtract 26 from 170... which gives 144 PSI.

So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire."

"Written by: Abraham Torres-Arredondo"

 

[SunlitComet]

engine must be hot. I know that is kind of hard right now. block throttle open, and minimum pressure is 100psi and lowest can not be less then 80% then the highest. saying that it will misfire if it is may be a bit much. it could indicate valve issues or worn rings but will not specify the valve just the cylinder with the issue. a leakdown test helps to find individual leaks but you need a constant air supply.

 

[99Vortec]

So more like 20% wiggle room?

Would the engine control module be able to interpret a backfire as a "misfire" properly?

(The Lean condition is what made me ask. I was summarizing that the mixture was being expelled through the throttle body.)

 

[SunlitComet]

vortecs can go to 70% of highest. the 80% is for the tbi's.

The misfire is set when he computer notices a momentary slowdown in the crank. so if the intake back fire were to cause the same effect then yes it would register. it is a little more trick with intake backfires tho for the misfire history as it sends your map and maf on a sudden and quick bit of inaccurate date. if you look at the misfire history in regards to intake back firing look not only at the cylinder indicated but the one before and after in the firing order as well. TBI's can not really identify misfires.

 

[youngjs1974]

No with the coolant sensor disc it still does the same thing no change. Also I have checked compression and all of cylinders are with one 10% of each other on the high side. Don't remember what the psi was. Yes I replaced the icm. There is a single wire under the dash you disconnect to check timing.

 

[SunlitComet]

Well if compression is good as you say with the same pumping sound and feel between each cylinder I would go back to looking for fuel or ignition issues. You said when you replaced the icm it work for a while correct? What brand do you have and did you use the thermal grease on it?

 

[youngjs1974]

Yes I used the grease. Brother came by and checked it out he is leaning toward fuel pump. Going to check the pressure with a gauge today

 

[SunlitComet]

thought you did that and said it was okay.

 

[youngjs1974]

Did not check with a gauge. When I had the tb off I turned the key on and saw there was plenty of gas coming out of the line. So I assumed it was good. Could not find a test kit any where close and was needing the truck for work.

 

[SunlitComet]

gotcha.