Cylinder #4 misfires (but no reason to?).

[Alpha one]

My 1995 LeSabre misfires on cylinder #4. In fact it almost never fires. The other 5 never fail to fire.
But the spark plug has spark, and the injector sprays.
It has new wires and new spark plugs, new coils and ignition.
Fuel pressure OK.
I switched injector with another cylinder, but #4 continued to misfire.
The car starts every time and it will drive places. I just don't want to until I fix that problem.

I did an incomplete cylinder compression test:
I attached a compression gauge to cylinder #4 and started the engine. I got 62 pounds pressure (Average, I guess).
Then, I connected the gauge to cylinder #3 and started the engine. I got 67 pounds pressure. A difference of 5 pounds.
Any idea why #4 doesn't fire? Could a little lower compression be the problem?
Thank you,
Antonio.

 

[MelsRegal]

Never heard anybody doing a compression test on a running engine.
Do you have code P0304 or are you OBD 1.5?
How did you determine it is not firing?
What does the plug look like?

 

[Alpha one]

No error codes. My 95 Lesabre has a pre version of "OBD". The OBD for the Lesabre started in 1996.
I do have a OBD like device that allows me to read codes and check how most sensors, etc. are working.
That's how I know that #4 is misfiring. I can see, as the engine is running, which cylinders are misfiring and are not. It shows Y (for yes) if it misfires and N if not. As I look at the display, I see it changing from Y to N depending on it misfiring or not. In my case, it shown #4 misfiring almost all of the time, and showing N (No) misfiring once in a while.
I also know #4 is not firing by listening and if I disconnect the wire to the plug nothing happens.

The plug is clean and if not firing. The others show signs of working. All plugs are new, changed about two weeks ago. But I also switched the spark plug and wire with another just to make sure they were not the problem.

Could it be bad valves? It is really strange since everything seems to be working.

 

[HotZ28]

Could be valve springs, valves, lifters or leaking head gasket between the two cylinders. Compression checks between 3&4 seem to be very low? Have you tested any other cylinders?

______________________________

 

[Homer Jay]

Could be a problem with the injector wiring (no power), check using a noid light.

 

[MelsRegal]

You do a compression test by removing all the plugs, attach the gauge to a cylinder, hold the throttle wide open and crank the engine using the starter. You do not attach the gauge to a cylinder on a running engine which causes those low numbers.

 

[Alpha one]

I took the intake manifold off and cylinder heads. The intake manifold gasket was bad. Water was going into the cylinder, but I don't think it reached the crankcase to mix with the oil.
The head gaskets were perfect. No leaks, nothing. But I had the chance to clean the carbon deposits.
New gaskets, no vacuum leaks, new ignition and good injectors, new spark plugs and wires.
However, the random misfiring is better but continues, mostly number 4, sometimes 3 or other, sometimes none misfires for a while. If I accelerate the engine a little they don't misfire.
I heard maybe I need to adjust the valves. There is no adjustment for it, except changing the pushrods length.
How do you adjust the valves?

 

[MelsRegal]

You pulled the heads and didnt send them to a machine shop to make sure they are not warped and the valve seats are OK?

______________________________

 

[imidazol97]

X2 on MelsRegal's comments.

You must change the oil, because likely coolant residue is in the oil which can damage bearings.