Low compression 5.0 back fires

[dean1roberts]

I have a 98 k1500 I have 60 psi in cylinder 3 all other cylinders average 150-155 psi. I don’t know if it’s Due to the loss of compression but this thing backfires like hell. The power drops off at 2500k and starts to backfire. Just trying to hear out similar problems and take pointers for what I’m looking for when I start to take this thing apart

 

[Schurkey]

Backfire out the exhaust, or backfire into the intake manifold?

If it's into the intake, look for burnt/bent intake valve, or an exhaust valve that doesn't open (wiped cam lobe, bent pushrod, etc.) An exhaust valve that doesn't open wouldn't lower the cranking compression pressure, though.

If it's into the exhaust, look for burnt/bent exhaust valve.

Expect the valve seat(s) to need work as well.

A cylinder leakdown test could confirm valve condition.

 

[Hipster]

Ditto on a leak down test to determine whether compression is going past the valves or rings.

 

[Road Trip]

Backfire out the exhaust, or backfire into the intake manifold?

A cylinder leakdown test could confirm valve condition.

^^ Like he said!

To further amplify what @Schurkey said, the beauty of taking the additional step
of a leakdown test prior to teardown is that you have a much sharper picture
of what you are wading into.

For example, if you are putting 100psi (at TDC on the compression stroke)
into the #3 cylinder and the hiss of leaking air is clearly heard when listening at
the tailpipe, but quiet at the throttle body, then you know that it's a
bad/bent/burnt exhaust valve and/or seat.

And if the exhaust is quiet but there's pronounced hissing at the throttle body,
then obviously the reverse is true. And how much/how little hissing is heard
at the oil fill opening will tell you just how good/how bad the ring seal is in the
wounded hole.

****

Speaking of which, how much (if any) coolant is being consumed? And how many
miles per quart of oil consumed are you experiencing? My reason for asking is
that if there's no coolant consumption & the the oil consumption is below the
threshold of pain then fixing whatever is indicated by the leakdown test makes
good sense.

NOTE: Before I added the leakdown tester to a normal compression test in my
troubleshooting toolkit I thought that it was a bit of a luxury to perform. Not
anymore -- with the price of everything these days I like to hedge my repair bets
as much as possible with hard diagnostic data *before* I (or whoever I'm helping)
decide to start down the repair path.

Let us know any more info that you've got, and if you can elaborate a bit more
on when it went from good to bad then that will help us further refine our educated
guesses. :0)

****

For what it's worth here's a photo of a cylinder head that I troubleshot down to
a burned exhaust valve prior to removal thanks to the *loud* hissing out of
the tailpipe prior to disassembly. By the way, thanks to the reading of the spark
plugs I already knew which hole was suspect.

Troubleshooting, Sesame Street style -- "One of these things is not like the others."

You must be registered for see images attach

Next, the compression test was good in 3 cylinders, but near-zero in the hole
where the discolored spark plug was installed. I then performed a leakdown test
on the bad cylinder, which confirmed my suspicion that the exhaust valve(s)
as the root cause.

Given at the time that this was my only vehicle, based on this info I decided to
minimize the time it took to go from 3 back to 4 cylinder power and ordered a rebuilt
replacement head, swapped that in, and then after setting the valve lash I
confirmed the fix with a follow-on leakdown test before firing it back up -- it was
excellent, all 4 cylinders now matched! It fired up flawlessly, and no further
issues for the remainder of the time that I got to drive the car.

To summarize, I definitely also vote for a leakdown down prior to teardown so
that you are going in with as much info as you can reasonably gather.

Forewarned is forearmed! :0)

 

[Schurkey]

the beauty of taking the additional step of a leakdown test prior to teardown is that you have a much sharper picture of what you are wading into.

Oh, yes.

For example, if you are putting 100psi (at TDC on the compression stroke) into the #3 cylinder and the hiss of leaking air is clearly heard when listening at the tailpipe, but quiet at the throttle body, then you know that it's a bad/bent/burnt exhaust valve and/or seat.

This can be done with a leakdown tester, OR a plain ol' compressed air hose if you've got a way to plumb it into the spark plug hole. Lotsa folks use a compression tester hose screwed into the plug hole AFTER removing the Schrader valve; and plugging the quick-coupler end into an Industrial Interchange (Milton "M") coupler body on the air hose. (Not all compression testers have a quick-coupler in the hose, and not all quick couplers are Industrial Interchange...but most of 'em are.)

Leakdown tester recommended, but not essential. The leakdown tester has a regulator to drop the air pressure to something manageable--60 to 100 psi typically. The higher the pressure, the more-likely you are to blow the piston down the hole, turning the crank in the process. It's fairly crucial to either get the piston to EXACT TDC compression stroke, or lock the flywheel.

The leakdown tester is not essential, because ANY leakage at a valve is too much leakage. If you hear a leak...the head is probably coming off.

And if the exhaust is quiet but there's pronounced hissing at the throttle body,
then obviously the reverse is true.

Again, this can be done directly from an air compressor, a leakdown tester is recommended but not essential.

And how much/how little hissing is heard at the oil fill opening will tell you just how good/how bad the ring seal is in the wounded hole.

Well...no. There's ALWAYS going to be "some" leakage past the rings. That hissing echos around in the crankcase, and in the valve cover. Little leaks can sound "big". It's essential to have a leakdown tester so that the AMOUNT of leakage can be indicated by the gauges.

Not mentioned previously--Pop the radiator cap when testing the cylinders, always check for air bubbles in the cooling system when each cylinder is pressurized. I check the radiator AFTER checking the tailpipe and throttle body, and the oil fill cap or other escape for crankcase pressure. That way there's plenty of time for the air in the cooling system (if any) to make it's way to the radiator and become visible. Air into the cooling system means a popped head gasket, or cracked casting(s)--head or block.

NOTE: Before I added the leakdown tester to a normal compression test in my
troubleshooting toolkit I thought that it was a bit of a luxury to perform. Not
anymore -- with the price of everything these days I like to hedge my repair bets
as much as possible with hard diagnostic data *before* I (or whoever I'm helping)
decide to start down the repair path.

YES! The downside is that an air compressor is required, and with the popularity of battery/rechargeable impact wrenches, screwdrivers, drills, etc. not so many folks want to invest in freshly-squeezed air.

 

[Road Trip]

Well...no. There's ALWAYS going to be "some" leakage past the rings. That hissing echos around in the crankcase, and in the valve cover. Little leaks can sound "big". It's essential to have a leakdown tester so that the AMOUNT of leakage can be indicated by the gauges.

***

Not mentioned previously--Pop the radiator cap when testing the cylinders, always check for air bubbles in the cooling system when each cylinder is pressurized. I check the radiator AFTER checking the tailpipe and throttle body, and the oil fill cap or other escape for crankcase pressure. That way there's plenty of time for the air in the cooling system (if any) to make it's way to the radiator and become visible. Air into the cooling system means a popped head gasket, or cracked casting(s)--head or block.

You make great points! Re: Rings always leak. This agrees with everything I've
observed as well. Best engine I've ever checked was ~4% leakage. Worst NA
stock motor was ~20%. The 4% engine was quieter at the oil fill cap...but it
*did* still hiss. (Could throw off someone new to this kind of test.)

Really like your comment about taking off the radiator cap & checking that also.

Good stuff! With my previous reply the OP would have actionable info with
access to a leakdown tester + compressed air.

With your additional recommendation the OP can get most of the same actionable
info just with plumbing compressed air to the spark plug hole -- sure, a little more
of a Go/No-Go test...but oftentimes that's all you really need anyway.

And to wrap all this up -- OP, as Schurkey mentioned be careful to really get as close to
TDC on #3 as you can when adding compressed air. Just a little off and the engine
will swing 180° in a heartbeat! (Ask me how I know this. :0)

****

Now to wait for more info from the OP & see where it leads...