Help me understand this compression test - 1 cylinder is HIGHER than others.

[CrackedHorn]

Did a compression test on my 15 GT with a cold engine.

All cylinders read at basically 212 psi except cylinder #5 which reads 240.

I repeated the test multiple times to verify. Wet test still increased readings verifying the increased reading isn't from oil.

All spark plugs looked basically the same. All plugs had a small amount of oil on the threads but also looked to be burning cleanly.

Car has been running fine. No detectable oil loss that I've ever seen.

So, am I possibly dealing with Carbon buildup? Sticky exhaust valve? Valve seals? Bad lash?

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[K4fxd]

Only way to know for sure is take it apart. Run a bore scope, if it's carbon you should see it.

 

[CrackedHorn]

Only way to know for sure is take it apart. Run a bore scope, if it's carbon you should see it.

Picture of #5 piston.

Other pistons show a little more carbon on them. #1 and #2 show slight rust colkred deposits? It's hard to tell

 

[CrackedHorn]

Piston 8

 

[CrackedHorn]

Piston 6 has heavy deposits

 

[CrackedHorn]

Piston 1?. Can't remember which this is.

Possible signs of detonation?

 

[K4fxd]

Yes seeing signs of detonation. I can't tell if #5 has build up or if it is a shadow. Mist some water through the engine while running. This should clean the carbon.

Is this engine tuned? What octane gas do you run?

 

[CrackedHorn]

Pistons 2 and 3, I believe .

 

[CrackedHorn]

Yes seeing signs of detonation. I can't tell if #5 has build up or if it is a shadow. Mist some water through the engine while running. This should clean the carbon.

Is this engine tuned? What octane gas do you run?

It definitely looked like carbon to me when taking the photo.

And carbon buildup in #6 looks wet, rather than caked on.

Car has been running/driving like normal. 91 octane, lund tuned.

 

[K4fxd]

Get it fully warmed up, open the PCV hose and spray some water or sea foam just go lightly. This will clean the carbon from the pistons.

Lund sucks for customer service but I would send picks of the detonating cylinders and ask them to add fuel or reduce timing in those cylinders.

 

[CrackedHorn]

Get it fully warmed up, open the PCV hose and spray some water or sea foam just go lightly. This will clean the carbon from the pistons.

Lund sucks for customer service but I would send picks of the detonating cylinders and ask them to add fuel or reduce timing in those cylinders.

Agreed.

My main concern now is that I am going to add forced induction in the coming weeks.

I'm now wondering if I should pull the motor. Ooof.

 

[K4fxd]

I'm now wondering if I should pull the motor. Ooof.

How many miles on it?
What is your budget?
Do you need the car back on the road ASAP?

 

[CrackedHorn]

How many miles on it?
What is your budget?
Do you need the car back on the road ASAP?

It has about 97k miles. I've owned it for 4 or 5 years and have treated it very well.


Budget limited. Under $3k right now.

I was planning on getting a forged shortblock at some point once this engine is retired. I was just hoping that would be another 1-2 years away.

This isn't my daily driver so no immediate need. That said, I love driving it and absolutely don't want to go all summer without it

 

[K4fxd]

I'd install the FI system and drive it. This winter or next I'd freshen it up.

With stock components limit it to one 60 to 130 every 10 minuets and same with the 1/4 mile, one hit to every 10 minuets.

Search Lund and make sure you want them tuning your car. I don't know what happened over there but they have gone to hell since the EPA raided them.

 

[K4fxd]

When you refresh it open up the ring gaps for boost.

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