Low Compression?

Hi there,

to diagnose this correctly, you can have a wet & dry comp test done or a leak down test, the wet & dry will show up any valve leaking as a wet comp test is where you add a small amount of oil to each cyl , this temporally seals the rings and if the comps stay low the leak is in the valves, a leak down test is where air is pumped into the cylinders and if thee is a leak from a valve you will hear it in the exhaust or inlet port.

These guides will help us fix it

https://www.2carpros.com/articles/the-reasons-for-low-compression

and

https://www.2carpros.com/articles/symptoms-of-low-compression

Please run down these guides and report back.

Hi, I'd request a "fuel injector relative flow" test also, - just to rule out - excess fuel could've washed cylinder walls causing low comp in two.

Hi there,

I would get this looked at ASAP as it will deteriorate and it may get to the point that I will let you down and cost more to repair.

Mark (mhpautos)

Nceagle, do the wet and dry compression tests. Put about a teaspoon of engine oil in the spark plug hole and do the compression test. If the causes the compression to pop up, you have bad rings. If not, valves.

One reason I like working on cars is that you can fix anything with enough money. The dealership lied to you. Valves and rings can be fixed relatively easily by any mechanic worth his tools. They're both labor intensive jobs, so they aren't terribly cheap.

YOU can do all of the re-ringing yourself with a motor repair manual. You can also do MOST of the valve job yourself with a manual. You'll just need to take the head to a machine shop to have the new seats installed and cut.

Hello,

This could be from a few different things. My guess without having the truck right here in front of me to inspect would be a blown head gasket, but it could be a bent valve, worn out piston rings, or a burnt valve. I have included a couple of links down below for you to go to:

https://www.2carpros.com/articles/symptoms-of-low-compression
https://www.2carpros.com/articles/the-reasons-for-low-compression
https://www.2carpros.com/articles/head-gasket-blown-test

Please go through these guides and get back to us with what you are able to find out. Once we get the problem narrowed down we will have specific instructions on how to fix it.

Thanks,
Alex
2CarPros

Typically the cel blinks when too much raw fuel is entering the combustion chamber and not being burned. If too much fuel had got into the cylinders it may have washer down the cylinders causeing the low compression and rough running.

I would start by pulling codes. I would shoot a litttle oil into each cylinder and see if the compresion returns. Oil probably smells pretty gassy.

Welcome to 2CarPros. Is the low compression all on one bank or is it split across them?
The 4.0 V6 has a nasty habit of breaking the timing chain guides and the chain get's loose and slips. They have been updated in the newer engines. They are not a fun engine to work on as they have one timing chain at each end of the engine with a jackshaft in the middle of the block that has another chain for a total of 3 timing chains. These are an interference engine as well so if any of the three chains fail while the engine is running you get valve damage.
The easiest way to verify this is to remove the valve-cam covers to see if the cams are moving.

If engine cranks then compression can be done. Have you checked for spark and fuel?

Since you are using the 95 head and cam, you should be using the timing for this engine.

If you are not able to get the correct timing, turn the # 1 piston to TDC and then use the valve overlapping method for cylinder # 4 to get the correct valve timing and compare with the marks.

No problem

2 people---will not take long--evil female in you house will do!

Disconnect the coil so it cannot start

take out #1 spark plug

watch out for the fan!

Stick your finger/ thumb in the empty hole tightly!

Evil woman "bumps" the motoror turn it in the correct direction with a wrench

(regardless, it will build compression, bumping with the key, sometimes lets the engine "roll or kick back" when the key is released. Sometimes using a wrench is better)

this is a 4 cycle engine--the crank rotates 2 times (piston up 2 times)for every one time it makes a cycle

one is exhaust stroke (pushing toward the top)

the other is compression stroke--this is the one we want!

This is the only upward stroke that both valves are closed and air/ fuel pressure builds up--when it reaches the top, this is top dead center (tdc)

so here's the deal--when she bumps it and you get a "spit of air" by your thumb----stop stop stop! (You may want to practice a time or two, we want the beginning of the spit--not after you pass it!)

you are headed up, on compression!---Beginning of the spit

now you must turn the engine by hand, in the same direction it normally turns---using a socket on the harmonic balancer nut and a ratchet/ pull bar/ pull handle

turn it slowly until your timing marks line up on "zero"

this is tdc!

This is my jeep cj 5 6 cylinder (258) on 8 degrees before tdc (btdc) and the next pic is tdc (zero) yours is different!

Remember this happens twice in a cycle (timing marks line up)--only once for the compression stroke. Compression will not build until both intake and exhaust valves are closed

with your modifications---if the cam(s) ain't timed to their correct open/ closed positions for "tdc"---this "old time method" just ain't gonna work!

The medic

No problem. It is always good to have a second opinion. :)