Renix Compression Test

First the background:

1990 4.0 w/ 228,000 miles

Blew the freeze plugs out about 3 months ago. Replaced the plugs and ran it about 3 months. It blew them out again and now it will not start. It turns over fine. No antifreeze in the oil at all

It is running extremely rich. (blowing black smoke and you can smell the fuel out the tailpipe)

I was suspecting a blown head gasket so I ran a compression check.

Here are the numbers.

#1 150
#2 130
#3 140
#4 130
#5 135
#6 132

These numbers surprised me for a engine with this many miles, but it still will not start.
What else am I missing? I have checked spark and I have good fire on all cylinders.

A lot of possibilities.
I've had weak spark that wouldn't fire a plug, one reason I always check spark with a plug in the cable, then to ground, opened up to around .060" instead of just the cable to ground. It always turned out to be water in something it shouldn't be.
Coolant is a good conductor and can short out most anything, better and faster than water or mud. A whole list of connectors that may have coolant in and on them, that could cause sensor problems. Really hot water and a small scrub brush works better for getting coolant off than spray or whatnot.
MAP vacuum line pops to mind, if it is broken, disconnected or pluggedi in the wrong hole, the motor will barley run if at all, the motor tends to run way rich.
Stuck IAC can flood your motor.
You mentioned when it ran, it ran way rich. So fuel supply problems may be less likely, but assumptions can bite you in the rear. Check your fuel rail pressure.

I was thinking on the line of MAP sensor and vaccum lines myself, but would that cause too much pressure in the crankcase? The freeze plugs blowing out is what is really confusing. It is acting like the head gasket is blown allowing pressure to build, but the compression check looked good to me.

Your compression values show an engine that's been maintained well. I've had my 88 since 2001 and change the oil regularly and use high quality oil (Castrol). My comp values, the last time I did this over a year ago, compare to yours.

As for your current problem, what do the plugs look like? Why are you blowing freeze plugs?

SCupstateXJ said:

I was thinking on the line of MAP sensor and vaccum lines myself, but would that cause too much pressure in the crankcase? The freeze plugs blowing out is what is really confusing. It is acting like the head gasket is blown allowing pressure to build, but the compression check looked good to me.

Click to expand...

When you get it going again and up to operating temperature, watch the top radiator hose, if it starts to swell and gets hard as a rock, It's likely your radiator is partially plugged.
i use a quality adhesive when I put freeze plugs in, Indian Head cement or the old style Permatex, not silicon.
Fluids don't compress much and a moderate fluid pressure can build a lot of force quick. Just a few PSI in a tire can lift your whole XJ.
The excess pressure should vent someplace, hopefully at the cap. Even the closed system cap has a pressure vent, it can stick shut.

The way I test for head gasket leaks is to pump air into each cylinder at TDC for that cylinder (hard to do unless you take the valve cover off), look for bubbles in the cooling system. When head gaskets leak into a cylinder they usually suck coolant that eventually finds it's way out the exhaust as steam or drops, as well as push pressure and hot gases into the cooling system.

My plugs are black, another indication that it is running too rich. As for the freeze plugs, I have no idea what is going on.

As for maintenace, I have no idea how the previous owner treated it, but I try to stay on top of it. I run Rotella syn oil and Napa Gold filters.

Man I want to get this thing back up and running, I've got a locked 8.25 w/ disc brake conversion and new leaf springs waiting to go in.

8Mud said:

When you get it going again and up to operating temperature, watch the top radiator hose, if it starts to swell and gets hard as a rock, It's likely your radiator is partially plugged.
i use a quality adhesive when I put freeze plugs in, Indian Head cement or the old style Permatex, not silicon.
Fluids don't compress much and a moderate fluid pressure can build a lot of force quick. Just a few PSI in a tire can lift your whole XJ.
The excess pressure should vent someplace, hopefully at the cap. Even the closed system cap has a pressure vent, it can stick shut.

The way I test for head gasket leaks is to pump air into each cylinder at TDC for that cylinder (hard to do unless you take the valve cover off), look for bubbles in the cooling system. When head gaskets leak into a cylinder they usually suck coolant that eventually finds it's way out the exhaust as steam or drops, as well as push pressure and hot gases into the cooling system.

Click to expand...

I used the Indian Head cement when I replaced the freeze plugs

I will try you technique for checking the head gasket. I always thought the compression test would show leaking head gasket, especially if the #'s were way off.

Thanks for the tip

SCupstateXJ said:

I used the Indian Head cement when I replaced the freeze plugs

I will try you technique for checking the head gasket. I always thought the compression test would show leaking head gasket, especially if the #'s were way off.

Thanks for the tip

Click to expand...

A compression test like the one you did is better for determining the health of your piston ring seals. The air test that Mud describes is a better exam for blown head gaskets.

Neither test is foolproof or conclusive. Head cracks, head gaskets, block cracks and leaky valves are all possible leak points in a compression and leak down test. You can do a similar leak down test of the radiator (and thus the head gasket) at a radiaotr shop, or buy the tester and do it yourself. Looks like a mini bicycle pump wih a radiator cap. But even it may miss a high pressure combustion leak.