"Normal" compression for beat 1989 4.0L?

Hey all -

I am at a tipping point with my beater XJ. Before I put more money in to it for maintenance (rear leafs, brakes all around, front U joint, full fluids all around), I wanted to get an idea of the strength of the engine.

This is a 1989 4.0L - non-HO. I can't find what are considered normal readings for the non-HO 4.0L. The engine has 197,800 miles on it - no idea if it's original or if it has even been rebuilt (my guess is no). I did a dry compression test while warm, using the starter (like a lazy mofo).

1 - 140
2 - 125
3 - 135
4 - 135
5 - 125
6 - 120

I realize there's a lot of variation, but they're all within 15% of cylinder 1. Anyone want to weigh in? I just want to have a decent idea that it's not a complete piece before I dump more money into it... if so I'll just sell it and get another XJ. :firedevil

Thanks for any input!

--Andy

The compression spec for the 4.0 is 120-150, with no more than a 30 psi variation between cylinders. Your numbers really don't look that bad for an 89 engine. Miles could be original.

Remember that if down the line you have engine problems, the engine is a COMPONENT. A large one, but one that can be pretty easily swapped out. If you like your vehicle and it isn't rusted badly and the drivetrain works well, I'd certainly invest in basic maintenance items. If you buy another XJ, you might be buying somebody else's problem.............

Thanks for that birchlakeXJ. That's all I really wanted to hear. I knew it was in good shape, I didn't think it was in THAT good of shape.

Time to continue the project!

--Andy

yup, those look fine to me, too. I'd keep it full of oil and fuel and run it till it blows.

Run a can of BG44K through it. It will clean the injectors and get rid of a ton of carbon. More than likely it will even help your compression numbers, but cleaning the injectors will yield a good bump in HP in most cases.

old_man said:

Run a can of BG44K through it. It will clean the injectors and get rid of a ton of carbon. More than likely it will even help your compression numbers, but cleaning the injectors will yield a good bump in HP in most cases.

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True story.

My 87 Wagoneer, has at least one cylinder down around 110-115 psi, with about 120-125 average, IIRC, and that was a test I ran 6 years ago, 50,000 miles ago, now at 178,000 miles, and it passes emissions tests ans does not use oil, but I use 20W50, down in Houston.

I have gotten mileage as high 18 mpg highway with it, but when I stomp on the gas, it does not have the same low end power, get up and go, my other renix jeeps have, that still have factory spec compression.

Ecomike said:

My 87 Wagoneer, has at least one cylinder down around 110-115 psi, with about 120-125 average, IIRC, and that was a test I ran 6 years ago, 50,000 miles ago, now at 178,000 miles, and it passes emissions tests ans does not use oil, but I use 20W50, down in Houston.

I have gotten mileage as high 18 mpg highway with it, but when I stomp on the gas, it does not have the same low end power, get up and go, my other renix jeeps have, that still have factory spec compression.

Click to expand...

Run some BG 44K through it and see what happens.

cruiser54 said:

Run some BG 44K through it and see what happens.

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LOL, you know you are talking to Dr. Snake Oil Mike here, Right?

LMAO!!!!:laugh3:

Of course I do. That stuff works though.

cruiser54 said:

Of course I do. That stuff works though.

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Hand grenades work too, LOL!

It is not near as aggressive as some others:
http://www.petrotech.co.il/PetroTech//userdata/SendFile.asp?DBID=1&LNGID=2&GID=470

Seems to be nothing more than stoddard solvent, and old exxon mineral spirits. Hell diesel fuel would probably do the same thing.

Seafoam is way over priced, good stuff if you eat caviar, LOL.

Gumout is cheaper and has acetone, MEK and xylene, plus the BG 44K ingredients, and

Berryman B-12 Chemtool is the best one I have found, low cost, and has Toluene, Acetone, MEK, methanol and 2-butoxyethanol with isopropanol, a much better formula for deep cleaning the carbon out. It is a formula I would use if I was creating my own formula.

I use Gumout and B-12 myself.