Souces of low compression?

About 10 years ago I checked the compression on my 87 Wagoneer - Renix and had about 130 to 140 psi on all but #1 and #2 cyls. Not sure how low they were, but they were about 5 and 10 lbs over factory minimum spec and all cyls were close enough to each other to be balanced. But I have fixed may issues since then and put about 60,000 more miles on the beast. Been running 20W50 all that time.

#1 and #2 have always had some oil fouling of plugs #1 and #2 over time while the other four plugs look new after 30,000 miles and up. I replaced the valve stem seals about 3000 miles ago on #1 and # 2 and got an improvement in the oil fouling issuing which had gotten way too bad at 285,000 miles. Had to clean the plugs every 250 miles at the worst point. I plan to run a wet compression test soon, but I was already planning to pull the head to get new valve guides and try to get another good 2-3 years out of her with just new valve guides. It is not a going to see a lot of miles. But my MPGs have dropped and I am wondering if it is time to do a complete rebuild or swap. The jeep has always been gut less with the low compression, compared to our 89 Cherokee.

So my question is this. I have no coolant or oil problems. Only oil problem is a recent increase in oil usage. Now using about 1 quart in 300 miles I think. No oil leaks, zero. Most of that could be the leaking valve seals and bad valve stem guides, as the oil usage the last 10 years with 20W50 was not bad. Maybe a quart in 1500 miles.

If I get low compression on my pending compression test on #1 and or #2, could the valve guide seals be so bad as to affect the compression test results? What results would say swap the engine and forget about the head job? Time is a constraint right now, so I am hoping to get a away with a head job for now. The engine does run smooth as glass until the #1 plugs get so fouled as to miss (takes 300 to 1000 miles, I am testing hotter plugs right now/next).

Hotter plugs, which by the way brings up topic 2.

Which plugs would you use to step up the heat range one step for #1 and #2 to burn off the oil and keep the plug cleaner longer?

Valve guides shouldn't cause a noticeable compression loss since it's on the other side of the valve sealing surface. Only way I could think of that would cause a compression loss is if tye guide is worn enough to allow the valve to tilt side to side and not seal properly

Valve seals will increase oil consumption and can lead to plug fouling, but will not affect compression.
The compression is set between the valve and the valve seat, and the rings. Rings, valves, blown/weak head gasket, or warped head will cause compression issues.

I have had the head off my 01 XJ twice and the head off my 97 XJ once. Typical interval is 160,000 - 180,000 miles. Compression always goes up when it goes back on.

You'll need to identify if it's the valves or the rings causing your compression loss.

If its the valves, the fix is easy, pull the head, send it to the machine shop, and put it back on with a new head gasket.
If wet compression test bumps up the compression significantly, a swap or lower end rebuild is probably in your best interest.

A quart in 300 miles is insane.

I have had valve seals riding on the valve stems and could go 5,000 miles before it was down a quart! I typically use 5w-40 synthetic & can go 12,000 miles with no appreciable oil level drop. I've been doing it for 300,000 miles on my 01 XJ and the engine still works great.

I suspect you have bad rings but check before reaching any conclusions.

Dragonlich1961 said:

Valve guides shouldn't cause a noticeable compression loss since it's on the other side of the valve sealing surface. Only way I could think of that would cause a compression loss is if tye guide is worn enough to allow the valve to tilt side to side and not seal properly

Click to expand...

That may just be the story here. The rocker was pushing the stem sideways on #1 and #2 when I got in there to replace the valve seals, and I could feel and see the sideways play in the valve guides on #1 and #2.

So if it has run this way for 10 years and 60,000 miles or more, the valve seats may be just about gone and thus may lower the compression. I have enough compression to run smoothly, but no where near the power, torque-guts it should have.

Good feed back, thanks. Your increase in compression could be due to the machine shop milling the head? I wonder how much compression that adds. I will ask my machine shop buddy (He races cars, great guy).

I may be able to get some extra compression back and buy a few years on this old beast before I am forced to swap the engine finally.

Never heard this before "If wet compression test bumps up the compression significantly, a swap or lower end rebuild is probably in your best interest."

What is significantly? I plan to do the wet test because I suspect miss fires have let unburned gas and and thus gas contaminated oil, de-oil the piston/cylinder(s) enough to affect the test results. New valve guides would fix this.

That leaves me with the next issue of cost of new guides, and maybe new rocker parts, versus trying to find a good used renix head near me?

md21722 said:

Valve seals will increase oil consumption and can lead to plug fouling, but will not affect compression.
The compression is set between the valve and the valve seat, and the rings. Rings, valves, blown/weak head gasket, or warped head will cause compression issues.

I have had the head off my 01 XJ twice and the head off my 97 XJ once. Typical interval is 160,000 - 180,000 miles. Compression always goes up when it goes back on.

You'll need to identify if it's the valves or the rings causing your compression loss.

If its the valves, the fix is easy, pull the head, send it to the machine shop, and put it back on with a new head gasket.
If wet compression test bumps up the compression significantly, a swap or lower end rebuild is probably in your best interest.

A quart in 300 miles is insane.

I have had valve seals riding on the valve stems and could go 5,000 miles before it was down a quart! I typically use 5w-40 synthetic & can go 12,000 miles with no appreciable oil level drop. I've been doing it for 300,000 miles on my 01 XJ and the engine still works great.

I suspect you have bad rings but check before reaching any conclusions.

Click to expand...

A wet test is used to check the sealing ability if tye Ring's. If compression increases then worn rings and judging my oil consumption possibly worn cylinder walls.
A full rebuild would be the only fix.

Yes, wet is mostly testing the rings. If you have a leak down tester you can do this too. Significantly is subjective but I would say any big jump. Typically if its rings its going to jump a bunch. Way more than 5 psi.. maybe 20-50. I attribute most of the compression jump to be going through the valves. If you go hang around shops its not uncommon for valve jobs every 150,000 miles to help out... and some I know put the head back down without even sending it out for planing because they didn't like the machine shops in town... It sounds like you are on the right path, know what to do, but are looking for some evidence to pull the trigger and spend the time and then money. So do the wet compression test and let us know. Any machine shop should be able to go through the valves pretty quick. Last head I had done cost me $135 cleaned, planed, and valve job. The was a good price, $300 is probably more common.

Wouldn't a weak or broken valve spring affect compression? Pretty sure I have a problem with a spring currently. Compression was down a little in #1 from the rest but still well within spec. Failed leak down miserably and have a misfire at idle.

It's a possibility. For a DIY job I suppose you could pull it and test. Usually by that point I am removing the head and sending it to the machine shop. Any good machine should test and let you know. In every case I've had valves have not needed to be replaced.