My compression test results... What now?

Valves are in spec. Engine is cold. Other plugs were in while running the test.

Cylinder
1 - 100 psi
2 - 105 psi
3 - 130 psi
4 - 135 psi

I read that the engine should be warm before testing... Is this true? I'll have to do a bit more work and get the bike running again. It has sat for about 6 weeks (drained the oil to change it, had the wrong filter, then got swamped at work with travel). Finally got the right filter and oil back in yesterday.

Looking at these numbers... What do the XJ gurus recommend as my next move?

 

You need the following to get good results:

- Fully charged battery
- Throttle wide open
- ALL spark plugs out

You also need to turn the engine over enough times to build pressure. I've got a gauge with a really long hose that takes a number of revolutions to build up.

You should either ground the plug wires or disable the TCI. Since it's easy to get to I suggest you simply unplug the TCI.

Run your test again meeting the above requirements.

THEN, if it's low, put about 1 teaspoon motor oil down each plug hole and run the test again.

Post both sets of results.

 

Thanks! I did unplug the TCI and let it run until the compression stopped building. Also charged the battery last night. I'll open the throttle up and try some oil in it tomorrow. Is a warm or cold engine important?

 

Cold is OK remove the air filter as well.
Put the spoon of oil just in the cylinder you are testing.

 

I'd disagree with doing this up front. This is a follow-on troubleshooting step to determine whether a low reading is more likely due to rings or valves.

1 and 2 are low, and are also significantly lower than 3 and 4. If your readings on 1 and 2 go up significantly after adding a bit of oil, then the problem is probably rings/piston/cylinder wear or damage.

If the readings don't change much with the oil, the problem is more likely valves (burned, bent, or otherwise not seating well), loose head nuts, or a damaged/leaking head gasket.

Hopefully the problem's in the second category, as those types of problems are usually a lot cheaper to fix.

 

Of course the dry test is always done first to get the base reading, see gearhead's first post.

 

The factory books recommend at least a slightly warm engine ("run engine for 2~3 minutes") for compression testing.

That being said, it sounds as though your methodology was a bit off; all 4 plugs need to be out, etc.

You should still get "representative" readings with the motor cold, at least enough to compare cylinders. It's the differential between cylinders that's truly important; rule of thumb is 10% max difference.

Re-test, carefully this time, first dry then with oil.

 

Dropped some oil in the cylinders and came back with 115 and 145 psi on cylinders 1 and 2 respectively. It looks like the head needs to come off. I am currently reading through the manual for the process. Does anyone who has done this before have some advice for things to be prepared for? What parts should I have on hand to make this process go smoothly?

 

No parts yet; you don't know what you'll find.

 

Fair enough. Any tips for keeping the elements out of the engine when I open it up?

 

Are you working on it outside???

(Not really recommended for open-motor surgery.)

 

If you must work on it outside I would recommend the use of wd-40 or a light oil on the open parts and coat some rags with some motor oil to put on top of the exposed area as well, then a liberal use of plastic wrap and some tarps to top it off just in case it rains. You will want to check it like a bandaid every few days as damaging moisture can still enter and build up quickly. WD-40 helps a lot to take any moisture out, I just wouldn't recommend forgetting about it for any length of time or it will most likely only compound your problems.

~ At least it has worked for me on car and truck engines for rebuilds - but inside is definately recommened.

 

Yes unfortunately I am working under an open carport. My other option would be if I pulled the engine and my girlfriend let me rebuild it in her art studio. Maybe if we can get a bedroom cleared out I could rebuild it in there. Doubt she'd be too pleased about either of those options...

 

Don't you know anyone who would give you a corner of the garage ?

 

I wish. I moved here (very small freaking town) a few months ago, not knowing a single person within about 150 miles. Most of my time is spent working in a small engineering office. Most of the guys here live in apartments, and I'm not really close enough to ask anyone to borrow their garage. Even the storage units around here that I've seen aren't really big enough to do any work in. Only person I know with a garage is the girlfriend's mom, and I try to minimize contact with her. She's a bit off...

 

Those readings aren't terminal. Unless your bike is desperately ill I wouldn't worry for the moment given your accn situation....anyone got a tent they can loan him? :lol:

I'd suggest taking more readings and get an average ie cold, warm, just after a long ride, etc etc

 

Open carport as in 30 cars attached to an apartment building or 1 car beside a house? If the house case, you could seal it with plastic or tarps for the duration of the work.

 

Thanks for the advice guys. Girlfriend is gonna let me break it open in her art studio. Just have to be careful not to make too big of a mess. Hopefully I'll find some time to tear into it this weekend.