which fuse for compression test?

budomove · 01-31-2009, 10:21 AM

I am thinking of testing the compression today, and am wondering do I pull fuse 18 (IMA/ECM) or fuse 7 (fuel pump) or both? I will unplug the harness on each of the ignition coils, but what else do I need to do to make sure nothing gets messed up?

Insightful Trekker · 01-31-2009, 10:51 AM

An open circuit fault (i.e. disconnected coil) will cause the PCM to turn off the injector pulse. 2 birds - 1 stone (fuel and "fire" both disabled). Just use extreme caution when doing a "wet" compression test (hydrostatic lock). Yah, you'd probably blow the gauge before bending a connecting rod, but its not worth either.

FWIW a percent leak down test can yield a more comprehensive picture of the engines condition vs. the wet and dry compression tests combine.

And remember either of the above must be done with a fully warm engine else the results can be skewed in favor of something bad.

HTH!

joecvt · 01-31-2009, 10:58 AM

Sounds like you want to disable IMA starting (and use the typical starter motor) to do the compression test. In the service manual, in testing the starter motor (page 4-4), they want you to remove fuse number 2 from the under dash fuse box (fuel cut relay, PGM-FI main relay, SRS unit) and then turn off the IMA battery by using the IMA battery switch (page 12-4).

JoeCVT = Just your average CVT owner

budomove · 01-31-2009, 03:20 PM

Thanks Guys, I unplugged the coils, and borrowed the gauge at autozone. I got 210 205 215 for 1, 2 and 3. Anyone know the compression specs? I am away from my service manual for the weekend.

Insightful Trekker · 01-31-2009, 06:40 PM

Reads like good numbers to me.

But for interpreting engine condition I like to see less than 10% leakage.

Still neither one is fool proof and is usually reserved as a diagnostic tool for a symptom. Interpreting either compression or percent leakdown as a maintanance tool is not reliable by itself.

HTH!

budomove · 02-01-2009, 10:02 AM

Cool, less than 10 percent leakage on the leak down test, or on the compression test? When I did the compression test, I tested each cylinder about 7 or 8 times. After 30 minutes or so of having the motor off, I did each cylinder again and it would go up to the previous pressure of cylinder ie 215 205 210, and then slowly leak down, whereas when the engine was warm the pressure would hold indefinitely. Is it normal when the engine cools for that to happen?

Insightful Trekker · 02-01-2009, 12:27 PM

Can't do a "leak down" test with a compression gauge. You need a differential pressure tester. Yes, neither test is valid with a "cold" engine and you will see different pressures or % of leakage when the engine cools as you describe.

Both the above are diagnostic tests for _when_ you have a symptom. You can't read too much into them as a predictor of future engine problems. Except to say that all is well for now, or not.

HTH!

budomove · 02-02-2009, 12:54 AM

Just got home and looked in the service manual to see the compression specs, and it says to turn the pack off and use the back up traditional starter to test compression. I will try it again that way this week to see if I get different readings. Manual says minimum is 128psi and 28psi acceptable variation. I wonder what a new insight's compression numbers look like. my readings using the dc/dc converter were 215-205-210.
fyi- I had thought I might be burning oil, that is why I decided to start by checking the compression. Let y'all know how it turns out later in the week.

Insightful Trekker · 02-02-2009, 02:31 AM

An oil consumption test is what should be done first. Simply monitor the mileage it takes to "burn" 1 qt. There are rules of thumb for such consumption, but nothing hard and fast. The reality with such losses is that there are no quick or partial fixes. Unless theres a leak (not true consumption) or the PCV system is clogged.

Most of the time with Honda engines significant consumption doesn't happen until the engine is well worn overall. Rings, valve gudes and stem seals are all contributing to the loss. Could be because of neglected maintenance, primairly oil change intervals. By the time consumption is serious enough to indicate a pending engine failure (something greater than 1qt / 1000 miles) its time for a rebuild. Nothing less will reestablish long term reliability (or eliminate oil consumption).

I've looked at the compression numbers on a couple of Insights using the IMA starter. Yours was spot on. So even with this assurance that the rings and cylinders are performing well, you can not expect to reduce consumption reliably by merely doing what was commonly called a "valve job".

Why you ask?

Because the engine is that well engineered. Which is to say that all the individual components that do their job to maintain engine performance are well balanced in the tolerances and materials chosen. Its _extremely_ rare to see a Honda Engine with oil consumption problems that can be fixed by addressing only one of the primary sources. I've personally seen it tried many times without success.

First rule of car repair: If it ain't broke, don't fix it.

HTH!

budomove · 02-02-2009, 11:25 AM

I had the valves adjusted last year and a couple exhaust valves were off by a tiny amount. I will confirm today with a cold engine what if any loss I have. The first two times it was a quart low after 300-500 miles which is a ton. I found one of the valve cover bolts had loosened it's way all the way to the point that it was just dangling out of the hole. Degreased the engine and have not had any visible leakage in the past 300 miles. Pcv tested fine, pcv tube looks clean. Breather tube has tiny bit of oil from crankcase pressure, but only a teaspoon pooled in the airbox breather inlet. If it appears to still be using oil when I check it dead cold later then I may remove the IM to check for deposits. Thanks for the insightful response!