Ric232

I just bought a compression test gauge and was trying it out on the boat tonight. I did not have much time so I only planned to test two cylinders and I didn't follow all of the instructions.

The parts of the instructions I ignored are:

1. Removing the flame arrestor (I just opened the throttle bodies using the shifter).

2. Removing all of the spark plugs (It seems to me that only the one I'm testing needs to be out).

3. Warming up the engine to operating temp. (It's dark outside)

I'm hoping that at least one of the above steps (#3 probably) is critical in order to get an accurate reading. Otherwise, I've got only 130 psi on cylinders 2 and 4 (the only ones I've checked so far). My last compression check was 2 years ago by my local dealer where everything tested between 155 and 170 psi.

Thanks for your help,

Luke_Chinewalker

warm up engine,remove flame arrestor and all spark plugs,ground ignition coil and have the the throttle wide open,perform compression #1,3,5,7,2,4,6,8 and repeat #1 again per SAE standard procedure,#'s should not vary by more than 10% between lowest # and highest #

blue thunder

The important part is the relative reading between cylinders. The pressures should be within 10% of one another. The good news is the two cylinders you checked were close. You really need to check all 8 cylinders to make the test meaningful.

BT

RumRunner

Ric232

Also pay attention to the first hit or tick of the gauge on each cylinder. When you first start to crank the gauge will jump to say 120, then climb with the following revolutions. Some people also check "Running Compression" buy check each cylinder with the engine at idle.

H2Xmark

also make sure you have a quality gauge, some of the cheap ones are about as accurate as me sticking my finger over the spark plug hole and saying "130lbs"

blown1500

The most important part is that they all be within 10 percent of each other. Removing the flame arrestor is not necessary. Removing the other plugs gives the starter less drag, more cranking rpm, etc. This changes the readings slightly, but if you only want to check for a problem, it is not significant. Warming the engine changes the readings also but, again, if you are checking for a problem, the readings will be lower on a cold engine and there are problems that will be worse when warm, but you can still get comparative numbers with a cold engine. Opening the throtle is important as it takes air to make pressure and if you don't open the throtle, the engine can't get enough air.
For the most accurate check, removing the other plugs and testing a warm engine are necessary. Just to check for a problem(burned valve, stuck rings, etc.) you can cheat a little with no serious problems.
A quality guage well sealed to the plug hole is ESSENTIAL!!!
Good Luck!!!

Ric232

Thanks for the great input. Unfortunately, I bought the $25 Actron gauge from Sears, so it probably isn't the most accurate. Hopefully it will work for what I am doing. I don't think I have any problems, I just bought the gauge to do a periodic check since I do my own maintenance work. I only have 150 hrs on the motor and it is naturally aspirated.

I'll follow the recommendations everyone laid out here just to make sure my numbers are as accurate as possible for future comparison.

Thanks again,

HyperBaja

You should be fine with only 150 hours...


Wondering if I should even think about running a compression check on the 1990 4.3 with over 1400 hours!!

Ric232

I'm only checking mine to be safe. I have on spark plug (#4) that looks like it's running quite a bit hotter than the rest. But the engine is running great.

blown1500

Ric232,
Especially on a fuel injection engine, you had better find out why you have one hot plug!!! Usually, the hot condition is caused by a lean condition. If that is true, head gaskets and/or pistons or more are in your immediate future. This condidion can be caused by many things among them are clogged or dirty injectors, wiring problems, and many other possibilities in the fuel system or even the ignition. The most common cylinder we see is #6, but that is not the only ones.
This is a serious problem and can be destructive and costly. If you need more info or help, call me at 704-574-4020 or pm me and I'll help all I can. At least, check it out or get help from someone with FI experience!!