I decided to do a compression test on my truck today just to see how everything was doing before I threw money into a new intake and other parts. This is the first engine I've done one on some I'm worried either I didn't do it right or something is not right. I rented the gauge from oreilys. I took all of the spark plugs out and disconnected the coil after getting the engine to operating temperature. I threaded the gauge into the first cylinder and had a friend floor the gas pedal and turn the engine over as I held the gauge and took readings. All of the readings came to right under 40 psi. I realize this isn't even close to where it should be. What's is going on?!? The needle on the gauge didn't hold at any pressure it just bounced. But it always topped out about the same. Is the gauge broke or should I start looking into a rebuild? Was my procedure correct? Any help would e greatly appreciated.

 

Procedure sounds good as long as you turned it over 7 or 8 times or to where no more compression could be had .. Gauge is suspect at this point

 

It sounds like the gauge is bad, specifically the valve core in the end that screws in is missing or damaged.

 

I agree with the others. If you have a spare valve core laying around replace the one in the guage and try it again. Your engine wouldn't run on 40 psi and if it did you couldn't keep oil in it very long

 

I appreciate all the comments. I don't have anything to fix it with and I don't know that I would want to work on because I rented it. I will definitely take it back tomorrow though and retry it. I didn't think it was right but I needed to know for sure! Thanks again! I'll try and keep it updated if anything else happens.

 

Is it imperative that I get the engine to running temperature before performing a compression test?

 

Only if want the test to be 100% accurate. You can do the test on a cold engine, but the results are usually a bit lower and/or vary more between cyls.

 

My results on a cold engine came out to be

Cylinder 1- 140psi
Cylinder 2- 140psi
Cylinder 3- 140psi
Cylinder 4- 145psi
Cylinder 5- 150psi
Cylinder 6- 145psi

So does it look like I'm going to be good to go ahead and start buying the performance parts I need? I do have a stutter off idle and almost seems like it's getting to much gas but my mixture screw is turned all the way in. I'm wanting to get the offenhauser c series intake as well as a 4 barrel carb with the efi exhaust manifolds.

 

Those compression numbers are swell. An Offenhauser intake and new exhaust will really wake it up.

 

That's great! I picked up my intake today but I couldn't find an exhaust manifold I was willing to pay for... Should I wait to put the intake on or go ahead? I still have to get a new carb also.