compression pressure diagnosis

[volver]

I have begun the M112 supercharger build. I have removed the MAF, TB, EGR, Upper intake manifold, Fuel rail. I have designed a set of adapter plates, they are going to be cut on Monday. I have replaced my fuel injectors with 39 pounds. I am currently waiting for the adapter plates.

I decided to do a compression test. Regardless of the outcome the supercharger is going on.

The driver side of the engine pressure reading ranged 160-180 psi the passenger side reading ranged 140-180psi except for on low at 120 psi.

I am wounding if this is a problem or if it is normal wear? What would have caused this? Cylinder wear, bad valve? How will supercharging effect this problem? Will I get blow by? I am running 5w30 oil, the test was run when the engine was cold.

 

[2000StreetRod]

Compression test results

Compression testing is more accurate on a warm engine but the relative differences between cylinders should still apply on a cold engine.

When the test was performed on the weak cylinder did the pressure rise with each compression and was the maximum pressure achieved in 6 compressions or less? If the compression built up slowly it could be leaking valves. I suggest that you repeat the test on the low pressure cylinder and then squirt a few shots of new motor oil into the cylinder and try again. If the wet pressure is 10 or more psi greater than dry your rings are probably leaking. If the reading is the same as without the oil than it's probably leaking valves or head gasket.

You may want to pull the valve cover on the weak cylinder side and check the valve clearance. I don't know which engine you have but there are lash adjusters on the SOHC.

I suggest determining the source of the weak cylinder before subjecting it to additional stress from boost.

 

[volver]

LOL. I am going to boost it any way, just not as much as I wanted to do before. I cant go back to the old intake, so i will have to get the supercharger on before i can retest the compression.

I will keep the pressure down. I will seafoam one bottle in the tank and one in the intake. (to clean the valves off). I will change the oil to 10w40. And then redo the compression test after i warm the engine.

 

[2000StreetRod]

V8 engine

You asked in one of your other posts about a manual transmission on a V8 so I assume that's what engine you have. I believe that the V8 heads are less likely to crack than the OHV heads so that's probably not the cause of the weak cylinder. I also recall that there is a specified collapsed tappet gap for the hydraulic lifters. If the valve cover comes off without too much trouble it might be worth checking the gap.

 

[volver]

So you think that the tappet could have 0 clearance when the valve is closed. or the tappet could be holding the valve open.

If i get a large reading then i could assume that the valve is stuck on some thing.

 

[2000StreetRod]

Specified tappet gap

According to my Haynes manual the collapsed tappet gaps are:

Desired: 0.091 to 0.151 inch
Allowable: 0.071 to 0.171 inch

 

[volver]

ok say it was a bad head gasket on the passenger side how would i tell?

 

[2000StreetRod]

Head gasket vs valve leaks

Its a little difficult to distinguish a head gasket leak from a valve leak. If there is white smoke in the exhaust after the engine is warmed up that's an indication that antifreeze is leaking into the combustion chamber. If the engine oil has a milky, coffee color to it that indicates coolant is leaking into the crankcase. If there are bubbles showing in the radiator top with the cap removed that indicates that combustion gas is getting into the engine coolant. A large manifold vacuum fluctuation on an idling warm engine may indicate a blown head gasket. Recurring drops of 2 to 4 inches of manifold vacuum on an idling warm engine indicate leaking valves.

 

[volver]

There is no water in the oil. The exhaust is going to be hard to tell because it is about -2 degrees out and everyone exhaust is showing white plus I cant run the engine right now. The radiator was a little low but the last time I checked it was 2 months ago. I was running 75% water, some of it could have evaporated. can't check to see if there are bubbles in the radiator right now. There is no coolant in the transmission.

The vacuum gauge reads 20 PSI when idling.

looks like things are pointing to a ring problem.

 

[ezrider440]

in response to the compression question, first off getting a truly accurate reading is an arguable process, the sop which i use is to have a warm engine, to take several reading and average them, since you can a have a slightly leak in your gauge or fittings, don't forget you're holding upwards of 100psi, plus the accuracy of the gauge, typicaly the first two cylinders and last two cylinders ( side to side) meaning 4 and 8 and 1 and 5 will show slightly highers amounts of wear than the middle cylinders so take that into considersation, if you have consecutive cylinders with low compression then start scratching your head,

before you do something costly and in depth check you measuring technique, use a Snap On or a Blue Point gauge not the Chinese 40 buck gauge and take as many readings as you can, to get a good accuracy

 

[volver]

i just used the gauge from autozone. It did get a leak but i fixed it then it worked again.