Mercruiser 7.4L compression test after valve job

[tpenfield]

I have done a few bench test runs of my 1991 Mercruiser 454 (7.4L) after getting all back together from the valve job and general refurbishment. Yesterday, I did a compression test to compare the results from before the valve job.

Here are the 'before & after' results and fancy graph in the image below. Overall, the compression increased by 9 psi on average, and the low compression in cylinders 3,4,5,6 are corrected. (before was 135 psi average, now 144 psi average). I am still concerned about the lower reading I got on the #1 cylinder, it sort of sticks out like a sore thumb among the other cylinders, that are now very consistent.

The columns headings in the chart are from the past 3 tests '2010' (over a year ago), '2011' is before the rebuild, '2012' is after the rebuild. The dashed pink line is from before the valve job, the solid blue line is after the valve job . . .




#1 cylinder was one of the better cylinders when I did a leak test before the tear-down, I am wondering if it is just a valve adjustment issue. I think I will run another test today after an extended warm-up run to see if I get anything different on the readings.

The compression is supposed to be 150 psi according to the manual, but I'll take the 144 psi, considering gauge accuracy, etc. Only 540 hours on the engine, so ring/cylinder wall wear is minimal, based on what I inspected during the rebuild.

Thoughts on the readings in general and the #1 reading of 136 psi?

TIA.

 

[90stingray]

Could be a rocker arm cranked down too much. But would definitely try to find out what's going on. Try and squirt some oil in cylinder 1 and see if it jumps up with the rest of them. Just an idea.

Sent from my DROIDX using Tapatalk

 

[scoflaw]

Re: Mercruiser 7.4L compression test after valve job

Painfully obvious that you should have reringed it as well. Your new valve job, will only create more pressure on your 20 year old rings and bearings. Valve job only, have always had this issue.

 

[tpenfield]

Re: Mercruiser 7.4L compression test after valve job

Today, I ran the engine for a full half-hour, then did the compression test. The results were better than yesterday; #1 is still the lowest, but well within 'range'.

12/11/2011 30 Minute Warm-up

#1 142
#3 149
#5 151
#7 144

#2 145
#4 149
#6 146
#8 144

Average = 146.25
Std Deviation = 3.1

The 2 cylinders with new valve seats are on top of the pack, but the other ones are in good shape.

 

[joewithaboat]

Re: Mercruiser 7.4L compression test after valve job

I bet you will be fine, not sure what the merc book says but it seems like i remember 10% being with in limits highest to lowest.
Glad to see you have it back together.
What method did you use to set valves?

 

[Bondo]

Re: Mercruiser 7.4L compression test after valve job

Ayuh,.... Looks like she's got Alotta hours left in 'er,....

 

[tpenfield]

Re: Mercruiser 7.4L compression test after valve job

Glad to see you have it back together.
What method did you use to set valves?

There are 2 methods prescribed in the owner's manual for this engine . . . Static (engine stopped) and Running.

I tried doing the 'running' method, but the exhaust set-up was too loud to adequately hear when I was reaching zero lash. So, I did the 'static' method. It seems to be fine.

 

[littlebookworm]

Re: Mercruiser 7.4L compression test after valve job

You're over-thinking. The difference between your highest and lowest readings is 6%. That's well within tolerance, especially for a stock marine engine. She should run fine; just go enjoy your boat. Hy

 

[dollarten]

Re: Mercruiser 7.4L compression test after valve job

You're over-thinking. The difference between your highest and lowest readings is 6%. That's well within tolerance, especially for a stock marine engine. She should run fine; just go enjoy your boat. Hy

Try checking the engine with a cylinder leakdown tester . This way you can hear where you are getting more leakage

 

[tpenfield]

Re: Mercruiser 7.4L compression test after valve job

Try checking the engine with a cylinder leakdown tester . This way you can hear where you are getting more leakage

Yup, my leak tester is out on loan for a while, but I will definitely run a full set of leak tests before I put the engine back in the boat. Before the engine was taken apart, I was getting only 8% leakage on cylinder #1. So hopefully it will be in the same ballpark this time.