Low oil pressure idle amp tapping AQ145B

"Hello,

I am new to this fo

"Hello,

I am new to this forum and I've run into a situation that I thought I might run by you.

I have a 1986 AQ145B engine that ran fine last year, but when I started it this year, I hear tapping coming from the #4 cyl and the oil pressure is around 30lbs at idle even when cold. I haven't changed the oil yet from last year, but it is full and is clean/clear on the dipstick. I have run the engine for 20+ mins and the tapping doesn't go away, but is diminished when I rev the engine (the oil pressure also increases to 50 - 60lb range). Since there seems to be a tie to the oil pressure, I suspect the oil pump is not able to get sufficient oil to the cam/valves over the rear cylinder? I haven't done a compression test yet. I know in old cars, sometimes "sludge" develops at the bottom of the oil pan and it can block some of the oil pump intake. Any thoughts or help would be appreciated. Thanks."

"You probably have a bad lifte

"You probably have a bad lifter; with oil pressure that good I would say that your oil pump is good. More then likely you have hydraulic valve lifters, as they wear they fail to pump up with oil and cease to be hydraulic. You may want to perform a compression check to ensure that you don’t have a stuck valve and or a bent push rod. I am not an expert on Volvo engines and I am not sure if you engine has an over head cam or not."

"Corey, there are no push rods

"Corey, there are no push rods or hydraulic valve lifters on the AQ145B or any of the 4 cylinder Volvo AQ series engines.

Bob, when symptoms like you describe happen the first thing I would do is to is to check that the oil pressure transducer and the gauge are in good working order (you may have to use a Bourdon gauge to compare readings). Then, if the readings are OK, the next thing to do is change oil and filter. If the problem does not go away, then you know you have to pull the engine."

"Hello,

Thanks for your res

"Hello,

Thanks for your responses. I did a compression test on the cylinders and found that the rear 4th cyl was at 85 lbs while the others were between 180 & 185 lbs. With this information, I pulled the valve cover and turned over the engine, everything looked good. I then pulled the head and inspected the valves and everything looks ok, they appear to open & close correctly and look tight. The piston looks fine from the top and the cyl walls are not scratched...no sign of damage. What could be causing the low compression and tapping in that cyl? I'm a half decent mechanic, but I hate to go any further if there is something simple I may be missing. I might be able to get the oil pan off and then perhaps pull the 4th piston for inspection.

Thanks for any help."

"Bob, I think you have missed

"Bob, I think you have missed one important step. When a compression test gives you results like the ones you have, the best thing to do prior to pulling anything is to conduct a leak down test. You could have something as simple as a valve not closing properly, or something not as obvious as broken piston rings. A leak down test would have pointed you in the right direction and you wouldn't be wondering now.

If you are going to work on the engine, I suggest yo may want to pull it from the boat. It may look like a hassle, but it will make your life much easier."

"Thanks El,

Agreed. I thin

"Thanks El,

Agreed. I think I will reassemble the engine so that I can perform a leak down test. A sticky lifter is highly suspect at this point. Note that I was pleasantly surprised at how clean the engine is inside and how all the bolts came out so easily. Although the engine is from '86, it was used primarily in fresh water all its life and only has about 500 hrs on it."

"OK,

I reassembled the engi

"OK,

I reassembled the engine & the leak down test indicated a piston ring issue. I also did another compression test with the same results (85lbs for cyl #4), then I squirted some engine oil onto the top of the #4 cylinder and the pressure went up to 185 lbs (matching the pressure in the other cylinders). Next step is to pull the #4 piston and inspect/replace the rings. I am hoping to be able to pull the oil pan & piston with the engine still in the boat, but have a boat mech linked up to pull the engine if necessary."

"Hello,

I was able to pull

"Hello,

I was able to pull the oil pan and the #4 piston with the engine in the boat! The top ring was totally compressed and held that way by a little bit of corrosion on the piston. The second ring was half stuck and the other half still had spring left in it (see pic). I removed (no small task) the old rings, cleaned the piston up and installed a new set of rings. These stuck rings were permitting blow by and causing the compression issue.

I have it mostly back together and should be able to test start it tonight.


AQ145B piston with stuck rings