I'm sure this topic has been brought up before, but I haven't seen a post here regarding this family of engine. I should preface this with saying that the engine is completely unknown in regards to mileage, work that he been done to it, or how much is was abused in its life, and I am perfectly comfortable if the final conclusion is to drive it til it makes noise! If and when this thing quits on me I'm probably going to swap in a 289 or something, but only if and when it happens and if I still own the car... For now I dig the six. Anywho, here goes: I recently picked up a little Merc Comet with a 200 six. It doesn't make much noise once warm, aside from the usual valve clatter of a six cylinder, but DOES have what sounds like a lifter tick when cold. I'm not sure if a bad lifter could cause these symptoms or not. I'll try to give as much detail as I can here in this post! The other day while driving home from work I saw the oil pressure drop, not to 0 or anything too alarming, but from 40 down to around 20. No change in engine speed, around 2,200rpm. The car was already good and warmed up; this was the tail end of my 45 min commute. At idle it dipped down to 0 for half a second and then up to about 10 like normal. I continued on for a few more blocks to watch the gauge- it would spike up to 40 when I got to around 2,000rpm, stay there for about 30 seconds, and then slowly fall down to 15, and back up to 20. I pulled over, checked the oil, and it was a tad low right at the "add" mark. Topped it off and continued driving, same symptoms. The next morning I dropped the pan thinking a clogged pick up, but the pan is spotless, pick up looks like new, oil pump seems ok visually. I changed to a different brand of filter, it had an STP on there and I was thinking maybe it was the same problem you hear about with Frams. I also checked pick up height from pan as best I could with a straight edge and ruler. It seems OK at about 3/8" but from what I've read it could be a little higher. The pan is very clean and in pretty good shape as far as dents in the bottom under the pick up, so I don't think its a cavitation issue - but can always give it a little more clearance if need be. Anyways, I got it back together and went for a drive. This time I drove out to the shop and all was fine on the 40 min drive, oil pressure stayed steady at 40psi spinning 3k on the freeway, and in town at round 2k. 40psi the whole time, and 10 at idle. I worked around the shop for a few hours and headed home. Steady 40psi on the freeway again until I was almost home, and then it dropped down to 20psi (still 3k rpm) and slowly danced between 20 and 30psi until I got off the freeway. Once on surface streets I had consistent oil pressure again, 10 at idle and 40 at speed. I pulled over to check the oil again and it was fine. I haven't tried a different gauge yet, but I plan to pick one up next week. I am currently running a Make Waves 2-7/8" guage, or whatever their large size one is. Copper tubing to gauge, straight from the port at rear of engine where the stock sender usually goes. I am not running the stock gauge, only the mechanical one.
If you were closer I'd lend you one, but Bob is right checking the gauge would be easiest. The other thing is there is an oil pressure relief spring or part that could be failing. I'm not familiar with your oil pump, but I had a big block Mopar oil pump do that once, I put a high pressure spring in it for additional pressure but eventually it fatigued and failed. New spring and was back in business.
Smart Ass answer : It knows it's going to be parked for the night, but wants to keep on going. LOL. I agree with last responder; try new gauge 1st, also check for any fresh leaks, & look at that COPPER line CLOSELY, especially at the fittings, as I had one years ago that had a split inside the edge of the fitting(age/vibration, or a combo?) that would only leak when I made a certain sharp left turn(Played hell finding it!!) I now don't use copper lines, & do make sure to use a flex line somewhere in the path from engine to gauge.
Thanks for the replies! There might be an old SW or Autometer gauge kicking around the shop I can try. I also found a procedure for checking the oil for airation in my shop manual. I might do that just for the heck of it. The pressure relief valve is a good idea to check as well!
Sorry for not posting back sooner, it want able to work on the car for a while. Anyways, I found that there was a bad leak at the gauge, which happened suddenly and after I had already been having symptoms. Fixed that, still had problems. I tried another gauge, new plastic capillary, both gauges running at the same time showed it was a true oil pressure problem. In the end, el Scotto called it! It was indeed a bad pressure relief valve in the oil pump. I didn't really dissect the oil one to see what exactly the problem was, but the old valve was frozen completely. I believe that it was in just the right position that pressure was allowed to build initially, but bled off quickly. Anyways, a new Mellings unit did the trick. While I had the pan off I noticed the bottom was slightly dented in, although from the outside it you xould hardly notice. I corrected this and thusly added a little more clearance to the pick up. While investigating I also found that it has adjustable rockers, which were adjusted horribly loose (maybe the PO thought it was a solid lifter motor?). The service manual procedure seemed way too tight, more than I felt comfortable with, so I just set them for slight drag when twisting the push rods like I do most everything else hydraulic. She seems a lot happier now, a little more peppy, a lot less noisy, and holds over 40psi of oil pressure at anything off of idle (30psi at hot idle). Thanks for the help and happy cruising!
You didn't say what year the car was but if it had a Load o matic distributor you sure missed a chance to up grade to a better unit.
Just a suggestion, change oil, look for a silver streaky in oil, not good, also cut oil filter apart, look for shavings, not good, by checking this you will be sure. Engine is not damaged.
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