Questions and Answers
Oil Pressure Problem
Q. I have a newly rebuilt 1971-1978 GM 250 CID in-line six. I purchased the short block from a motor replacement company in Tucson AZ. I installed the motor into the pick up truck drove through the break in period around town the way it was recommended.
A short time later I needed to drive the truck on the freeway for about a 200 mile trip. I had driven about 40 miles and I felt the motor lose power and start to slow down. At that time the temperature gauge also started to climb up from it's normal range. I pulled over and let the truck sit until it cooled down and limped back home. I continued to drive the truck around town with no apparent problem for several more days before I tried the freeway trip again. Once again I got about 40 miles and the same thing repeated only this time I developed a motor knock.
I returned to the place I bought the motor from they said it sounded like the fuel pump went out making for a lean mixture that would have overheated the engine and collapsed a piston but I could drive it for a while with no further problems. I did this for about four months with the only side affect being a noisy motor. The motor now has about 6,000 miles on it since it was put in the truck. Finally the head gasket blew out near the rear of the motor creating an exhaust sounding leak, no water in oil was found.
I now had to remove the head so I thought I would also replace the bad piston while I had the head off. I determined that number five piston was the bad one, and replaced number five piston, rings and bearing. I now have new intake, exhaust, head gaskets, rings. bearing, oh yes I also put on a rebuilt carburetor just for good measure. I went through the break in period once more before starting out on my freeway trip. Once again on the freeway about 40 miles into the trip I noticed the oil pressure starting to drop a little and heard "rattling" noise under the hood. The engine never boiled over before I shut it off but the knocking noise was back. I towed the truck home and took it apart again. This is not the most fun when you are in Arizona and the daytime temperatures are above 105° so I could only work morning and evenings.
I disassembled the bottom end once more to fine that #6 piston had seized a wrist pin. the bottom of the piston was dark brown from overheated oil it did have 4 corner scoring on it also. This time I pulled out all six pistons for inspection. #6 was the only on that looked bad. I replaced #6 piston, ALL rod bearings, inspected several main bearings, plasticguaged all the clearances and reassembled the motor. I started the motor and everything was fine until the operating temperature started to move into the "normal range" then the oil pressure started to fall to 0 on the gauge. I shut off the motor before and thing else went wrong.
I was told one of two things could be wrong so I replaced the distributor with a Mallory duel point because of detonation problems and replace the oil pump, which only has 6,000 miles on it, in case it was malfunctioning. Put it all back together last night drove it about 30 miles the oil pressure went away again but the motor never got out of the normal range. You could hear lifters and bearing raping loudly with no oil. I now have a motor knock and lifter noise. Can you help me with what might be wrong?
Thank you,
KenA. Sounds like you have gremlins in the motor. Won't the rebuilder do anything for you? This sounds like a call is needed to see what they will do for you. I would have done this before anything else. I see no need to replace pistons and bearings for a motor that was rebuilt by someone else. Does it have the correct oil filter on it? Oil pump screen clogged? Oil passages clogged or too much clearance somewhere? Defective oil pump? Water pump turning the right direction? Let us know what happens.
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