BigBrother-84 Posted May 24 Share Posted May 24 Hi people, During Big Brother’s last « long rides » (highway 150-300 miles trip), I noticed that oil pressure was slowly going down to the lower portion of the gauge "normal" zone. This does not occur for short trips, oil pressure stays in the middle of the normal zone. Maybe this is a normal behavior when oil is heating up and becomes more fluid? Big Bro is going soon to my friend’s garage, in order to replace the oil pan gasket (I am not equipped to lift the engine). I am wondering if I should take this opportunity to replace the oil pump, pump drive shaft, screen, and dipstick. Any advice will be welcome! Link to comment Share on other sites More sharing options...
Gary Lewis Posted May 24 Share Posted May 24 It is normal for the oil pressure to drop a little bit when you put 25 or more miles on as everything gets completely warm. Hasn't it always done that or is this new? If new, have you changed the type or brand of oil recently? As for changing the oil pump, how many miles do you have on the engine? It won't hurt to change the pump, but that's not normally necessary if the oil has been changed regularly. But most builders I've talked to don't recommend a high volume pump, just the normal one. And when changing the pump it sure won't hurt to change the driveshaft, although I've reused them many times. But I don't see a need to change the screen. Clean it thoroughly and it should be good. Ditto the dipstick. Link to comment Share on other sites More sharing options...
mat in tn Posted May 24 Share Posted May 24 first, how many miles are on the engine? this matters because the most common cause of this symptom is bearing wear. the first thing I would do in this case is add a mechanical oil pressure gauge. either under the hood as a means of spot checking or as a second opinion in the cab, even if set up as a temporary. the factory gauge is a very general one and not the best for diagnosing. hot oil is thinner, we know this. thinner oil not only draws seals the pump better but every bearing also. I had this case a year or so back on a 351m. pulled the engine and replaced crank bearings, seals and timing chain. Very important to do the chain also. this was a couple hundred in parts and about ten hours labor. a very reasonable option compared to a reman in both time and money. Link to comment Share on other sites More sharing options...
BigBrother-84 Posted May 24 Author Share Posted May 24 Hasn't it always done that or is this new? If new, have you changed the type or brand of oil recently? Well, have to admit I never payed too much attention to the gauges, until Big Bro’s restoration. He was “just an ol’ truck”. The engine was rebuilt in 2015, and Big Bro doesn’t have high yearly mileage. So the engine “renewed” mileage isn’t high. I don’t know the oil pump mileage, cause I can’t confirm if they replaced it in 2015. The first time I noticed this was during Big Brother’s Big Tour. But the needle always stayed in the Normal zone, even very low. And now, each time we hit the road, I give an eye to the oil pressure gauge. About the oil, well it’s always 10W30 regular one. Link to comment Share on other sites More sharing options...
mat in tn Posted May 24 Share Posted May 24 Hasn't it always done that or is this new? If new, have you changed the type or brand of oil recently? Well, have to admit I never payed too much attention to the gauges, until Big Bro’s restoration. He was “just an ol’ truck”. The engine was rebuilt in 2015, and Big Bro doesn’t have high yearly mileage. So the engine “renewed” mileage isn’t high. I don’t know the oil pump mileage, cause I can’t confirm if they replaced it in 2015. The first time I noticed this was during Big Brother’s Big Tour. But the needle always stayed in the Normal zone, even very low. And now, each time we hit the road, I give an eye to the oil pressure gauge. About the oil, well it’s always 10W30 regular one. this opens up for more questioning. if the engine was "rebuilt"? how was it rebuilt? did it get a new crankshaft, machined crankshaft or was it just cleaned and lined in new bearings? these are just a few of many possibles. I have done all of the above after measuring and inspecting based on the case at the time. and then there are the cam bearings. not always replaced and often assumed to be good by some. the oil pump wear is not as prevalent due to harder materials and a little less load sensitive, they do wear! but they are not the weak link here.they have a pressure regulator because they have more potential then needed and this adjusts for other tolerances up to a point. at least for an inspection, while the pump is off, have the number 1 main cap removed and the bearing inspected. number one carries a tighter tolerance than the rest and should tell the condition. Link to comment Share on other sites More sharing options...
viven44 Posted May 24 Share Posted May 24 I have monitored oil pressure on a number of trucks... perhaps most of these are high mileage vehicles and using 10W-40. The oiling system / pressure can vary depending on big block/small block 302/351W Cold start --- oil pressure can be in the 45-55PSI range When fully warm and idling at ~800rpm --- oil pressure can be in the 20-30PSI range When fully warm in peak summer and idling low at ~650rpm --- this is usually when the AC kicks in and you are in 'D' at a stop light and the idle rpm drops (clearly I wasn't running high idle solenoid) I have seen the oil pressure drop to as low at 10PSI.. maybe even lower. On both 302 and 351Ws. These same engines still met the spec as there was good pressure rise at higher RPM. Usually at 1500RPM, I have seen pressure get up over 40PSI. Never had any valvetrain noise or anything. I would put the vehicle in park if I was stopped at traffic to keep oil pressure up. 460 Cold start --- oil pressure is usually in the 60-75PSI range When fully warm --- oil pressure is usually in the 20-30PSI range, even when the rpm is quite low. 351m/400 I do not have direct gauge readout on these.. but based on the cluster gauge the behavior was similar to the big block oiling system Definitely agree its time to install a real gauge. If you are running electric fuel pumps with an oil pressure switch, you may have to forgo the cluster gauge to find a tap for the oil pressure gauge. Link to comment Share on other sites More sharing options...
mat in tn Posted May 24 Share Posted May 24 I have monitored oil pressure on a number of trucks... perhaps most of these are high mileage vehicles and using 10W-40. The oiling system / pressure can vary depending on big block/small block 302/351W Cold start --- oil pressure can be in the 45-55PSI range When fully warm and idling at ~800rpm --- oil pressure can be in the 20-30PSI range When fully warm in peak summer and idling low at ~650rpm --- this is usually when the AC kicks in and you are in 'D' at a stop light and the idle rpm drops (clearly I wasn't running high idle solenoid) I have seen the oil pressure drop to as low at 10PSI.. maybe even lower. On both 302 and 351Ws. These same engines still met the spec as there was good pressure rise at higher RPM. Usually at 1500RPM, I have seen pressure get up over 40PSI. Never had any valvetrain noise or anything. I would put the vehicle in park if I was stopped at traffic to keep oil pressure up. 460 Cold start --- oil pressure is usually in the 60-75PSI range When fully warm --- oil pressure is usually in the 20-30PSI range, even when the rpm is quite low. 351m/400 I do not have direct gauge readout on these.. but based on the cluster gauge the behavior was similar to the big block oiling system Definitely agree its time to install a real gauge. If you are running electric fuel pumps with an oil pressure switch, you may have to forgo the cluster gauge to find a tap for the oil pressure gauge. all good points. on the 351m it is comparable to the 460. without the electric pump interrupter. on that i just removed the sender and connected the mech gauge there temporarily until I was satisfied if the results. swapped it back and noted where the gauge read compared to immediately before with mechanical. on the Windsor that would need to be done behind the power steering pump instead of top rear of the block. even though it's on the front it can be a pain to get to, especially without the factory extension. if using a plastic lined gauge be sure to keep it away from exhaust. Link to comment Share on other sites More sharing options...
FordFETruck Posted May 24 Share Posted May 24 all good points. on the 351m it is comparable to the 460. without the electric pump interrupter. on that i just removed the sender and connected the mech gauge there temporarily until I was satisfied if the results. swapped it back and noted where the gauge read compared to immediately before with mechanical. on the Windsor that would need to be done behind the power steering pump instead of top rear of the block. even though it's on the front it can be a pain to get to, especially without the factory extension. if using a plastic lined gauge be sure to keep it away from exhaust. This is normal. My tired 351W gets 60-65 PSI at a coldstart and HOT idle in gear its about 20-25 PSI with 5W30, 10W30, 15W40 and 0W40. Driving it's 40-50 PSI hot. My factory oil pressure gauge varies a lot more by the day than the mechanical gauge does. Link to comment Share on other sites More sharing options...
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