I have a 1948 Chevrolet truck with the 235 cu in engine. Just about 10 days ago my pressure gauge has started showing pressures of around 10 psi at highway speed and around 5 psi at idle. I've taken the truck to my "shade tree Bubba". He says they'll run a manual pressure check and determine if I have actual low pressure or a defective sending unit. He's also said that if I have that low of pressure, an engine overhaul is the solution. Couldn't it just be that my 70+ year old pump is getting weak and needs to be replaced? The truck is running great. I never drive it faster than 50mph. I run it on surface streets only- no interstate travel. The truck starts easily regardless of the number days between use. Opinions???
If that yruck has an original engine it would be a 216 and hot oil pressure at 50 MPH uld be about 14 pounds. If it has a 235 engine adapted to it it it could have an early low oil pressure engine with the same oil pressure as a 216 or a later 235 with full pressure oiling and oil pressure should be 30 to 35 poungds at 50 MPH. The first thing I would suspect it that the oil by-pass valve in the pump is stuck with anty of the engines. The oil prssure gauge would be he last thing I would check. The main bearings will seldom fail suddenly and you would hear them pounding it they were that loose.
First, it would be good to know if you have a 1948 235, or a later full pressure engine. Normally, in 1948, 235's were supplied in larger trucks (2 tons) with smaller trucks having 216's.
Even if you still have the low pressure 235, 10 and 5 psi is low. What were the driving and idling pressures before, and how suddenly did the drop occur?
Oil pumps on 235's (and 216's) almost NEVER wear enough to be noticeable.
And unless you can hear a lot of knocking, I wouldn't think a sudden drop in pressure would indicate a need for a full overhaul. The solution is quite likely much simpler and cheaper than that.
The oil pressure gauge just goes from 0 to 30psi. Before this drop, the running psi was around 15psi and the idle psi was around 8-10 psi. The manual I have says these readings are normal once the engine warms up. The cold engine on starting would read between 25 and 30 psi. On cold starting now, the gauge reads just above 15psi. I don't think the engine is original to the truck. It is a 235. I couldn't say if this engine is a full pressure engine or not. There is no knocking or rattling noise coming from the engine. Like I said in my original post, the engine starts and runs well. Sometimes, I haul hay in the truck- 15 bales usually. I bought the truck in 2019 and have put almost 5000 miles on it since then. These low pressure readings just started suddenly. One day, I have normal readings and the next day I see what I'm seeing now. Makes me think it's a bad sending unit.
A little over 15psi at running speed and around 8-10psi at idle. The manual I have says these are normal readings. The gauge just goes from 0psi to 30psi.
Yes, thAT IS THE CORRECT PRESSURE FOR A LOW PRESSURE CONNECTING ROD DIPPER ENGINE. The only trucks that came with a 235 engine in 1949 was the two ton 6400 series trucks. What are the raised casting numbers on the lower right side of the block? 3835253 or 3692682 for a 216 or 3836309 for a 235.
The truck isn't at my house right now and I won't be able to go check the engine numbers until later this week. My "Bubba" did a manual oil pressure check and found that my gauge is giving an accurate reading. We'll replace the oil pump and see if that solves the problem. The truck might need an overhaul, but it would seem that to me the truck wouldn't be running well and would be making odd noises if an overhaul was needed.
While the oil pressure is a little low you do still have oil pressure so everything is getting lubricated, there isnt a major hurry but does warrant investigation. If you do pull the oil pump i suggest checking main bearing clearance while you are in there which is another cause of low oil pressure. Tony
If it had any connection to the main bearings it would not have come on all at once. Just loose mains would not have caused it, would take burning out of the babbitt to drop the oil pressure. The sticking of the oil pump pressure blow-off valve is the most common cause of a sudden oil pressure loss event. The valve is sping loaded and its function is to limit the maximum oil pressure to 70 pounds - which could happen with cold oil and high engine RPNs, esecially with poor oil. In the dealership years ago we would find this during cold weather.
I got the new oil pump installed in the truck. The oil pressure is more normal now. The truck is running great. I researched the options for an overhaul and they weren't good in my area. I was looking at a 9 to 12 month wait time and cost close to $5000. I'm not willing to go that route. I'll drive the truck and monitor the oil pressure. If an overhaul is unavoidable, I'll put the truck up sale as street rod project.
