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Where could my oil be? Stumped!

1974 Ford F100

Somehow my truck has used up 7 quarts of oil in the matter of 3 days and only 50 miles of driving, but I can find no major leaks and am not seeing signs of smoke from burning oil!

Here are the course of events leading up to today…

A couple weeks ago I had an oil change, drained all the oil, added 5 quarts of oil and drove home (10 miles).

On my way home, I noticed that the oil pressure gauge was reading low and made an appointment with the garage at my local Ford dealership. I checked the dipstick when I got home and it read “Add 2”. Next day, I added 2 more quarts of oil and drove it a block to the mechanic. They diagnosed the truck with a bad sender. I also had them run a complete diagnostic check and the only things that were red flagged were burnt out back up lights, suggested automatic transmission flush, and minor oil leaks (not bad for a 36yr old truck!).

Drove the truck to work and back the next day (20 miles total) with no problems.

Got to work just fine yesterday, and on the way back home it started riding a little rough (another 20 miles).

Then today the oil pressure started dropping as I pulled into my workplace and the engine was noisier than usual. I knew something was up, so I arranged a ride home, but thought I’d check the oil before I left. Low and behold, the oil was barely registering on the dipstick. So, I went and bought 4 more quarts of oil and slowly added them one by one. Each quart moved the dipstick reading and finally by the 4th quart it read “Safe”. Drove home and the car sounded fine.

If I am losing this much oil I should be able to see it, right? There are a few drops on the ground where I normally park, but not enough to make up this kind of loss. Had a friend drive behind me and there were no visible leaks while driving. The truck is not smoking, so I don’t think it is burning off anywhere.

Any ideas? I plan on taking it back to the shop and questioning the diagnostic test if something this major wasn’t noticed, or could it be something that arose from the work they did?

Try idling it in the drive for 10-20 mins. If you are losing a quarter every 7.5 miles this might give you some leads. You can also put in in neutral and hold it at 3000 RPMs to see if it is caused under load. Finally, and I’m not 100% sure of the physics here, check your coolant. If you’ve blown a gasket, it could be seeping into the coolant.

Ok first, don’t try to blame this on the shop that looked at your truck…

Neumann5 has a great idea. Run the truck without driving anywhere and with an oil leak that size you should have a puddle pretty quickly. If the oil were being burned you’d have a huge blue cloud following you everywhere.

“Any ideas? I plan on taking it back to the shop and questioning the diagnostic test if something this major wasn’t noticed, or could it be something that arose from the work they did?”

Diagnostic test on a 36 year old pickup?? I would have loved to have watched that…

Yeah, check and see if your radiator is full of motor oil…

I had a friend with oil consumption like this. Turned out to a pebble flew up and poked a hole in her oil filter. The oil was squirting up into the sub frame and “disappearing”. New filter and oil change good as new. It was hard to find cause it only really leaked when the engine was revving and oil pressure was elevated. I vote for the driveway as well.

I also had a bad oil filter. I had just changed oil and filter on my car (don’t remember which car0 and drove to work-5 miles. I had checked under the car with the engine running and not a drop of oil on the ground. Before I got to work my oil light came on. Checked and no oil on the stick.Bought 4 quarts at a gas station and put them in. Looked underneath and still no oil on ground or smoke from tailpipe. I went to brush away what I thought was a dirty piece of straw or grass that looked like it was stuck between the filter and frame and it was a small diameter stream of hot oil going directly into a hole in the frame that carhaulers use to chain down the cars. I had 4 quarts of oil inside my cars frame.

Is there a large(3/4 in diameter) vent hose from the valve cover to the air filter housing?

An unrestricted vent. Not the PCV.