Low RPM and oil pressure only at idle in drive

[Geekman]

Any ideas on what might be causing an issue with my truck? I was driving home and hit a medium size pothole (not big enough to damage tyres / wheels etc) and my oil pressure suddenly dropped, along with the RPMs (doesn't have a rev counter to give an exact figure, but idle sounds a lot lower). The pothole could have been a coincidence, but the issue happened at the exact same time so I thought it was worth mentioning.

In park or neutral, the oil pressure and idle is fine, likewise when under acceleration in drive, but if I sit at a stop in drive with my foot on the brake, oil pressure and RPMs are very low. I was only 2 minutes from home, so I've just parked it up to avoid damaging anything, and I won't get the chance to take a look at it until tomorrow. Any thoughts as to what my issue might be? The truck is a 1978 F150 with the original 5.0 (302) engine.

Thanks

[Mave]

I'd have a good look underneath for something come loose or leaking. Have you checked the oil level?

[stevieturbo]

Mechanical gauge ? electric gauge ? other ?

Did it empty oil everywhere after this incident ?

Did you smash the sump up into the pickup ?

[Geekman]

Mave said:

I'd have a good look underneath for something come loose or leaking. Have you checked the oil level?

Nothing loose or leaking that I can see, oil level is full and was changed less than 1000 miles ago.

stevieturbo said:

Mechanical gauge ? electric gauge ? other ?

Did it empty oil everywhere after this incident ?

Did you smash the sump up into the pickup ?

It's an aftermarket gauge, pretty sure it's electric. No oil was emptied or anything smashed - the pothole was relatively small, of the kind most people hit maybe every 6 months or so? The truck is lifted and has thick tyres, so the sump didn't get anywhere near the ground. I'm wondering if it might be something to do with the carb adjustment but I know nothing about these things.

[gazza285]

Oil pressure is related to tick over speed, so if the tick over is low then the pressure will also be low. If it does this with your foot on the brake, then it could be related to the servo and vacuum, if air is getting in, then tick over speed will drop.

What happens when you are in drive, but with the handbrake on instead of the foot brake?

[stevieturbo]

idle speed is one aspect.

But first and foremost you need to get a known working and accurate gauge, and determine if what you are seeing on your existing gauge is actually correct.
Or pull the sensor and get a known pressure source and test the existing gauge setup.

[Geekman]

gazza285 said:

Oil pressure is related to tick over speed, so if the tick over is low then the pressure will also be low. If it does this with your foot on the brake, then it could be related to the servo and vacuum, if air is getting in, then tick over speed will drop.

What happens when you are in drive, but with the handbrake on instead of the foot brake?

The handbrake doesn't work unfortunately, so I can't say for sure.

stevieturbo said:

idle speed is one aspect.

But first and foremost you need to get a known working and accurate gauge, and determine if what you are seeing on your existing gauge is actually correct.
Or pull the sensor and get a known pressure source and test the existing gauge setup.

OK, but surely if it was just the gauge, the idle speed wouldn't have dropped? I'll see if I can get it tested anyway.

[stevieturbo]

Geekman said:

OK, but surely if it was just the gauge, the idle speed wouldn't have dropped? I'll see if I can get it tested anyway.

Idle speed will not be an effect of oil pressure.

but lower oil pressure could be an effect of a lower idle speed.

So depends what is going on the two could be linked, or separate.

[321boost]

check vacuum hoses too