Erratic oil pressure readings • The Dodge Challenger Message Board

by **DINGBAT** » 15 Jun 2009 9:55

Hello Gentlemen, my compliments on a great & informative forum. :s017:

My name is Pat, I,m located in Dublin Ireland
I have a query & any ideas would be appreciated.
I recently bought a 1972 Rallye Challenger 340 c/w Sixpack setup.
The car runs fine with no serious issues other than the usual little niggley bits loose this & that, oil leak here & there etc.
I am familiar with the car for about 2 years and know the previous owner well.
When testing the car prior to purchase I did notice the oil pressure reading on the dash guage only went to the first marking i.e. the 1/4 mark at all times either hot or cold and never moved irrespective of engine rpm. Apparentley it always read this way & was never a problem.
I changed the oil & filter last weekend & fit new chrome rockers, breathers, & pcv valve.
The pressure reading is still the same but I did notice on a run that when the car is braked to a stop the needle slowly runs down to the bottom of the guage & when you pull away again it will slowly creep back up to the 1/4 mark. It will do this every time you brake.
I understand that senders & guages in these old cars can act intermitantley & give false readings.
There are no unsavoury knocks or rattles from the engine so I can only assume that oil is circulating & hitting the right spots.
Any ideas. :hmmm:

by **dave-r** » 15 Jun 2009 12:07

Fit a mechanical pressure gauge and see what the actual pressure is.

Blown head gaskets can make the oil pressure go up and down. So can a worn oil pump.

by **fbernard** » 15 Jun 2009 12:25

You need to know one thing about the dash gauge. it's SLOW.

Amazingly so.

That's because it's based on a very simple principle, a heated spring wire that moves the neeld when current flows through it. Cheap to make.

While this may be OK for water temperature (which does not change fast), and desirable for the fuel level gauge (for which you actually need an average level information), it plain sucks when it comes to oil pressure (which can vary greatly in a short amount of time).

A mechanical gauge, or a modern electronic one (electric full-sweep from Autometer for example) will tell you what happens.
Oil pressure going down when hot at idle is normal. You just need to know how far down it goes, and how far up when you're in gear.

by **Eddie** » 15 Jun 2009 15:19

Are the heads on the engine the original ones? If, after hooking up a mechanical gauge, the problem persists, I would investigate my oil pickup tube.

by **DINGBAT** » 15 Jun 2009 20:16

Thank you for the help gentlemen, I have ordered a mechanical guage
Fingers crossed.
Cheers :thumbsup:

by **Goldenblack440** » 16 Jun 2009 11:20

HI Pat, welcome to the forum. The best set up and easiest to use forum in the world i think. My surname is Whelan so i gort a bit 'o Irish in me as well. Good score that deal, picking up a 340 6-Pack. Best Detroit SB on the planet. Well done. Can we see some pics sometime? When you get the mech oil gauge, just hook it up first to the sender unit hole and you can monitor it while you are manually revving it by hand. We always use one when priming the oil galleries with an electric hand drill before a first start on a rebuild. 55-65 psi is good. The factory sender units and gauges were notoriously erratic and inaccurate. But i'm with Ed, could be a half junked up pickup tube.

by **DINGBAT** » 17 Jun 2009 14:50

Installed a 3 pod Sunpro guage set today.
Oil pressure cold was 75 at idle 900rpm.
When warmed & running at 3000rpm pressure was 60-65.
At idle when warm just under 25.
Looks like it was a dicky sender or guage in situ.
Happy Days. :lol: