Reaaaally odd problem with oil pressure and temperature

[jmigueldepriego]

I've been strugglin' with my 1981 351M Bronco at the point of having no answers.

Like two months ago the engine started to knock and overheated, (or maybe it overheated and then knocks), and we could'nt find the way to tune it up to its 6º from TDC. Weeks later we detect a problem with the distributor and get the correct spark, but days just before, the engine started to loose oil pressure (as low as the "N" letter in the NORMAL reading of the gauge). The oil pressure was fine while cold, but as it got warmed, it started to drop, and went high again while throttling as much as near the half of the gauge (almost vertical needle). This got complicated with the temperarture which started to go higher than ever (as much as the vertical position in the gauge).

We first change the oil and filter, clean the oil sender unit path and connections and got no good results.

Then we replace the oil pump and got a liiiittle improvement but still too low.

We got to the point of replacing all the main and rod bearings and got the highest pressure ever within the NORMAL range but, as it gets hotter, drops again to the N described in the preceeding parragraph.

So, what could you think is going on? We can't figure out what is the deal with the oil pressure in this engine.

And of course, it gets hot above normal operation temperature (i've never got a reading above the first border line in the NORMAL range) and as well, the engine continues to knock.

Any input will be very much appreciated, so please, don't hesitate to share.

Thanks a bunch.

Jose Luis

 

[Bully Bob]

Timing chain., or Dizzy problem I would suspect... B)

 

[muddrivermike]

I had the oil pressure drop problem with mine as it warmed up...After a high volume oil pump it had great pressure,higher than Ive ever seen it,But once again as it warmed up the pressure dropped,even to zero in gear.What it was the oil pressure sender/switch,As it heated up the resistence would change giving me a false reading.The sender was like $13.Replaced that about 4 months ago And its been perfect even on hot days.Your overheating is another issue,Clogged radiator,Bad thermostat.I dont know but.If its knocking it might be on the way out.Why not get another engine and replace the rad and water pump?I would like to think it would take care of it.

 

[Bronc76]

The overheating of the engine could cause the low oil pressure because of the excessive oil temperature. If you solve the overheating problem you might see normal oil pressure. You stated you replaced main and rod bearings, you may also have worn cam bearings that could also cause low oil pressure. Might be time for rebuild.

 

[Justshootme84]

The overheating of the engine could cause the low oil pressure because of the excessive oil temperature. If you solve the overheating problem you might see normal oil pressure. You stated you replaced main and rod bearings, you may also have worn cam bearings that could also cause low oil pressure. Might be time for rebuild.

I agree, the knocking sound is likely due to one or more spun bearings. Just replacing with new bearings won't do much good if you've already worn down the rod or crank journals, and the camshaft bearings are likley worn, too. Replacing the oil pump is a short-lived solution, as metal filings from any previously worn bearings/journals will quickly "corrupt" the new pump. That is what Melling quoted on my original 84 block when it went bad.

You have another serious issue with the overheating. I'm currently overhauling a diesel engine on one of the farm tractors that had a prolonged overheating problem, Changed the oil on a regular basis, but the engine would get so hot that it melted the plastic cowling around the engine. Upon teardown, there is at least two inches of sludge or "cooked" oil in the bottom of the oil pan. This eventually built up to the point where it clogged the pickup tube for the oil pump, starving the engine and scoring several bearings. It even seized up the pistons on two cylinders. JSM84

 

[Bully Bob]

If you're convinced the timing issues are solved & timing is spot on....

Pull one plug wire at a time...if the knock'n drops down or goes away., you've found the sick cylinder.. :-&

 

[AdamDude04]

Maybe the baring in the water pump pully is going out, hence the knocking which relates to a slight overheat problem?