View Full Version : Low oil pressure help

08-05-2015, 09:50 AM
(Copying here from v6p since i've attempted to follow the supercoupe blower/intake/etc for my Taurus mods, maybe there's some engine specific concerns you guys might know about?)
Ref: http://www.v6power.net/vb/showthread.php?p=542014

It's been several years since I put this engine together, and I haven't run it more than maybe 10 miles in that time (tuning/eec/trans issues). I've forgotten what the clearance values were however.

It's always had pressure a little lower than I'd like, around 20PSI cold idle for those years.

Now this spring/summer after getting back into this project, i've found that the pressure is even lower. Even after new oil pump, 10w30 oil and filter, priming the pump with a drill only gets up to about 25PSI. It idles pretty smoothly around 800 RPM, but at those speeds right after startup it peaks at about 20PSI, and drops to <10, actually reading zero on the autometer electric gauge as the engine reaches operating temp.

One more bit of info, when cruising the pressure does build up to about 40PSI but the as stated above, when the speeds drop back down to idle (in gear or in neutral).... <10PSI. And course lifter tap tap tap at that point.

When I read about many of you hear seeing upwards of 35 PSI at idle, I know something must be wrong, any ideas?

Since i'm pulling the engine back out in a few days i'm just going to replace the rods and mains and triple check the clearances (again). But is there some other obvious thing I should check into during all this?

Taurus (fwd) w/
99 Windstar block
New 4.2 crank
RPM 6.125 rods + matching pistons

Switch to 10w40 or 10w50 weight oil. BTW, 10psi at warm idle is OK AS LONG AS when you step on the gas, the oil pressure jumps up instantly. You should also be seeing 60psi of pressure at WOT. Ideally, higher is better at WOT. Heavier weight oil will help a lot. An easy fix. The reason for low oil pressure is that the main crank and rod bearings have too much side to side clearance which causes some oil to sling off the rotating parts with the motor running.

Had the exact opposite problem on my new build. 95+ psi with cold startup even with 20 wt oil. Ultimately, had to do a 4AN bypass beading off some oil back into the pan 100% of the time. It is working well. Still using 20 wt oil, but can go higher to raise pressure at any time.

The low idle pressure doesn't rise very quickly unfortunately, when rev up to to about 1800 RPM, the pressure builds over the next 5 seconds or so to ~30PSI (starting from about the "zero" psi resting point).

I'll be grabbing a few things at the parts store today so I can pick up a few quarts of 10w40. Am I wrong in feeling like that's a bandaid rather than a proper fix? What pressures (and at what oil weight) are you guys seeing after performance build?

thanks again, I hope i'm just being paranoid but that tick-tick-tick when the pressure falls is a little unnerving!

Go with a 15w50. 10psi at idle is way to low imo. I see 20-25psi with 10w 30 and around 30psi with 10-40. Bearing clearance dictate oil weight so it's not a band aid. If the thicker oil does not fix your issue then it's time for you to pull the motor apart and investigate. Maybe get another gauge on it first though.

Who built the motor? Whats done to it?

As an experiment i drained and refilled with 10w40, the pressure took several seconds at 1200 rpm for pressure to even register, once it did though idle pressure jumped up to about 30-35PSI and stayed there and most of the lifter tapping went away.

Once the engine was fully warmed up though the slowly dropped lower to <10 and the lifter noise returned. I imagine i'd have better luck with the 15w50 based on these results but since i've already got the new bearings in the mail i'll just go that route.

As for the engine, I had a local machine shop bore/hone/etc and replace cam bearings. I myself did the assembly, taking what I thought were adequate precautions and double checks but it seems i'll have a chance here in the next several days to see what I must have missed.

Am I right in shooting for .002" target main bearing clearance this time? I understand .001 is far too tight for a performance build, and .003 too sloppy. How about the rods?

I should also mention these are RPM custom forged pistons, and I was under the impression forged = a little noisier when cold?


Think the best bearing clearance would be .0015" for both the mains and the rods. It is those clearances that control oil pressure or lack there of. Also, if you are going back with main and rod bearings, suggest you go with the NASCAR rated bearings which are a little harder which will last longer with adequate oiling. Another issue that controls oil pressure is the side clearance between the outside edges of the bearings and the journal. The tighter, the higher the oil pressure. Another area that controls oil pressure is how well (close) the journal radius and the bearing radius are matched (chamfered).

I would try and get the clearance to 0020-0025 and run the 15-50 oil. Every one has their preferences though. To me I'd rather have the bearings looser and run the heavier oil weight than on the tight side and have to run a low weight oil.

I'd be checking the clearance on those cam bearings too.

Re: Cam bearings

I must have just gotten lucky in the past, having swapped camshafts (regrinds) probably 4 or 5 times in the old 120,000 mile engine with zero problems.

The new engine was from a 99 Windstar, it only had ~3000 miles on it but I had a local shop bore+.030/clean/balance/etc , as part of that process they installed new cam bearings too, I didn't even think to check that.

All I've ever used is plastigauge anyway, swapping multiple cranks and rods as to experiment and never hard an issue before, definitely not top quality equipment by any means but it's never failed me. I might have to see about renting or checking HF for a dial bore gauge, at least for the cam bearings.


Re: Clearances

I was targetting something near what the thunderbird supercoupe engines use since i've for the M90 blower load and stresses too. They call for clearances of .0009-.0026" for journals #1,2,3 and .0014-.0032" #4, with that my target of .002 would be weil within the recommended ranges.

Rods call for .0010-.0014"

Does this change any recommendations?


08-05-2015, 11:30 AM
That's all I did. I used plasti-Gage for the bearings, and standard feeler gages for side clearences. You did check the oil pump clearances? Relief isn't stuck open?

08-12-2015, 12:44 AM
The problem appears to be the pickup tube itself - specifically, the bolts loosened up on it so much so that it was just lightly pressing against the block, my guess is it was sucking up some air along with the oil.

Since I've got the engine out and new standard bearings on the shelf I went ahead and continued with the teardown and checked the old bearings, am I reading the bearing clearances right? They look like ~.001, maybe slightly less. The bearings themselves all looked fine, same goes for main and rod journals.

I got away with standard bearings/clearances for years with the stock Taurus crank and also after swapping in a supercoupe crank, am I really putting the engine at risk if I just left the .001 clearance? It's right within the .005-.0023 range Ford recommends. If that's definitely too tight, what are my options?

XR7 Dave
08-12-2015, 08:20 AM
It will run like that, but it's very tight. Ok for long life, not so much for high performance use. You don't have any options because no one offers +.001" bearings for the 3.8L. The only solution is to grind the crank for bigger clearances but that means complete disassembly and a specialist capable of doing it right. There aren't many crank grinders who can clearance a crank accurately enough to achieve that without going an additional .010" undersize. If it's a stock build I'd just leave it.