Oil pressure drop through adapter, oil cooler, lines

**Tom, F&L GoR** · 02-07-2015

In cold starts (25F and lower) the oil pressure runs up to 60 psi at idle but behaves somewhat like the oil is not returning to the pan quick enough. At higher than idle, the oil pressure may go up, down or stay the same. I verified this with a mechanical gauge (which indicated the dash gauge reads about 7 psi lower than actual.) It never goes low enough to worry about, but doesn't act normal.

I changed only the filter, no difference.

I added an extra quart, no difference.

I changed the oil from 5W-40 to 0W-40 to 5W-30, no difference.

I took the pan off and there is nothing floating to block the pickup screen.

I removed the oil filter adapter, inspected the oil cooler bypass valve and spring. They are fine.

I removed both oil pressure relief valves, replaced them with ones from a '97. (I bought a timing chain cover to see what I was getting into. The pressure relief valves in my car seemed to have more burrs, so I put the '97 ones in. All are tight.) Still no difference.

I made a hydraulic hose to bypass the cooler - so from filter out to engine in: oil pressure was 90 psi. Interesting, since the oil pressure gauge sending unit is on the return from the cooler, meaning the oil filter sees a lot more pressure than the gauge shows.

I removed the bypass hose, reinstalled the oil cooler hoses and oil pressure back to 65 psi. So the oil cooler (and the OEM oil lines) combine for a dramatic oil pressure drop.

Has anyone had a failed cooler or oil line - not a leak, but a blockage? Does a 25 psi drop seem correct (doesn't to me.)

It's a '98 GTS with 21K, bought this summer with 15K. I assume the car sat for long periods due to the age and low miles. Never had a hint of this when the weather was warm. Anybody have helpful comments or experience. No, not driving in the cold is not an option.

**99RT10** · 02-07-2015

Hey Tom,
It sounds like the issue is in the oil cooler. If you want, I could send you one to switch out and see if there is a change with the swap. I would think a 25lbs drop from "in" to "out" might be a bit excessive.

Have you checked to see if there was any kind of blockage in the lines?

**C.J** · 02-07-2015

Originally Posted by Tom, F&L GoR

In cold starts (25F and lower) the oil pressure runs up to 60 psi at idle but behaves somewhat like the oil is not returning to the pan quick enough. ...

What is the behavior that makes you think oil isn't returning to the pan quickly? Thanks.

**Tom, F&L GoR** · 02-07-2015

Thanks, Mike, may take you up on it. The hose I made eliminated the cooler and two hoses. Now I've made a fitting to connect the ends of the two oil lines in the grill - that will test the hoses and isolate the cooler. Let you know. (Not sure the cooler can be removed without removing fascia - ugh.)

CJ, cold oil that isn't runny enough when cold will not return to the pan quickly. If enough oil hangs up in the heads and block, the oil pump sucks a hole in the lesser quantity in the pan, same as sucking a straw in a cold milkshake. In my case, start up is fine, driving away is fine, then after a few minutes (literally) the gauge will drop pressure. Idling seems to return it to "normal" pressure, however if it was really oil that was too cold, this phenomenom would happen right away and not after a few minutes. It made me think the relief valve or cooler bypass valve was sticking because the car sat for a while. I can't yet guess what the cause is, but now it has to be either the lines or the cooler, and only when cold. Weird.

The "W" number of an oil is determined by lab tests that simulate 1) cranking and 2) slumping. In the days of carburetors, the engine had to crank fast enough to draw suction on the venturis and pull fuel into the mixture. A choke would assist and make sure enough fuel entered the mix. So the important criteria was the "Cold Crank Simulator". Since the use of fuel injection, it doesn't matter how fast the engine cranks - the fuel pump delivers fuel to the injector. An injected engine will start and the oil needs to run back to the pan. This is measured by the Mini-Rotarty Viscometer. The technical societies (SAE, ASTM) have determined criteria that safely cover engines and their cold temperature needs.

http://media.noria.com/sites/archive...l_can_tab2.gif

A 5W-xx is supposed to be safe down to -25C and 0W-xx down to -30C for cranking; another 10C lower for pumping. 25F is -4C, so oil viscosity should not be the issue.

**Bluecoupe** · 02-07-2015

Is the car sitting still when the oil pressure drops off above idle. If it is moving forward, the oil cooler is doing it's job and cooling the oil colder than ambient. If it occurs with the car running in the garage, then this would not apply. Just a thought...

**Tom, F&L GoR** · 02-07-2015

Dan, both. I first noticed it driving, but letting it warm up in the driveway it still does it.

**99RT10** · 02-08-2015

Another possible culprit, bad sending unit and/or gauge. I could send you a gauge too. Try replacing the sending unit first though. Probably a lot easier than taking off the facia.

**Tom, F&L GoR** · 02-08-2015

I replaced the sending unit and while doing it, have installed a "T" at the sending unit location. The mechanical gauge reads ~ 7 psi higher than the dash gauge, but otherwise they go in the same directions. Today I'll be able to connect the ends of the hoses, so that will tell me if it's hose or cooler. I just can't believe there is something inside either hoses or cooler that would do this - seems very odd.

**AZTVR** · 02-08-2015

25psi drop across the oil cooler surprises me also; however, I am an EE, not an ME mentality. One would think that the oil cooler would be designed with no pressure drop, but, maybe that is an artifact of what has to happen to get the desired cooling effect ? I don't know. I've never read a discussion such as this before; so, it's a very interesting topic !

**Tom, F&L GoR** · 02-08-2015

I bypassed the cooler with all plumbing - AN-10 to pipe elbow to pipe elbow to AN-10 in the grill, and startup pressure was 85 PSI, close to the 90 psi given today was 28F instead of 22F. So the hoses must be fine and the cooler is the cause for the 20 psi drop.

BTW, the oil cooler can be removed going up. In front of the radiator is a removeable plastic cover. Underneath is a two piece cover shrouding the hood latches. I removed the driver side (one Christmas tree). The oil cooler can then be passed upwards and out under the hood.

I don't know that I can test the cooler, but will think about this. At least I can drive the car since I don't need to cool the oil for a while.

**Camfab** · 02-12-2015

I'm trying to get a mental picture here. If the oil pressure sending unit is on the output side of the oil cooler and the oil cooler bypass is prior to the cooler. Logically, and I'm sure I'm missing something here.......but wouldn't the oil cooler bypass opening up, cause the gauge to read lower than normal? In other words, bypassed oil is returning to the engine while a certain amount of oil is still flowing through the cooler at reduced pressure? Would that not explain why pressure isn't increasing with RPM as it's being held steady by the bypass valve in the cover? I know the ACR timing cover does not have the bypass. The owners manual I believe has a warning about operating the ACR in cold temps.

**Steve M** · 02-12-2015

I was going to ask what size the lines were (I've known some others that made them too small), but -10 should be more than sufficient.

What type/brand of oil cooler is it? From what I've seen researching them once upon a time, not all are created equal. You want enough dwell time in the cooler so that heat actually gets exchanged, but you also don't want a massive pressure drop.

**Tom, F&L GoR** · 02-12-2015

Originally Posted by Camfab

I'm trying to get a mental picture here. If the oil pressure sending unit is on the output side of the oil cooler and the oil cooler bypass is prior to the cooler. Logically, and I'm sure I'm missing something here.......but wouldn't the oil cooler bypass opening up, cause the gauge to read lower than normal? In other words, bypassed oil is returning to the engine while a certain amount of oil is still flowing through the cooler at reduced pressure? Would that not explain why pressure isn't increasing with RPM as it's being held steady by the bypass valve in the cover? I know the ACR timing cover does not have the bypass. The owners manual I believe has a warning about operating the ACR in cold temps.

The oil cooler bypass does the bypass feature at low pressure. Normally, pressure from the oil pump outlet moves the shuttle valve against a weak spring (someone quoted 17 psi oil pressure) and closes off the bypass port, forcing all the oil through the cooler. Only when the pressure is low (i.e. hot oil at idle) does it bypass the cooler. Seems backwards, right? But then the reason for it was to keep the oil cooler from degrading air conditioner performance. I've had my adapter/bypass apart so I can verify this is what it is supposed to do. Once the oil pump closes the bypass, it remains closed at any higher pressure/RPM.

Originally Posted by Steve M

I was going to ask what size the lines were (I've known some others that made them too small), but -10 should be more than sufficient.

What type/brand of oil cooler is it? From what I've seen researching them once upon a time, not all are created equal. You want enough dwell time in the cooler so that heat actually gets exchanged, but you also don't want a massive pressure drop.

The lines are stock AN-10 lines.

The fitting I made bypassed the cooler only in the grill (oil flowed through the stock oil cooler lines) and the pressure was 85 psi, so I don't think the lines are problematic. The oil cooler on the car is a factory part, although with suffix AB. According to the parts manual, a '98 should still have the original version with no letters; AB only started in '99. After that there was an AC version, which I now have one of. The AB is 2.5 lbs, the AC is 4 lbs. The AB bottom tube is straight, the AC bottom tube curves and hugs the bottom of the heat exchanger. The AB has 9 rows of tubes, the AC has 7. So the coolers are easy to tell apart, although there is no way to see if the AB one has a restriction.

It'll be cold here in NY for a while, so I am looking forward to putting the "AC" cooler on (ironically) and seeing if that makes the difference. It has to, since I mechanically isolated the old cooler as the restriction, but the engineer in me is having difficulty picturing how it is doing this.

**Tom, F&L GoR** · 02-13-2015

This is the difference between the AB and AC version. The upper one is the AC (newer). Correction: AB has 12 rows and AC has 9. The newer one is still all aluminum, but everything from end tanks to the mounting tab seems to be thicker material. I would be curious what 1992-1997 owners have as the original version.

A boroscope view inside the one that came on the car does not show any sign of deposits, failures, plugging, etc and actually looks pretty clean. So, no smoking gun as to why there is a 25 psi loss.

20150212_195505.jpg

**JonB ~ PartsRack** · 02-13-2015

I just saw this thread:

One of the issues with early Gen 2 'shallow' pans was inferior castings. The cast-in oil return line is small and rough inside, not larger and well-honed as with 2000+ hi-cap pans. Back at VOI-7 I had a display in our PartsRack booth of both pans. Very obvious improvement. In fact, some of the early G2 pans had casting 'flash' debris that broke loose, and did not depart the oil return channel but blocked the channel. This was only discovered (rarely) at VEWC in MI, and one of the reasons for the improved pan. (Along with oil starvation)

You might try to fish plastic tubing or other flexible probe into-thru the return line and see what you find? Good Luck Tom

Oh yes: The "Shuttle Valve" on the oil filter adapter is a line-restriction to the oil cooler line. Gen 1, and G2ACR do not have this mechanical valve. The valve prevents oil from going thru the cooler at low pressures and idle, so as not to overhead the A/C condenser ! Not having a valve results in +5~7 PSI pressure at low RPMs. [Id rather have the oil cooled ALL THE TIME rather than protect the AC from tripping off line] Since the G2ACR was 'designed' without AC, Team Viper {Charlie Brown III and Dick Winkles} eliminated the shuttle valve in favor of better Oil Pressure in the ACR. Bless Them, and for TELLING us track rats about it !

Those ACR Adapters are long since sold out and discontinued. I don't know if the valve can be removed. You CAN use a G1 adapter on a G2 motor, but it makes the oil filter point the wrong way and difficult to change.

**Camfab** · 02-15-2015

As a point of reference, I have the AB version on my 2000 ACR.

**Tom, F&L GoR** · 02-18-2015

Installed the AC version - actually quite easy to do from the access panel in front of the air box. The hoses are easy to reach and tighten through the grill if you do them first and mount the cooler last. Another hint is to loosen the hoses at the filter adapter to allow the hoses to rotate so you can get them onto the cooler.

On the day I installed it, the temperature was 20F and oil pressure was stable at 70 psi, meaning there is still a 20 psi pressure drop through the new cooler. Oil pressure increases with RPM, did not decrease. I didn't drive the car (couldn't get over the 1" snow mound around the car.)

Today was -5F according to the Weather Channel, but backyard thermometer said +5F. Take your pick. Started fine, oil pressure to almost 60 psi on the dash gauge, which I now know means 70 psi on the mechanical gauge. Let it idle a few minutes (this isn't warming the oil at all) and then off to work. Oil pressure does not rise with RPM and seems to wander between 50 and 60 psi on the gauge until the coolant is up to thermostat temperature. When warm, the minimum is 65-ish psi and max at 70 psi.

Conclusions and Observations:

Gen 2 constant oil flow through the oil cooler is a bad idea. It's a massive flow restriction when the oil is cold, reducing oil pressure and in extreme cases, some oil flow. Gen 1s had an oil thermostat, which is a much better system. Not only did it allow better oil pressure to the engine when cold (I saw 90 psi bypassing the cooler) but oil needs to reach the boiling point of water to drive off any moisture or fuel accumulation. Those of you with an oil temperature gauge know it takes 30 minutes for the 8.5 to 11 quarts to get that warm and every trip you take isn't 30 minutes, nor does everything boil off the moment you reach 30 minutes.

The oil cooler bypass valve in the non-ACR Gen 2 is fine to leave as it is. The valve is not a restriction since it is not between the oil filter outlet and the oil cooler inlet, it is another outlet that gives the oil a second place to go. When oil pressure is low enough for the valve to open, this second path is exposed, leading directly back to the engine. It does not close off the entrance to the oil cooler. Therefore the oil can go either through the cooler or back to the engine and since the diameter of the two paths are roughly equal in diameter; you can guesstimate that roughly half the oil goes through the cooler anyway and the other half back to the engine. That the ACR version claims higher oil pressure, it must be smoother or larger internally - you can't make the non-ACR version perform better by any combination of springs or removing/changing the valve. (This bears some more checking since externally the casting doesn't appear that different.)

If you really want better/higher oil pressure/flow, don't obsess over the adapter and get an oil cooler with less restriction.

**Viper Specialty** · 02-18-2015

FYI... consider an upgrade to the Gen-3/4/5 heat exchanger, much more stable oil temp, less restriction, and no cooler blocking the condenser.

**Ron** · 02-19-2015

Hi Tom -

It's been a long time!

I think your point is demonstrated by this image:

**Tom, F&L GoR** · 02-20-2015

Thanks, Dan, but my #1 is to get oil temperature up, not keep it down. I have a Mocal oil thermostat on order that comes equipped with AN-10 fittings. I'll make two more short AN-10 hoses from it to the adapter to complete the circuit - should be clean and simple. And much better for the oil and engine. I guess Dodge never figured anyone would be out below 32F... Sorry, then it takes too long to get my 100K miles.

Hi Ron, yes it has. My new job supports Andretti, so a visit to Zionsville Road is in the cards. See you then. Your drawn lines are correct. The valve in the non-ACR version opens a second path, but never closes off the main path to the cooler. So the fork in your drawing is exactly correct during the low pressure scenario. What surprises me still is that the two OEM oil coolers I tried have such a large pressure restriction, and owners never know it because the gauge measures pressure downstream of the cooler. I will post again when the thermostat is installed.

**Viper Specialty** · 02-20-2015

Originally Posted by Tom, F&L GoR

Thanks, Dan, but my #1 is to get oil temperature up, not keep it down. I have a Mocal oil thermostat on order that comes equipped with AN-10 fittings. I'll make two more short AN-10 hoses from it to the adapter to complete the circuit - should be clean and simple. And much better for the oil and engine. I guess Dodge never figured anyone would be out below 32F... Sorry, then it takes too long to get my 100K miles.

Tom... I know what you are trying to accomplish [and your issue], hence why I have suggested it

The Gen-3/4/5 cooler is a HEAT EXCHANGER between the coolant and oil, not an air/oil cooler. It will pull your oil temps up to engine temp and stabilize them far faster than any other option, while un-cluttering the air supply to the radiator and condenser. No need for an oil thermostat in this case, in fact it will be counter productive. Coolant temps rise far faster than oil temps, causing it to actively pull oil temperature up faster than it would rise on its own, even compared to cutting the cooler loop out totally. As we all know, air flow through the cooling module of the Gen-2's and A/C performance has always been of lack-luster performance, and the oil cooler being the first in front has always been a mistake for Street applications IMO. Just too many blockages before the radiator.

The above, combined with the proper thermostat and ECU flash to bring fan temps in line is pretty much the ideal route.

**Tom, F&L GoR** · 02-21-2015

Dan from Buffalo: I looked up the parts and yes, that would have been ideal. It's more plumbing than the old school fix I will end up with, but the proper way to do it. I am still feeling a little let down by Dodge than Gen 1's had an oil thermostat (because Gen 1 owners would drive in sub-freezing cold?!) and when a roof was added the oil thermostat was removed... Meanwhile I have the cooler bypass hose installed again until the parts arrive.

**Viper Specialty** · 02-21-2015

Originally Posted by Tom, F&L GoR

Dan from Buffalo: I looked up the parts and yes, that would have been ideal. It's more plumbing than the old school fix I will end up with, but the proper way to do it. I am still feeling a little let down by Dodge than Gen 1's had an oil thermostat (because Gen 1 owners would drive in sub-freezing cold?!) and when a roof was added the oil thermostat was removed... Meanwhile I have the cooler bypass hose installed again until the parts arrive.

Tom-

In all honesty, its easy as hell to plumb. -10 O-Ring In/Out for oil, and the coolant side hooks right into the Heater Core return on the passenger front of the engine. You could mount it to the front cross bar, and have less than 1.5 foot hose connections all around.

**Tom, F&L GoR** · 02-21-2015

Originally Posted by Viper Specialty

Tom-

In all honesty, its easy as hell to plumb. -10 O-Ring In/Out for oil, and the coolant side hooks right into the Heater Core return on the passenger front of the engine. You could mount it to the front cross bar, and have less than 1.5 foot hose connections all around.

You're killing me. That's the kind of clever upgrade fix I would like to do, but after buying a front chain cover to have oil pump and pressure relief valve parts, then getting another oil cooler in case mine was plugged, and with the oil thermostat and hose connections in the mail, I'm going to finish it this way and try to get a little satisfaction that I diagnosed it correctly. Meanwhile, let's all agree cold weather sucks.