Oil weight and season/temperature

I'm having an hot idle oil pressure problem that I know how to attack from searching but I don't know what weight oil is ok to be used at what temps

I'm in New England and its still getting cold at night and in the morning

I have 6.5 quarts of 10W40 in there now with the oversize Mobil 1 filter and that boosted pressure all around except hot idle after extended driving

Should I will it gets warmer out to try out the 20W50 or can it just be run year round??

any insight as to what weight oil is ok to use during what seasons/temperatures would be awsome

Thanks a lot

Bobby

If you run synthetic it will flow freely at all temps and does not thicken up when it hits below freezing. Take a bottle of 20W50 standard oil and mobil-1, let them sit out all nite below freezing then pour them both, you will see the difference.
I run 10W30 Mobil-1 in all our jeeps and 5w30 in the wifes oldsmobile, all year. I stopped using organic motor oils back in 95 or so.
You might want to check your sender connections, the one on my TJ was corroded/tarnished and would read high then when warmed up read low and cause the 'check gauges' light to come on at idle. I cleaned the contacts on the sender and then used a burnishing tool to clean the socket on the plug that goes into the sender, problem fixed.

hmm I was having wierd readings and I replaced the sensor which fixed them BUT every time I start the Jeep the check gauges light comes on for a spit second then the gauge jumps up, think that means anything?

Before the days of Mobil 1 and multiviscosity oils, new engines got straight 30 wt oil. Not sure if they thinned it out for winter up north, as I have lived in Houston all these years. It very rarely gets under 20 F here in the winter so straight 30 weight was fine.

I ran staight 30 wt and straight 40 weight oil until just a few years ago in all my vehicles. I put nearly 400,000 miles on a 1978 dodge station wagon with a 360 V8 using straight 40 wt the last 300,000 miles. I started using multi viscosity oils about 3 years ago.

I would think that 20W50 would be OK. It is available here in a synthetic now, from several companies.

The real answer is to try it. If it solves your hot idle problem, and does not slow down the cranking on cold mornings I would say go for it.

How many miles are that engine? What cold, warm, hot, cruising and idle pressures are you seeing.

EBXJBOB said:

hmm I was having wierd readings and I replaced the sensor which fixed them BUT every time I start the Jeep the check gauges light comes on for a spit second then the gauge jumps up, think that means anything?

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The check gauges light is normal during the POST of the jeeps system, just like a PC, mine started after an oil change. I tried again a purolator Pureone and that would not build pressure after about 25 miles on an oil change, went back to a K&N as both I and the stores were out of Mobil-1 filters. It was fine for a while afaik. I alternate jeeps so it was a couple of weeks till I used it again. Heading down to jersey on interstate 80 at 70mph the oil pressure was 5lbs and man, I was sweating it all the way to the house down there. Got it in the garage, popped the hood and unplugged and replugged the sensor, oil pressure shot up to normal, +20lbs idleing hot, took some tension on the plug and the pressure dropped, pulled the plug and opened my tool bag, pulled out a GC burnishing tool and cleaned the spades in the socket on the connector, shot some dielectric in there and plugged it back in and it's been fine ever since [knock on wood]. This happened on the TJ.

142,000 miles on it

It has fine oil pressure on startup and crusing, I just did an oil change with the larger filter and 6.5 quarts of 10W40

I have start up pressure of around 50-55 and its a little above 40 when crusing

The hot idle pressure starts off fine, around 30, but the more I drive the more it drops, Eventually to 21, which is fine

The problem comes after a highway trip or off road for a while, it starts to go down to about 10, it hasn't hit that 10 line yet so it's prob right at 13, which I know is in spec, but still I don't want it getting that low

before the switch to the 10W40 once the engine was that warm the oil pressure would be about 40 when accelerating and drop to about 30 when it went into overdrive and the RPM's dropped, the 10W40 brought this up even though it still gets low at hot idle

usually, wear on bearings, or a faulty sender or gauge cause pressure fluctuations, possibly a bad pump. thicker oil won't fix either one. in fact, thicker oil just causes your oil pump to work harder, which causes extra strain on the pump. running synthetic oil may give you the best protection, even if it is just a sender gauge. synthetic oil is great for cold weather climates, and also for high temps. in the summer. changing the sender unit would be a cheap and easy place to start, mine went bad at 126 k.

yeah mines new as well, thats the first thing I did

Book says 5w30, I run that, I also substitute a quart of oil for a quart of Lucas Oil Stabilizer. I too startup with about 55-60, and then after I get off the Interstate, I notice it drops down just below the 40. I keep great pressure on the road, I just don't seem to have the lower pressure until I am close to oil change time. I also run Slick 50 about every 50k, and she is about due anyways.

Jeff

Yeah if mine was dropping down to just below 40 that would be great, except its dropping to just above 10

So basically i'm trying to see if running 20W50 all summer would be fine and hopefully boost the pressure till I can do the bearings, which i'm not sure I want to do if the 20W50 gets it up even a little higher

BTW would/should I be replacing the rod bearing, main bearings, or both

and I've also read something about turning the crank as that could cause the problem as well

The other nice thing is the Jeep I gave my dad with almost 180,000 on it holds great pressure

BTW would/should I be replacing the rod bearing, main bearings, or both
All the bearings, cam bearings too.

As far a turning the crank you won't know until you tear it down and then a whole kit, re-ground crank and bearings is usually cheaper.

Did I hear 'slick50', god is that crap still around ? Man that stuff is bad news.

Why?

if your oil pressure is dropping to just over 10 I would be concerned. my engine with over 250K miles doesnt drop below 30 after wheeling all day in the 110 degree desert here in cali. I dont think 20w-50 will solve your problem

Neither do I, But it might boost it a little until I can either get the time/space or the money to do the bearings

Anyone have an idea what that would run me to have a mechanic do?

dzraces said:

if your oil pressure is dropping to just over 10 I would be concerned. my engine with over 250K miles doesnt drop below 30 after wheeling all day in the 110 degree desert here in cali. I dont think 20w-50 will solve your problem

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20W50 should solve or at least help his problem considerably. He has plenty of oil pressure (50-55) at startup and crusing speeds.

I had the same problem last year that he is having (and several others have had the same problem and solved it by changing to 20W50,... plus 1 quart of Lucas in the summer months) and we have all switched to 20W50 with a single quart of Lucas swapped out for the 20W50 during summer months and we got our hot idle pressure after a long hard highway run back up from 10 PSI or less to at least 20 or over 20 psi, with the cold start up (50-55) and cruising pressure (30-40) still running at 30 to 55 PSI.

I started out puling the oil pan on mine, checking the crank shaft, the mains, rod bearings, replaced all the mains (although they had no copper showing yet), the rod bearings looked and plastiguaged as new, the crank shaft looked and tested as almost new, no oil consumption (less than 1/2 quart in 3000 miles) so the rings are like new (mine had 225,000 miles on it when I bought it so I think it may have been extreemly well cared for or had already had a head, ring and rod bearings job done before I bought it). I also replaced the oil pressure sender and the oil pump with a high flow pump.

After all that work I still had 13 psi at hot idle, so I switched to 20W50 plus Lucas, and now I get about 22 psi at the notorious hot idle after a long hard highway run. Interestingly, if I let it idle for about 15 minutes after a long hard highway run the oil pressure comes back up from about 22 to about 30 PSI.

The curious thing is that the hot idle problem only develops after a long hard run on the highway. I can let sit and idle in the driveway, hot for hours and it holds 30 PSI.

I am still baffled as to what is causing the low oil pressure at hot idle, but obviously Jeep knew something we don't because the FSMs all say 13 psi at hot idle is OK!

I just can't bear to see it get that damn low on mine. Also, 20W50 acts like straight 20WT oil cold at O degrees F (as I recall) so it is not that thick! It only acts like a 50WT oil once it is hot (about 200 F) at which point a 50 WT oil is about as thin as a 20WT cold oil at 0 degrees F.

Slick 50 is a bad idea in my opinion too. Two problems, it has solid solids in it, even if the solids are teflon, they are solid. Secondly, if the teflon ever gets hot enough, like around an exhaust valve maybe, it can breakdown, oxidize and form Hydroflouric acid.

Ecomike we have the same problem, my oil pressure only drops that low after highway trips, and I also don't like seeing it that low, I actually put it in N at stop lights and give it steady gas to keep the rpm's up and the pressure up

around town it stays between 20 and 30 at hot idle

I just changed the oil with 10W40 so I'm going to run that for a while since it did increase my start up and cruising pressure as well as hot idle as long as I don't take it on the highway

When I get out of school in may i'm going to switch it to the 20W50 with a quart of Lucas (its Lucas oil stabilizer correct) and hopefully it will raise it to 20 ish, if not then I guess someone really neglected/beat the hell out of this 4.0

EBXJBOB said:

Ecomike we have the same problem, my oil pressure only drops that low after highway trips, and I also don't like seeing it that low, I actually put it in N at stop lights and give it steady gas to keep the rpm's up and the pressure up

around town it stays between 20 and 30 at hot idle

I just changed the oil with 10W40 so I'm going to run that for a while since it did increase my start up and cruising pressure as well as hot idle as long as I don't take it on the highway

When I get out of school in may i'm going to switch it to the 20W50 with a quart of Lucas (its Lucas oil stabilizer correct) and hopefully it will raise it to 20 ish, if not then I guess someone really neglected/beat the hell out of this 4.0

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There are several others besides us with the same exact problem. And yes, it is the 30 minute highway trip that triggers the low hot idle pressure problem. Mine also stays at 25-30 around town.

Yes it is "Lucas oil stabilizer".

I have a few theories as to the cause, but my best guess is that under heavy load the oil itself is getting just a bit too hot somehwere in the engine, maybe a local hot spot under heavy load in our engines, and that the oil is thinning out too much when it gets that hot. I have even toyed with the idea of an added an oil cooler, but I probably won't go that route as it would not be a simple undertaking.

yeah, I am currently trying to strategize my way through low hot idle as well, mine reports between 10-14 after a nice ride.

My plan is to switch to synthetic oil with a quart of synthetic lucas, and see if that does the trick. If not, I will probably switch to the heavier weight oil, which I guess 20w50 is the recommended oil.

DaveKerwin said:

yeah, I am currently trying to strategize my way through low hot idle as well, mine reports between 10-14 after a nice ride.

My plan is to switch to synthetic oil with a quart of synthetic lucas, and see if that does the trick. If not, I will probably switch to the heavier weight oil, which I guess 20w50 is the recommended oil.

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If you have any miles or age on it I would avoid the synthetics until you have replaced every seal, o'ring and gasket on the engine! I used a mix of 2 quarts synthetic to 4 quarts dyno recently and my 1/2 quart / 3000 mile oil consumption went to 2 quarts / 100 miles, and the only seals I had not replaced were the oil filter adapter o'rings.