Low oil pressure at idle

[3wheelingCJ]

I recently noticed that once the engine is warm/hot the oil pressure at idle is only at about 10psi which is lower than what it used to be. The oil level seems good. The pressure at idle gets lower as the engine warms up.

Also, if i re-start the engine after it have been running for awhile (gas station, short break on the trail, etc) it is usually harder to start than in the morning and requires the gas pedal to be pushed a few times to keep it running. Doesn't happen when the engine is cold.

Any idea what might be going on?

Thanks

 

[ijeeper]

You could check the specs, Ohms, requirements for the oil pressure sending unit, it may be failing. If that's not it, then check the gauge. Gauges can easily be verified by getting an affordable aftermarket gauge and installing it. Also check the sending unit wire, it pass by some hot items and could give a bad reading if shorting out. If the pressure returns to what you expect, then you've found the problem. Beyond the gauge components, you're into changing the pump, which isn't too bad of a task, just dirty.

As for hard starting when hot, you need to understand the differences in circuits used, both electrical and vacuum, of cold verses hot. When cold, your distributor gets vacuum advance then turns off once it warms up. On California CJs, you only get distributor vacuum advance in high gear, otherwise it's off. So, a bad vacuum hose on the warm cycle could be a contributor, but not as likely as an ignition issue.

An ignition coil will always run great when cold until it out right fails. As the coil gets warmer, the spark gets smaller and you'll have issues with it starting and running. Pull the coil wire off the distributor and clamp it about 1/2" away from any metal on your engine and turn the key. You should see a big fat spark. If it's hardly noticeable, change it out, they run about $12 and up. Any standard coil will due for testing.

I've also suffered an issue with a vacuum leak under the carburetor that only appeared when the engine was warm. You can spray carb cleaner around the base of the carburetor when the engine is running. If you the idle changes at all, you have a leak. Keep a fire extinguisher, wet towel or garden hose handy in case there's a fire from the spray. In my case, the base of the carb was warped.

All these engines need is gas, air and spark to run. Air is usually a given, so the last thing is gas. Usually it's a pump issue. The mechanical pumps allow vapor lock which occurs when the engine is hot. Install an inline electric pump with a pressure regulator and return line and vapor lock will never happen again. If you have vapor lock, you shouldn't smell flooding from the carb. Pull the aircleaner off and have someone turn the engine over while you watch for fuel to spray into the carb. First look at it when cold and compare when it's hot. Should be the same.

 

[73CJ]

A couple other thoughts

What weight oil are you using? I run 10-40 year round.

How many miles on the motor? Worn rod and main bearings are very common causes as well.

 

[3wheelingCJ]

I am using 10-w30 oil.
Not exactly sure about the milleage ont the motor. I bought the jeep a few month ago and it has been restored somewhat recently (~5yrs). that included a motor rebuilt. the odometer is showing 36K.

 

[Fastatv]

An increase in oil viscosity will provide a slight increase in oil pressure. Ten psi at hot idle doesn't sound bad. You might research and find out what the minimum pressure is for the AMC 304 , completely warmed up. My 360 only has 10 psi at hot idle, but no knocks, no rattles. My neighbor has an AMC engine book, and it indicates 10 psi is ok. As someone mentioned, as the bearings wear, oil pressure will drop. In regards to the "somewhat" hard to start after warm up. I would think that issue would more than likely be carburetor related. The idle air/ fuel mixture, float height, choke slightly closed, and so on. Hope this helps you.

 

[zackstheking]

its a normal situation could be a 4 year old engine like your or a 40 year old engine like mine once it warms up look for 8 to 15 psi constant at idle its just the way they are been through multiple oil pumps including high volume through all the passages in the block and heads its just the way they are and the oil is definitely not what it used to be:dunno:

 

[3wheelingCJ]

First of all I would like to thank everyone for the responses and apologize for not responding sooner.
I did make a few repairs and observed the jeep's behavior under different conditions. All work was done two weeks ago and here are the observations:

I changed all the plugs in hopes to improve the starting issue. I don't really feel like it helped much. So I will try a few other troubleshooting ideas that were suggested. I did notice however, that after starting it now, cold or hot, it runs a little rough with idle going up and down to a point of almost dying. That seems to go away after about 4-5min.
I also noticed that in the last couple of days that got better.

I also changed the oil with high mileage oil and put in a new oil filter. Turned out that the oil filter that was on the jeep is actually a Transmission filter from some other car. Not sure how that affects it, but guessing it can't be good.
At first the oil pressure stayed at the same levels as before. however yesterday while on trail I noticed that it was higher, at about 20psi. I think being in 4low made a difference, but it seemed higher than in similar situations prior to the oil/filter change.

As far as the oil gauge, it's not electric, but has a copper pick up tube that runs from the engine to the gauge. It was touching the head in a couple places to I moved it from it. Didn't see any bends or kinks.

I will keep an eye on things and also rebuilt the oil pump. got the kit from autozone for under $20.

Still open for any other comments/suggestions.