Brief Knock at Start-Up, Then Low Oil Pressure at Idle When Warm

My 2007 Dodge Grand Caravan with the 3.8L V6 has 226,000 miles on it. It recently developed a 2 or 3 second knock at start up, then when fully warmed up will show the oil pressure light when at idle. As soon as I begin accelerating, the oil light goes off. I run 5W-20 oil in the vehicle. I changed the oil pressure switch with a new MOPAR unit and still get the low oil pressure light, so I am rasonably sure oil pressure is low.

A friend suggested replacing the connecting rod bearings to eliminate the knock and restore the oil pressure. This is possible to do with the engine in the vehicle. He also suggested running a thicker oil, such as 10W-40, if I have to operate the vehicle before replacing the bearings. Or he said just skip the bearing replacement and run with the thicker oil and see if that “fixes” the knock and the low oil pressure.

I put tires, shocks, and struts on the car in April, and was hoping to drive it another two years. Not sure if a DIY rod bearing change is throwing good money after bad. Maybe just go with a higher viscosity oil and drive it till it drops? If you have experience with a similar situation, please share any advice you may have.

Thanks

You need to connect a mechanical oil pressure gauge to the engine so the actual oil pressure can be measured.

But from what you describe, I would start looking for a replacement vehicle.

Tester

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Yes, sounds like a worn engine, just replacing rod bearings won’t fix that. You could try the thicker oil, but it will at best be a band aid, not a ‘fix’.

Of course, it is possible to replace the connecting rod bearings from below, with the engine still installed. The question, of course, is if the crankshaft journals are still “close enough” to round that new bearings will last, however at DIY prices this is not a huge risk. We are talking less than $200 for a set of new standard-size bearings, a tube of engine assembly grease, a new oil pan gasket, a couple cans of brake cleaner/carburetor cleaner (to clean the oil pan, once removed), new oil and oil filter. Depending on how accessible it is, I might change the oil pump too, especially if a higher-flow standard-pressure version is available.

If you have access to a lift, that would make this job very easy, but if not, you can use a floor jack and place a truck wheel under each tire. That should get you enough clearance to climb under there and remove the oil pan. A battery-powered impact wrench will come in handy to take apart the connecting rods, though reassembly will require a torque wrench with angle. The hardest part of this job is going to be laying on your back under the car trying to use the torque wrench.

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Survey question - how many times has anyone known of a case where replacing just the rod bearings fixed a knocking problem? I don’t remember anyone posting about it.

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+1
After spending the time and money to overhaul the worn-out engine, I hope the OP realizes that the 226k mile transmission will be the next big-ticket item that will need to be overhauled. Personally, I wouldn’t spend the money to overhaul the 15 year old engine when the equally-old transmission is at–or beyond–the limits of its design life.

Well, I know of at least two regular posters here who tried that unsuccessfully…

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I remember those. And the chance that replacing the rod bearings would solve the oil pressure problem is even smaller. All the other worn bearings are contributing to the low oil pressure.

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Yes, agreed the engine is worn out.
To the OP, heavy oil would be a band aid. If you wish to go with heavier oil, I would say 20-50 during the summer, you may get a few more weeks out of it.

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It’s a personal financial decision, of course. This is a bad time to be buying a new vehicle.

But something else to note is everything else on the vehicle also has 226K miles on it. Except for the stuff you’ve recently replaced, but those are just wear items.

Personally, I think I’d try some of the “easy” and cheap fixes mentioned, and just plan on driving it until it drops. The van probably doesn’t have much value right now with the age and mileage, otherwise I’d suggest trying to trade it.

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IMO anything over 200,000 miles is bonus. When my Town and Country got 200,000 miles I only replace accessories like alternator, radiator and the like . And that’s because I do the work myself. If I had problems that your having I’d bandaid that puppy and drive til it drops. Knowing it could leave you at the most inopportune time. Start saving for another vehicle. Mine has over 250,000 miles and performs perfectly, but I’m ready for it to drop dead any day. I just hope it don’t happen until the car prices stabilize.

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I am one of them. I wanted to replace the bearings in a 1993 Sundance from below, but unfortunately one of the crankshaft journals was so ovalled out that I didn’t need a measuring tool to see that it wasn’t even close to round. So I ended up pulling the motor and transmission to replace the crankshaft, etc. And now, nearly 4 years later, the car is still sitting in my carport, with the hope that I will find time to rebuild it. Maybe this year I will have time over the winter when my work slows down.

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And I am the other! Spun a rod bearing in my race car. Dropped the pan and fixed the bearing… for a short while. Ended up regrinding the crank.

There is a potential problem with running a higher viscosity motor oil. Depending upon the amount of bearing wear and crank journal wear what can happen is that a heavier oil can cause a bearing shell to change sides.
That is going to lead to a catastrophic bang and trashed engine.

Changing rod bearings could help but if this is done the mains should be changed also.
The issue there is that at 226k miles the crank journals are likely nowhere near tolerance so how long those new bearings last is sketchy at best. Best you can hope for in a case like this is buying a little time while not taking any road trips.

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My memory’s hazy, as far as your 1993 Sundance goes

Does it have sentimental value?

Could knocking at start be a bad anti drain back valve in the oil filter? I have known of a few cars with the oil light at idle but ran quite a while.

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Not saying no but no one I have known that replaced bearings had much long term success.

An oil filter is not going to have any effect on rod bearings or hydraulic lifters that bleed down; bad drain back valve or not.

Most oil pressure senders open at a low oil pressure; often 5 or so PSI. The light may go out but that does not mean there is enough oil pressure to protect the engine.

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Probably the most common failure mode when an engine starts to wear out from normal usage is the oil path begins to develop many small internal leaks at each point along the crankshaft, connecting rods, rings, camshaft etc. While theoretically possible, in practice it seems unlikely that repairing only the connecting rod portion of the leaks would resolve the oil pressure issue. Sort of like if your swimming pool has ten leaks, repairing just one of them likely wouldn’t be much help. So what to do? Worth a try to increase the oil viscosity, might do the trick for a while anyway. If you are feeling adventurous, just want to see if you can do it, replace the connecting rod bearings, then provide the curious among us here with the before and after oil pressure measurements. A less-work-for-you alternative, measure one of the existing connecting rod bearing clearances.

What has your oil & filter change-out regime been over the years? Consistently every 5,000miles? Always used the recommended oil and good quality filters? What about the driving conditions? Mostly freeway? A lot of low speed stop and go?

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Back to the Buick with 520,000 miles and no engine noises. Oil changes every 3000 miles. Car or maintenance or both, who know?