'99 4.0L, no oil pressure at idle - Jeep Wrangler Forum

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Old 05-01-2009, 10:09 PM   #1
Join Date: May 2009
Location: Dallas, TX
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'99 4.0L, no oil pressure at idle

My first post here, thanks in advance for all your help. Also here's a pic of the Jeep on the shoreline of the lake by my house:


I just bought this '99 TJ Sport about two weeks ago, and after wrenching on it for the last two weeks for various things (tune up, soft top zipper maintenance, removing the light bar and associated wiring), I noticed a problem with the oil pressure when idling after the engine has warmed up.

After doing a little research, I saw this thread:

Cracked 0331 cylinder head castings - JeepsUnlimited.com Forums

About a chronic problem with the 0331 cylinder head cracking. I don't know yet if I'm losing coolant (haven't driven it enough), but the oil pressure problem is pretty much exactly the same as everyone else is seeing.

I'm not 100% sure that I have the 0331 cylinder head. I went outside last night with a flashlight but I couldn't find the casting # on the head, I'll try again today when the sun is out. I also opened the oil filler cap and didn't see an obvious crack, which from seeing the photos it is pretty obvious.

Here's where I wanted to get everybody's advice. The plan going forward is to:

1. Verify during the daylight hours which cylinder head I have.
2. Fill it with coolant, drive it all weekend and see if I lose any.

If I don't have the 0331 cylinder head, that doesn't mean that it still couldn't be cracked, but I'm guessing that lowers the chances of it. If I'm losing coolant and having the oil pressure problem without the 0331 head, does that pretty much point to a blown head gasket? What's the parts&labor on that? (that's a bigger job than I'd do on my own)

Also, I plan on driving this Jeep only 1-2K miles/year. I got it mostly as a tow vehicle for my jet ski, and for dirt road / outdoor fun on the weekends. If the problem is either a cracked cylinder head or a blown head gasket, would I be better off buying an extended warranty (like 12 months / 10000 miles), waiting two weeks and then taking it to the dealer for this problem? If a warranty is cheaper than a head gasket job, worst-case I'm coming out ahead, and best-case the cylinder head is cracked and I'm coming out WAY ahead.

Again, thanks for all your help in advance!

'99 Wrangler Sport
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Old 05-01-2009, 10:11 PM   #2
Join Date: May 2009
Location: Dallas, TX
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Priced warranties today, 3 yr / 36K mi powertrain $1500, 5 years BtB $1900. Not sure it's worth it if the problem ends up not being a cracked head.

Bought some radiator / coolant system fixer stuff today (the goo that is supposed to seal up leaks) and added it to the radiator, as well as filling the coolant reservoir to the full line. After it warmed up I drove for a bit, it didn't happen as soon as it reached operating temperature, more like 10-15 minutes after. Not only that, but it did something really strange first: the oil pressure spiked to 60-80 and kept doing that for a good 10 minutes, and THEN it started having the low pressure problem. I'm not sure why the spike happened if the problem is coolant getting into the oil.

After about 50 miles round-trip I checked to see if it lost coolant, and it hadn't. Also, there was no coolant apparent inside the valve cover (I opened the oil filler cap and checked), and the oil isn't showing any coolant in it at all.

I'm starting to think that this is a traditional sending unit / oil pump problem, but I'm not sure why it wouldn't start acting up until the oil gets hot.

Also, I'm still not sure if I have the 0331 cylinder head, I'm having trouble locating the number that identifies it. I'm not sure if where I'm looking is just covered in too much gunk and I need to clean it off, or even if I'm looking in the right place. Can I see it by looking at it on the top, or do I have to look from the side? I'm not sure how much I can see of the head from the side, the intake manifold seems to be covering it completely. Is there anything else I can look for on the head that would indicate the casting #, some kind of external attachment maybe?

At this point I'm seriously considering bringing it to the local shop and having them do the sending unit and/or oil pump. I was hesitant to try this before since it's just wasted money if the head is cracked, but except for the oil pressure dropping I'm not seeing any of the other signs that indicate a cracked head.

Any ideas?

'99 Wrangler Sport
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Old 05-02-2009, 01:05 AM   #3
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just go to the parts store buy the sensor and replace it. it is easy. the sensor is only 40 bucks. you should change your oil while you are doing that too.
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Old 05-02-2009, 04:28 AM   #4
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Originally Posted by jimmy-buffett View Post

I'm starting to think that this is a traditional sending unit / oil pump problem, but I'm not sure why it wouldn't start acting up until the oil gets hot.
Things expand when they get hot. Poor connection inside and the sensor case expands and opens the connection. I would replace the sensor and see what happens. If you replace the sensor and its the head you've only wasted about $40. If you replace the head and its the sensor......well I'm sure you can see the difference.
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Old 05-02-2009, 05:58 AM   #5

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What do you mean by low oil pressure? Is it zero? Half way etc. My bet would be the sending unit.
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Old 05-02-2009, 09:20 AM   #6
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Jimmy, I'm new to this forum but I'm an old dog with Jeeps. This story is about an '01 Cherokee with the time honored 4.0. One summer day at a stoplight my oil pressure dropped to zero and the warning light came on until I gave it some gas to bring up the rpms. This began to happen at every stoplight. I freaked out and thought my oil pump was worn out. I replaced the oil pump. It did it again! I replaced the oil sending unit. It did it again! Somewhere on the net a guy said to use a Napa Gold oil filter(I've always used Fram). I thought that advice was bullcrap but out of frustration I used the Napa Gold and my oil pressure light never came on again. The oil pressure still dropped to about 10 or so at a stop but never bottomed out again. I don't believe everything I read but try the Napa Gold filter. Good luck. Jim, '04 Wrangler X
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Old 05-02-2009, 11:12 AM   #7
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If I remember correctly - reading on here - the oil pressure gauge really isn't a direct "sensor to gauge" like you'd think. Instead the sensor feeds into the PCM, the PCM "interprets" that signal, then registers it's "opinion" on the gauge.

Try disconnecting and cleaning the connections on the PCM.
Do a search on here for oil pressure or gauge problems, try to find the original threads about it.

If you can't find the info, try putting a mechanical gauge on it to see if what you are seeing is real. Mechanical gauges aren't expensive.
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Old 05-02-2009, 11:16 AM   #8
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The very first thing I'd do is just swap the oil pressure sender. They have a reputation for going bad, mine did as well, and it only takes a few minutes to swap. Anything else you could do would take longer and the odds of it not being just the sender are pretty low.
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Old 05-02-2009, 04:56 PM   #9
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Originally Posted by crazyredneck View Post
just go to the parts store buy the sensor and replace it. it is easy. the sensor is only 40 bucks. you should change your oil while you are doing that too.
The sending unit ended up being the problem, and yes you got the price exactly :-). I did the fix myself and once I figured out how to get the clip off, the whole process took ~5 minutes (about as easy as changing a spark plug).

For whoever does this next, to get the clip off you loosen the sending switch and turn it so that the red partial end is facing toward you. There is a little clip there you have to push toward the cabin, this will let you push the retaining clip through the wire housing (it doesn't come completely out). After that just push the tab at the back and pop the whole thing off. Unscrew and replace.

After the fix I drove about 30 miles, and the pressure fluctuated between 20 (idle) and 50 (highway speeds). I never saw the pressure spike as before, and the pressure then never went to zero at idle.

Sure am glad this wasn't a cracked head...

'99 Wrangler Sport
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