1987Comanche Posted October 31, 2009 Share Posted October 31, 2009 I was running errands in the '90 that I picked up last week and I noticed the oil pressure gauge acting a bit odd. I just swapped the cluster and put in the used sending unit from the '87, which always read fine with that gauge. Anyway I noticed right off the bat that the gauge wasn't nearly as active as it was in the '87 (lower pressure at a stoplight, increasing with revs). It read around 40psi at cold start up and dropped to about 17psi as it warmed up, with no difference in pressure reading whether stopped or moving at 45mph. At the last errand (Auto Zone appropriately) I started the truck and the gauge was at zero psi, with no change. I drove about 0.3 mile, saw it was still at zero. I pulled over, shut her down, and checked everything. Oil was clean, everything looked ok. I got back in and went another mile or so and still had no reading on the gauge. At this point I stopped and had her towed back home. Fortunately the oil looked clean when I stopped the 2nd time. Once I got home I pulled it from the wrecker to the driveway and the gauge read 20psi. Perhaps I was being overly cautious getting her towed but my question is WTF? Bad sending unit, clogged pickup screen, electrical problem, or was it actually at 0 PSI? Thoughts? Link to comment Share on other sites More sharing options...
wallisek Posted October 31, 2009 Share Posted October 31, 2009 Seeing that you just recently swapped clusters I'd vote for an electrical issue. Wouldn't hurt to invest in an oil pressure gauge for testing purposes. I picked one up at harbor fright for $10. Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted October 31, 2009 Share Posted October 31, 2009 Two things come to mind. 1St. The factory gages are notoriously inaccurate. Go get a real gage and install it. 2Nd. You twice mentioned clean oil. Did you recently change the oil? If so whose oil filter did you use? IMHO the ONLY one to buy is a WIX filter. But the one to ABSOLUTELY AVOID is FRAM. Fram filters will cause pressure problems. Nearly the same thing happened to my in my TJ. I was on my way to a trail run when all ages went out, nothing no speedo, temp, oil pressure, battery or tack.... Mine was a common problem with TJ's a bad plug at the back of the cluster. nothing to do with our MJ's just similarly feeling of urgency. CW Link to comment Share on other sites More sharing options...
Pete M Posted October 31, 2009 Share Posted October 31, 2009 The last Fram I ever used caused a 0psi issue on the freeway. I was none too pleased. :fs1: Link to comment Share on other sites More sharing options...
comanche09 Posted October 31, 2009 Share Posted October 31, 2009 Ya know Fram has been bad mouthed for years on the web (and rightfully so) you would think they would get their act together. Guess there are ALOT more non-enthusiasts out that that don't know any better. Where would one find a good accurate oil pressure sender and where would you put it? Is there a way to get the oil sender to read on the OEM gauge? Thanks Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted October 31, 2009 Share Posted October 31, 2009 What we are up against is a at least 17 years of use/abuse. Things happen with time. Its on my list of things to do is to add REAL, accurate gages to my rig too. I don't trust my temp or voltage/amp meter gage. Oil pressure seems OK, but who knows. CW Link to comment Share on other sites More sharing options...
1987Comanche Posted November 1, 2009 Author Share Posted November 1, 2009 Both the cluster and oil pressure sending unit were transferred from my '87 (sold today...) where they worked perfectly. I won't say the wiring in the new truck isn't to blame but I know the cluster works pretty well. I changed the oil as soon as I got it home from the PO's house (it was nasty). I used a Purolator filter on it. This is, however, the first day the truck had seen the highway and 30+ miles of driving in probably 6-9 months...based on when the inspection ran out. I'm thinking the safest course is a high vol oil pump, new pickup, clean the pan out and see what happens. Since the engine oil seems clean the bearings should be ok, correct? Link to comment Share on other sites More sharing options...
glundblad Posted November 1, 2009 Share Posted November 1, 2009 What we are up against is a at least 17 years of use/abuse. Things happen with time. Its on my list of things to do is to add REAL, accurate gages to my rig too. I don't trust my temp or voltage/amp meter gage. Oil pressure seems OK, but who knows. CW :hijack: What would be really cool is if CWLongshot could install new, more accurate oil/volt/temp gauges yet still allow the old ones to work so the truck was kept original. Then he posts the "how-to" on the forum. :brows: Link to comment Share on other sites More sharing options...
Eagle Posted November 1, 2009 Share Posted November 1, 2009 What we are up against is a at least 17 years of use/abuse. Things happen with time. Its on my list of things to do is to add REAL, accurate gages to my rig too. I don't trust my temp or voltage/amp meter gage. Oil pressure seems OK, but who knows. CW :hijack: What would be really cool is if CWLongshot could install new, more accurate oil/volt/temp gauges yet still allow the old ones to work so the truck was kept original. Then he posts the "how-to" on the forum. :brows: Temperature would be more difficult, since the sender has to be in the coolant, but on a Renix it would be easy enough to change over to an HO thermostat housing with the port for a temp sender. Leave the one in the head for the stock gauges, and use a new sender in the t-stat housing for the supplemental gauge. CAVEAT: They won't be measuring at the same part of the system, so 100% correlation should not be expected. I keep thinking about adding a supplemental idiot light for oil pressure, and that's easy. Remove the sender, install a tee, then reinstall the original sender in one port and add a sender for the idiot lights in the other side of the tee. Wire the idiot light sender to a big red light somewhere on your dashboard where you'll see it if it lights up, and you have a redundant system. You check the gauge to see what the system is actually doing, but if there should be a sudden, catastrophic loss of pressure the light would tell you immediately. Link to comment Share on other sites More sharing options...
tugalo Posted November 1, 2009 Share Posted November 1, 2009 Mine recently did the same thing....oil pressure to zero. Drove it 2 miles to home, took it to the shop, fired it up again, and the pressure was at 50. tore it down and found a LOT of carbon. Best guess was a piece of carbon got into the pressure relief valve and caused the drop. Replaced the pump and cleaned the screen anyway. good to go now. Could be a carbon issue for you also...... Link to comment Share on other sites More sharing options...
1987Comanche Posted November 1, 2009 Author Share Posted November 1, 2009 Since the truck sat for 6-9 months I could definitely see that. The pressure relief is integral with the pump on these, correct? Would there be any reason to pull the oil filter adapter? Other than pulling the sending unit and cleaning it I hadn't planned on pulling the adapter, just the pan and pump. Link to comment Share on other sites More sharing options...
jpnjim Posted November 1, 2009 Share Posted November 1, 2009 Something to think about next time you end up with zero oil pressure, engines get very noisy without oil pressure, clanging, knocking, rattling, they also start to run like crap, sluggish, nonreponsive, and usually overheat shortly after (since all that moving oil removes heat from the bearings, and no oil = lots of extra friction heat). If none of these things are happening when your gauge reads zero (not so much as a new lifter tap), I usually lean towards a gauge malfunction, than anything else. FWIW, when someone does have zero oil pressure, the first thing I check is the filter. I've seen engines lose oil pressure after a 'Quicky Lube' visit, only to find a brand new, perfectly clean (no oil) plugged up filter on the engine. Second thing I would check is whatever drives the oil pump (shaft, or tab from the distributor usually). Link to comment Share on other sites More sharing options...
1987Comanche Posted November 1, 2009 Author Share Posted November 1, 2009 Other than noisy lifters I didn't experience any odd sounds, smells, overheating, etc. To be fair the lifters have been noisy since day one. I did notice the oil was surprisingly clean. When I bought the truck the oil was filthy, which is why I changed it before I limped her home. After driving 5 miles home the new oil looked almost as dirty as the old oil. After driving on the highway yesterday the oil is now quite clean and the color I'd expect it to be, considering it only has 60 or so miles on it. Could it be the Purolator filter did it's job and is now completely plugged with garbage, forcing the bypass to open and, in the process, bypassing the sending unit? Link to comment Share on other sites More sharing options...
jpnjim Posted November 1, 2009 Share Posted November 1, 2009 Could it be the Purolator filter did it's job and is now completely plugged with garbage, forcing the bypass to open and, in the process, bypassing the sending unit? The bypass shouldn't bypass the sending unit (but to be honest, I really don't know either way), but the instance I posted about the perfect/clean filter & no oil pressure, something similar happened. (SIL's Dodge Omni) The clogged, brand new filter (defective) forced the bypass to open, owner left 'Jiffy Lube' thinking everything was OK. After driving around for a day, or so, the bypass must've failed, because the pressure went to zero, and that's when my SIL asked me to look at it ("um, there's a big bright red light on the dash"). I swapped the filter, and everything went back to normal (normal for a POS Omni anyway). Link to comment Share on other sites More sharing options...
1987Comanche Posted November 3, 2009 Author Share Posted November 3, 2009 I put a mechanical gauge on it last night and here's what I saw. On cold startup I'll hit 40psi and, as she warms up I'll drop down below 20psi. I'll bottom out at around 13-15psi and, at this point, bringing it from idle to 2000RPM doesn't affect oil pressure at all. I'm thinking the pickup screen is clogged and the next logical step is to drop the pan and put in the high vol pump. Any other ideas? Also how hard is it to change the rear main on the 4.0? I was going to do that while I have the pan off. Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted November 4, 2009 Share Posted November 4, 2009 Changing the rear main seal is a doable job, but CAN be a PIA. Your oil pressure is just fine for a vehicle with some miles on it. Having approx 20# @ idle when warmed up is FINE!!! Once it drops down to 10 less then start looking at a rebuild. the bearings in the bottom end are wearing and now have more clearance. this extra clearance prohibits the pressure from building in the oiling system. A clogged oil pickup causing this issue is unlikely. Pull the valve cover and see how it looks inside. Is it all sludged up? If so MAYBE you have a partially clogged pick-up. As for the valve cover, yours, (Like 99% of them) is likely leaking anyhow. At least a portion of so called rear main seal leaks are simply valve cover leaks running down the back of the block. CW Link to comment Share on other sites More sharing options...
Eagle Posted November 4, 2009 Share Posted November 4, 2009 I put a mechanical gauge on it last night and here's what I saw. On cold startup I'll hit 40psi and, as she warms up I'll drop down below 20psi. I'll bottom out at around 13-15psi and, at this point, bringing it from idle to 2000RPM doesn't affect oil pressure at all. I'm thinking the pickup screen is clogged and the next logical step is to drop the pan and put in the high vol pump. Any other ideas? Also how hard is it to change the rear main on the 4.0? I was going to do that while I have the pan off. I'd say you are leaping unerringly to a couple of illogical conclusions. As CWLONGSHOT noted, 20 psi at idle is withing spec. However, the pressure should be around 40 or better (warm) at 2000 RPM. The factory spec is 13 psi (minimum) at idle, and 37-to 75 psi above 1600 RPM. So your pump is producing pressure, as evidenced by the idle pressure, but it can't boost the pressure above 20 psi. That doesn't sound like a clogged screen to me. Even if it did -- you could just drop the pan and clean the screen, it doesn't mean you need a new pump, and it CERTAINLY doesn't mean you need a new high volume pump. Your problem is pressure, not volume. Then you ask about the rear main seal. The RMS has nothing to do with oil pressure. It just prevents the oil from leaking out of the engine. By all means, if you have the oil pan off that's the time to replace the RMS. But that's not going to cure your oil pressure problem. IMHO, you need to replace the bearings, not the oil pump. It can be done with the engine in the truck. Link to comment Share on other sites More sharing options...
CWLONGSHOT Posted November 4, 2009 Share Posted November 4, 2009 I put a mechanical gauge on it last night and here's what I saw. On cold startup I'll hit 40psi and, as she warms up I'll drop down below 20psi. I'll bottom out at around 13-15psi and, at this point, bringing it from idle to 2000RPM doesn't affect oil pressure at all. I'm thinking the pickup screen is clogged and the next logical step is to drop the pan and put in the high vol pump. Any other ideas? Also how hard is it to change the rear main on the 4.0? I was going to do that while I have the pan off. I'd say you are leaping unerringly to a couple of illogical conclusions. As CWLONGSHOT noted, 20 psi at idle is withing spec. However, the pressure should be around 40 or better (warm) at 2000 RPM. The factory spec is 13 psi (minimum) at idle, and 37-to 75 psi above 1600 RPM. So your pump is producing pressure, as evidenced by the idle pressure, but it can't boost the pressure above 20 psi. That doesn't sound like a clogged screen to me. Even if it did -- you could just drop the pan and clean the screen, it doesn't mean you need a new pump, and it CERTAINLY doesn't mean you need a new high volume pump. Your problem is pressure, not volume. Then you ask about the rear main seal. The RMS has nothing to do with oil pressure. It just prevents the oil from leaking out of the engine. By all means, if you have the oil pan off that's the time to replace the RMS. But that's not going to cure your oil pressure problem. IMHO, you need to replace the bearings, not the oil pump. It can be done with the engine in the truck. Amen.... :D Link to comment Share on other sites More sharing options...
Akula69 Posted November 5, 2009 Share Posted November 5, 2009 I put a mechanical gauge on it last night and here's what I saw. On cold startup I'll hit 40psi and, as she warms up I'll drop down below 20psi. I'll bottom out at around 13-15psi and, at this point, bringing it from idle to 2000RPM doesn't affect oil pressure at all. I'm thinking the pickup screen is clogged and the next logical step is to drop the pan and put in the high vol pump. Any other ideas? Also how hard is it to change the rear main on the 4.0? I was going to do that while I have the pan off. Question: What did you use to get the mechanical pressure (where did you get it?) and how did you hool it up? Changing the RMS is easy once you get the pan down. There are a number of very good articles already herer in the forum, but I used this one: http://jeepin.com/features/rearmain/index.asp Link to comment Share on other sites More sharing options...
1987Comanche Posted November 5, 2009 Author Share Posted November 5, 2009 I picked up the gauge set at Harbor Freight and hooked it in place of the oil press sending unit. I agree that the RMS isn't related to low oil pressure...rather I have one that seems fairly leaky and was going to replace it while I had the pan off. Do you think it's possible I have bearings going bad after 117K miles? The inside of the valve cover is pretty dirty b/c the PCV system doesn't appear to have worked correctly in years. The oil I drained out didn't have anything abnormal in it though. When I changed the oil pump on my Beretta I saw a 20psi increase across the board, which is why I was thinking it might help out here. If the bearings were worn would I be able to affect the oil pressure while revving during warmup? In that case how difficult are the bearings to change with the engine in the truck? I'm assuming I'd get standard bearings since I wouldn't be doing anything else. Link to comment Share on other sites More sharing options...
Eagle Posted November 5, 2009 Share Posted November 5, 2009 Be sure to check the code on the side of the engine to see if the bearings were .010 under from the factory. If so, there's a letter code stamped in a boss on the side of the block next to the distributor. Link to comment Share on other sites More sharing options...
FxRacing282 Posted November 6, 2009 Share Posted November 6, 2009 you could always try thicker oil. general rule of thumb, for every 1k rpm you want 10psi of oil pressure Link to comment Share on other sites More sharing options...
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