Posted: 4/12/2004 11:34:44 AM EDT
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I have a V8 Jeep Grand Cherokee. Yesterday it started doing this: When you brake to come to a stop the oil pressure gauge sometimes drops instantly to zero, and the [CHECK GAGES] idiot light comes on. If you accellerate the gauge immediately pops back up to 40, and the light goes off. The oil level is full. When the gauge reads zero there is no valve rattle or unusual noises, although I haven't let it run like that for more than a very short time. When cruising down the highway the gauge always reads steady at 40. (So far) There is no smoke, oil leaks or oil in the anti-freeze. The Jeep runs fine as far as power, performance, etc. It seems to me it is most likely the guage, wiring, or oil pressure sender; but I'm pretty paranoid about it. I don't want to drive it and ruin my engine ($$$) if it is in fact the oil pump, blocked oil passage, etc. I'm also concerned that if I take it to a shop they will sell me a high dollar repair, even if the problem is only an electrical connection or sender. Any ideas? |
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Quoted: I have a V8 Jeep Grand Cherokee. Yesterday it started doing this: When you brake to come to a stop the oil pressure gauge sometimes drops instantly to zero, and the [CHECK GAGES] idiot light comes on. If you accellerate the gauge immediately pops back up to 40, and the light goes off. The oil level is full. When the gauge reads zero there is no valve rattle or unusual noises, although I haven't let it run like that for more than a very short time. When cruising down the highway the gauge always reads steady at 40. (So far) There is no smoke, oil leaks or oil in the anti-freeze. The Jeep runs fine as far as power, performance, etc. It seems to me it is most likely the guage, wiring, or oil pressure sender; but I'm pretty paranoid about it. I don't want to drive it and ruin my engine ($$$) if it is in fact the oil pump, blocked oil passage, etc. I'm also concerned that if I take it to a shop they will sell me a high dollar repair, even if the problem is only an electrical connection or sender. Any ideas? start with the sender and gauge, first. is it a manual oil pressure gauge or is it the electrical type? |
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Your power brakes booster is failing, and you may have one or more other vacuum leaks. The brake booster uses engine vacuum to assist with braking. What's happening is that the vacuum leak is nearly stalling the motor out, causing the RPMS to drop lower than typical idol speed, such that the oil pressure generated by the oil pump is below acceptable levels. This is causing the sender to trigger a warning light and the guage to show pressure dropping. To confirm diagnosis: Get a can of starter fluid. Applying the brakes at idol, have someone spray a mist of starter fluid near the big diaphram (round thing) on your brake booster. The engine, if there is a vacuum leak, will suck in the fluid and you'll notice a change in the RPMS. |
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It could be a broken oil pump pickup. The oil pan has a sump in the rear for the oil pump to pickup the oil from (via tube and screen). If the pickup (tube/screen) is broken off, then it's possible that as long as the oil pump is submerged in the oil sump area, it's maintaining oil pressure. When you hit the brakes, some of the oil will slosh out of the sump area and into the front of the pan. This may uncover the pump enough to suck air... I'd have the oil pan dropped, and be sure...Otherwise you will burn up the bearings. |
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Some of that is a little (or a LOT) overboard. The Jeep Grand Cherokee has a HIGH failure rate on the Oil Pressure sending unit. I had to replace mine on my 1996 JGC about 2 years ago. Jeez, it's like a $15 part and takes about 5 minutes to replace. Just replace that. What is likely happening, is that you engine (stock) runs a little over 40psi at operating speed, and ~20 to ~35psi at idle speed. Your sending unit is likely just messed up, so that it cannot register the lower than 40psi pressures.... so it reads zero. Very typical on Cherokee/Wrangler vehicles. I am so confident that will fix your problem, I wont even go into replacing the gauge, oil pump, etc..... I doubt it is related to your vacuum brake line at all.... as a momentary loss of idle speed on a V8 would not allow the oil pressure to drop that low, for that long. I doubt it is related to your oil pump pickup tube, since if that breaks, you would likely always have zero oil pressure... as the oil pump is not submerged. If the screen was clogged, you would see the PSI "affected" at all rpms, not just "zero" at idle. Go change your sending unit. |