electric!sheep

Car: 1985 Nissan Maxima SE.

I was driving on the highway today when I noticed that the oil pressure guage was pegged at zero--a marked change from 60 psi when heading out from home (max is 90 psi). Strangely, as I slowed down on the side of the road, the pressure steadily rose to around 30psi (14 psi is the minimum), but as I headed back on the highway again, it dropped back to 0. Not liking this situation, I returned home, and noticed the same pattern as I was accelerating up a small hill in 2nd (oil pressure drops, then rises as I let off the gas pedal at the top), but didn't see as pronounced an effect on a longer grade (~1/4 mile) in 3rd, until about 3/4ths of the way up, when it started dropping to 10psi, before recovering shortly afterwards. It does not misbehave at idle, as far as I could tell, nor is any other unusual behavior apparent.

What the hell is going on? I checked the oil level yesterday, and it was 75% full.

maximase86

iTrader: (24)

Hmmm, are you noticing any unusual noises from the valve train of the engine. How is the car running? If everything is fine, it could be the oil pressure sending unit, or the guage could be going defective. Does this seem to be happening when the motor is as a certain RPM range?

S

electric!sheep

Nothing has changed.

No better or worse than in the past.

Those are the two hypotheses I've seen mentioned on other forums, along with the relief valve.

Strangely, I don't think so. I can cruise along at 2500 in third and it'll be fine, but once I head on the highway in the same RPM range, the gauge pegs at zero. Maybe I'll roll it around the neighborhood again...

electric!sheep

I figured it out, or at least partially. Regardless of gear (neutral works too), if I accelerate steadily, the psi will rise until around 2200, then start dropping, and the car starts making a faint "bom bom bom bom" sound. I knew about the sound before, and blamed the throttle (it has problems with regularity, esp. when cold), so it may be quite coincidental.

maximase86

iTrader: (24)

hmmm, you might have someone rev the car to 2200 revs while you check around and see if you can locate there that bom bom bom noise is coming from....it may not be a good sign.

S

electric!sheep

Well, it could easily be mistaken for a rough throttle, which is what I've always thought of it as. I do believe it is coming from up front, but I'll try to get some specifics tomorrow. Hell, on a cold morning the car positively CHUGS in first and second gear until warm. It doesn't seem to be directly correlated with the oil pressure problem.

Anyway, I'll get back to you with a better diagnosis, and I also plan to call my friendly mechanic tomorrow to see what he thinks.

maximase86

iTrader: (24)

cool cool, lemme know how it goes, I hope it isn't anything to worry about.

S

MaxWgn

iTrader: (1)

To me, it sounds like a possible oil pump failure...could also be the cams and/or cam bores in the heads are worn or damaged. These engines don't use cam bearings...

electric!sheep

The mechanic says it's most likely the sender unit, so I don't need to worry much. In his words, Nissans have had gauge problems since the beginning of time.

Geza

2 years ago once I also had the same symptom with my 88 Maxima the day after I changed the oil and the filter. I was driving up to Mt. Wilson from Azusa when I noticed, that the oil pressure is dropping continuosly. (and the water temp was raising)

Of course, I was assuming first that I must be loosing oil at the filter or the oil plug.

I checked everything, but I couldn't found any leak, oil level was normal. All this was happenning on a hot summer day, driving uphill on a mountain.
I decided to turn around and have the oil pressure checked out at a repair shop, the problem was, that it was all happening on a Sunday morning.

Anyway, finally I found a shop that was open.
(btw, the only out of usual was around the oil change the previous day, that very first time I installed a FRAM filter that was equipped with a back pressure valve.)

So, at the repair shop I asked the mechanic to replace my brand new FRAM oli filter with another one, any generic one. The guy was in disbelief, and kept saying to me, that it will not fix the problem for sure, but I insisted.

Well, after I had the filter replaced, and added the missing oil, I turned around and left for Mt. Wilson.

This was the first and last time that this oil pressure dropping incident happened to me. I don't know what caused it or how it could be related to the oil filter with the back pressure valve, but since this incident I became a believer and the I swore that I will never use oil filter with back pressure valve. (I have been using regular FRAM oil filter since 1989, since I came to the US.)
Maybe you can relate your incident to mine, I don't know, I just thought I will share my story with you guys.

Geza

Bleach

The 280ZX/1gen's always have the oil pressure sender go bad. There are many different common readings that you get on your gauge. This is one of them. On my Z, the pressure would read fine below a certain rpm. Above that rpm the gauge suddently drops to Zero. A $50 sender unit fixed the problem. I got mine from Nissan because i heard it was much more accurate than the $30 aftermarket units.