Oil pressure anomolies... hope I didn't just destroy my car - PeachParts Mercedes-Benz Forum

Shortsguy1 · #1 02-09-2013, 12:15 AM

On my drive home today, the oil pressure in my 87 TD, OM603, 190,000 miles, started acting oddly. Normally it pegs high when the engine is cold and then once the oil is warm, it drops down to 1.5-2ish at idle, and re-pegs as soon as you raise the rpm. It has regular DELO in it (15-40?) and the oil level is fine. Today, having not driven the car for 2 hours, I started the car and drove a short distance when I looked at the oil P. It was at 2, which surprised me because the engine was cool. When I sped up, it only slightly increased the P, but it mostly stuck at 2. There was no way to get 3 or pegged this afternoon. At idle, the P was 1 bar. I had to drive home to get my son, but I did so knowing I might be destroying my "new car".

As soon as I got home, I turned off the car and the P dropped to zero. I didn't think to open the hood and look for oil flow inside the filler cap, as I just wanted to turn off the engine ASAP. But when I turned the key back on, the oil pressure jumps to 1 before you even turn the starter. So something is definitely wonky (to use a technical term).

Based on my research just now, it looks like it is probably one of five things:
1) blocked oil pickup in the sump
2) malfunctioning oil pump
3) malfunctioning oil pressure sensor (sender)
4) malfunctioning oil pressure gauge on dash display
5) malfunctioning wire between the sensor and dashboard

I am hoping it is 3 as this seems the most likely and easiest to fix. 4 or 5 wouldn't be bad, and obviously I would prefer it not be either of the first two.

Based on my reading, when the sending unit fails for many of you, it pegs high. But that obviously isn't the case here. Does that suggest that my problem isn't likely to be the sender?

Once in a rare while, I would notice when I got into the car (no yet running) that there was a pressure indicated on the gauge. When I hit the starter, the pressure would drop to 0 very briefly, and then peg a few seconds later. If you get an indicated pressure when the car isn't running, does this suggest the sender or the gauge is more likely to blame?

And while unlikely to be relevant, a lot of the warning indicator lights on my dash are very dim when they are tested with the key first turned. It seems doubtful, but maybe my dash has some common problem with the lights and the oil pressure gauge. It was rainy today if grounding can ever be an issue with these things.

Would you start the car to see if this problem was a one-time electrical gremlin? Or would you replace the sender first (or install a mechanical pressure gauge) and then risk starting it up?

Any other thoughts are appreciated. Thanks for your time. And sorry about misspelling "anomalies"! ha ha.

compress ignite · #2 02-09-2013, 08:50 AM

1.Check the dipstick.first
2.Look for evidence of loss of oil.

As Rudimentary as the Oil Pressure information system is
(It's more a "Representation" of what the oil pressure Might be)
IF that sucker shows anything out of normal,Do Whatever is necessary to cease RPMs...10 seconds ago.

97 SL320 · #3 02-09-2013, 09:36 AM

Quote:

Originally Posted by Shortsguy1

Once in a rare while, I would notice when I got into the car (no yet running) that there was a pressure indicated on the gauge. When I hit the starter, the pressure would drop to 0 very briefly, and then peg a few seconds later. If you get an indicated pressure when the car isn't running, does this suggest the sender or the gauge is more likely to blame?

Any reading before the key is turned on can be ignored but, if you turn the key to on and the motor isn't running, the needle should show 0.

A test gauge is the only real way to make sure you have enough oil pressure.

If you have a service manual, there may be a pressure to resistance chart. Use these values to test the in car gauge by putting resistors across the wire.

If the oil pressure regulator is stuck part way open or the spring has broken , there will be a drop in oil pressure.

A complete loss of oil pressure is a bigger deal then a drop ,so you are probably still OK as to motor condition.

LoosBenz · #4 02-09-2013, 09:39 AM

I just went through this with someone who has a W124 with the OM602. Happened suddenly and out of the blue. He installed a mechanical gauge setup under the hood and it showed the oil pressure to be fine. The sending unit was bad.

However, I agree with Compress Ignite and 97 SL320. Check all that, but don't panic. I'm seeing more and more sending units fail in the manner that you describe.

Stretch · #5 02-09-2013, 09:41 AM

Quote:

Originally Posted by 97 SL320

...

A test gauge is the only real way to make sure you have enough oil pressure.

...

X2 You need some confirmation that the oil pressure is OK

mach4 · #6 02-09-2013, 11:37 AM

I think the simplest way to check the actual oil pressure would be to drill and tap the filter cover and install a manual gauge, kind of like this:

This particular image shows a 617 but it should be no different on a 603 with a gauge fitting. Ideally you could find a oil filter top at the junk yard, but they are not nearly as plentiful as the 617 ones. When you're done testing just screw in a plug. There are several threads on the subject and all recommend drilling from the bottom to avoid the ribs.

barry12345 · #7 02-09-2013, 11:52 AM

Your oil pressure gauge is part of an electric circuit. With the key off there is no electrical power in that circuit. A good gauge should always read zero I would think. In your description the movement may be sticking for example. Does the gauge normalise if you tap the dash?

At least it really does not sound like you have low oil pressure issues in the engine.

Shortsguy1 · #8 02-09-2013, 10:12 PM

Thank you everyone for your helpful replies. The car has not lost its oil based on the dipstick. And there were no obvious new leaks or anything too suspicious under the hood. I didn't have much time today, but I removed the wire from the pressure sender. With it disconnected, and the key on (but no engine or starter running), the indicated oil P pegs (so that is a good sign that the gauge and wire may be okay). I didn't even get a chance to ground it, but my understanding is the indicated P should go to zero. In my research yesterday, I did find a reference to resistances I can install between the wire and ground, so verify if the wire and gauge are okay. Visually, the wire is a little beat up. The external insulation is cracked, but the internal colored insulation looks okay. I couldn't easily tell where the other end goes, but I think I want to avoid having to replace the wire if possible.

I really like the suggestion of putting a mechanical pressure gauge on the oil filter cap. I haven't yet figured out whether I will do a long term solution like that, or try to temporarily rig something to the port where the sender normally goes. But the bottom line is I will not start the car until I have a redundant pressure gauge installed. Thanks again for those suggestions.

Oh, and regardless of the outcome here, while poking around I noticed that the fuel line downstream from the prefilter had a leak. So I tugged slightly on it, and it was able to be easily slid off its connection at the pump. I feel so lucky to have found that. I know diesel is less flammable than gas, but I lost an old VW to fire years ago, and I would not want to risk losing another car to a major fuel leak.

Peachparts forum members rock!

pawoSD · #9 02-10-2013, 09:58 AM

I've had the same thing happen on two of our cars, in both cases the sender had become faulty.

Shortsguy1 · #10 02-19-2013, 12:24 AM

Thanks again for all the great advice. Here is the "followup/conclusion" post. I bought a new VDO pressure sender via peachparts and decided to try that first. As an aside, it didn't come with a crush washer, which was a bit annoying. I jacked the car and located the sender from underneath, but was unable to get it to budge. I have a short MB 17mm wrench, which I could get onto the sender, but there was no room to turn it, no matter what I tried. I don't know about you, but I find these sorts of things rather frustrating. I considered cutting down a cheap 17 mm wrench, but didn't want to spend the time and effort. Fortunately for me, I found post #10 from Chris_87_300D in this thread which saved my sanity:
603 Oil Pressure Sending unit
It gives simple instructions of what to do to access the sender from above, and it really was easy. He says it takes him 30 min, but it probably took me an hour. Most of that time was trying to remove the washer fluid reservoir for the first time. Those plugs were very "stuck." Ironic that removing a water bottle was the most difficult part of the job.

The old sender had something physically broken inside it (it rattled). I used a overpriced crush washer from a chain car-parts store and the new sender installed easily. The wire connector has broken hooks/fasteners, so I just crimped the metal sleeve a bit and hope it holds.

I also changed the o-rings on the center bolt through the oil filter, in case they needed it. I went with 6 mm on the lower one and 7 mm on the slightly higher one (I see there is some discussion on those sizes). And I put in a new plastic fuel filter and replaced a few fuel lines that needed it. I filled the plastic fuel filter with diesel purge, and the car started right up. No problems with priming. After 5 seconds, oil pressure pegged at 3. Yea!!! Once warm, it came down to 2. Yea!! It was a small victory, but a victory none-the-less. Thanks again.