350SDL oil pressure fluctuates--confirmed bad w/mechanical gauge
 

Hi, I am new to the forum. I have a 1991 350SDL that has suddenly developed a problem with the oil pressure. I have changed the oil and filter--Mystik 15w50 synthetic blend and a Wix filter. I also replaced the oil pressure sending unit. None of the above changed the problem. I know the full history of the car but won't go into detail now. It had a new engine installed by MB at 150,000 miles.

I had just returned from a 1700 mile trip, and the car performed flawlessly. The day after I returned, I was pulling into traffic and noticed the oil pressure dropping as I accelerated. I let up off the pedal and it crept back up. Pressed the gas and it dropped again. It seems to happen under load at slow speeds, and returns to normal once cruising speed is reached. Oddly, if I accelerate in reverse the pressure jumps up, Forward, it drops. I have removed the filler cap and verified that oil is flowing to the cam.

I am including a video link. This was shot while accelerating from a stop, engine at full operating temperature after a short freeway run, and on a sharp incline over a viaduct. If I let up and allow it to shift the pressure jumps back. The first few fluctuations in the video show it going through the gears; the last one I gunned the engine to see the effect.

https://www.youtube.com/watch?v=TD7XJ1l_plQ&feature=youtu.be

Any ideas?

===========
1991 350SDL
1974 VW Safaré Camper

That looks concerning. I'd want to get another oil pressure gauge on the engine to verify the oil pressure isn't faltering.

-J

Yes, my neighbor has a mechanical gauge, that's the next step.

Whoa! Oil Pressure
 

1.Did you replace the two rubber O rings on the Oil Filter cover Shaft?
2.Did you replace the Oil Filter Cover Top O ring?
(It seals between the cover and the Canister body)

At 1800 rpm the O.P. gauge should almost be "Pegged"

This problem appeared before I changed the oil. I did replace the O-ring on the filter housing, not so the ones on the shaft.

My oil pressure has always been good, pegged the meter immediately at startup and stayed there, maybe dipping slightly at idle when hot.

This new problem came on suddenly.

I had a similar problem in my 1990 350SDL. It was the connector and the 10" of wire leading to the electrically-powered oil pressure gauge.

I put a manometric (manual) gauge on the vehicle, checked at idle, 1000 1500 2000 2500, cold and hot.

Everything was fine.

After cleaning off the contact and the wire, and replacing a portion, my oil pressure now reads like it used to.

It's really scary when your electric signal to the gauge is lessened, and you think your oil pressure is 4 psi at 750 rpm!!! In reality it was 29!

I do have some other issues in the instrument panel; the tach quits working sporadically, and often the ABS light comes on when the tach quits and goes out again when it kicks back in.

I will use a mechanical gauge to confirm the oil pressure, if it is OK will proceed with electrical checks.

If the oil pressure is truly fluctuating as shown on the gauge, what might be causing it? It seems to lose pressure inversely proportional to the load on the engine. Except in reverse, when the pressure jumps up to full when accelerating. It seems to come up when making a RIGHT turn also. Not so when turning left.

It is a ground based sensor like most of the gauges. Mine has always read high, new engine, new cluster... same wiring.

I have had several people indicate that this might be an electrical problem. Replacing the sending unit had no effect, same symptoms. The oil pressure is not erratic per se; the needle does not jump around at random. It is very consistent as to when the pressure drops; i.e. under load while accelerating. The video shows the pattern clearly, it is predictable. It gets worse as the engine heats up.

Is there anything electrical that could cause this? I am not very knowledgable about these systems--I cut my mechanical teeth on air-cooled VWs which are, of course, much simpler machines.

It is my fear that the oil pump is failing. Or has already failed. Could that cause the problems I am having?

As I stated earlier, I plan to use a mechanical OPG to confirm the actual pressure. If it indeed has pressure problems, which I suspect to be the case since it behaves identically with two different senders, where should I start looking?

Many thanks.

Use the mechanical gauge for confirmation, but I also think the problem is electrical. You could have problems on the gauge end of things. Perhaps acceleration is causing the wiring harness to move affecting the connection at the instrument cluster. It might be good to use some electronics contact cleaner on the plug at the cluster.

X2 confirm with mech gauge that it is not actual oil pressure problem. Accelerating in reverse does not lower oil pressure makes no sense so good bet it's electrical. Check supply voltage to sender and gauge for abnormal swing when reving engine to 2k rpm. Swing should not go past 13.8 v or so. The gauge circuit is suppose to be a bridge from what I have read that is immune to voltage fluctuations. If there is some wiring faults, the bridge no longer functions correctly and could produce strange readings. Check, clean and tighten the elec connections including grounds for the sender and gauge and see if that fixes it.

Good information, thank you X2! I will post the results.

Here's a pic, it's a SWEET ride!

The news is NOT good; it would appear that the dash gauge is reading correctly. (shudder!)

VID 20140616 201004 - YouTube

(shudder!)

NOT an electrical problem. Where to look now? Oil pump? Bad mains? Blown seal? Used car lot?

Drop the oil pan and you are going to find out what is the problem