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  #1  
Old 06-16-2003, 11:32 AM
Wendell Allen
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Question Oil Pressure Problems

Hi All,

I'm working on my '89 300E. She's a doll, and I've had her since '95. She's got about a quarter million miles on her right now. And I've noticed over the past few years that the oil pressure gage dances around a little. Nothing major, but some small intermittent fluctuations that I've just kept an eye on. Since it's never gotten wild, I didn't worry too much about it. I'm using factory oil filters, and run Castrol 20-50 normally. The car is burning oil, but I kind of expected that with this many miles on her. The water temp never gets high at all on the car. Always a cool runner.

But things changed last Friday. I went to start the car to go home for lunch, and it was acting as if the battery was dying. It struggled to turn over, but finally fired and I drove it home. But she stalled twice on me after she came up to temp. I didn't see anything abnormal at the time. So I went on home and checked the atlernator and battery, and it all checked out fine. So I decided to give it a shot and took her on back to work. But the stalling thing was getting worse. And I noticed as I was pulling into the parking lot at work that the oil pressure was pegged all the way up, with only very slight fluctuations downward. And it stayed that way even at idle.

So I'm wondering: Do I have a plugged oil passage that's causing the internal components somewhere in the motor to heat and swell enough to cause the motor to try to lock up?

Part of what is encouraging me to think this is that I went ahead and drove it home. And since it was cool, and I only live a short distance from work, I thought that maybe since the car had a chance to cool down, it wouldn't be on the road long enough to heat up and stall even if there was a plugged galley. And that's exactly what happened. I drove all the way home with the oil pressure pegged, but it never stalled.

She's in the driveway right now and I'm not starting her till I find something out. I love this car and don't want to see her dead. But frankly I'm not in a financial positon to do a rebuild right now, so I'd like to explore other less invasive methods of getting my babe back in shape.

Any help at all would be appreciated.

Thanks,
Wendell
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  #2  
Old 06-16-2003, 05:30 PM
G-Benz's Avatar
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I'll start with the basics.

Oil gauge fluctuations are part of the MB character. Unlike other vehicles where the oil and temp gauges have the needle hold steady at some "automotive karma" position, the MB gauges actually provide useful info. The numbers on the gauges indicate pressure in units known as "bar" (a bit different from atmospheric pressure, but close). You should typically see the needle pegged at "3" after starting, and when driving under load. Otherwise, the needle goes down a bit when idling (between 1 and 3 depending on engine configuration and condition).

I don't think your problem is oil related. A major leak would set off the oil level light on your dash, and pressure seems to be fine from what you describe.

If you were experiencing an overheating condition as a result of oil starvation somewhere, your coolant temp would still register the anomaly, and your engine performance would increasingly worsen as you drive.

Some other info would be needed...does your car idle smoothly? Does it start fine warm or cold, of differently depending on whether your engine is warm or not? Do you hear the whirring sound near the rear passenger tire? (That's your fuel pump.)

Also check your OVP (overvoltage protection relay, LOTS of threads on this), especially the fuse.

My thought without seeing your car? The timing chain. When's the last time the timing chain was replaced? If it's stretched, it may have jumped a tooth or two causing the timing to change a couple of degrees...perhaps enough to make starting difficult, and hamper overall performance.
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  #3  
Old 06-16-2003, 05:49 PM
Wendell Allen
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Thanks for the reply, G. Here's what's I understand and what's happening.

I'm glad to know about the slight fluctuations. I feel better about that now.

As for how the gage works, I understand that, and that's how my car has acted in the past. Pegs 3 upon start up, and drops once the car warms up, usually to just under 2, slightly higher in the cooler winter temps, and slightly warmer in the hot summer months. And always slightly higher when the oil is newest like right after an oil change.

As for how the car starts, it's always started best when cold, seldom taking more than one revolution to catch. When warm, it's seldom taken more than two, maybe sometimes three, revolutions to catch. And it's always idled at about 650 RPM. Depending on who's tach I use, I get a slight variation, but I assume that's due to calibration/manufacturer differences. But it's always close to that. The only time I get a different reading on the idle speed is either when the engine is cold (only a very slight increase) or when the A/C compressor is on. I've never had a problem with the car stalling either. Ever.

What's happening now is that even once the car is warm, and I mean in 90 degree plus ambient temps, the oil pressure never comes down. It always stays pegged at 3. No fluctuations at all on the needle. And that's even at the idle speed of 650 RPM. And the car is hard to start, always taking ten or more revolutions to start. And as noted, it's stalling regularly now. But once it's started, and as long as it's not stalling, it runs fine - very smoothly as always, but what seems to me to be slightly down on power.

Fuel pump is fine as far as I can tell. I'm familiar with the whirring sound your talking about, and it's not doing that. I replaced it two years ago as well. As for the OVP, I'll check that out. But if you're talking about the voltage protector that keeps the main computer from getting blown from voltage surges, I had that replaced by a Mercedes dealer a year ago. But again, I'll double check that.

The belt is a new one to me. I'll check that, as I put a new one on earlier this year. But if there's something else wrong, maybe I just haven't looked close enough to see it. I put a new compressor on a year ago, but recently converted the car to R-134, so maybe something happened there. But I normally turn the A/C off when I stop the car, and on only after it's started, so it would have to be the clutch if this the problem with the A/C. Also, I just had the alternator rebuilt a few months ago. And the water pump was replaced early last year.

The timing chain is a new idea to me as well. I've not replaced it, and it's been about 100K ago that I bought the car. I like this idea (you know what I mean - lol). Is the tensioner adjusted with oil pressure? Could this somehow tie the increased oil pressure and hard starting/stalling problem together?
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Old 06-17-2003, 11:49 AM
G-Benz's Avatar
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Again, your oil pressure is not the issue here...and the idle variations seem normal, as there are relays that adjust idle (like when the A/C compressor cycles).

Difficulty with hot starts sounds like the OVP, but since you have had it replaced recently (replacements have been corrected), I am now wondering if it is a fuel-related issue.

You have replaced the ancillary components, so it's not probably that any of them are "seizing" up and causing erratic stalling problems...besides, you would see premature belt failure by now, or at least a lot of belt material coating ther rest of your engine.

Sounds like perhaps there is a vacuum leak of some sort...on or near your fuel distributor.

Also check your ignition components, specifically, the cap, rotor and ignition coil...
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  #5  
Old 06-17-2003, 12:12 PM
Wendell Allen
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Thanks for keeping up with me G. I guess the combination of these symptoms aren't a common problem.

I spent some quality time with the wife last night, so I didn't get to look at it. I'll do so later this week and let you know off line (as well as here) what I find out.

But I'm still trying to figure out why the sudden and drastic change in oil pressure though. It's never done this before. Oh well, guess I'll find out soon enough.

Thanks Again,
Wendell
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  #6  
Old 06-17-2003, 02:08 PM
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Location: Portsmouth UK
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Wendell - Hi

I preface this by saying - I am no expert.

For what its worth and from your description of the symptoms I would say you most likely have two problems and they are unrelated.

For the moment I would ignore the Oil Pressure Indication.

It is feasible that there is something wrong with the indication but at least it is up there in the safe zone. If you were over pressurising the oil system I suspect you would have leaks galore and oil consumption would rise pro rata.

Concentrate your efforts on the stall issue - which unfortunately could be a myriad of things. A search on Stall will identify the usual culprits.

Good luck.
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