95 E320: Rough idle above 4000RPM

[EricSilver]

Car: 1995 E320
165K miles

While revving the engine in idle this morning, it became rough just after 4000rpm. From 0 to 4000 rpm, the engine is smooth and imperceptible. But as I ease past 4000, it behaves as if one or more cylinders are not firing in sequence.

Ancillary Observations:

* Check Engine light is on (has been for a long time – 3 years -- with no discernable cause – but I am hoping it is related to this discovery).

* Oil pressure needle, after a fast highway drive, hovers just above 0. Shifting into neutral puts it just above 1.0. (Planning an oil & filter change today).

Any ideas?

[EricSilver]

Also:

On hard acceleration in Drive, i.e. when merging onto the highway, there is no roughness at/above 4000rpm.

[Arthur Dalton]

<
Normal in P or N
Torque converter RPM safety cut-out.

[Arthur Dalton]

< >>

Why don't you just press the code button and see what code is stored that tripped ther CE
Search Code tool

[EricSilver]

Thanks Arthur.

And look what I just found, and was about to post, after doing some more reasearch:

12-23-2005, 09:16 AM
Arthur Dalton
Location: Florida
Posts: 3,521

The car has a 4K rev limiter for P/N to protect the torque converter from operating at high RPM w/no load. It works by dropping a couple of cylinders. It is over-ridden by a speed senor when the car is moving.

You beat me to it.

Any thoughts on the oil pressure issue?

I plan to do filter and oil this afternoon and see if it persists. The car has been largely undriven since March, which is also when it had its last oil change (still less than 3000 miles since) and I suspect the filter may be mucked up a bit, even though the oil appears to be very clean.

Also, I have noticed that the oil pressure needle, at full, does not enter the white space of the 3-bar line, even though I know it is at max level. (On my previous car, I believe it did enter that space.) Also, on pre-start, before cranking, the needle moves to approx 0.1 or 0.2. I don't see why, since no oil is flowing -- or is that a diagnostic to show that it is working?

As for the CEL, I am new to this car, and that type of diagnosis, and still learning about both. I do have your LED diagram and code lists, however.

The previous owner told me that the Service shop (a really good one) that has maintained the car has not found any emissions-related issues connected to the CEL. That does not mean, of course, that other issues are not causing it.

Before I purchased the car, the shop's owner stated that they could not track down the specific cause. It went away after replacing the wiring harness, but then returned, and he said that codes sometimes refuse to clear.

I trust his knowledge, experience and judgment, however, if the CEL is illuminated, even for a mundane reason, it is like dirt swept under a rug and I cannot sleep soundly, knowing it is there.

I will search Code Tool to learn what to do. Thanks again.

[Arthur Dalton]

Car has a stored DM code if CE is on..
Te button for retrieve is in front of the battery

[EricSilver]

Originally Posted by Arthur Dalton

Car has a stored DM code if CE is on..
Te button for retrieve is in front of the battery

OK. I will have a look there while changing the oil filter.

[EricSilver]

Changed oil (Castrol 20W50) and filter (Bosch).

When engine is hot (90C-100C) pressure is now between 0.5 to 1.0 with car in gear, and 1.5 with car in neutral. With engine at approx 83C, pressure is 2.2 in gear and 2.5 in neutral.

The old oil I removed ran very freely and quickly. I suspect it was a lighter, winter weight since it was last changed in March (car was not driven much since then). I also am wondering if lighter oils keep the engine generally cleaner, and if my 20W50 is too heavy....

To be safe, I will have the sender checked this Friday, and perhaps have the service shop change the oil yet again to whatever their standard is, since they have maintained the car in great condition for the past 7 or 8 years. I can live with the low pressure indication if I know there is not a problem.

[fastpakr]

What is the recommended weight for your engine? 20w50 sounds ridiculously high. I'd have expected something more like 5w30 winter, and 10w30 summer.

[EricSilver]

Castrol 15W40 has been used for the past 7 years or so. I will be reverting back to that on Friday.

[EricSilver]

Also, because it has been so hot here (Wash. DC Metro area) this summer, the heavier oil makes good sense.

The 20W50 appears to be causing cavitation, evidenced by the occasionally erratic pressure guage needle, as described in another oil thread. Reverting back to a lighter weight will resolve that (my guage did not do that before).

Since the below-1.0 pressure readings are normal, so long as pressure shoots straight to 3+ on acceleration, I will not worry about it.

[fastpakr]

You really need to stop worrying about the air temperature and focus on the oil temperature and pressure directly. It's not that hot outside. If you're getting good pressure with the 15w40, then it's counterproductive to go higher. Again, what was the original recommended pressure?

[EricSilver]

On the Washington Beltway, in slow rush hour traffic, it is that hot outside -- at least as far as the engine is concerned.

With 15W40, oil pressure at idle was near 0 at approx 95 degree coolant temp, and with drive engaged. Shifting to neutral raised it to between .6 and .8. At lower coolant temperatures, pressure was higher -- 1.0 and up.

Since this appears to be normal for this engine, I am not going to worry about it. 20W50 thins out quite significantly when heated, to almost the same extent as lighter oils, so I will stick with the 15W40 since that is what has been in the car forever.

[fastpakr]

I would strongly encourage you to get a second gauge to check your readings. What viscosity was the engine designed for? If you're getting zero oil pressure with 15w40 in ANY climate found on earth and that's the factory oil weight, then you have a problem and you're only masking it temporarily with the 20w50 oil.

[EricSilver]

I am not getting zero, just near zero, i.e. ,0.3, which is within design parameters. I never saw that on my other car, thus my curiosity and unfounded concern.

Still, when I have the oil changed (again) tomorrow, the sender and gauge will be checked, and corrected, if there is a problem.