Poor running after sitting a few days

I'm puzzled by this one. '93 Mercedes 400E with nearly 200k miles on it now (!) Here's the basics of it - let's say I've been using it as a daily for a week or so. It will be running fantastic - engine is butter smooth, barely know it's running at idle, no squeaks or rattles, shifts well, propels you forward on a never-ending wave of torque. It's wonderful.

Now, this is where it gets puzzling. Let it sit for 24 hours, no problem as far as I can tell.

Let it sit for 48-72 hours - starts right up, idles great. But once you're doing like 40mph and you give it some gas, it bogs down. Gas pedal can be floored and it just sits there at like 1500rpm, ever so gradually climbing to 1600. Manually downshift it to 3rd, it improves, but there's still a bit of a shudder. After maybe 20 miles and/or 20 minutes, the problem is completely fixed. Back to its old self. You can actually feel it improving as the minutes tick away.

Let it sit for say 5 days, and it still starts right up. But now it hunts at idle. It might stall at a red light. It will bog leaving an intersection, even if shifted into 3rd or even 2nd. 40mph to 60mph is glacial. This time it takes about 30 miles/minutes to fix itself. But it again returns to its old, wonderful self. If you literally drove it everyday, I don't think you'd know this problem even existed.

I changed the auto trans filter and fluid right after I bought the car (about 8k ago) but that didn't make a noticeable difference. It has a very slight trans leak. A few drops over the course of a week.

I can't imagine anything involving electronics or cables would behave like this. I will freely admit I really have no clue how autos work. I keep an eye on the fluid level, but if it was low, I don't see how it would fix itself. The only explanation I can conjure up is that somehow air is working its way into the trans due to the leak, and the longer it sits, the more air gets in. Once you start driving it, it will be terrible until it gradually self-purges the air and then all is well again.

Thoughts? The car is incredible when at 100%. I'd like to see if I can keep it at 100% more often. As it is, it forces me to drive it often, which I'll admit isn't really a bad thing.

NorseDave wrote: Let it sit for 48-72 hours - starts right up, idles great. But once you're doing like 40mph and you give it some gas, it bogs down. Gas pedal can be floored and it just sits there at like 1500rpm, ever so gradually climbing to 1600. Manually downshift it to 3rd, it improves, but there's still a bit of a shudder. After maybe 20 miles and/or 20 minutes, the problem is completely fixed. Back to its old self. You can actually feel it improving as the minutes tick away.

Fuel delivery, starvation issues? I don't think it's transmission related. Put a meter after the fuel pump and see it has the correct pressure, then see if the pressure fluctuates.

Curious to see how this ends.

Dan

My Classic Beetle is like this as well, I think it's a sticky float valve caused by the new gas blends found at all gas stations nowadays. My theory is that modern gas reacts with the old parts and after soaking in it for a few days, the float sticks a little until I run it for a while. The more a drive the Beetle, the better it runs.

My first thought is that I have the same problem. Not with my car but me. If I sit for a few days I can't run at all till I get limbered up.

Regarding your car, I think the first thing you need to do is to try and get a handle on if the problem is engine or transmission. Based on your description I think you've got a running issue that you can mask by shifting the transmission manually.

EGR valve? Maybe it's sticking open and needs a little coaxing to close after it's been sitting?

I have to admit, I had indeed been assuming this was trans related. Probably because of the 4 vehicles I have regular access to, this is the only one w/ an automatic, and its the only one that behaves like this.

Anyway, good thoughts here. I will start investigate. APE, you're right, I do indeed need to figure out if its engine or trans related. And yes, sitting for extended periods is at least as bad for humans as for cars!

Charge the battery before you drive it and see if that helps. Also check your grounds, some cars don't like low voltage, iffy grounds can get better with driving.

The EGR theory sounds solid, except for the lack of performance.

My subaru idled rough for ever until one day it got a new alternator and battery, after that it was smooth as a non subaru. All of the subaru owners told me they all idle like that.

Wet ignition components?