I could use some opinions on this one before I go dump a ton of money.

I have a '90 300E 3.0L. 135k miles that we bought about 8 mos ago. I had the Head rebuilt 6 mos ago because I found a dead cylinder - burnt valve. New plugs, wires, cap & rotor, and replaced a couple of injectors at that time also. Car has been running great.

About a week ago, I started noticing a couple things about the way it was running: 1) gas milage seemed to be down a bit, as did power. Watching the economy gage, I could tell that I was having to use more throttle than previously to maintain a given speed. 2) I started noticing a surging while maintaining a steady throttle setting. It wasn't terrible, but it was noticable.

Last night it got noticably worse. Acceleration is very poor and is rough. If it was a carb motor, you'd be pumping the accelerator to get it to accelerate. The surging is much more noticable. I thought - fuel filter.

I bought a new filter today. Jacked the car up, put her on stands and climbed underneath. After removing the cover over the fuel filter/pumps area, I caould see that one fuel pump looks new. I would say easily less than a year old. The other looks ancient - might well be original. The fuel filter looks moderately new...not REAL recent, but not ancient either. The hard lines that connect the pumps and filter look fairly new as well. Looks like maybe the PO had this issue before. Now I'm thinking the older pump is probably on it's way out.

Not sure what the life expectancy of these pups is, but 18 years and 135K doesn't sound too shabby to me, so it makes some sence to me that the old pump becomes the primary suspect at this point.

Someone also suggested the O2 sensor, but if there is one under there I sure couldn't find it. I see what looks like a front pre-cat, a flat oval shaped cat and a muffler at the back. Nowhere downstream of the pre-cat can I find a sensor. Didn't look forward yet.

Sure would appreciate any comments from those more experienced than I.

 

To find the O2, its easiest going under the car from the drivers side.

The O2 sensor will have a cable that runs through a grommit. You may want to pull up the passenger carpet and find the cable that runs under the center console to see if your O2 sensor is present.

 

Final update:

Problem was not fuel, it was electrical. Replaced the coil yesterday and the problem so far appears to be gone. Not sure if the coil issue is related back to the alternator puking the week before or not.

 

check out your fuel pump relay and the OVP relay. notorious for failing, right about at your mile mark.

 

UPDATE...

Well the coil replacement took care of 95% of the problem, but not quite all of it. Most of the time it runs fine now. No more severe missing or surging, or lack of power. But it still stumbles starting out from a stop sometimes, and other times is fine.

You can tell how it is going to run as soon as you start it up. If the car starts up and idles smoothly, it will run fine. If the car starts up and idles rough, it will stumble when you hit the throttle from a stop, then pick up and run ok. Until the next time you stop at a light or something. It will keep doing this until it has reached temp, been shut it off for a min or two (like a stop for coffee), and then restarted. Then it will usually run fine.

I have in my mind it sounds like the engine temperature vacuume switch, or OVP. Anyone have any thoughts?

 

Yes go replace the OVP if it hasn't been replaced yet.

You will also want to do a vacuum test.

I would recommend going to a local pepboys or something and get them to look up any error codes you are getting from the ECU. Usually they can do it for free or a very small price. This could save you alot of time and money.

 

What do I need to be able to read the error codes ?

 

Pepboys or other local stores have Error Code Readers (idk exact name off the top of my head) which can do a very basic read if you have the correct list of codes for your year/model/engine (you will have to search these) and they run anywhere from $30-$300. I would say though if you don't want to invest, its cheaper just to get them to/pay them to pull the codes for you. It's going to tell you if a sensor is bad or anything alike. With these cars though this was the beginning of the whole error code production so it is very basic....but its better than nothing. I have a coolant temperature sensor error which I am fixing in a few hours when its light. No clue why I am up at 6am

 

I am beginning to develop a love/hate relationship with this car. Seems like as soon as I get one thing fixed another jumps up and says "hey, I need some attention too!"

So the coil replacement fixed the power and acceleration issue, except for some intermittant flare ups that I think the OVP may be responsible for (one on the way.) So now it's running pretty good for the most part.

Yesterday, I'm coming home from work and it didn't seem to be going into overdrive. Was going back and forth between about 40 and 50mph in moderate traffic and noticed the tack was staying at about 3000 rpm. Counted shifts leaving a stoplight and counted 2 shifts, but the rpm's were too high and I couldn't feel any slippage. Seemed to kick down ok. Trans was serviced maybe 8 months ago. Checked the fluid - fine. Checked the vac line on the modulator valve - fine. Fluid has a little smell to it, but not burnt.

It has always not wanted to shift when you first run it in the morning (for the first 100 yards or so, then it's fine) but not even a HINT of anything else wrong from the trans until yesterday.

This morning it was fine again.