I picked up this 300TE a few months ago and over all haven't had to many problems with it. It has occasionally had an issue where at idle or while driving it would start stumbling and running rough then would clear up. Each time that this happened the engine light would come on and go off once the problem cleared.

The car has 95,000 miles and recent work performed include new Bosch Alternator, relays, fuel pump relay, new cap rotor, plugs and wires(all Bosch).

I have done a bit of reading up online and in a manual I have, I have consulted with a few shops that I deal with and narrowed down to troubleshooting several specific things.

Recently the problem worsened, the rough idle became permanent and would stall. The air flow sensor plate was out of adjustment, after correcting that it ran better but the problem came back shortly after and the engine light came on permanently. Soon after, cold starts became an extremely difficult ordeal and it continued to run rough and lack power during driving.

A visit to a local shop resulted in an adjustment of the fuel air mixture, it ran significantly better and the engine light went off. However the fuel/air was not able to be adjusted within spec on the tester indicating that the oxygen sensor may need replacing. The existing oxygen sensor was replaced as it looked to be original. The car seemed to run better after that however the problems returned on the drive home.

The following morning the car would not start. I finally got a very low rough idle. I adjusted the fuel/air a little until the idle picked up and smoothed out better. It still idled rough, it did not transition well from idle to higher RPMs and it would occasionally stall. I did some poking around and disconnected the plug from what I think is the EHA valve on the side of the fuel distributor and the idle dropped slightly and smoothed out although it is still a rough idle. It runs significantly better at all speeds with the plug disconnected. The engine light remains off more than on. This morning it had a very rough idle until it warmed up and then it ran great afterwards.

I have checked for vacuum leaks and found none, I have run this car on premium gas as suggested although I do not know what the PO used. The E.H.A. valve shows no signs of leaking fuel.

I feel like I am on the right path in figuring out what this issue is but I figured that now would be a good time to see if anyone has had a similar problem and take suggestions from the more knowledgeable members.

Thanks in advance.

Does your car have an ovp relay? These are classic symptoms of a bad one.

the OVP relay was replaced as well. I recently inspected the fuses and they were good, however i guess it doesn't hurt to recheck.

thanks

Bump!

OK. I am hoping someone can give me a few hints here as this problem is persisting.
Considering I had a noticeable difference in behavior when I unplugged the EHA valve, I replaced it with an almost new unit that I bought from PS2CHO. teh existing valve looked like it was a replacement and it turns out the seals were leaking slightly.
Installed the valve , plugged it in , adjusted the Fuel/air screw to lean it out a bit and it ran great. this lasted for about a day before it started acting up and after about 3 days I had to unplug eh EHA valve and enrich the mixture to make it drive-able again.

Any suggestions?

Thanks in advance

It seems that basically when you are letting the system try and enter closed loop mode its struggling to meet a normal A:F ratio...

I know you have said you have checked for vacuum leaks, but have you physically replaced the idle control hoses? If not, do it...I bet money they are hard as rocks and they are impossible to check the one that tucks down under the intake manifold. They are cheap enough. When I got my 88 300TE @ 64k, they were rock hard and replacing them helped me rid of a vacuum leak I had.

I would start there first

What did you set the potentiometer to? It should be as close to 0.8v as possible. Anything more than +- .1v is too far out of adjustment. So basically you need to be dead on .8v otherwise its gonna idle like crap or stall during different conditions.
Also, have you checked for oil consumption?

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

So turns out it was a vacuum leak!
I checked the lines again, all my vacuum line and the rubber hose connectors are in good shape, soft and pliable.
I jacked the car in the air and crawled under it to find the ports under the intake manifold, which I didn't realize were there. Sure enough one of the lines, the one connected to the larger black line, had came off the port. It was little difficult to get my big hand in there and slip it back on but after doing so everything seemed to smooth out.
Adjusted the mixture a little leaner to get the idle to smooth out. It still runs rich and the idle is not as smooth as it could be. I'll take it over to the shop I've been dealing with and have the potentiometer adjusted to spec.
Hopefully this is the cure , we'll see.
Thanks for your help. it was the hint I needed.

now to fix the odometer, wheel bearings and shifter linkage bushings.

Start by resetting the pot then work on the mixture.
Good job

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

The problem is back again.
It drove great for the last 3 weeks until this Sunday.
Same issue. Unplugged the EHA valve to make it to my destination. I replaced all of the vacuum line connectors and checked the odd shaped ones that I didn't have replacements for as well as the one at the transmission too. Plugged the EHA valve back in and it ran great again for about half a day.
I checked the vacuum line for the pressure regulator, the hose in the regulator it definitely needs to be replaced so I doubt that the suction is working well enough to operate what ever the mechanism is in the regulator.
If this is an issue with the regulator, I am unclear what the relation with the EHA valve is and how it will be "fixed" after small repairs only to act up again.
I am reading up on 124zone to hopefully gain some insight and figure it out.

anyone have any ideas to add?
thanks

Doesn't happen to coincide with rainy weather does it?
If the OVP gets wet, it can cause erratic symptoms. Check the drainage on that side and make sure everything is ok.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Actually, this time it does coincide with rain however not always before. I'll check the drainage and dry out the relays.
Excellent website BTW. After reading some of the articles I think I am going to replace the idle control hoses as well as some of the other rubber items as a preventative if not repair measure.

Thanks

If you haven't replaced them yet, do it. They can crack at any place and its just not humanly possible to see every part of those hoses. Its just imperative you do it before anything else at this point.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Parts are on order, thanks for the help.

X2. You will be surprised how often it's just the little things.. Change the hoses, ( I hope you bought genuine BTW) and dump a bottle of redline in the gas tank next time you fill up.

__________________
Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2

yep , I ordered original with the exception of one which is Contitech

so here is an update, in the spirit of black humor its is somewhat funny.

I order all the parts, specifying that I want MB OE hoses.
Parts come in, I don't have the time to do the repair until Saturday
Everything comes apart easily and the whole job reveals itself to be rather simple and easy.
Turns out the idle control to take hose is not MB but CRP brand or some similar 3 letter name. I find out Saturday afternoon while scanning the forums that this particular brand is prone to cracking sooner than later. So in a delima I decide to wait until Monday morning to see if the local MB dealer has it in stock, which it has to be Monday as I am going on Family vacation to Washinton DC on Tuesday at 5 am.
No go on the part Monday so I use the hose I have with the idea that I will replace the hose later.
I replace the throttle body gasket, pressure regulator, distributor intake boot, all Idle control hoses and put it all back together .
After reassembly, it ran significantly better, it had one hick up on its first test drive but other than that it was way better.
On the way to D.C. it had one hard start after refueling but ran good the rest of the way. On our second day there, on our way into the city in a large traffic jam next to the Jefferson memorial it started running rough, stalled, and wouldn't restart. Awesome!
We pushed it over into the break down lane, unplugged the EHA valve and it started right up. wound up having to finish the rest of the trip with the valve unplugged.
I just ordered the control to manifold hose and will tear everything down again next week to see if that was the problem.