Hello,

I have an idle situation that I need help with. I have a 1990 300se/M103 with 190,000 miles. Between the time that the car is first started and reaching operating temperature, something happens that causes a change in vacuum and a rough idle. Here is what happens. I start the car and warm it up for a few minutes OR start cold and go. All is normal and the economy gauge is to the left in the range it should be. I drive a short distance to the next stop and the economy gauge SOMETIMES will jump up to the middle of the range and the car will idle rough consistent with a loss of vacuum. Revving does not change the condition. Then I continue driving, and for the rest of the stops and starts over a 14 mile commute all is normal.

Other than this annoyance, the car runs beautifully. Everything that is prone to failure has been replaced. New head gasket, ignition parts, OVP, EHA, fuel pump, filter, injectors, O rings, and seals. I also cleaned the idle control valve. I have had the system checked for vacuum leaks by a Bosch certified shop and there are none. At the same time I had them set the fuel distributor pressure since I had to install a new EHA. About the only thing I have not replaced is the airflow potentiometer. I checked the black and white vacuum check valves for proper function and I am assuming correct functioning of the vacuum switches both fender wells. So, has anyone experienced such an intermittent rough idle/vacuum loss situation and what might be the solution? It seems like it is triggered by something that does not occur again once the car is fully hot. Except for this little problem, the car runs like a turbine. Any thoughts?

Thank you,

Lloyd

I didn't see mention of the O2 sensor being replaced, and it sounds suspect in your description. More often than not the O2 sensor needs to be replaced in tandem with the EHA.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

Thank you for your reply. The O2 sensor was replaced this past summer.

Probably the easiest thing to do is to examine all vacuum lines in the engine compartment and replace any of the larger black vacuum lines which are cracked. Be sure vacuum connections are snug.

Also be aware (in general for all vehicles) that certain vacuum sections may be switched on /off under certain operating conditions. Like hot /cold, exhaust recirculation turned on or not, etc.

Hello,

Thank you for your reply. All vacuum lines and connections have been replaced and I have had the system checked for vacuum leaks by a Bosch certified shop and there are none. Its that "on/off during certain operating conditions" that I need to narrow down. I was just wondering if what I described was an obvious symptom of some component acting up.

Then what I would do next is learn about how each section of the vacuum system works - what each gizmo using vacuum does - learn when things are switched on/off.

With that knowledge, you could then disconnect and plug certain portions of the vacuum system, then see if the problem goes away or not.

For example maybe a line in the engine compartment goes to the climate control system inside the car. That could be disconnected in the engine compartment, climate control turned off, then see if the problem goes away or not.

I have been trying to solve a similar problem on my 1989 300se. I have the rough idle/ low vacuum problem in my car. It begins soon after starting and it always clears up once the engine temp reaches about 70. In addition to the rough idle during this time period the car has little power and wont accelerate to highway speed until the engine temp reaches 70. Then it surges a few times and breaks out of it. After that it runs just fine. Like yourself I have replaced all the usual culprits and am trying to figure out what is not working right under pre-operating temperatures. Hopefully one of us figures it out or someone else has figured this out that can enlighten us.

Thanks for mentioning your issue. It sounds similar except that my car runs normally with good power and no surging (except for the effort required to get the cold transmission going). I am going to have to research this when I have a chance to get the car running in this state, and then see if I can isolate the cause. I know that there are no leaks, so there must be some switching thing or something sticking that is causing this. I also have a code reader so I will check that aspect as well. I made my initial post with the hope that someone would have had and fixed this problem so I would not have to do real work. Now I guess I will just have to figure it out! I will keep you posted.

In addition to everything you did I swapped out the MAF sensor and there was no difference. That would rule out the potentiometer assuming that you were talking about the pot on the MAF sensor. It was a junkyard part but still there was no difference in what the car did. I will also try and dig into this. I am in the process of disconnected electronics and checking connections - everything looks good so far. Good luck - I'll keep reporting any progress.

I haven't figured it out but now the problem is worse. Even after it warms up now it runs poorly. I just drove 10 miles at a top speed of 20 mph. I still have never seen the check engine light but I know it works because it lights up when starting the car. Now I suspect fuel delivery - it sounds like the car is starving for gas. Once/if I find the problem I will post the results.