I just bought this car heat worked great driving around on side streets ,when I was test driving it.on the way home I noticed that when ever the rpm is above about 2000 rpm the heater" actualy the defroster thats a different problem may be, air only comes out the defroster vents no matter what button is pushed eccept for the one with the o then it shuts every thing off" back to the heat above 2000 air temp goes cold,let off the throttle and it returns.Also found that the rad cap is bad did not notice it before but it is missing the seal and relief spring that seals in the presure in the sytem could the pump be cavatating due to no presure.I had removed the cap and smelled the over flow bottle to see if it had any exh smell and it does not.Car runs great has lots of power temp gauge ran at 80c or just a little above the whole time although it is cold out 40 to 45 deg.Also any one know what these motors are governed at what should be rpm at 65mph.One other question there is a bypass valve of some kind in the exh that is between the in take and the exh there is a pot on it with a little pipe that looks like a vacuum line could go onto, and what looks like a adjutment screw is this a egr valve or a manifold boost relief.

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89 Ford F3504x4CC 7.3 IDI turboed Intercooler and running on wvo now powered by a 1997 Cumins
87 300SDL on bio and wvo killed by chevy truck 5-24-06
1999 E300TD black with gray int.

Welcome to the SDL club.

First, I would make absolutely sure you have a proper cap and the coolant is filled up to the cold mark when fully cold. Without this baseline, all other efforts are for naught.

Then, if you still have no heat, it's time to pull the monovalve and check the diaphragm. There are many posts on this procedure if you are not familiar with it. Search "monovalve" using the search function. It's not difficult to pull the diaphragm and usually that is the cause of the problem.

The 603 engine will be turning about 2650 rpm at 65 mph. It does rev quite easily and can easily reach 4600 rpm. It does have a governed maximum, but I don't remember it offhand. The maximum horsepower is at 4600 rpm, so it doesn't make much sense to go above this figure.

Depending on the location of the valve, it could be the EGR valve or it could be the ARV valve. I can't determine which from your description. The EGR valve is right up front and close to you. The ARV valve is down on top of the turbocharger, not as easy to see or access. Post a photo and we can confirm it.

I picked up a cap from a independent shop that carrys MBZ parts today it is a 100 the one that was on it had a black alum sticker covering the whole cap and was marked 140 peeling the sticker back and it says 120 I am thinking this is the presure of the cap maybe it is located on one of the little round handles.I think it is the egr valve located on the very top and feeds in to the intake runner must be okay to disconect I know a lot of the TDI guys do it. Okay I did not relize this engine could rev that high I am used to my truck and never take it over 3000 most of the time its 2800.Thats about what it looked like for rpm at sixty five.

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89 Ford F3504x4CC 7.3 IDI turboed Intercooler and running on wvo now powered by a 1997 Cumins
87 300SDL on bio and wvo killed by chevy truck 5-24-06
1999 E300TD black with gray int.