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1985 MB idles great but not under load

1985 380 se runs great until engine warms and put under load. Replaced fuel pump, fuel filter, fuel pump relay and the gasoline… help

coolant temperature sensor?

Make sure the catalytic converter isn’t plugged.

This should be a CIS injection car and there are always 2 things to check first when any performance problem exists on this curse to humanity. (The CIS system, not your car.) :slight_smile:

Check for vacuum leaks; preferably with a vacuum gauge. CIS does not tolerate vacuum leaks well.

Fuel pressure system check. This requires a specialized CIS pressure tester as a normal fuel pressure tester will not work. The diagnostic steps have to be followed and pressures checked as per a chart. Pressures vary by ambient temperatures, type of car using the CIS system, and so on.

The CIS pressure tester will have to be purchased or one can be home built. Failing that, you may have to have this test performed by a Benz shop or a shop which specializes in VWs, BMW, and so on as they used the same system.

If not vacuum leak exists then about all I can do is wild guess at the control pressure regulator. I’m not a Benz guy but have dinked around a lot with CIS on other European cars and the CPR was a common fault.

thanks, will do.

This is what the little critter looks like below and as you can see by the price it’s not something to take lightly on a replace and hope repair. Note it’s out of stock also.

Out of all of the CIS problems I’ve encountered the CPR was the most common after air leaks.
The reason why the air leak thing is so critical is because the air sensor plate relies purely on the suction of intake air to raise it against the fuel pressure on the plunger in the fuel distributor.
As an analogy, an air leak in a CIS system would be similar to trying to suck soda through a straw that has a crack in it.

Those pressure testers used to sell for about 400 dollars (ouch) so I just made my own.

What kind of load do you put on it while idling??

All above great comments. On thing I’d add for a 1985 would be to rule out a compression problem with a compression test. A fairly easy to do inexpensive test. If the compression is off, that has to be fixed first, before mucking about with the car’s various bolt-ons.

Thanks for the help everyone. This is the OP’s boyfriend, I’m trying to fix her car for her. We have replaced the fuel pump and filter because I suspected it might be the fuel pump (because she recently ran out of gas). Also replaced the fuel relay. Same problems still exist. Let me attempt to give you a better description of the car’s symtoms:

Car starts easily. Car runs perfect, for about 10 minutes. When it reaches operating temperature it starts to cough, sputter, and want to stall. It will still idle fine, but if you give it any gas it sputters and won’t go. But again, it runs GREAT when it’s cold. Even high speed, 70mph no problem. But soon as it’s warm, it then doesn’t want to go.

Last piece of info, this problem came on suddenly. It didn’t get worse over time. One trip to the store it was fine… the next trip to the store it had this problem.

Thanks for any and all help.

I’m afraid that I can’t be of much help as to specifics simply because of the unknowns regarding potential vacuum leaks and fuel pressure testing. In a shop setting, checking both of those would be the first steps and as stated earlier this requires a vacuum gauge and the specialized CIS fuel pressure tester along with a chart outlining the pressure test steps and specs.

The reason I mention the Control Pressure Regulator is because the CPR is time based and affects what is called the cold and warm control pressure. I’ve attached a link below with a diagram of the CIS system. In a nutshell, CIS is basically simple and can be a pain in the neck at the same time. Vacuum and CIS problems can also occur at any time so the fact that it came on suddenly doesn’t mean much.

Wow 4450, thanks for all the info and diagrams. We really appreciate your help and will keep you updated on the status. :slight_smile:

Don’t overlook a possible ignition system problem. I would check the plug wires and coil.