This has happened twice in the last few weeks, for a few hours in a day each time.
The engine starts running and idling rough (Mercedes with straight-6) and if I have to stop in traffic ( as for a red light) I put the car in neutral so it won’t stall. After I’ve had the engine shut off a couple of times, when I park to go into a store, for example, it goes back to normal. Also hot weather has continued here but the temperature gauge does not read high…what’s going on?
You need to hook it up to an OBD-II scanner.
What’s an OBD-II scanner?
OBD is on-board diagnostics. OBD II is an improvement over OBD I. In the U.S. OBD II has been required for emissions standards since the mid 1990s. If your car was built after the mid 1990s, it has an OBD II port in it.
An OBD II scanner plugs into your car’s engine computer and reads error codes and other types of data from it.
You can buy an OBD II scanner from ye ol local auto parts store.
A lot of auto parts stores like Autozone will connect an OBD II scanner to your car and will read out the error codes for you for free, knowing that if you need to buy parts you’ll likely buy them from that store since you are right there at the time. I believe California prohibits this, but I think Autozone in any other state will read the codes for free.
Well, it’s a 1994 Mercedes C-280 (the guy at the parts desk at the Mercedes dealership claims it’s a '93, and an independent mechanic claims it’s one of the prototype cars for the C-280 model, one of the first 30,000 or so manufactured anywhere).
Perhaps also Firestone, where I had the brakes worked on and which also does other auto repairs, might do it also…
Possibilities.
If it has one, it might be the idle air control.
Air leak. Check any rubber parts on the intake manifold system. May be some crack or loose connection. A vacuum leak can cause things to get out of balance. Usually shows up worst at idle or low RPMs. Smooths out more at speed.
There can be a whole bunch of air control valves, check valves etc. They do age and fail. The tubing can break and crack too.
An EGR problem is another possibility. EGR is Exhaust Gas Recirculation, and basically the engine takes part of the exhaust and puts it back into the cylinders to reduce nitrous oxide type compounds during combustion (it’s an emissions thing). Something like a bad EGR valve or some other problem in the EGR system often causes a rough idle that is typically worse when the vehicle is warmed up. So that matches up with the symptoms of letting it sit while you go into a store and having it work a lot better when you come out of the store again.
There are other possibilities as well. Any error codes from the engine computer would be very helpful in figuring out what it is.
Note that the ODB II error codes don’t necessarily tell you what is wrong. They only tell you what the engine computer thinks is wrong. It’s a useful diagnostic tool, but sometimes the failure isn’t exactly what the engine computer thinks it is.
Many times just preforming scheduled recommended factory maintenance at the various mileage intervals will resolve this type of problem.
That is replace spark plugs*, distributor/rotor, filters including fuel and air filter, change oil, etc.
*IMPORTANT NOTE: If Mercedes says to use a particular spark plug USE IT! Don’t try to re-engineer your car by using different parts. The sales person at an auto parts store is not a Mercedes engineer - follow the advice of the Mercedes maintenance manual instead.
Also Mercedes with Bosch Jetronic mechanical fuel injection systems are very reliant on the fuel pressure and volume being what it should (if your car has that system?). If all else fails, check the fuel pressure, volume, and check for internal fuel system leaks. Of course replace fuel filter. (In factory service manual if this applies to your specific vehicle.)
distributor/rotor,…? So last century.
Often this is because the engine temp is not reaching the desired level due to a faulty thermostat. Also possible, but less common, is a faulty temp sending unit (input to the gauge only), or on some systems a faulty coolant temp sensor (input to both the computer and the gauge).
Dopers may remember my thread about the trip to Riverside–in the car, on June 20. (The exterior temperature gauge, on the dashboard, read 108 degrees Fahrenheit when I got on the freeway in Riverside to come back, about 1 p.m.) For much of the 60 - mile trip back home the engine temperature gauge read 90 degrees (Celsius), but the engine did not act up; in fact, the only apparent effect was that the oil light went on the next day, so I added some oil. The light went out. The oil light lit one more time after that; again I added a little oil.
Wait a minute – when you said the gauge does not read high, do you mean it doesn’t get up to normal temp (what I thought you meant from the OP) or it doesn’t read excessively hot (what you seem to be saying here). Please understand that with a properly operating engine cooling system the gauge will read essentially normal (approx. 200°F, 90-95°C) regardless of the outside temp. Unless the symptom changes with the outside temp, said temp is irrelevant and mentioning it when the engine is at its normal temp is unnecessary and confusing.
Well, usually it runs at about 80 degrees Celsius.
From what I’ve seen lately, and advice by a former neighbor (who helped me find the car in the first place), I now suspect the spark plugs or their wiring, or perhaps both. Are the spark plugs readily accessible on a Mercedes C-280 (1994)?
If it’s been raining lately or very humid, spark plug wires, distributor cap, rotor, or even the coil could be the problem.
We have had very little rain–California is suffering from a prolonged drought. Rain has been quite sparse this year.
Yeah sounds like spark problems, perhaps just a hole in the lead’s insulaton. Or a spark plug come lose.
Inspect the leads and spark plugs.
Forgive me…I’m not sure what “lead” means as you use it here…please elaborate.
It’s Britspeak for spark plug wire. Also called a high tension cable.
Thanks 