My 1988 420sel has just developed a problem where when I am accelerating at lower speeds the engine seems to sputter and surge. High speeds are not a problem but it seems to hesitate and surge all the time during acceleration. The spark plugs and wires are all new, car has been fully serviced and tuned up. Starts right up, idles at 1000rpm when parked 7500 when in gear. Any ideas? Thanks

For the idling problem most commonly it's the idle valve.

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find out if you have a vacuum leek! it will cause you this type of problem till you reach a higher rpm where the loss of vacuum is no longer a problem.JNT

That is the funny part, I always keep the idle control valve clean and did clean it again just incase, didn't change anything. I have traced all the vacuum lines, nothing out of place. I was thinking perhaps this might be a problem with the fuel supply?

This symptom can be caused by the airflow plate position sensor. It is a pot (variable resistor) mounted on the passenger side just below the fuel distributor. They wear out and get noisy. This device is used to tell the computer to add fuel when it senses acceleration or movement in the air flow sensor plate. Much like the way carbs used to squirt fuel into the engine when you stepped on the gas.

They used to be available but I think the source has dried up. There are other threads that discuss this device.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

I took it apart and cleaned it out, no change at all. I am thinking Fuel Pump may be going? Has anyone had any experience changing it on this type of car?

This thing is electrical it operates as an electro magnet, just cleaning it will only make it look good. Test the pulse settings to see if you are close on electrical flow,if it gets good current, If so and it still will not let it idle at proper speed,Replace it, you can get a used one from another member.JNT

Wait a minute, are you talking about the air mass meter or another part that may be causing the problem? I am starting to think that I was working on the wrong part.

Throwing money at it? Do the electrical test of the pot and the EHA and o2, If you think you are in fuel flux then use a tester to see if the upper and lower chambers of the fuel distributor are at proper value,I would also suggest you look into the dampener,lots of post on all of these subjects. JNT

If any of you have a picture, that would be very helpful. By the way I drove the car long distance today, performed amazing on the highway and in the city it was sputtering something terrible on acceleration.

I thought you were talking about something else. I tested all the vacuum lines again and found that there is no leak. The Idle Control Valve when unplugged causes the idle to race, I am not sure what further testing I can do. I really have no idea where to start on that. Is is possible that there could be another simpler problem asides from the fuel pumps? My mechanic tells me that it sounds like the fuel pumps are going so I am not sure what I can do.

Could this be caused by the idle control relay and is it possible that these parts are causing the sputtering at low speeds.

Idle control valve,you said you cleaned it! OK, now after it warms(Eng.) what kind of idle do you have? Next it is of no use to clean the potentiometer, thats the electrical device on the right side of the air mass meter. That one little toy is what tells the eha to richen the fuel in a manner so that you do not have the sputter. In a carburetor-ed auto it would be called the accelerator pump,it gives the engine the little little boost of fuel. your system is not working.If you go to posts in my name you will find pictures and info from other members who helped me threw this very problem.NOW for the bad news that part seams to be discontinued,you may find some one who has one that is still good, Or you may have to replace the entire unit, if so I would try to locate one from a salvage yard. JNT

Thanks for your reply. I did clean it out again just to be sure about it. When in park or neutral it idles between 1000 and 1250 rpm. When in drive or reverse it idles at about 750 rpm. So I need the potentiometer? I will have to start looking for one. The car is amazing for an old 300k mile beast, I have no problems at all that I haven't been able to fix asides for this which has to be the most annoying thing I have ever dealt with. Hopefully we can get it fixed up.

Go to posts in my handle and you will find pics of intake job as well as some other work,as Dave said you would benefit greatly by replacing all of the rubber parts in the intake area,your idle is far to high cold start 1000 then settles to 650 or 750 tops.so if the intake rebuild dose not bring this down then you most likely will need to replace the icv. JNT