88 420SEL Surging During Acceleration - Page 3 - PeachParts Mercedes-Benz Forum

dpetryk · #31 06-06-2007, 01:06 PM

I agree with Dave's diagnosis on the fuel distributor. I see alot of people fighting these idle, rough running, surging, hesitation problems. I have gone through it all and it has turned out to be the fuel distributor in 3 out of my 4 cases. I hate to suggest it because it is so expensive to replace and you want to be sure all of the other issues have been checked first.

Also I have discovered a mystery about the famous idle control valve. I have had a number of these things not work correctly or have the idle slowly creep up. I installed a new one and it fixed the problem. I wanted to understand what failed in the ICV. I then checked the bad one on the bench under controlled conditions by driving the coil with known voltages and currents. I compared the physical movement of the plunger in both the new and old one and could not find any observable difference between the two. So why did it not work? The mystery I now believe is known.

It seemed like the bad valve did not have enough "authority" to close off the air supply enough to make the engine idle at the design point. The valve would do it if it could get enough current. But the control module controls the current and there were no adjustments there so I concluded the valve must be at fault even though I could not find any measured differences between good and bad valves. So the differences must be very small.

It turns out that these things are adjustable. There is a spring that is factory set - but adjustable, and is used to vary the pressure on the valve actuator electromagnet. I figured that if I could reduce the spring pressure then the actuator would have more "authority" for a given amount of current passing through the coil. I adjusted my "bad" valve and it worked perfectly.

Here is what you do;

Pop a small cover off the center of the plunger. You will see a small threaded brass insert in there. Find a screw to fit the thread (I could not find the right one) or as I did, use a screw of the appropriate size to "get hold" of the brass piece. I think I used a 10-32, which grabbed the threads nicely. If the Idle is too high, which seems to be the majority of the cases, pull the center piece out a little bit. Test and re-adjust if necessary. Don't move this thing very far. The balance for optimum performance is very narrow. If you go too far the idle will be too low, just push it back in.

Give it a try and let us know if it works for you.

just-n-time · #32 06-07-2007, 10:30 PM

Great to know I have one and will give it a try mine was to high also.I bought one from another member (used) and it fell to 750 static just wright.

RC2007 · #33 06-07-2007, 11:24 PM

Actually, the idle is just right about 750rpm. The car drives just fine now when you slowly accelerate but when you floor it or need to accelerate quickly it hesitates terribly.

saumil · #34 06-08-2007, 01:37 AM

Did you measure the vaccum pressure ? What is it at idle ? Get a simple vaccum gauge. Low vaccum at idle can indicate problems with the catalytic converter if there are no leaks, it could be clogged. Clogged catalytic converter will result in lack of power and thus sluggish acceleration. Check the resitance of the EHA, should be low, if high then the EHA's coil is blown and you will need to replace it (will also need its gaskets). EHA is incharge of providing the fuel enrichment needed during acceleration. Does the speed increase proportionally when the RPM increases ? If not, then there could be some problem with the transmission also (slippage could occur in a variety of driving conditions, specially on non-freeway type driving), check transmission oil, may need to be topped off.

brandob9 · #35 06-10-2007, 12:07 AM

Look into the Water Temp Sensor. I am making the assumption that your 420SEL runs CIS-E like my 300E does. It's a commonly overlooked item that can cause the engine management to run the wrong fuel mixture.

When I pulled mine it turned out to be almost crimson from the corrosion, while the new ones are brass. After replacing that part my practical rev limit moved from 4500 to 6000+.

burton · #36 06-10-2007, 03:22 AM

Verry nice post! I want to mention that I have similar problems with my 86 420 SEL. Short history, for over 1 year had rough idle problem, and the the idle was high too aprox. 900-1000rpm, I changed every thing (ruber part) in the intake including fuel injectors, all the ignition parts and stil rough idle, until finely replais fuel distributor and now is smooth idle. But stil 1000 rpm. When engine is cold, idle seems to be normal~750 rpm, after few minutes goes high 1000rpm even higher. I checked throtle position switch, engine temperature sensor, if I disconect them idle speed goes higher. I chenged idle speed solenoid with one from my 88 560 sel, and stil did not fix the problem. I assume the solenoid is ok because the 560 sel runs smooth. I chenged the idle speed control box from 560 sel to 420 sel (they look similar but part no. are different) and still high rpm. Anyone can tell me if this control boxes are intercengeble from this two cars? Finely, I have this idle speed solenoid from an 84 380 se, is smaller then 420, but I"ve tryed it anyway. For some reason the idle speed is ~750 rpm and smooth! I'm puzzled and don't know what could be wrong! I appreciate for any help!

dpetryk · #37 06-10-2007, 09:02 AM

Read post #31 and follow the procedure outlined. It works.

burton · #38 06-10-2007, 04:14 PM

after two fine adjustments it works!!! Finely I can take the car for a drive test. Thank you very much for the input! By the way, what's the explanation an idle speed solenoid works in one car and not in the other? I'll keep you posted if enything comes up. Best to you all!

RC2007 · #39 06-14-2007, 07:21 PM

Just an update. I haven't measured the vacuum pressure but will. The catalytic converter is new and does not leak, the transmission has no slippage, shifts perfectly, just like new. I did replace the Oxygen sensor with a new one, it appears that it was the original one installed in the car. The interesting thing that I did notice is when I first start the car and drive it, there is no sputtering at all even when at full throttle, this hesitation and sputtering starts and gets worse as the temperature of the vehicle gets up to normal levels. Now I am really getting even more curious as to what this could be. Again, thanks for all of your help.

dpetryk · #40 06-14-2007, 07:28 PM

Have you tried the fuel distributor?

RC2007 · #41 06-14-2007, 07:31 PM

Not yet, I will be doing that soon.

saumil · #42 06-14-2007, 07:42 PM

Someone had mentioned the coolant temperature sensor, also called the fuel injection temperature sensor. It informs the fuel computer about the temp of the engine so that the computer can adjust the mixture accordingly. It is the last sensor on the engine, one closest to the windshield in my 87 260E. With engine off, disconnect the connector from the sensor, the resistance of the sensor should be about 2.5 to 3.5 Kilo Ohms at 20 deg-C (these number are for my 260E but should roughly apply to you also). Then run the car, get the temp of the car to about 80 deg-c, then turn it off again. Measure the resistance across the sensor terminals, it should be about 250-350 ohms. The resistance should drop by about 8-10 times as the temp changes from 20 deg to 80 deg. If this does not happen, replace this sensor.

Do you have a mechanical or electronic fuel injection ?

RC2007 · #43 06-14-2007, 07:44 PM

The thing that really puzzles me is that when the car is cold, it runs just fine. Is this an indicator that the fuel distributor could be a bad?

RC2007 · #44 06-14-2007, 07:52 PM

Are you referring to the sensor that is located I believe on the thermostat housing? I think it is blue with two prongs.

saumil · #45 06-14-2007, 08:20 PM

I dont know the layout of your car, on my 87 260E, it is the sensor on the engine block, on the intake side, and it is the last one, the one closest to the windshield. It should have two terminals. On my car it is close to a metal part from where one lifts the engine. It was very difficult for me to do it myself, I had to get that one done by a mechanic, just did not have the right socket.

If the car works fine for 10 minutes when the temperature is going up, it is less likely to be the fuel distributor.

It could be some other fuel component but first check the temperature sensor.