My 72 sl surges when the engine is cold - PeachParts Mercedes-Benz Forum

Captain sl · #1 05-30-2004, 08:02 PM

It stops after 5-10 min.
Any advice?
Thanks!!!

Greg in Oz · #2 05-30-2004, 09:59 PM

A search should turn up some info for you. A common problem with the D-Jetronic V8s. The M116 was very prone to this. It is caused by the engine idle speed (or engine speed on overun) reaching the fuel cut-off speed. Revs then drop suddenly until fuel injection resumes, thus repeating the cycle. When hot, engine speed is lower. Many faults or a combination of faults can cause this. Commonly it is the auxiliary air valve allowing too much air when cold or not closing down soon enough. Vacuum leaks can also contribute to the problem. An easy fix many use is to place a restriction in the hose leading from the throttle body to the auxiliary air valve which results in a lower cold idle speed. Many resort to this after even a new auxiliary air valve does not cure the problem. You can experiment with different restrictions to achieve the desired result. For mine I have a restriction with only about an 8mm (5/16") diameter hole. Just make sure that whatever you use cannot be ingested by the engine! I believe a better fix (that should have been designed in originally) would have been to disable the overrun fuel cut-off until the engine approached normal operating temperature.

cth350 · #3 05-30-2004, 09:59 PM

A 3.5? (the yellow injectors), not a 4.5 (the blue injectors)?

there is a bug in the 3.5 circuit, that when fast idle is over a certain threshold, the box makes a mistake and lowers it, which of course, has it boost it, to make the mistake all over again.

What to do about it is in the archives here and at the veteran's list. Also, Frank Mallory would remember off the top of his head, but he's gravely ill, so I'll ask, but I may not get an answer.

-CTH

Captain sl · #4 05-31-2004, 01:06 PM

Will share this info with my mechanic.

Tomguy · #5 05-31-2004, 07:19 PM

Is it a 3.5 or 4.5? And when you say cold, how cold are we talking? Or, perhaps this is a 6-cyl?

Captain sl · #6 05-31-2004, 10:10 PM

Hi Tom, I am pretty sure its a 4.5, it surges right after I start it, lasts for 5-10 min. until engine is warmed up.

Tomguy · #7 05-31-2004, 10:53 PM

My guess is a misadjusted throttle position sensor. It isn't closed in the "idle" position and someone used the air speed screw to adjust the idle instead of the ECU. The engine gets too fast and because the TPS isn't saying it's in the "idle" position, it assumes you're off the throttle so it cuts fuel until the engine slows. This repeats itself until it warms up and the engine comes off cold mix and doesnt race.

To check your TPS: with the engine off and the air cleaner off, you can see it on the throttle body. Pull the plug. The pins are marked 9, 20, 12, and 17. Check the resistance between 12 and 17, it should be close to (if not) 0 ohms (closed circuit). If there is very little or no resistance (infinite/open circuit), then it needs to be adjusted.

To adjust: Loosen the (two) screws and turn it CW until it moves 1 mark on the indicator. Tighten it a bit, move the throttle a bit, and test again. Keep going until it measures resistance. If it doesn't ever measure resistance, you need to replace it. Once it hits 0 ohms (it might measure like 0.8 or something) then open the throttle a bit, it should measure no resistance. Then open it a bit more and close it slowly, the point at where it first measures 0 ohms can be a little before fully closed to save some gas.

Captain sl · #8 06-01-2004, 01:44 AM

Hi Tom, Thanks for the advice!

psfred · #9 06-03-2004, 01:06 AM

Does it surge while driving or while idling? Usually the cause is too high an idle speed (you will get fuel cutoff at 1500 rpm so the engine races, dies, races, dies, etc). Lurching and lunging at light throttle, on the other hand, is bad trigger points.

They usually check out fine with an ohmeter, but you will discover that the rubing blocks are worn. This results in some of them not opening reliably when they are cold since the rubbing block is short. When the dizzy warms up, they start to work properly.

Since failure to open and close the points results in no fuel injected on two cylinders, it surges as they go one and off until the dizzy gets warm, takes about 5 minutes usually.

My christams present to myself this year is a new set of trigger points for the 280.

Worn dizzy shaft bushings make this worse, by the way.

Peter

Greg in Oz · #10 06-03-2004, 01:35 AM

Quote:

Originally posted by Tomguy
My guess is a misadjusted throttle position sensor. It isn't closed in the "idle" position and someone used the air speed screw to adjust the idle instead of the ECU. The engine gets too fast and because the TPS isn't saying it's in the "idle" position, it assumes you're off the throttle so it cuts fuel until the engine slows. This repeats itself until it warms up and the engine comes off cold mix and doesnt race.

Just a slight correction here if I may. If the idle contacts were not closing then in fact you wouldn't get the surging or hunting. The reason being that until the idle contacts close, the ECU can not go to fuel cut-off (above the required engine speed). It is the continual cycling above and below the fuel cut-off speed that causes the surging or hunting when the idle contacts are closed. This condition actually indicates the idle contacts are closing (ie. functioning correctly). You can demonstrate this by very slightly opening the throttle (such that the idle contacts open) when the hunting is occuring (either stationary or on the overrun while driving). As soon as the idle contacts are opened the hunting ceases and the engine remains at a high idle speed. Incidently, only auto transmision versions will do this on the overrun due to the torque convertor allowing the engine speed to vary without road speed changing. The reason that the hunting ceases once the engine warms up is that the idle speed is then sufficiently low not to reach the fuel cut-off speed. Something I am tempted to try one day is to fit a temperature operated switch that will only bring the idle contacts into circuit (ie. fuel cut-off) once the engine has warmed up.

Captain sl · #11 06-03-2004, 11:47 AM

Bringing it to the shop today, will keep you posted.

Tomguy · #12 06-03-2004, 09:31 PM

Greg: If that's the case, I'm assmuing that someone used the air speed screw to adjust the warm idle faster (opening it more) and then it lets too much air in on the cold engine to cause this?

I was thinking that the fuel cutoff occured when you were off the gas, not nessecairly at idle. If you look at the switch, it can tell if it's being accelerated or held at a consistent speed, or if the throttle has been let off and it's coasting, before it hits the idle - I thought this triggered the fuel cutoff?

So if the former is the case, screw the air speed screw in about a turn, maybe more, when it's warm, and adjust idle mix with the ECU screw and NOT that screw (if possible, screw the air speed screw in even more, if the ECU leaves the room for adjustment. It's supposed to be max speed @ max vacuum, but my vac doesn't change as I change the screw).

Captain sl · #13 06-15-2004, 02:09 AM

Hey guys, My mechanic caped off the Aux air valve and the car runs great! No more surges!
Thanks for all your help!!!

Mark

cth350 · #14 06-16-2004, 08:03 PM

What do you mean by "Capped off"? -CTH

psfred · #15 06-16-2004, 10:02 PM

Now you need to find and fix the vac leaks that actually caused the problem. Check ALL the idle air hoses, check the brake booster and hose, and the door lock system (and tranny modulator) for leaks, and test the seals between the upper and lower half of the intake manifold. A common leak point is the "Y" hose between the idle control screw and the intake manifold, it cracks in several places.

You will not get fast idle with the aux air valve plugged, and this will cause cold running problems in the winter on top of not correcting the actual problem.

Peter