560SEC surging/modulation
 

Hi guys. My new to me 1990 560SEC has a surging issue while going down the freeway. It is more noticeable the faster it goes, starting at about 65-70mph and getting worse at 80 or 90.

When I push on the throttle it accelerates more than it should, and when I lift it drops faster than it should. Making it hard to modulate.

Worse on cold days. It has been in the 30's lately and it is more marked then.

It feels almost like a wandering vacuum advance, a fuel pressure issue or sticky injectors. Is there something that needs cleaning? A sticky solenoid?

Plugs/wires are good, rotor/cap looks OK. I have run about 1/2g of sea foam thru it, and a quart of mystery oil.

Thanks!

Are you certain it has up-shifted all the way?

Wait wait wait !!!

I got it!

Kickdown switch is stuck or needs to be cycled? :)

It is not a transmission issue, the trany shifts fine.

It is like the throttle is overly sensitive.

Even w/ the cruise control on, it either has too much throttle or too little. The car surges forward or falls back.

Also, when it is acting up, the vacuum gauge sinks farther into the red than usual. Give it a tiny bit of throttle, and the gauge swings into the red. So I am thinking a vacuum issue.

This is an intermittent thing, worse on cold days.

If the car was delivered outside the USA, there is an outside chance that it is equipped with ASR (anti slip, aka traction control) and that that circuitry and the throttle actuator is misbehaving.

If it is a US model, first try removing the link between the cruise control actuator and the rest of the throttle linkage (pop off two ball ends). then check that the throttle linkage moves smoothly thru its range. Drive with the cruise control link removed. Any improvement?

Thanks for the reply.

The car does not have traction control (And who doesn't appreciate a little wheelspin now and then?).

I checked the linkage the other day, it felt smooth enough. I might grease it this weekend. The car does run like it has a sticky linkage (on/off nature of throttle) but it doesn't drive like it, if that makes sense. Everything feels smooth physically and mechanically.

My CIS 928 has a WUR that occasionally would act up. I ended up modifying it to be externally adjustable. Is there something similar in the J jetronic system? Cold start valve sticking? How do I check that?

I have a half a mind to just inject some Mystery Oil into every vacuum actuated device under the hood to loosen things up.

Also, I have heard weird stories about what a failing OVP can do... does this sound at all plausible? Really it doesn't feel electrical in nature, but who knows. I could switch it out w/ the one from my wife's 300SE, but she would kill me if she found out.;)

The car didn't act up at all yesterday, but it wasn't too cold. The intermittent nature of the problem makes it tough. Car either runs fine the whole commute, or acts up the whole commute (20miles one way, 90+ mph at 4:30AM)

From the previous post. Did you try driving without the CC link?

"If it is a US model, first try removing the link between the cruise control actuator and the rest of the throttle linkage (pop off two ball ends). then check that the throttle linkage moves smoothly thru its range. Drive with the cruise control link removed. Any improvement?"

Why would that be only for a US model?

EN:

I've been approaching your problem from the standpoint of mechanical control of the position of the throttle plate.
In rereading your original post, it seems that you may be describing a change of mixture (or possibly timing).
The primary control of mixture in the KE-jetronic system, as with all K-jets, is mechanical, via the airflow sensor plate and the control plunger. Secondary control in KE is via the O2 sensor, the ECU, and the electro-hydraulic actuator (EHA). The current to the EHA is also modified by air temp, barometric pressure, water temp, vehicle speed, rate of throttle opening, and more, but those adjustments are small and gradual.
It is possible that the feedback signal from the O2 sensor to the EHA is causing a large enough change in mixture to be felt as surging.


INSIDIOUS:

The reference to US models is in my first post, and was to distinguish them from ROTW cars that might have been equipped w/ARS. ARS actuator linkage differs from USA cruise control linkage.

Thank you.

I will poke around the O2 sensor (I think I have a spare one lying around), and will google up the EHA and see what I find.

There is an occasional high-idle issue, where the car idles at about 1000 when warm (no AC engaged). Probably not related though.

EDIT: I think I have a EHA off my wife's '88 300SE lying around as well, I was trying to fix a rough start issue. Will it work on the 560, or will it have different settings?

Eric, you might check the action of the throttle bore air valve with the engine off; be sure it's moving smoothly throughout it's range, and that it retains resistance to downward movement for awhile after the engine is stopped - that's a quick check for retained fuel pressure. Also, a check of the O2 duty cycle via the round diagnostic pins 2&3 might give a hint towards the problem. (if you haven't done this, do a search herein for exact procedure using a Sears meter). Check it with the engine cool, and well warmed up, at idle and at 2500. Then post results and see if it's abnormal, and what to do.
DG

I unfortunately have come down with a nice case of H1N1. I can't seem to stay vertical for over 10 minutes, let alone work on the car. :puke:

Will post results after I get better.

Thank you all for your inputs so far!

I am becoming more convinced that this is a solenoid-operated vacuum issue. The car will be down on power when it acts up, the vacuum gauge will swing much more than normal with small changes in throttle position, and the transmission will hold a lower gear longer (as if it thinks I am giving the car more throttle than I am). It does not smoke or smell, which would indicate a too rich mixture. I think it is simply allowing too much air into the mixture, leaning it out.

A few times, the car will spontaneously switch to normal and stay that way, after it has warmed up well. It has done so a few times driving down the freeway, with no change in pedal position. The car just kind of "clicks" and starts accelerating, then I look down and notice the vacuum gauge is normal again.

So I am thinking this is an airflow issue, instead of a fuel issue.

Anyway, I will look into it more this coming week.

Thanks guys!

I marinated the car for a few days with Sea Foam sucked down the brake booster line, and shut off before it was burned. No change.

The vacuum lines are brittle but seem intact upon inspection. The car holds vacuum well after being shut down.

Problem is worse on cool mornings, less during warm afternoons.