Power ONLY at Full Throttle - PeachParts Mercedes-Benz Forum

#1 01-09-2003, 07:35 AM

Well, I have run through everything except plug wires and coil wire on the secondary side of the ignition, but my intermittent problem is back.

This started several months ago and when I described the problem Steve Brotherton said it is very likely secondary ignition, maybe coil or rotor. I replaced the rotor and cap and coincidentally replaced the plugs. It ran fine for several weeks and then damp weather came seemingly the cause of the problem to return.

The damp weather kept leading me to believe it was a secondary ignition problem.

The behavior is this;

It never seems to happen when the car is first started up from being cold.

After warmed up, sometimes 10 miles, sometimes 60 miles, it starts losing power almost like you turned off the ignition, then once the throttle pedal reaches the floor, sometimes very close to the floor, it takes off with full, healthy power.

I now have replaced; plugs, coil, rotor and cap. Every time I replace something it seems for awhile it is fixed, but then it rears its ugly head again.

For historical information you can go to my thread of several months ago entitled: Bosch KE Problem in this forum.

In that thread Steve Brotherton said I could disconnect the EHA to see if it has any effect. The thing of it is, I would have to drive it several hundred miles to tell.

I'm really at the end of my rope with this one. Any help you can offer will be greatly appreciated.

I appreciate any thoughts you may have.

Have a great day,

#2 01-09-2003, 01:14 PM

A thread has been started by evoboy entitled "A not funny problem..." It seems that we both are experiencing the same unusual symptoms.

Please help,

stevebfl · #3 01-09-2003, 03:28 PM

It does look like we are almost beyond simple testing.

I think that I would want to see the fuel pressure while the event occurs although the fact that the thing improves at full throttle screws with most fuel delivery senarios.

When you floor it and it recovers, is it the kick down that causes the improvement?

#4 01-09-2003, 03:55 PM

Symptoms sound like an intermitent O2 sensor problem, since at cold start it is closed loop, than at full throttle it would go into closed loop again.
Would not hurt to scope it.

#5 01-09-2003, 03:56 PM

Steve, thanks a MILLION for the response.

No it's not the kickdown, this is a manual transmission car.

I have a fuel pressure gauge, but I will have to research where to connect it on one of these systems. I will investigate what it will take to rig a fuel pressure gauge that I can read from the cabin.

Does the Bosch KE have an open loop and closed loop mode? I am very familiar with this on the electronic injection cars with individual electrically operated injectors, but with the mechanical fuel distributor, I didn't think the O2 sensor operated in such a way. The KE is a much different animal than my '88 Vette with which I am much more familiar.

One other clue. Sometimes when this is happening the engine idles normally and pulls away from a dead stop normally, then acts up when the car is at cruise. Other times it idles so bad that it almost dies and has lots of trouble getting untracked. Again, this is a manual transmission car.

Thanks again,

stevebfl · #6 01-09-2003, 04:49 PM

The O2 sensor is used as input to keep mixture in check. The mechanism of fuel metering changes but the act of the O2 sensor is the same for most cars.

The difference between open loop and closed loop just refers to whether the system is in control. On your car if you are worried about it then get it warm and disconnect the EHA or the O2 sensor - should give the same results.

ctaylor738 · #7 01-09-2003, 06:42 PM

This might have something to do with the way the control unit is interacting with the throttle valve switch and/or the deceleration shut-off microswitch. The throttle switch has idle, run and full throttle positions. It shares a common ground with the control unit and the deceleration microswitch which is also an input to the control unit. Plenty of room for mischief.

So maybe you are driving along in closed loop there is a malfunction in the EHA. You floor the throttle and the throttle switch kicks the system out of closed loop and somehow the EHA gets its act together. Or maybe the decel switch is sending a signal telling the control unit to shut off fuel. Then full throttle grounds tells the control unit to stop this nonsense. And maybe all of this is caused by moisture?

I am struck by the coincidence of the "full throttle cure" and the full throttle switch in the injection system.

Richard Wooldridge · #8 01-09-2003, 11:16 PM

Hi Larry,
Sorry to read that you're having troubles! You might also think about connecting a high impedance voltmeter to your O2 sensor and monitoring the voltage while you are driving - you would surely see at least the result of what's happening as far as a rich/lean condition goes. It sounds to me like the engine must occasionally starve for fuel, and the function of operating the full throttle switch gives it enough fuel to run again.
By the way, I'm thinking of removing the dual rochester side drafts on my '77 Jag XJ6 and installing TBI from a 4.3L engine in their place, just to have a better running vehicle. (The Jag really starts hard in the winter with no real enrichment system)
Good luck on the problem!
Richard Wooldridge
'82 300D/4.3L V6
etc...

#9 01-10-2003, 06:36 AM

Richard,

Once you pull off the sidedrafts, I'll pay the shipping if you'll send them to me. I'm ready to put carburetors on this thing to keep from having to troubleshoot an intermittent problem.

I drove to the house last night and back to the office this morning, a 180 mile round trip. The car ran perfectly all the way except for roughly idling at a stoplight that is in a little town about halfway home. The engine made power fine while pulling away from the stoplight.

Yes, I could monitor O2 voltage no problem. The connector is even in the passenger compartment. In fact, this makes it easy to try Steves suggestion of disconnecting the O2 sensor after warmed up. No one rides in this car with me, so I will pull up the passenger carpet and I expect I can reach the O2 connector while driving.

I have measured O2 sensor voltage before. I welded a fitting in the exhaust pipe for an O2 sensor on my '78 Ford big engine 4X4. I then ran a wire to a test jack underneath the dash. I can connect a multimeter and monitor the voltage while driving. It was very useful while "dialing in" the carburetor. This was a very inexpensive mixture meter. I got the idea when I saw a mixture meter sold by Crane in the Summit catalog. It was easy to see that all it was doing was reading the voltage from an O2 sensor, so I improvised.

Thanks for all the suggestions. Please keep them coming as I work through this mess. It's a good thing I'm already bald or I would end up pulling my hair out over this one.

Have a great day,

BobK · #10 01-10-2003, 08:00 AM

I think the idea of the full throttle switch is very interesting. KE-Jetronic uses a valve to modulate fuel presure when you are in closed loop mode. That is how they fine tune the mixture. For a simple test, I would hook up another switch to mimic the throttle switch, but have it in the cabin by you. When the car begins to act up you could just hit the switch without stepping on the gas any harder. This would limit how many things you are changing at one time and help figure out where the problem is. Having the meter to monitor the O2 sensor at the same time could be real instructive. I love K and KE Jetronic systems. Only thing I hate about them are the prices of parts.

#11 01-10-2003, 08:16 AM

Larry,
During my surfing for information on things K-jet , I came across this company http://www.interject.com.au , they have some interesting test and calibration equipment for K-jet and also some interesting info. The link takes you to the home page and just look for the K-jet button.
Of course , you may already know this site , but it may be useful to others.

L8r

stevebfl · #12 01-10-2003, 08:31 AM

You all place too much stock in the capability of the electronics to make these mood swings.

The absolute best way to monitor this car's fuel control is to view the EHA current. Everyone that owns a KE car and wants any info about control should creat the proper test harness. Once wired the test can be done with little effort. The hardest part is removing the aircleaner.

What you must make is a harness with a male and female connection that allows you to unplug the connector at the EHA. One part of your rig will plug into the EHA and the other back into the connector you removed. The harness is wired such that the two wires that jumper the circuit have one wire broken and wired to plug into the ammeter sockets + and - . This is so that the current flowing must pass through the ammeter.

Make the jumpers long enough that you can view the current from the drivers seat. The added capacity of this testing is that by pulling one of the + or - leads you have opened the circuit and have assured zero current (which is the limp home position - coincedentally).

Now once you are monitoring you will notice that it will take a huge number of ma to make much difference. The only instance I can imagine would be the equivalent to decel cut-off. In decel cut-off the current goes to -60ma. This shuts off the motor by dropping the differential pressure to zero. This is easy to view. Go watch decel cut-off..... When the event occurs it will take a similar current (I would presume at least 30 negative ma) to get the drop in performance I am imagining from your description. If the current isn't close the elctronic fuel control is not the problem. You will notice that all other driving when warm never causes more than a 10ma change. The greatest change occurs on quick full accel. A combined effort of full throttle and rapid movement of the airflowmeter position potentiometer.

Get some perspective by viewing this, watch the road. All aspects of electronic control including closed loop can be monitored. I wish all aspects of control could be monitored so easy as KE.

If you need more info on this jumper/break-out lead send me a fax number and I'll draw it out. A picture of its use on a cheap RS meter is in my DIY article.

Anyone making diagnostic decisions on this system needs to do this testing. Not only is it the easiest test imaginable, it gives one of the best systematic views of control I can think off... everything in plain view.

#13 01-10-2003, 10:53 AM

Steve,

I will make such a harness and start studying the current when the car is running well, then it should mean more to me when it acts up.

I did a little reading last night and understand a little more about the theory.

Your time and effort is greatly valued.

Have a great day,

Mike Richards · #14 01-10-2003, 11:16 AM

Larry:

If you like, you could buy one ready made, part # 102 589 04 63 00 07. Part available from this sites Part Shop - $18.00. Photo in this post:

M103 EHA/Lambda Adjustment HELP