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2phast 06-21-2003 07:31 PM

Running EHA in FOM to fix hesitation?
Car is 1992 190E 2.6 (m103 motor)

Alright, heres my dilemmna.....

Bad hesitation upon acceleration, goes away at WOT or by feathering the accelerator. Idle is fine and this problem does not happen when engine is cold only warm/hot. As outside temp increases, problem gets worse. No problems with idle or starting (either hot or cold)

This is what I have done so far....

Replaced O2 sensor, plugs (correct OE plugs), plug wires, rotor, dist cap, replaced all vacuum lines (no vacuum leaks) adjusted air/fuel and checked OVP.

Problem is not related to these components, diagnosis keeps pointing to EHA unit so I unplugged it to run in FOM mode. I am aware that this will affect cold start and cause the car to run rich, but right now the car runs better than it ever has, hesitation is gone, acceleration is strong and gas mileage has improved. This confirms that EHA is either bad or is receiving incorrect signal, so...

What are the long term affects of running the EHA disconnected (FOM mode)?

Since I am having trouble finding the item that is causing the EHA to reduce fuel (assuming that the EHA is not defective), anyone have any concrete idea what else (other than the items mentioned above) that could be causing the hesitation?

I have searched this forum and read every post about hesitation and the EHA, nothing I have found provides solid answers. So far, the work I have done was justified as standard maintenance but I don't want to throw money at this problem.

These are the other possible items that could cause this problem, please feel free to add comments if you have experience with this items causing hesitation.

1. Coolant temp sensor (b11)
2. Throttle actuator
3. Air temp sensor
4. Defective EHA
5. Defective ECU

I am tempted to replace the coolant temp sensor, as its not that expensive but I have not found enough information about this item to really know if it could be the culprit. The Throttle actuator is quite expensive, so I definitely don't want to replace this unless I can pretty much ascertain if its the problem.

As for testing, I am not equipped to test the majority of this items but if we can narrow down the possible causes, I will drop the cash down at the local stealership for them to test these items.

thanks for the assistance.

psfred 06-21-2003 09:16 PM

It sounds as if you are going lean on throttle actuation.

Check for vacuum leaks before jumping into the EHA. Also, you need to check the EHA current, it must correspond to the duty cycle or you may have a wiring or computer problem.

Certainly check the vac connection to the computer, if you have one -- bad info here will do funny things to the mixture.

2phast 06-21-2003 11:24 PM

I hear ya, no vacuum leaks, like I mentioned, all the lines have been traced and replaced. Of course I have not ruled out the possibility of the ecu being bad, just trying to narrow down the possibilites first.

If it was a vacuum leak, would it be correct to assume that disconnecting the eha and running it in FOM mode would have no effect on the hesitation? Seems to me that a device somewhere is malfunctioning and senind an incorrect signal to the ecu which in turn is telling the eha to cut delivery of fuel

BENZ TEK 06-22-2003 03:06 PM

Check your airflowmeter for bad spots especially around the idle zone.

mhingram 06-22-2003 11:58 PM

I am in the same boat on a 91 560, I believe mine is the air mass flow, my stall goes away with the eha disconnected but it is still hesitates and is rough when cold and a beast to start. But when warm it is almost perfect although still perceptable on acceleration.

I replaced the temp sensor although I knew it was un-necessary it only effects the start up and the manual provides a great test that is 100%. There is also an easy test for the throttle switch.

Make sure your duty cycle and EHA are in sync it is an easy test, and will help the narrow down the system issue.

Potomac Pat 06-23-2003 10:49 AM

I wasn't sure if you got my email or not, but here is the breakdown...................

sbourg 06-23-2003 11:37 AM

Like others have posted, I would suspect the airflow meter pot. You say thatyou feel the symptom is caused by leaning of the mixture from the EHA, but if it simply doesn't provide proper enrichment when you come off idle, it will be lean. This signal is from the pot. If the deposition has completely worn away from the pot where the wiper sits at idle, enrichment may come too late - or the ECU may simply be ignoring its input any more.

Removing the pot for inspection is very easy - four T-15 screws. Just make certain you first scribe an alignment mark across the top where the pot housing contacts the body - for alignment on reassembly.

Alternatively, pop the pot connector cap and check the voltage on the three pins at idle. I understand it should be 0.7 V measured to ground on pin #2 (center pin).


2phast 06-23-2003 01:07 PM

Ok, just so I am on the same page. The device directly under the throttle body is the throttle actuator (or so that is what MB says) is this the device were referring to? (aka air flow meter)

If were on the same page, then I will procede with testing of this device, assuming I have the necessary tools for testing.

Also, even with the EHA disconnected, I have no issues with the car starting, it starts right up. It does seem to be running a bit rich (as per the nose meter) but no black residue in the exhaust, on the ground or under hard acceleration. Will be checking the plugs in a few days for fouling though.

Never mind, just checked Pats picture, the throttle actuator is the air flow meter. Expensive item to replace. Is the point of failue here the air mass potentiometer? Anyone know what is involved in calibrating this item?

sbourg 06-23-2003 02:13 PM

You can checkout this thread:

Note that the mechanical assembly path for the intake is air cleaner - airflow meter - throttle body - intake manifold - intake port. The large plate in the airflow meter is in no way linked to the throttle - it moves as the engine demand for more airflow causes it to move, and the throttle plate position is only one factor.


2phast 06-23-2003 05:15 PM

Thanks for that info and link Steve. Will check into getting a unit from your source.

Also aware that the device is not actually related to the throttle, the term "throttle actuator" is what the local stealership used, not a term I came up with. Although it did confuse me when they told me, as I was trying to figure out what kind of actuator was conntect to the throttle linkage haha!

mhingram 06-23-2003 05:42 PM

I order one from olympic, but am still having problems losing the current right at idle so it I have a new airmass flow from ebay on its way to test it.

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