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Old 01-31-2002, 03:56 AM
Posts: n/a
Question EHA confusion

Hi All!

OK....I've been following all of the threads on hesitation/rough idle problems.

I appear to have the "classic" symptoms". Cold start rough idle, poor acceleration from stop, etc.

This evening in an attempt to do some troubleshooting without the use of test equipment, I warmed the car up - drove perfect with one exception (hard stop followed by poor acceleration) then disconnected the EHA valve.

Car started very rough and any and all acceleration was very poor. It appears that my condition got worse.

So, the question is (without the use of any multimeters, etc), is the EHA valve functioning correctly?

I did notice some weapage on the underside of the EHA valve from the 2 holes in the valve.

From previous posts by others I have the impression that by disconnecting the EHA valve, conditions improve, thus there is a potential problem with the EHA valve.

I do acknowledge all of the recommendations previously posted for continuing troubleshooting directions. I've already changed plugs, rotor and cap and will seriously consider the ignition coil. The coolant temperature sensor checks out ok, although the temperature meter sensor is questionable.

Thanks for any and all input.


1991 300CE 108,000 miles
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Old 01-31-2002, 08:25 AM
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Electrons can do anything
Join Date: Sep 2001
Location: Houston, Texas
Posts: 1,071
The purpose of the EHA is to adjust the mixture of the engine and keep it at the lambda point. Lambda is where the air fuel mixture is at its optimum point. There are a lot of variables that cause the engine not to operate at this point so the oxygen sensor is the device that measures the oxygen content of the exhaust gases and the engine managenment systems adjust the air/fuel mixture to keep it at lambda. The EHA has a limited authority as to how far it can affect the air fuel mixture. So if the engine basic adjustments for air fuel mixture is too far off or there is some other problem the EHA cannot compensate. You should first disconnect the EHA and perform all the basic adjustments to be sure that the engine is as close as possible to being perfectly set without the lambda system operating. Then connect the EHA and that will insure that the EHA has as much authority as possible to keep your engine at the optimum operating mixture ratio.

In theory disconnecting the EHA should not change the engine operating characteristics . The fact that it does indicates that it is having to work to keep your engine in lambda and I would conclude that your engine needs to be adjusted. When the EHA is connected a closed loop control system is now adjusting the air fuel mixture and will compensate for any adjustments that are not correct. Also the fact that the valve is leaking fuel would indicate that it needs to be replaced.

I hope I have not confused anybody. My explanation of the lambda system is an attempt to help provide an understanding of what is really going on with the EHA and is purpose in life.

Any car that has an oxygen sensor is using this basic closed loop system to maintain the air/fuel ratio. Different injection systems have different methods if controlling the fuel delivery. But the principal is the same in all cases. In your case the EHA is the device that changes the air/fuel ratio.

Before you start the shotgun approach and start replacing parts, I would do the basic adjustments and go from there. Look at the on-off ratio and that will tell you a lot about the mixture settings.
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Old 01-31-2002, 09:36 PM
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Location: Youngstown, OH
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By disconnecting the EHA you have set a default/preset Fuel:air ratio. In other words there is NO fine-tuning of the fuel:air ratio for various engine loads, similar to an old mechanical fuel injection.

This fine tuning is directed by the computer which gets input from many sensors, like the O2 sensor, engine temp, etc. You've done some isolating of your problem. If your problem was solely fuel:air metering it probably would have smoothed out your hesitaion during acceleration (rough starts with an other-than-WARM car is NORMAL with the EHA disconnected since there will be no start enrichment; that's one of the functions of the EHA). I think you ought to look at the weeping (of fuel, I assume) from the EHA.

The EHA alters the fuel:air ratio by altering the fuel pressure between the upper and lower compartments of the fuel distributor, A leak at the EHA will probably make ANY further trouble shooting invalid. I suggest you replace the two o-rings between the EHA and the fuel distributor.

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Old 02-01-2002, 05:25 PM
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Thumbs up

Understood. Have ordered the o-rings already. Checked the ignition coil yesterday (resistance), both the primary and the secondary read OK. I am going to continue troubleshooting tomorrow when the o-rings arrive.
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