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Old 07-29-2000, 03:36 AM
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Join Date: May 1999
Location: Sacramento, CA
Posts: 1,416
Hey folks,

We've got a 1991 300E that has a couple of issues going on with it. First, it's getting pretty miserable gas mileage - between 12 and 15 mpg city. At first I thought it was because of all of the boxes and assorted belongings we keep in the car, but after rifling through the trunk, there's at most about 40 or 50 pounds of dead weight back there. Which brings up another issue - the car used to be a pretty spritely performer (it has the 1st gear start). Now it drives like there's about 300 pounds of weight in the car, to the point where it's about as torquey off the line as our 2nd-gear-start '87 300E. This is fairly recent, within the last six months to a year.

Also, (this may be related, I'm not sure), the car is very hard to start when cold (and half the time when warm). Starting involves one or two 5 second cranks, after which the engine slowly comes up to a normal idle speed. The idling starts out rough, but smooths out once the engine hits about 500 rpm. This has been happening for years.

In searching previous posts, I came across mention that the EHA could leak fuel and fuel pressure, possibly causing both symptoms(?) (There IS a strong gasoline smell in the exhaust after start up, but not before.) What does this part do, and is there any way I can check if it's malfunctioning? I couldn't find mention of it in the shop manual, but a previous post noted it was under the air cleaner.

Other posts mentioned leaks around the fuel pump area; there doesn't appear to be any gas dripping from around the protective cover, nor can I smell any gasoline from that area of the car. The pump does whirr noticeably with the engine on, is this the buzzing that occurs before the pump fails?

So many questions...

thanks in advance,
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Old 07-29-2000, 07:06 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Your situation sounds like a classic OVP relay (overvoltage protection)problem. The OVP feeds the engine management system which uses the EHA (electro-hydraulic actuator) to affect all electronically controlled mixture adjustments such as: cold start enrichment, warm-up enrichment, acceleration enrichment and lambda feed-back controlled enrichment. Take away all these enrichments and the system works relatively good warm on the same system that came out in 1976 CIS - K-jet.

Checking this is very easy if you are prepared. It can be done by reading the integrator results on pin# 3 on the drivers side diagnostic socket. It also can be confirmed by reading the current flow through the EHA. It should be 20ma key on engine off. If you have current then you don't have a OVP problem.

I suppose a qualitative test could be done with no tools by trying the starting process with the EHA unplugged. This should make a difference in cranking and starting cold. If it makes no difference real testing will be required.

The EHA is a black box about half the size of a pack of cigarettes fixed to the side of the fuel distributor. It has two wires. (there is a second black box on the side of the airflow meter that is the acceleration potentiometer; it has three wires and is not what you are looking for).

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 07-30-2000, 11:32 PM
Registered User
Join Date: May 1999
Location: Sacramento, CA
Posts: 1,416
Thanks for all the info Steve, I'll check both the socket and the EHA as soon as the car is in the garage long enough for me to get under the air cleaner.

Incidentally, any tricks to removing/disassembling that part? I've noticed that the top half is secured by several clamps, as well as what looks like four locking nuts, which are torqued down pretty hard. Is there anything besides the air filter in there that would necessitate such a secure housing? Also, does the hose running from the air cleaner to the engine cover detach from the air cleaner, the engine, or both?

thanks again,

[This message has been edited by anthonyb (edited 07-30-2000).]
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Old 08-01-2000, 03:31 AM
Registered User
Join Date: May 1999
Location: Sacramento, CA
Posts: 1,416
I got the air cleaner off, and checked the current running through the EHA. I did get current running through the unit, but not 20 mA - on the 500 mA scale my multi-meter needle was pegged. (No idea on that one, I'm going to try again as soon as I can get my hands on another meter) I also checked the diagnostic socket; I don't have a digital multimeter so I can't configure it to read the lambda output, but I gathered from another post that it's some form of DC V, and I read zero volts out of pin #3. I took a look at the OVP relay while I was at it, other than a dent in the side (??) it looks fine, fuse is good.

Any clues on this one?

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