eha question 124
 

What actually fails with the eha.

my 103 is a bit hesitant on start up...and until it warms up there is a bit of hesitation

i can't find a vacuum leak.... and i believe my cap and rotor are fine... i do have a new rotor on order..

From working on my 91 300SL ( M104 24 V ) I found the fuel distributor plunger to be gummed up causing all sorts of really bad behavior.

The motor will sometimes go lean since there is more resistance on the plunger, if reved up, held then let back to idle, the motor was rich.

Cleaning consisted of removing the fuel distributor from the air flap housing then spraying carb cleaner in various ports, applying air to the control pressure port ( be careful of spray from another port ) and spraying cleaner directly on the plunger. At least with the car I was working on, spray and air worked just fine in cleaning up 3 year old fuel.

Removing the dist consisted of removing all lines and removing 3 screws towards the center of the dist. There are other lines around the edges, don't take these out as they hold the dist together. Supposedly some plungers will fall out so be careful during removal.

There is a how to take the distributor completely apart done by a Porsche 928 guy floating around, but this is flawed as it recommends using gasket shellac / aviation sealer on the gaskets. Neither of these are fuel proof and expect gum to leach in to the fuel system.

Have a close look at the dist cap dust cover, ( the dog dish that is removable after the rotor is unbolted ) these supposedly can carbon track causing spark leakage.

The EHA relies entirely on O2 sensor output, so before even considering touching that thing other than replacing the 2 o-rings for leaks, replace the O2 sensor. To save $60 splice in 1990 Mustang GT sensor. Identical unit minus the connector.

Never taken one apart, but could just be a combination of wear on the magnet from ethanol fuel and deposits over time. Only option is replacement, and its not always drop in and go since over the years the mixture is adjusted (wrongly) to compensate for wear. Once the new one is in, everything is thrown out of whack again.

thanks.... replaced the o2 sensor some time ago..

perhaps it is one of those dreaded hard to find vacuum leaks..

i did get a new rotor yesterday... will put that in and examine distributor cap

Well, the EHA does rely on 02 sensor input, but only after the engine warms a bit, and the 02 becomes active and goes into closed loop. Upon cranking and initial warm up, the EHA is programmed with a set value depending on temperature. During cold cranking, the EHA sets at +20Ma, very rich. Then when the engine fires, the current drops according to the engine temp, usually about 4-8Ma. When the 02 goes closed loop, the EHA will modulate, optimally around +-1Ma. So, yes the EHA could cause lean running during warm up if it were faulty and not responding properly to the requested current.
But, if it were faulty, it would probably also indicate a warm-engine adjustment quite a bit from the nominal 0 Ma or 50% duty cycle, as it tried to follow the 02 sensor inputs.
But, as mentioned in the earlier posts, a lot of other things could cause poor cold-engine performance; the #1 culprit probably being injectors.

EHA's seem to be pretty robust, the biggest fault being fuel leakage, which is very obvious. First test might be to check warm duty cycle on the diagnostic connector and see if it's in spec at about 50% +- 10, at both idle and 2000 RPM (do a search for the procedure). If it is, then the EHA is probably OK, as well as the air valve setting. If it's not, then the search begins; vacuum readings, air valve adjustment, fuel pressure, then actual EHA current monitoring.

DG

great info, thanks... i don't think I have an eha problem

Lee,

I have tried searching some of the really old posts here with no luck, but I think it can be done somehow. If you can manage to search back about ten years, try to search for threads by "eha" and then look for threads that Steve Brotherton started, I think he was "sbrothertonfl." In some of those threads he references some really comprehensive articles on the EHA that were published in some auto service magazines. These articles are short, easy to read and very concise.

I'm sorry I can't offer very much, but I hope this helps.

One of these days I will learn....ovp
 

I had a spare in the trunk... all is well now...

the one on the car did not blow any fuses...it did not rattle.... go figure...

no idiot lights were coming on.... just hard start when cold.....

like I said one of these days I will learn.... at least i always keep a fuel pump relay and an ovp in the trunk