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E-Class (W124) 1984-1995: E 260, E 300, E 320, E 420, E 500 (Includes CE, T, TD models)

Wonky problems with fuel pressure

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Old 03-18-2017, 02:00 PM
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Wonky problems with fuel pressure

So, I'll start here. I have been chasing down weird starting/running issues with my '87 w124. I have replaced the injectors, air flow meter(used) and pot, O2 and temp sensor, ecu(used), plugs, EHA(used), TPS and body, rubber hoses. I have cleaned the fuel filter, tank, and lines.

I was originally getting very erratic running and it would stall occasionally at stops. I then replaced the O2S and it began to run very weird or wouldn't start. That's when I replaced the rest of the stuff. Since then, I have chased wires and replaced a few. Still nothing. Now I've focused on the fuel pumps and system pressure. This is the weird part to me. I am getting very high pressure from the pumps. I am testing at the junction of the line with the fuel distributor. I am getting enough pressure to tack out the self made gauge which is 100psi.

Is there any reasonable explanation for this high pressure? Also, this might be a side note, I blew my meter testing the current between pins 7&8 on the FPR. This circuit is fed by the OVP, which is limited at 10A. I can't imagine that I'd get more current than the OVP fuse could handle.

Anyways, could the pumps cause a very high pressure without blowing the OVP fuse?
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Old 04-15-2017, 02:48 PM
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A lot of these problems are related to the ovp, so i read in the manual .But as this is for low fuel pressure i dont know By the way PM sent .. ..May be if its gone open circuit , the relay only works for a few seconds as you turn on the ignition switch. Pump should not run all the time, it runs just a few seconds before engine starts and relay stops it if engine is not running.
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Old 07-16-2017, 10:03 PM
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@optimusprime m1,

Thanks for the PM about the manual. I actually found that one right after I last posted. haha. well, I got the pressure of the first pump in line, which is the one that I was supposed to test. Not to mention, my gauge was bad, so I got another and I'm getting good pressure out of the pumps. As far as the out of spec reading at the FPR, that seems anomalous.

I've recently gotten the car to run stably, but it's having some difficulty.

This may be a dumb question, but is the air meter plate supposed to relax to its lowest point allowed by the spring when depressurized, or should the spring be tight enough keep the plate seated up against the top position when depressurized?

I know that I haven't posted in a while, i've had so much going on with work, etc, but if I should repost, then I can.
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Old 07-17-2017, 07:55 AM
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The plate is inj the uper most and flush with the mouth of throttle body all the time till you reve the engine up .
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Old 07-17-2017, 08:05 AM
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The throttle plate is closed and in the upper most part of the throttle body until you rev the engine up , then it will close after you take your foot off the pedal .
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Old 07-17-2017, 02:38 PM
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OK, but is the af plate always up due to just the pressure in the distributor pushing against the pin or the spring on the af meter arm? Say, if I relieve all pressure, should the plate drop a little and relax against the spring pressure, but not still be up in against the bevel?
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Old 07-17-2017, 02:39 PM
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Or, while totally depressurized, would the spring keep the plate tight in the fullest up position?
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Old 07-18-2017, 04:12 AM
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There is a way to test if flap its working ok. Not sure i think you need to bridge 7and 8 on the FPR relay ,. Do this after you have removed all feed tubes to injectors and the injectors them selves .Put injectors on to the feed tubes [ pipes ] and each injector in to a jar .With ignition on ,then at the flap or trap door if you like,just push it down gently a little , the injectors should spray in to the bottle .Just get a second opinion on this before you do it .
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Old 07-18-2017, 04:20 AM
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The flap should centre on the walls of the throttle equal and no binding .If you look at the flap and its not central this is the problem .Spray the whole throat around the trap door and hold it open as you do it ,clean of all old crud aroung the mouth ,so it opens and closes .I use carb cleaner to clean the old rubbish off .Dont use force to open it you only need to get the fluid in to the part it close to. Talk to the shop tell them what you want if you dont have any.
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Old 07-18-2017, 05:15 AM
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The flap is never in any other position than up , and closed .It only opens as you rev the engine up. Its held up with the spring. .. If its down all the time ,then you have found the problem
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Old 07-18-2017, 10:05 AM
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Originally Posted by optimusprime m1 View Post
The flap is never in any other position than up , and closed .It only opens as you rev the engine up. Its held up with the spring. .. If its down all the time ,then you have found the problem
OK. I was hoping such. See the thing is that I had gotten a new af box and had taken it off the af box to clean it all out and I had loosened the screw for the spring and I had put it back in the configuration where it is tighter and not looser. I was worried that it was tight enough as to not retract from top rest as the vacuum from cylinders pulled it down.

I wanted to know how tight the plate should be in the up most position.

Another question might be, do you know how the two contacts behind the af potentiometer work with the pot? The reason that I ask is because the old box started to give crazy readings from the pot. So, I replaced it and still got weird readings. The new box doesn't give me problems in that respect
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Old 07-18-2017, 10:12 AM
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Also, would you happen to know what the brass flat head screw on the end of the arms pivot rod right behind the pot?
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Old 07-19-2017, 04:08 AM
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Originally Posted by dvergurinn View Post
Also, would you happen to know what the brass flat head screw on the end of the arms pivot rod right behind the pot?
You have a manual i sent to you that will give you more information that i know about ..I would read it .The info is in the ezl section Trev.
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Old 07-19-2017, 04:19 AM
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fo

Two to look in to .For these you will have to google search for it just put in the search box -
Bosch CIS-E Airflow Sensor Potentiomiter Replacment And Adjustment .

And another one is - 80-300 SE Start Stall,Start,Stall,Start Stall
Let me know if it helps .This ;lst one is a 300se but like ours...Trev
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Old Yesterday, 10:42 PM
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Very good. Thanks for the suggestions for the manual. I've found w124 Performance pretty useful. Also, the autolib.diakom.ru site has the most accurate diagrams for my car. It's a decent site if you know Russian or have translate, I guess.

I was just curious about the spring on the AF arm.

Thanks for all of your help guys
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