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Discussion Starter #1
Thought I would post here because there is a larger M110 owners.

I have a M110.989 in a 1984 280se.

Just recently renewed a long list of parts on this car and engine.

Was running fine, and has developed a hot start issue. Will start fine when cold or hot (unless the car has sat for over 15 minutes or so when hot it will then be very difficult to start, takes probably 10 seconds of cranking to start, then runs rough for a maybe 30 seconds before returning to normal.

System pressure is 5.2 bar.
Hold 2.6 bar after 30 minutes.
control pressure has been set to 3.6 bar ( rebuilt WUR)
New fuel pump, accumulator, one way valve, strainer, filter etc.
Air metering plate pulling down at start up, idle screw has been set, no binding, centered and moves freely.
Has all new injectors, although the spray pattern at idle isn't the best, none of the injectors are leaking on test bench.
Cold start injector checked, not leaking, not spraying at hot start, doesn't change starting if connected or not.
Ignition, new rotor, cap, leads, plugs and coil, timing set

(Note: above parts weren't used to solve hot start, just part of a overhaul after 14 years in a shed, also a lot of the parts had been replaced by mechanic before I bought the car. Only thing I didn't like was the aftermarket fuel pump, I would usually put a new Bosch pump in. However it flows in excess of 3 lpm and is maintaining 5.2 bar system pressure.



Have just got off the phone with a known CIS specialist and he is out of ideas also. Short of driving a few hours to see a specialist I will try a couple of small things, but not that confident in solving this on my own now.

Any ideas, fire away, very keen to here what issues I may have overlooked.
 

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Discussion Starter #2
Just and update, I checked the fuel tank purge valve tonight.
It isn't blocked, but the valve flows both ways, so no longer working as a valve.

Also the hose connected to it was quite perished. I have had a lot of positive pressure in the tank lately noticeable when removing the filler cap. So next test will be to trial a hot start with tank cap removed.

I'm waiting for my injectors to return from flow testing, once I have them I can test as above.
 

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Check the micro filter on fuel distributor where supply line connects to, to eliminate any restriction since you’ve addressed all other components.

Also you say you used a non Bosch fuel pump...did it come with a check valve or did you use your old one from other pump? Spins in on the exit side.
 

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Discussion Starter #4
Check the micro filter on fuel distributor where supply line connects to, to eliminate any restriction since you’ve addressed all other components.

Also you say you used a non Bosch fuel pump...did it come with a check valve or did you use your old one from other pump? Spins in on the exit side.
Hi and thanks for replying,

Fuel pump was some aftermarket one, but identical to a bosch K jetronic pump. Check valve, I have tried two, one was from a new Bosch pump. Didn't make any difference. New accumulator also made no difference. No pressure lost when engine switched off, holds around 2.6 bar for 30 minutes or so.

Fuel distributor micro filter is brand new and replaced when I rebuilt the FD.

Do you think excess fuel tank pressure could be an issue ? I currently just have tank vent tube connected to a vacuum port, original canister was too old and shot.
 

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What about the thermotime switch or auxiliary air valve? Hoses tight? The green lead to distributor to ignition control box? Stiff? No corrosion?
 

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Discussion Starter #6
What about the thermotime switch or auxiliary air valve? Hoses tight? The green lead to distributor to ignition control box? Stiff? No corrosion?
I have tested the thermotime switch with a multimeter and it was in spec. I also disconnected the cold start injector, but that had no effect on hot start. Cold start injector not leaking.

Auxiliary air valve has been removed and cleaned and 'seemed' to be working ok when put in hot water.
However, the cold start idle was a lot lower after the WUR rebuild maybe 1200-1300rpm, Coincidence ? not sure, but i have suspected that maybe the auxiliary air valve may be sticking sometimes. Wouldn't this cause cold start issue too ?
Hoses on Idle hose assembly has all been overhauled cleaned, new clamps etc. Hot idle is 800 rpm and steady.

The green lead, didn't notice any corrosion when removed, I had some issue with the timing advance on the distributor and managed to repair that. I'm always under the assumption that ignition issues would show up with hot or cold engine and hence have focused on fuel for diagnosis.
 

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Hmm, there is an O ring that can fail on the fuel distributor that can cause issues. Will lose vacuum but maintain pressure. This sure sounds like vapor lock. Perhaps sort the gas tank vent first.
 

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Discussion Starter #8
Hmm, there is an O ring that can fail on the fuel distributor that can cause issues. Will lose vacuum but maintain pressure. This sure sounds like vapor lock. Perhaps sort the gas tank vent first.
All the o rings on the FD have been replaced. Which specific one ? Certainly will look at that again.

Yes I will sort out gas tank vent and go from there.

I'm also thinking I might be testing the pressures wrong ? do you leave the valve open at the WUR end of the line or do you shut it off to test for pressure loss ? I have been testing with valve closed.

Thanks again !
 

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Discussion Starter #9
Ok, sorted gas vent out. Compressed air though tank end, and also from engine end with valve removed.

Further research appears fuel tank vent valve is a two way valve designed to maintain positive pressure in tank, so all appears ok there.

Back to pressures and injectors, put my old injectors back in while new ones have been getting checked by CIS Specialist.

Long story short, looks like I have a faulty fuel pump relay. As soon as I start with pump bridged when hot it fires straight up, replace relay and back to long start and rough running for a few seconds. Bridge pump good again, relay back in hard start again. That's all in a space of a few minutes so not even requiring a heat soak situation.

My only question is maybe the Relay is getting a weak signal and it isn't the issue ? Does it run off an ignition signal ?

My tacho decided to stop working today also, stuck at 3000 rpm, don't think that is related as this has been fine before.
 

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Fuel pump relay also powers the WUR and AAV and thermotime switch. How are those wires/terminals going from WUR to fuel pump relay?
 

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Discussion Starter #11
Fuel pump relay also powers the WUR and AAV and thermotime switch. How are those wires/terminals going from WUR to fuel pump relay?
I haven't checked those, guess I will be tomorrow ;)

I'm going to run a temporary standard relay for the fuel pump just to rule that out completely and also until new one arrives (which might take awhile, considering shipping these days)

Do you know what signal tiggers the FPR ? I was thinking that's the only other possibility of a false positive for FPR fault is a dodgy trigger signal ?? I'm assuming it has multiple inputs and controls rev limit.

I have heard people resoldering the FPR, however mine looks like new inside, has 11.1.89 stamped so assume it was replaced at some point since new being an 84 car.
 

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I just replaced the fuel pump relay in my 280CE with an OEM one and even though it had to come from Germany I was surprised how quickly it arrived.

Were you a little shocked at the price of the relay? I know I was.
 

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Discussion Starter #13
I just replaced the fuel pump relay in my 280CE with an OEM one and even though it had to come from Germany I was surprised how quickly it arrived.

Were you a little shocked at the price of the relay? I know I was.
Yes shocked, very expensive. I'm sure you could retrofit a newer relay from another car, but then again, possible a big hassle.

What were your symptoms before replacing it ?
 

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My car is new to me and not running well. The relay was replaced trying to narrow down why the car is low on power. Has new fuel pump and such and the next thing going on the car is a rebuilt fuel distributor from CIS Flowtech. The shop feels someone messed around with the one currently on the car.

The relay was cheap compared to the rebuilt fuel distributor.
 

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Discussion Starter #15
My car is new to me and not running well. The relay was replaced trying to narrow down why the car is low on power. Has new fuel pump and such and the next thing going on the car is a rebuilt fuel distributor from CIS Flowtech. The shop feels someone messed around with the one currently on the car.

The relay was cheap compared to the rebuilt fuel distributor.
OK, gotcha, always handy to keep one in glovebox either way I hear.
I might try my luck on ebay.de
 

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Discussion Starter #16
I actually have a few spare w116 280se fuel pump relays. However these appear to be just standard Bosch relays nothing special. Fuel pump prime and switch off function must have been changed on the W126. Does anyone know the difference between late 70's and mid 80's fuel relay's. I'm assuming the systems carried over between models.
 

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Discussion Starter #17
So tried a couple of more ideas this afternoon. Ran a Relay setup like this.
Screenshot 2020-11-22 192729.png

Compared it to starting hot ( after heat soaking ) with the factory FPR, and honestly hot starts were no better.

I think its hit and miss, leading me to think that was the issue but it isn't. Maybe bypassing the relay completely and running the pump for 5-10 seconds before starting masks the issue somewhat.

I am out of ideas, this one sort of has me beat.

I have my new and tested injectors to go back in, but they came back as not leaking, but under idle conditions they weren't the best ( pity considering they were new) However Colin from K JET believes they wouldn't cause the hot start issue.
 
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