trouble on some hot starts 3.5 L M117 engine

[rutger kohler]

Hi My 2.80SE 3.5 is sometimes hard to start when hot.  When I say hard to start, I can depress the accelerator (gas pedal) to the floor and it starts ok after a short time.  Back in the day with manual chokes this would suggest it is flooded. It is not a biggie and if there isn't a quick and easy fix I would rather leave as the engine runs beautifully apart from this.

[Marrs]

Try holding the key in the accessory position for 5-10 seconds to run the pump and purge the fuel lines of air/vaporized fuel, it should start right up. This is actually listed as the proper starting procedure for the early cars.

[hkollan]

Hi,

I think you've already given yourself a good answer. If everything else is running well I'd leave it as is. I think depressing the accelerator on hot start is pretty much std. Procedure on most Injected MBs from that period wether D-jetronic or mech.inj.
The FPR on a Djet car (k-jet as well) will only activate the fuel pump for one second upon turning the key to pos.2 From what I understand was done to prevent a defective injector from filling a cylinder with fuel. So the advice given won't bring much I suspect.

Hans

[rutger kohler]

Hi, thanks for both your replies. I always do wait for the pump to stop on position 2, when UI first start the car but can't swear that I do when it is hot.  Will check that out and count how many seconds before it cuts out.  I think it might be more than 1 second but less the 5.

thanks again

[Marrs]

I find this interesting as my fuel pump will run as long as the key is in position, I can hear the faint whine coming from the rear. This is also an MFI car correct, not a D-jet? I can see the later versions you mentioned having a cutoff, but am unfamiliar with that function on a Mechanical Fuel Injection.

[hkollan]

Hi Marrs,

On your car with MFI that is the correct behaviour, the fuel pump is running continously  when the  key is in position 2, no Fuel Pump Relay is present to control the FP.
The 280 SE 3.5 with a D-jet electronic injection system, has a completely different
setup with very little in common with a MFI such as the one found on all pagodas.

Hans

[Benz Dr.]

I would check to see if the cold start valve is working. Use a 12 volt test light to check it. Vapor lock can be a problem on any of these old cars but normally they start well.

[cynewan]

  If everything else checks out okay, try this :....
So what if a new pump fixes nothing? The 3.5-liter cars also tend to develop hot-start, but a new pump won’t always fix the problems. Our old cabrio had a hot-start glitch as well. For two hours after shutdown, that car would not start at all. The fuel pressure was at 34 PSI, which is high but not excessive. I tried everything from a MAP sensor to a new fuel pressure regulator and drilling a small hole in the output line of the fuel pressure regulator, as suggested by Mercedes technicians “to allow vapors to vent from the fuel distribution rail,” but all in vain.
Finally, frustrated, I decided that another 5 PSI on the fuel pressure regulator wouldn’t cause a major catastrophe, so I went out on a limb and made the adjustment.
That car started and ran like a champ. Apparently, the fuel wanted to vaporize at anything under 36 PSI. Perhaps too much heat was rising from the engine, or perhaps the car had some valve-sealing issues, but at the end of the day, it started well at all times. We did have to tighten some injector hose clamps after the repair, but hopefully your mechanic will watch for leaks.

Referenced from the Star Magazine 'Myths and Magic with Bosch D-Jetronic Fuel Injection'

[rutger kohler]

Hi there, that last post about uprating the fuel pressure is interesting. I have got the car apart at the moment, getting the odomenter repaired, and I have repaired the indicator lever, on the column switch myself.  When I get the car back together, I will first try the earlier suggestion of ensuring I let the fuel pump stop on hot starts, but could also check this. Where would you tap into to put a pressure gauge plse?

[rutger kohler]

Just an addendum to the last post, where would you adjust the fuel pump pressure?

[cynewan]

id upload a picture of it tonight . I did it on my 3.5 coupe and its been working ever since

[JPMOSE]

I'd be curious to see the picture if you have time to post it.  Thanks!!

[rutger kohler]

I have checked the fuel pump pressure on my car and it is slightly low (26.5 psi).  Can the pressure be adjusted, and if so whereabouts plse?

[cynewan]

Please see attached.you would need 2 different size wrenches ; a size 13 to hold the other nut in place, and a size 10 mm to adjust the small but to increase the pressure.
Apologies for the delay in replying

[rutger kohler]

Hi, thanks for the photo.  there appears to be two of these on my engine, one after the other?

When I tried to restart my car, after carrying out the repairs to the odometer and steering column switch it wouldn't start. To cut to the chase the spark plugs were badly gunked up and this probably explains why the car is hard to start when hot ( I have replaced the spark plugs).  The gunk up however needs chasing down, I have noticed, for some time, that the inside of the exhaust tail pipes are very black suggesting the car is running too rich.

I am going to check that the cold start valve isn't leaking  during running, and if it isn't will find somone who can do a CO/02 reading and if necessary adjust the fuel mix on the engine electronic box. I am having a bit of trouble finding out the correct CO readings at idle. My engine is a M116-980-22-o123388. Should the CO reading, at idle be 1-2%, or 3-4%?

[cynewan]

Rutger, does this help ? See attached

[rutger kohler]

Hi there, it certainly does thanks! Andy Burns has loaned me his Vane exhaust gas analyser. Do you know how to adjust the idle RPM plse?

I will also be checking the dwell angle which I understand is 30 degress.  Do you agree?

[GGR]

These D-jet systems are known for vapor lock on hot days. I dealt with it by having the pump run continuously when I put the contact on. For hot starts on hot days I let the pump run 10 to 15 seconds to replace vapor by fresh fuel and build up pressure. Then car usually starts OK.

Fuel pressure should be 2 bars, or 30 psi. Increasing it a bit is OK, though you may run a bit richer.

If your car runs rich while your pressure is lower than it should, then you have an issue. Check your injectors and make sure they are the correct ones. They should be yellow on 3.5s and blue on 4.5. I've seen yellow ones swapped by blue ones. But the latter flow much more than the yellow ones.

Before fiddling with the fuel side make sure your ignition is all up to specs and adjusted correctly. Only then you can start acting effectively on the fuel side (if not all your adjustments will be out of whack when you finally adjust the ignition correctly). 

[rutger kohler]

Hi, thanks for that.  have checked idle RPM and Dwell angle, also checked distributor points and replaced rotor ( the spark plugs were replaced several weeks ago). 

I have also found the idle adjustment but didn't have to alter as the idle RPM was within spec. The air/fuel ratio however was not. At idle with the engine hot the analyser showed a CO reading of 6%.  I have gradually reduced this at two clicks at a time, with 5 minutes in between each adjustment, to 4%. The Chilterns manual I have got, specifies 1 -2% CO at idle for the M116 engine however I am going to leave the present setting as we are going on quite a long trip (for NZ) shortly and i will see how it performs before lowering even further.

[GGR]

The MaP sensor (the round thing on the firewall connected to the air plenum by a hose) can also go bad on these D-jet systems. The membrane inside gets damaged and the sensor cannot hold vacuum as it should. As a result, the engine runs richer than it should. So make sure that sensor holds vacuum and that the hose is not damaged or porous.

[rutger kohler]

Hi thanks for that will also check the MAP sensor