Car backfire, need help!

[n/a]

Hello,

The fuel pump in my 1964 230sl was just replaced. The car now backfires after you start the car and disapears a few minutes after driving. The pump was replaced with the correct new Bosche pump.

Does anyone know what this could be?

Look forward to hearing from you guys out there.

Thanks very much,
jason

[n/a]

I am worried that the replacemet of the fuel pump could have messed something else up or maybe change pressure in pump? No idea, I am looking for any idea as to what this would happen.

Thank very much,
jason

[Cees Klumper]

Probably you are referring to the fuel injection ("FI") pump (rather than the fuel pump that sits underneath the fuel tank in the back of the car).
Myself, as shade-tree mechanic, would check all of the basic adjustments like ignition timing/dwell, spark plugs + wires, cold start systems (CSV on the intake manifold and the fuel/air enrichment on the FI pump, the CO values, the air/fuel linkages, the valve clearances (retorque the head first) and also check for false air possibly coming in from the brake booster, a loose intake or exhaust manifold etc etc. Check the fuel flow also (rate and pressure).

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[n/a]

Cees,

Thank you for the reply. I am actually refering to the Fuel Pump that sits in the back of the car. It was rusted and leaking fuel.

Would this change you suggestions?

Thank you very much.

Jason

[rwmastel]

Where the fuel line returns back to the fuel tank, disconnect it and run it into a container.  Turn on the ignition (do not start the car) for 15 seconds.  The pump should pump 1 liter (about 1 quart) of fuel through the system in 15 seconds.  Doing the test at this point tests the whole system, reducing risk of hidden fuel system blockages.  This tests the pump's rate or volume.

There's also a pressure test, but I did a search on this forum and I can't find any procedure listed, just a lot of posts saying to check fuel pressure along with volume.  Can anyone describe in detail a fuel pressure test?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[Naj ✝︎]

Static fuel pressure can be tested by connecting a pressure gage to the fuel feed line at the cold start valve. Reading should be 0.8 to 1.1 atm.(Same procedure as fuel delivery: ignition on and fuel pump operating but engine not running)

naj

P.S: Rodd, see page 00-14/2 in your 230 BBB

naj

65 230SL
68 280SL

[Benz Dr.]

No mystery at all. This a problem at the injection pump cold start system. The slide valve is stuck in the warm position and is a pretty common problem. Your car pops and backfires until it warms up and then runs fine, right?

You wil have to take it apart and free up the slide valve and put a bit of sealer around the thermostat. Failure to do this important step lets a bit of coolant seep into the valve area and it will cause it to jam up again just like yours did.

Daniel G Caron

[hands_aus]

Hey Dan,

Can the air slide valve and also the walls of the housing be smoothed?

I ask because my valve was slightly grooved but I never smoothed the walls because I was afraid I would remove too much material and make it leak air all the time.

What do you recommend to use to lubricate the air slide valve?

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[Benz Dr.]

I use a bit of high temp grease. Maybe yours is worn a bit and jammimg in the housing. The lube will make it slide better so it should work OK.
You can test it by pushing on the slide with the thermostat removed.

Daniel G Caron

[hands_aus]

ok, thanks Dan

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto