1985 Mercedes Sl Stumble and Hesitate

[samonline]

I drive the car for two to three minutes , the car start stumble and hesitate , if I press on the accelerator , the car died and stop , I have to wait five minutes to drive it back again for two or three minutes to start again hesitating and stumble.
I did ground the wire going to the "oil temp sensor "unit to bypass the the o2 sensor and keep the flow of gasoline higher , no result still doing the same. The gas pour very well to the fuel pump . ( Thanks to Chuck for all these tests) . Any more ideas ??

[meltedpanda]

Misses and Hesitates under a load
1. Fuel pressure incorrect:
Test fuel pressure.
2. Fuel injector clogged:
Same as above #9
3. Control Plunger Binding:
Same as above #10
4. Incorrect ignition timing or faulty ignition system:
Same as above #12
5. Clogged Fuel Filter:
Check the fuel filter. Replace if necessary.
6. Faulty injector:
Test all injectors for quantity and leaks.
7. Vacuum leaks:
Inspect all hose connections and clamps. Replace all old and loose fitting hoses.
8. Fuel Pump not working:
Check for voltage at the pump. If no power check the fuse and the relay.
Replace if necessary.
If there is voltage at the pump and it is not running replace the pump.
9. Warm Control Pressure too high:
Check for at least 40 PSI with out vacuum on the warm up regulator and 46 PSI with vacuum on the warm up regulator.
10. Warm Control Pressure to low:
Check for at least 40 PSI with out vacuum on the warm up regulator and 46 PSI with vacuum on the warm up regulator.

[jplinville]

What is the economy gage doing while the car is running?

[samonline]

The economy gage move normal , when I press on the accelerator the needle move to red , when the car stumble it move down to white.

[Kimolaoha]

Originally Posted by samonline

I drive the car for two to three minutes , the car start stumble and hesitate , if I press on the accelerator , the car died and stop , I have to wait five minutes to drive it back again for two or three minutes to start again hesitating and stumble.
I did ground the wire going to the "oil temp sensor "unit to bypass the the o2 sensor and keep the flow of gasoline higher , no result still doing the same. The gas pour very well to the fuel pump . ( Thanks to Chuck for all these tests) . Any more ideas ??

Have you eliminated the OVP relay (or its 10amp fuse) as the culprit?

[Bill Adkins]

If the car starts and idles when cold the fuse is good as that circuit enables the fuel injection control module for enrichment as well as idle stabilization. The car will run without it but very poorly.
Bill

[samonline]

I checked all the injectors one by one , all good and spray perfectly.
I checked the pressure , It was around 46 psi with vacuum on the warm up regulator .
I don't see any major vacuum leak so far. But car still hesitate and stumble after 3 to 5 minutes of driving especially when I press on the accelerator .
The fuel filter is new.
Fuel pump is new, also the accumulator is new.
New spark plugs.
Deos the ignition has any thing to do with this problem.
Why the hesitation and stumble start after five minutes of driving perfectly.Then when I let it park for one to two minutes the car run perfectly again for 3 to five minutes and start stumbling again.

Thanks for your help .

[meltedpanda]

is the pump aftermarket, perhaps it is not provding consistant pressure while running hot

[samonline]

Yes It is After Market , May be you are right,But don't you think that the pressure will go lower after 10 minutes of running the engine on idle. Thanks Sam

[Kimolaoha]

Originally Posted by Bill Adkins

If the car starts and idles when cold the fuse is good as that circuit enables the fuel injection control module for enrichment as well as idle stabilization. The car will run without it but very poorly.
Bill

OK Bill, but after a battery replacement my '83 380SL started and idled fine while cold, but after about 3 minutes it began to stumble and finally die when the accelerator was applied. It would restart, but stumble until the next cold start. Sure enough, the OVP fuse had blown when I had changed the battery. A new fuse and reseating the relay cleared up the problem. Just checking to see if he had actually verified that the fuse was still intact, cause his symtoms sound exactly like mine.

[samonline]

I will check the OVP fuse tonight. Or it could be the OVP relay bad. I will let you know tonight . Thanks

[samonline]

The ovp fuse is good, I remove the OVP relay and start the car, the car run perfectly for 10 minutes. The engine stop twice during that 10 minutes but start immediatly and run without stumble or hesitate. What does the OVP relay role??

[Bill Adkins]

Kim, what you experienced is what I described, the OPR is to protect the electronics from voltage surge or spike. It eliminated the electronics thatmade the car run properly. When I ran a dyno I carried several 10a fuses in my pocket all the time due to the amount of cars brought in with a tow truck after a battery jump.
Bill

[meltedpanda]

so the pump may be the only variable not ruled out?? You have checked fuel plate ( plunger) - free to move , checked hot and cold, Fuel regulator, metered slits at .2MM?
Fuel regulator working properly
My thinking is if it is only when hot it must be whatever is in the fuel delivery side - pump -regulator -injectors - things expand when hot, power draw - increases as well

[samonline]

Thank you for your support, I tied all the connectors and I cleaned them all and tied all tubes and vacuum lines. It is working fine so far . I drove the car for half an hour with no problems at all. Thanks for all the members that help me especially Chuck and Meltedpanda .