Why does my car sometimes idle rough and hesitate during accelloration?

CWBCAW

MEMBER
1988 MERCEDES BENZ 300SE
189,000 MILES

When I start my car and let it warm up, sometimes I come out to it idling rough. It seems to happen once the engine begins to warm up. If I press the accelorator to the floor the engine rpms up and smooths out for a minute then goes back to rough idle, it also smooths out if I shut the engine off and restart it. Also when I try to accelorate it hesitates but if I put the accelorator to the floor the trans downshifts and the engine usually backfires and regains power and accelorates as it should. I have recently had the fuel pump and filter changed. I thought this would take care of the problem but it hasn't, it seems to be happening more frequently.

1 Reply

DRCRANKNWRENCH

MECHANIC
3,380 POSTS

I am going to provide you with a diagnostic walk through that covers both rough idle and backfire issues.
This is the best way to go through this as it is a process of elimination and it goes into depth and explains things in a way that might lead you to realize you may have an idea of what is wrong already from the symptomatic behaviour that you will read about in the walk through.
You will come to a part where either there is a reference to the fuel pressure being tested and the trouble codes being pulled. You have to go into a lot of deatil to pull the codes on an older car like yours, so unfortunately you may find that the limited information you can get, which is done by reading off how many times the check engine light or an LED on the ECU blinks, will not give the kind of inormation that newer cars can get from readers lkike the OBD system.
However you can test the fuel pressure and you can get the tool to test it at any Advance Auto or Auto Zone on a lonaer basis. It comes with adapters for the test port on the fuel rail.

Just follows the links below to get to the diagnostic walk through;

https://www.2carpros.com/articles/engine-misfires-or-runs-rough