At least I'm pretty sure it's fuel related. Think I've eliminated most other things.

So, the other day I came to my car started it up all fine and after 30 seconds it starting running horrible and very unresponsive throttle. I could get it to increase by feathering the pedal. Turned it off and back on. Again all good so went to drive from car park and same problem.

Disconnected the maf and tried to run it. Helped maybe a small bit but overall same problem. It is barely drive able. It will drive but only with very light use of throttle to increase revs and definitely no boost

No CEL for problem either. Have checked all vacuum pipes. Maf. Air filter. Changed fuel filter. Changed fuel pump. Cleaned throttle body and iacv. Coming to a dead end now.

Noticed that on startup the fuel pump pumps harder/ faster to start with for about 30 seconds before cutting back to a 'Low' mode almost. It is at this exact point when it starts running horrible.

Does anyone have any ideas as to what could be causing it. I'm pretty sure it's an underfueling problem but may be wrong.

Thanks.

 

Thats a strange one.....

Did you get the fuel pump new or second hand?

 

I swapped it out with a mates old one as he got an uprated one. The thing to me saying it's not the pump itself now as on startup it runs it's high mode but I assume once gone to low mode upon using the throttle it should go back to high mode to give more fuel?

That's why I'm thinking maybe something not telling it to do what it should.

I don't know maybe I should get a new pump in there for peace of mind.

 

Check the tiny vac pipe from the fuel regulator to the front of the inlet manifold

 

Got it to give a code p0171 system too lean.

 

More findings. Short term fuel trim running at 25.9 rather than around 0.

O2 sensor voltage. 0.035 ish rather than a normal average around at least 0.4 I think.

 

Fuel pressure regulator or the vacuum hose that connects to it.
02 sensor won't give an accurate reading until it's warm/hot.

 

So the problem has only started since changing the fuel pump?

 

Yes since fuel pump change. A few weeks down the line. Original change wasn't sure if weak or fuel pump on way out as was struggling to start now and again. New fuel filter was put in at this time also. Since changed back to original pump.

I think the problem is now a bad O2 sensor given the sub 0.5 voltage readings and the constant high positive short term fuel trim

 

Scott that reading was at normal operating temp.

 

Check the small rubber hose that connects to the fuel pump.
These can split, more so the new one supplied in the kit then the old one.

02 sensor won't cause this as you can run without one and the ECU will use the base map.