Hi all.

I have a 53 plate st170 that has now covered 78k.

All has been well but of late noticed it's abit lumpy on idle, but clears if i raise the rpm slighty.

I also notice at 2500 rpm it feels if it's slighty holding back underway, but again if you put your foot down it clears.

Any idea's?????

Thanks

Dan

 

Sounds like a lean mixture. Could be an ageing air mass meter or a small air leak.
Get the fuel trims read in live data. If they are not around zero then there's a fueling problem.

 

There is a well known flat spot in the ST170 map at around the revs you state.

 

Looks like Lambda sensor 1 as running lean!

 

Changed sensor - no difference?

Unplugged maf sensor and idled the same but once plugged back in light came on, is that correct?

 

if you unplug any sensor on the 170s and then plug them back in the E M L will come on every time as the ecu thinks there is a fault ( disconnect the battery and leave it for 15 mins then connect it back up and the light will go off and the ecu will reset it's self) as for the flat spot try cleaning out the throttle body as they get caked up over the years and hence a flat spot and rough idle done mine a few months ago and been fine since

 

I have replaced the first Lambda sensor, but engine still running lumpy and this code cam up again..

P1131Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1

 

Oxygen sensor is not switching and reporting a lean mixture.
Could be an ageing air mass meter or a small air leak (both on the intake as exhaust side).

How much fuel is the engine using? Normal or more?

 

I can't hear any hissing around engine and I've had a good look around, anyway of testing the Air mass meter, thats bolted to air box?

When the light came on this morning I deleted with fault and started up the engine and did smell strong of fuel.

 

Not sure if relevent but when I fitted the new sensor I notice that the cat looks like its had some spot welding carried out...!!

 

If the cat has any air leak in it this will fool the engine into thinking its lean (your ecu says thi sis the problem) but in actual fact it is not lean (as the excess air in the exhaust is tricking the lambda) So your ecu will richn it up to compensate, but your engine will now run rich (and lumpy) I think the fault goes away at higher throttle loads as the pressure in the exhaust stops the ingress of contaminating air, and stops the fueling problem. All this is assuming the hole in the cat is the problem, BTW it takes a VERY small hole to cause this. The only other thing i can think of is that there maybe a broken wire to the ecu from the front lambda, and as also mentioned the air mass meter (but my money is on a leaking cat) let me know how you get on cuz

 

Strong smell of fuel does suggest a leak in the exhaust.
But I would concentrate more on the exhaust manifold and manifold gasket, rather than the cat. Because the cat is after the upstream oxygen sensor and a leak there would influence only the downstream sensor.
I do not think there's a wire broken from the front oxygen sensor. That would trigger other fault codes. All the ecu is saying is: I am seeing a signal from the front sensor (not a short circuit), but it is always lean and not switching to rich.

 

thats not correct, it will dilute all the exhaust gases swirliing about before the cat, certainly at low throttle openings.

 

Chaffe, can you deny that a leaking exhaust manifold gasket would influence the oxygen sensor? I don't think so.
So, the exhaust must be checked from the beginning, not starting half way.

There's a simple method to detect the problem. Hook up a gas analyzer and check the percentage of O2 in the gasses. High percentage of O2 (more than 0,5) suggest a leak.

 

Wow, wow, wow, wait a fucking minute, i never said that. I said that a leak post sensor can still fuck up the readings. Of course a gasket problem will cause this problem, but the OP is suggesting a corroded cat, therefore i would be checking this out first, no?

 

I found the problem a pipe off under the throttle body, anyone any idea what should be on it there is no loose pipe present i can see, I have blanked it off and car running perfect.

Any help would be appreciated thanks, it is the one that is blanked off with the grey bung.