lets talk lambdas!

[Oneeyeblind]

hey everyone. quick recap... I have posted a few times about a odd idling problem I have. I have gone into depths about it a few times so a quick description! basically the idle is always a little erratic. between 800 and 1k. i can live with that BUT if I hold in on boost for more than 5 seconds (hard full throttle) the idle goes nuts. starts hunting between 800 and 2k. Switching engine off and on immediately fixes it. very annoying.
I have checked and adjusted TPS, seconds hand IAC, second hand MAF, cleaned throttle body, given it a carbon clean, reset the battery, smoke tested it (found a small leak and was super confident I had sussed it. no. )
I am now considering finding someone who can check/repair/replace my ECU which is obviously going to be £££ but I am getting pretty mad about this problem. it appears to me the IAC is just going insane after prolonged full throttle.


I am lucky enough now to have access to a ramp so put it in the air earlier to find a vibration I've got, unrelated, but I noticed I only have 1 lambda sensor, on the turbo downpipe, and nothing by the cat. But I do have a bunged hole though, where a lambda could go? As far as I understand. the 1st lambda will deal with any fuelling adjustments and the 2nd lambda is supposed to just read emissions? i may be wrong though. not very familiar with old Volvos! I hunted everywhere for a disconnected lead and also couldn't find one.

do these only have the one? could that first one potentially be my problem!!! should I just sell the wagon and daily my partners Hyundai i20.......

sorry the the essay. any advice appreciated!

[griston64]

Can't see what year model or engine it is but,if your car is a 940 turbo it only has the one on the downpipe

[GingerMagic]

I had a recent new exhaust mid section, thanks to some toerags that stole my cat, however the new exhaust had 2 holes for lambdas.

My 1998 only has one lambda, which is normal, so my mechanic mate just bunged an old broken lambda into the 2nd hole ( farthest away from the manifold ) with a random one he had laying around. It's not connected or anything, just acting as a bung.

Is your exhaust original?

I still have a hot start issue but I'll tackle that in the summer.

[Oneeyeblind]

Originally Posted by griston64

Can't see what year model or engine it is but,if your car is a 940 turbo it only has the one on the downpipe

Oops my bad, it's a 940 turbo! 10 points for you

[classicswede]

2 bungs, 1 lambda - both bungs are pre cat just pic which bung you want to use

[360beast]

Originally Posted by classicswede

2 bungs, 1 lambda - both bungs are pre cat just pic which bung you want to use

I wouldn't fit the lambda in the bottom bung as it will hunt at idle, 50 shades had this as I fitted it lower down because of how long the wires are. When I moved the lambda up to the original position after trying everything else I could think of (by the turbo) the hunting stopped.

[griston64]

Originally Posted by 360beast

I wouldn't fit the lambda in the bottom bung as it will hunt at idle, 50 shades had this as I fitted it lower down because of how long the wires are. When I moved the lambda up to the original position after trying everything else I could think of (by the turbo) the hunting stopped.

It comes out the factory in the top bung so makes sense to put it there

[360beast]

Originally Posted by griston64

It comes out the factory in the top bung so makes sense to put it there

Only reason I did was because the lambda was for a non turbo (which 50 shades started out as) which goes in the cat so I thought ah seeing as it's got so much wire rather than bunch it up in the engine bay I'll stick it in the bottom location... I ended u running the excess wire around the strut tower twice after moving it to by the turbo.

[Rversteeg]

The TPS actually has two switches, one closing at idle to ground and the other closing at full throttle. The last series 940 LPT/HPT only uses the idle switch, not sure about earlier models.
The full throttle switch is used for "high power system (accessory)", I guess the Turbo+ system. In my 940LPT this switch is not connected. Could it be it has connected in some way by the previous owner? What happens if you disconnect the red/white wire?
The fault could also be in the TPS itself, grounding the idle switch at full throttle.
Worth a try before looking for a replacement ECU.

[Forrest]

I would suggest that the most likely cause of idle problems is a vacuum or air leak. Coming off WOT the engine will be running very lean. If extra air is getting into the system the lambda probe will detect a super lean condition and the ECU will increase fuel to the engine to compensate. This will in turn increase idle speed. I think the car has to be stationary with the throttle closed for a few seconds before the ECU will recalibrate idle.

In my experience a prime location for a hidden vacuum leak which affects idle is the turbo bypass valve membrane. If you ever get any mysterious stalling coming off throttle and straight onto the brakes definitely take a look at this. A smoke test is unlikely to detect this as it is all internal and it only takes a very small hole to mess things up.

If you have adjusted the TPS have you followed the correct method to do this precisely? This involves using an accurate tachometer and feeler gauges. Unfortunately, people tend to mess with the TPS as a means of compensating for other faults. It should be one of the last things you check when everything else is within spec.

Good luck getting to the bottom of the problem.