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Lambda Sensor Voltage?
briarswood57 - 14/1/21 at 07:15 PM

Evening all, after a bit of information which I am sure someone on here will know.

After a year of procrastination I am trying to get my Saab 9-3 B207 engine to run properly. Broadly the engine runs fine when under load but has a persistent misfire at idle - it throws the P0300 Random Misfire on the ODB2. I am gradually working my way through the seemingly endless list of potential causes and am looking for some information about the lambda 1 voltage.

When I measure the lambda 1 voltage on the ODB2 reader it seems to fluctuate between 0.06v and 0.8v at idle - my question - is this what I should expect or does this point to a faulty sensor.

Any help info much appreciated.

Thanks

coyoteboy - 14/1/21 at 07:36 PM

Yes, assuming it's a narrowband sensor (older type) then yes, it fluctuates between 0 and 1v. At 0.5v it's stoic. It will fluctuate one to the other at about 1-2 cycles a second if the ecu is properly controlling mixture. Many cars don't do that at idle. It could still be an error though, if the heater has failed or the sensor is too sluggish to respond.

briarswood57 - 14/1/21 at 07:53 PM

thanks for that - yea it seems to fluctuate 1-2 times per second so that would concur. I'll perhaps try and get a cheapo off ebay and see if that makes any difference before I start chasing the myriad of other potential causes.

Cheers.

Replicar328 - 14/1/21 at 11:09 PM

I'm not an expert but I've been researching how ECUs work and I believe that the two sensors which provide the main information to control the on time for the injectors are the MAF mass air flow sensor just after the air filter and the MAP sensor measuring manifold pressure. Information on the detail to distinguish the purpose is sketchy but my view is that the MAP sensor largely determines the fuel demand when the throttle butterfly is closed or nearly closed because the pressure in the manifold drops whereas the MAF sensor is better for controlling the fuel mix when the flow is higher as the throttle is open. The lambda sensor is adding feedback to the system to finely adjust the fuel mix but the engine should continue to run okay although it may be running lean or rich.

In this case I suspect the MAP sensor. The MAF sensor cover for a malfunctioning MAP sensor to some extent, but with relatively low air flow it's not so accurate.

I'd welcome feedback on this - I was brought up on Webers, Solex and SU where you could see and understand what was going on!

Slimy38 - 15/1/21 at 08:16 AM

I used to own a Saab 9-3 turbo, fantastic car but it did have some niggles. An intermittent misfire was one of them. They are supremely sensitive to the coilpacks used, a lot of aftermarket ones cause no end of issues.

Check your coilpacks are genuine Mitsubishi, like these;

https://www.ebay.co.uk/itm/152235562119

About £40 a pack is about the right price. Don't go for anything other than those ones. Along with that, also check the plugs. Again, lots of options out there but I only ever managed to get the proper NGK plugs to work properly for any length of time. PFR6T-10G.

But even with the genuine packs and plugs, they still 'hiccup' every so often. It's not the smoothest idle I've ever seen from an engine. It shouldn't bring up an engine code though.

If you want some more ideas, drop an email to Karl at Noobtune, http://noobtune.co.uk/. He lives and breathes Saab engines and will be able to suggest options.

briarswood57 - 15/1/21 at 01:01 PM

quote:
Originally posted by Replicar328
I'm not an expert but I've been researching how ECUs work and I believe that the two sensors which provide the main information to control the on time for the injectors are the MAF mass air flow sensor just after the air filter and the MAP sensor measuring manifold pressure. Information on the detail to distinguish the purpose is sketchy but my view is that the MAP sensor largely determines the fuel demand when the throttle butterfly is closed or nearly closed because the pressure in the manifold drops whereas the MAF sensor is better for controlling the fuel mix when the flow is higher as the throttle is open. The lambda sensor is adding feedback to the system to finely adjust the fuel mix but the engine should continue to run okay although it may be running lean or rich.

In this case I suspect the MAP sensor. The MAF sensor cover for a malfunctioning MAP sensor to some extent, but with relatively low air flow it's not so accurate.

I'd welcome feedback on this - I was brought up on Webers, Solex and SU where you could see and understand what was going on!


Thanks for the info - that sort of makes sense that the Lambda is making smaller adjustments so it might be a MAP / MAF sensor. Its also good feedback from slimey38 about the coil packs and plugs. I had previously changed the plugs but pretty sure they were not the NGK ones as ECP were out of stock. I have also swapped the coil packs for some used mitsubishi ones but no difference.

I'll get a new set of plugs, work my way round the sensors and then if no difference I'll probably bite the bullet and go for a brand-new set of coils. At this rate I'll have spent more on plugs and sensors that the car actually cost :-)

Thanks for the help.

Replicar328 - 15/1/21 at 03:25 PM

Thank me when you've solved the problem and my suggestion showed you the way!

Slimy38's recommendation to use only the correct coil packs and plugs is good. Checking the condition of the plugs is the lowest cost and simplest check to make first. In my view, if the coil packs are not doing their job I'd be surprised if they suddenly recover at higher engine speeds, but if the plugs are not sparking properly you may just not notice it at higher engine speeds - so the plugs that are mis-firing may be blacker than ones working fine. Cleaning plugs with a wire brush and setting the spark gap is worth doing if they are out.

If the misfiring at idle goes away when the engine is warm it may be that moisture is allowing the high voltage spark is leaking to ground due to moisture - a common problem in winter, possibly a problem with aftermarket coil packs

Have you tried taking one plug connection off (briefly) at a time to check the difference? It may point to one cylinder only. The lambda sensor will be seeing too much oxygen so it will try to enrich the mixture and, failing to achieve lower oxygen, could get confused. Watch out to avoid a nasty HV bite!

Above all else, I'd try hard to find the source of the problem before incurring the expense of buying replacement parts. Getting your friendly local garage to put it on their analyser might be the quickest and cheapest option and these days, although I've never tried, I've noticed that they aren't too expensive to buy. I might have to buy one as I've stripped down my MX5 including the wiring and I do wonder if I will ever be able to put it all back together correctly....

briarswood57 - 15/1/21 at 05:34 PM

quote:
Originally posted by Replicar328
Thank me when you've solved the problem and my suggestion showed you the way!

Slimy38's recommendation to use only the correct coil packs and plugs is good. Checking the condition of the plugs is the lowest cost and simplest check to make first. In my view, if the coil packs are not doing their job I'd be surprised if they suddenly recover at higher engine speeds, but if the plugs are not sparking properly you may just not notice it at higher engine speeds - so the plugs that are mis-firing may be blacker than ones working fine. Cleaning plugs with a wire brush and setting the spark gap is worth doing if they are out.

If the misfiring at idle goes away when the engine is warm it may be that moisture is allowing the high voltage spark is leaking to ground due to moisture - a common problem in winter, possibly a problem with aftermarket coil packs

Have you tried taking one plug connection off (briefly) at a time to check the difference? It may point to one cylinder only. The lambda sensor will be seeing too much oxygen so it will try to enrich the mixture and, failing to achieve lower oxygen, could get confused. Watch out to avoid a nasty HV bite!

Above all else, I'd try hard to find the source of the problem before incurring the expense of buying replacement parts. Getting your friendly local garage to put it on their analyser might be the quickest and cheapest option and these days, although I've never tried, I've noticed that they aren't too expensive to buy. I might have to buy one as I've stripped down my MX5 including the wiring and I do wonder if I will ever be able to put it all back together correctly....


You make some valid points and have just reminded me - when I do a cold start the engine will idle relatively smoothly at about 1100rpm for the first 35-40 seconds then when the idle speed drops to 900 rpm it starts to sound rough and will eventually throw the P0300 random misfire error.