Hi all,

I'm trying to figure out what is causing fault codes P0172 and P0175. I have changed my coolant temp sensor as part of changing the thermostat housing as I had a bad coolant leak but I now have these fault codes. I had a rough idle and engine vibration and rich running which is why I replaced the housing incase the coolant leak or a faulty temp sensor was the cause. Even after changing this, I still have an engine vibration at about 1400 to 1800 RPM. When I drive the car over that rev range there is not engine vibration and the car is powerful and quick. It just runs rough between those revs and is now throwing 'Restricted Performance' as soon as I start the car and the car is running really rough at idle or when started cold.

I'm now checking my MAF sensor incase the issue is being caused by that. Dirty or faulty perhaps.

However, I don't know what readings I should be expecting to see. The readings I've got so far, at idle and once the car has warmed up to operating temperature, seem to fluctuate. I'm not sure if that is how it should operate. I would expect that the readings would fluctuate a little but the readings I'm getting seem to vary quite a lot and fluctuate at the same time that the revs fluctuate.

Can any of you guys comment on what mass air flow rate (g/second or lb/minute) readings you are getting when your cars are running at idle and at temp? And do you see any fluctuation or a constant reading?

I have captured the live MAF readings data which I will upload once I get them off my tablet.

Thanks,

Chris

 

With the codes the car stores lots of "freeze frame" data to help you see why - things like engine load, speed, rpm. Might be good clues!

 

Thanks JagV8. I'll grab the Freeze Frame data logs as well.

The below are some of the readings when the car is idleing. You can see that the RPMs are clearly jumping around along with the MAF readings.

Thanks,

Chris


RPM versus MAF readings at idle and when car is fully warmed up.


RPM, Coolant temp, MAF and Air Intake Temp readings at idle and fully warmed up.


RPM, Coolant temp, MAF and Air Intake Temp at idle and warmed up.

Now going to get the MAF out and do some voltage readings before in case this is a connection problem.

Thanks,

Chris

 

Hi JagV8,

Here is the freeze frame data whilst driving in traffic.


Freeze frame data


Thanks,

Chris

 

Is that freeze frame when the DTCs flagged? Doesn't look to be.

 

It's currently got 'Restricted Performance' on the dash but no fault codes or DTC's pending nor stored. The EML is also off at the moment but the runs rough at idle and vibrates at the same RPM as before.

 

If you cleared the codes you may now be in Catch-22 where the OBD monitors cannot set and so most codes are suppressed.

If so, diagnosis gets tough and you may need to go back to guessing e.g, try a smoke test, swap parts, etc.

But do bear in mind that by law many codes can only be flagged after 2 warm ups.

 

Thanks JagV8.

I've been looking through the MAF readings again and I'm pretty sure my readings are way too low.

I've done some calculations and it seems that I should be getting approx 4.2 G/sec at 500 RPM.

Now my car does not idle at 500 RPMs. So at 648 RPM (which my car very roughly idles at) I am getting 4.69 and I should be getting more like 5.4 G/sec.

At 1260 RPM I get 5.8 G/s and should be getting approx 10.5 G/s.

And at 1480 RPM I'm getting 7.2 G/s and should be seeing 12.4 G/s.

So I'm going to try cleaning the MAF first and see what that does and I'll check the connector on the MAF as I've read some owners having issues with the connector causing rich running conditions due to inconsistent connectivity.

Thanks,

Chris

 

i was informed a while back the 4.2 STR and XJR MAF range at idle is between 3 and 5 G/S
idle being 650rm . I Imagine the NA will be the same at idle

 

Hi Chris,

I think you may be missing the diffence in units in which MAFS readings can be expressed. From your freeze frame, your MAFS reading was 4.66 pounds-per-minute (lbs/min) at 1975.5 revolutions-per-minute (rpm). This is equal to 2113.74 grams-per-minute (g/min) or 35.229 grams-per-second (g/sec). My impression is that this is plausible and not low to any significant degree.

However, your graph of the MAFS signal certainly appears erratic. I assume you have cleaned the MAFS/IATS elements and electrical connector? Have you cleaned the ground point referenced by the ECM, on the inner fender inside the right engine bay compartment where the cabin microfilter and ECM electrical connector are located?

I have to get to bed, but I note that your STFTs are Lean and your LTFTs are Rich. I'm pondering these numbers while viewing your O2 sensor readings.

Restricted Performance with no DTCs could suggest an electronic issue somewhere. Does your scan tool have the ability to view Network (U-prefix) and Chassis (C-prefix) codes? I also like JagV8's suggestion that you may be in a Catch-22 until DTCs recur.

Cheers,

Don

 

Thanks Don and Datsports. I haven't yet had a chance to look at this further. The car now is sat on the drive as it stalls after starting and runs very rough up until it stalls. It will only run for about 2 minutes and then stalls. Going to look at this today and hopefully it will be something simple.

 

Good hunting

 

If you can get it properly hot (by driving) i.e. so the cats are hot, see what the trims (especially LTFTs) are at idle then rev to about 2500 and see what they are then.

If the LTFTs start quite big but drop at 2500 you've an air leak and when bad enough you get stalling. The MAF would then not be likely to be #1 suspect.

 

I finally got to look at this problem. Tested the MAF voltages and continuity and it all seems to be fine. It has good ground and the right voltages.

So I cleaned the MAF sensor and also cleaned the connectors with electrical contact cleaner.

Started the car and it ran rough as normal and displayed the 'Restricted Performance' and no EML. After about 5 minutes of running at idle the 'Restricted Performance' disappeared and the car ran perfectly. Before the clean and before these problems you could feel the engine at idle. The engine was now smooth at idle and you could see that the engine was not vibrating. It was running much smoother and almost completely silent when in the car.

I took it for a drive and it all seemed ok. I drove it for about 30 or 40 minutes of mixed driving. The car didn't hesitate and acceleration was immediate.

I had no codes and the 'Restricted Performance' warning was off.

I went out in the car this morning and as soon as I started the car the 'Restricted Performance' displayed again with no fault codes. The EML was off. I scanned the car for codes and it showed P0175 and P0172 as pending codes. I cleared them and the RP disappeared for a few seconds but then comes back.

So I drove it about 10 miles and the RP stayed on all the time and drove really rough. Stopped the car and went shopping for a few minutes. Restarted the car and no RP warning. It was off.

The car then ran perfectly and was smooth and quite again. I drove the car for about 20 minutes and still no RP warning. I got the car home and switched it off and then back on again and no RP warnings.

It looks like the car is now throwing the RP when the car is started when cold and until the car has reached full temperature. It was throwing the RP all the time before I cleaned the MAF. Once it's at temp it looks like the RP goes off and the car runs perfectly.

 

Once it's at temp it looks like the RP goes off and the car runs perfectly.
Sounds like me most mornings when i wake up

 

So these are the readings from the car for fuel trims. This is when the car is fully at temp.


Fuel trims at idle


Fuel trims at 2500 RPM - the drop off at 62 is when I released the throttle.

 

Yes it's looking that way WIngrider.

 

Thing is that by clearing the codes you've cleared the OBD monitors when what you want is to see if those rich codes flag and if so see why.

Clearing codes is stopping you getting useful data.

Give it a chance to learn what's happening, and see if all the OBD monitors manage to set.

Then give it a chance to flag codes.

 

Thanks JagV8.

So I need to let it throw the fault codes and then see what the fuel trims are whilst the codes are not pending? Is that right?

 

The car is definitely running much better now that I have cleaned the MAF.

Whilst I was there I did a vacuum leak check (using carb cleaner). I sprayed all over the air intake tubes/connections, air filter housing, throttle, egr, brake booster lines, oil return lines, injectors, all around the manifold and the RPM did not change at all. I've also done a smoke test and found no leaks.