Hi, Cool engine runs rich, no good for the wallet, engine or MOT.
Hoggy

 

As you have VCDS, compare the coolant temperature seen by the engine ECU with the temperature shown by the dash/49c.

If it is actually running cold, it is probably the thermostat.

 

Thanks David, I'll switch the coolant sensor over in the morning and take it out for a 10-15 min stretch and measure the temps via VCDS to rule it out.

It'll be a bit of a pain if it's the thermostat as I'm off work until Monday but won't have time to get it, do it, and then get the retest before I'm back at work ☹

Side note: is 49c not the same temp the ECU sees? I thought 49c was used to rule out a dodgy dashpod because it's "always" correct (unless the sensor is dodgy)?

 

Hi, When engine is warm/hot, A/Con OFF while stationary, hold revs at 2k for 5 mins or so, if coolant temp rises >82 replace thermostat & use OEM 87 degree stat part No.050121113C
Hoggy

 

Thanks again, 87 degree thermostat replacement was on the list of things to do while the cambelt was being changed but seems like it might be being done a bit sooner...

Edit: just looking through the service history - the thermostat was changed in 2019 at 74k miles, coolant temp sensor in 2017 at 71k, car is currently on 75k, is a genuine thermostat likely to fail that quickly? (1,000 miles?)

Edit 2: after a bit more Googling, someone else on the forum seems to have had a similar issue (though their HC was high too) and fixed with a new temp sensor (MOT Failure on Emissions).

Will still explore all options, but hoping for the easiest solution 🤣

 

Hi, Genuine OEM Coolant temp sensor Earley Square, 078919501B. Later round, 059919501A
Hoggy.

 

How have you decided the coolant sensor is at fault? You havent scanned it?

Lean is more air less fuel eg > 1 lambda (14.7:1 AFR)

Rich is more fuel less air eg < 1 lambda (14.7:1)

So a faulty coolant sensor, egt or maf will run default fueling 0.75 lambda (11:1 AFR) you'll get some wafts of black smoke unburnt fuel, and knock out your O2 sensors.

Vacuum leaks will draw in un-metered air, which will mean the o2 sensor will see more air versus fuel > 1 lambda say (15.5:1 AFR) that is lean.

If you have VCDS clear all the faults, even if there isn't any and watch the Short Term and Long Term Fuel Trims, to see if you have a big air leak.


Do you have a stock Catalytic Converter? Or a sports car or decat fitted,?

I can see your lambda is 0.95 - 0.96 but that isn't lean that is rich. Its actually lovely for the engine, just bad for the environment...

Why don't you log the car, to see what the ECU is saying the lambda is? Is this a BAM or APX ?

I would still look for air leaks maybe the ecu is seeing lean and compensating by adding fuel? You won't get a too rich issue for 0.95 lambda, a decent map and supporting hardware will go to 0.85 lambda (12.5:1 AFR)

 

How have you decided the coolant sensor is at fault? You havent scanned it?

Thanks for the info, Stuart! The engine is a BAM with a stock catalytic converter.

I figured it was a coolant temp sensor fault because there's been a few threads online with people having the same MOT issue & low coolant temps, and changing that seemed to have fixed the emissions.

I also found a thread here where someone had similar issues and changed their sparks back to OE ones and once done, the excess fueling/CO out of the exhaust was no longer an issue so I changed my sparks back to the standard Audi ones today too. I forgot to mention that the BKR7E's that I mentioned in my original post were gapped to .7, so that could have been one of my issues?

After replacing those and the coolant sensor, I held the revs @ 2700rpm logging blocks 001 / 031 and 034 on 01 Engine - the engine coolant temp got to ~75 degrees, egt(?) was ~500 degrees and lambda readings fluctuated around 1.008 and 0.992, so that seems to be a improvement?

I've checked all of the PCV piping today and nothing is split - one pipe is a little spongy and will probably need replacing soon, but it doesn't compress when revving. I didn't get a chance to log the short term/long term fuel trims, as you suggested though.

Annoyingly, I just left to take the car for a run to get it a bit hotter & the Oil Pressure light flashed up 3 times in red, on the dash, so I had to bail out. I ran a few quick tests with VCDS & it's stating "Oil Pr .2 <min" until just over 1750rpm, then says "Oil Pr .2 OK" to anything above that - I'm assuming this won't have anything to do with my above issues?

It's not throwing any fault codes up which I find unusual, but now means I probably have to get the sump off and replace the pick up pipe / sensor just to be on the safe side before I'm able to drive it to the test station again? 😩

 

Revving a car on a driveway will not get much in the way of EGTs as there is no load... you should log it whilst booting it in a straight road, you will expect to see more like 800 - 900 egt on boost.

Sorry, I didn't notice, your original low reading coolant sensor.... I got a bit lost in factual diagnosis and speculation

 

Sorry, I didn't notice, your original low reading coolant sensor.... I got a bit lost in factual diagnosis and speculation

No worries, it's useful information to know either way - planning on a remap once she's tip-top!

Revving a car on a driveway will not get much in the way of EGTs as there is no load... you should log it whilst booting it in a straight road, you will expect to see more like 800 - 900 egt on boost.

I would have taken it for a short spin but the oil pressure light came on just as I was leaving my work's yard. Lambda seems stable @ 1.008 / 0.992, so I can only hope that changing back to PFR6Q fixed the carbon monoxide levels - I'm going to go for the retest in the next 2 weeks & will update here.

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In the meantime, I've got to sort this oil pressure light before driving anywhere, so ordered all this:

06A 919 081 J - Oil pressure sensor
058 145 757 A - Turbo pick up gasket
06A 115 561 B - Oil Filter
N 0282222 - Pick up pipe O-ring
N 90813202 - Sump plug
06B115251 - New pick up pip (just in case)
(& need to pick up some good RTV silicone)

Because I can't see any reference to the sensor being changed or the pick up pipe being cleaned over the car's 74k mile/22 year history, so I'm going to pull the sensor and test it (similarly to this from "Larcs Workshop") & might as well drop the oil/clean the pick up in the same breath, though I'll have to pick up a oil pressure test kit though if this doesn't help. The car was sat for a few months before I bought it and was only driven a couple of hundred miles every year for the past few year & the oil dipstick pipe was broken, then has again been sat for ~5 months, so I assume some gunk/sludge/plastic could be blocking the pickup pipe... hopefully (I'm hoping it's not the cluster/anything more serious).

 

Just thought I'd resolve this thread:

Booked the retest for this morning @ 8am, but on my (test) drive home last night I noticed the coolant temps dropped from 90°c to 75°c, and even went down to 50°c at some points - scanned her with VCDS and was showing:

• P1296 / 17704 - Error in Mapped Cooling System (usually temp sensor or thermostat).

Which had never come up prior to replacing the coolant temp sensor, so I got a mate to pop around & we pulled a good thermostat off of my old Golf 1.8t as was too late to find a new one anywhere & fitted it to the TT - engine stays between 91/99°c on 49C & needle stays at 90°c on the dashpod 🙌

The thermostat we removed wasn't visibly stuck open, but was it definitely leaking coolant from the engine when it should have been closed & left a huge puddle under the car after replacing it. I did this on the Golf before, but forgot how easy of a job it was - it could have easily been done in 15 minutes (or so) if it wasn't dark & I was just fitting a new unit 😆

The MOT retest passed with flying colours - gave the car a good 20 mile run before hand, and seemed to get the EGT up to temp on VCDS, but tester didn't give me a new printout of the emissions.

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To summerise the whole thread:

• Changed spark plugs back to OEM (NGK PFR6Q) - unsure if this had a effect on emissions.
• Replaced coolant temp sensor (059 919 501 A) - seemed to stabilise the lambda reading to 1.008 on VCDS.
• Replaced thermostat (050 121 113 C) - seems to have fixed the carbon monoxide (CO) output of 1.3%.

An added benefit of fixing this means I was getting a lovely 42mpg on the dual carriage way this morning, not bad for a 225... especially in rush hour!

Next step is to source a new genuine thermostat/coolant temp sensor, as they were just swapped as a temporary fix.

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edit: forgot to say: I changed the oil pressure switch on Saturday & that resolved my oil pressure warning - I haven't dropped the sump/changed the oil/cleaned the pick up pipe yet as I didn't have time, but do have all the parts, so it's definitely a job for the near future.