Trying to understand fuel trim from rich running

reFRence

Active Member
Jan 23, 2022
21
4
So I've had this car almost half a year and ever since I got it it's been running pretty rich (evidenced by strong petrol smell).
I've been trying to find the issue for a while now. Initially I thought it was related to the decat, but I learned that the decat would only affect the downstream sensor reading - giving a cat below efficiency threshold code and a check engine light, but on most cars, not affecting fuel trim (is this true for 1.8Ts)?
I've changed my MAF as the car was initially almost dying on cold starts and had to be revved to be kept alive - the new Bosch MAF solved this issue.
I've also changed the PCV system as the hoses above the oil filter housing were rotten and the petrol smell was getting into the cabin.
I can't find any leaks although I can hear hissing nearby the hose leading from the intercooler to throttle body, the clamp on the intercooler side had come loose so I retightened that today - hissing still present.

More to the point, I drove around today while watching my short term fuel trim and the numbers seem very weird:
-While cruising (foot off throttle), the trim stayed in the positive between 0 and 10%.
-It went negative when accelerating, then positive when easing off the gas.
So far these seem normal to me, here's what I found weird:
-When stopped at lights (foot on brake, car in netral), the fuel trim would jump above 10%, sometimes as high as 25%, and only coming down very slowly - jumping between say; ~7% and ~18%. Doesn't this mean the engine is dumping fuel as if under hard acceleration?
-When stopped on my driveway, (handbrake, car in neutral), the fuel trim would stay between -10% and 10%.
-When cold it was just flipping between -1.6% and 1.6%, but I could smell petrol intermittently.

Has anyone seen this pattern before? I'm pretty confused.
Sidenote: I haven't changed the N75 or the O2, the EVAP Purge Valve is fine and spark plugs have been changed for NGK ones recently.

Another question; I've heard that rich condition is sometimes caused by the thermostat (don't really understand this as of right now, but), on my OBDII as it was reading an Engine Coolant Temp of 98, the temperature gauge on the dash was sitting at 90. Is the gauge supposed to indicate the Coolant Temp or something else?
The reading rose with the gauge as it was warming up, but the gauge didn't show any values past 90.

If I missed out any information you need to get a better picture of the situation please let me know, thanks.
 

Alexis27

Active Member
Dec 20, 2009
1,828
294
Manchester
I had a petrol smell and it was just the fuel pump wasn't screwed down tight under the back seats. Maybe worth checking?
 

reFRence

Active Member
Jan 23, 2022
21
4
I had a petrol smell and it was just the fuel pump wasn't screwed down tight under the back seats. Maybe worth checking?
My petrol smell is coming from the exhaust, and the fuel trim readings clearly show out of spec air/fuel ratio compensation. I'll give it a check anyway to be sure.
 

reFRence

Active Member
Jan 23, 2022
21
4
lamba sensor ? but your OBDII should pick that up unless intermittent fault
The reader gives me an Amperage value for the lambda sensor and it is fluctuating between positive and negative so I think at least it's working to some degree. What would you expect to see if it was not working?
 

reFRence

Active Member
Jan 23, 2022
21
4
High fuel trim numbers can also be caused by a vacuum leak. The upstream sensor sees too much oxygen in the exhaust and it cranks up the fuel to compensate.
So if there's a vacuum leak, the sensor would see too much oxygen and ECU adds fuel - would it over-compensate? I've attained the Long Term Fuel Trim value of -7.8%.
 

Nam-uk

Active Member
May 11, 2011
879
183
lancashire.
The reader gives me an Amperage value for the lambda sensor and it is fluctuating between positive and negative so I think at least it's working to some degree. What would you expect to see if it was not working?
not using the reader a smell of bad eggs and high fuel usage that's a good sign of intermittent and full fail it may be ok just the wire issues on the lamba, only time i have been successful with a reader is if they worked or failed even finding fault, you may have to trail and error replace parts seeing above posts
 

mty12345

Active Member
Jun 17, 2011
3,538
352
bristol
You're correct in thinking that the decat wouldnt affect the mixture. !00% of the fueling is decided by the first sensor before the cat

Short term fuel trim is usually all over the place to some extent, the long term numbers are more useful in finding issues.

I would start by sorting the leak around the intercooler. If air is being measured by the MAF and then goes missing before the engine under boost, the mixture will be too rich and the lambda will have to compensate. But then at idle (vacuum conditions) the same leak would have the opposite effect and cause unmeasured air the get in, causing a lean mixture
 
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