Negative Fuel Trims, hesitation during boost

Maurice EVO · Post by **Maurice EVO** » 12 Sep 2017, 04:01

Hi,
A short intro on myself, in case I haven’t done this already. I’m Maurice Comans and I’m from The Netherlands and I’m a proud owner of a Volvo V50 T5 AWD from 2006 with the B5254T3 engine.
I’m owning this car for 2,5 years now, has now 170000 KM on the clock and since the time I bought it was always a bit hesitant under boost, which means I can rev easily until 5500 – 6000 RPM however along the way you notice small hick up’s in acceleration.

I’ve went in total 3 times to the dealer with this, while the car was still in warranty, and they said this was perfectly normal and all T5 engines were doing so, of course no CEL is lit. When I told them I find this strange as this engine is supposed to have a flat torque curve they came up with the second “lie”. Now it was the Haldex system making adjustments in the power delivery towards the 4 wheels as the torque is “so enormous” that the Haldex system is adjusting for this and that can be felt by small hesitations in the drivetrain. Despite the fact that the car definitely has some kick I find it really hard to believe that 320NM torque is more than 2 high quality front tires can cope with (during dry conditions) and that the 2 back wheels are taking some of the torque.

Now of course the car is (long time) outside warranty so they are not doing anything about it and I let it be until 2 months ago. 2 months ago I treated myself with a little present and ordered a car specific tune to increase the BHP and most importantly some additional torque. Despite the fact that the power increase is definitely there, the hesitation became even worse.
So in order to pinpoint the cause I’ve hooked up my OBDII scanner and went for a spin. Before the run I have a LTFT of around -4 (so negative, rich conditions) and STFT is when idling with warm engine 0 with sometimes a lean condition of +0,78 STFT but that only happens like once every minute during idle. So I don’t see any reason for vacuum leak here.
When driving normally (without making a lot of boost) the STFT stays close to 0 and LTFT is going up a little towards zero. When you are making boost (let’s say you want to overtake a car) the STFT goes a little negative to around -4 or -5 and, depending how long you request some boost, LTFT goes down a bit again to around -4 or -5 so all makes “perfect sense” that LTFT is around 4 or 5 negative.
However when your making full boost (WOT) you see the STFT go negative fast until it maxes out at -25% and the moment that happens you feel the hesitation in acceleration. This happens due to, I guess, retarding ignition and lower boost levels and this process continuous until you stop with WOT. So this means the first let’s say 1500-2000 RPM you have full power as STFT is still capable of leaning the mixture sufficiently however after that it starts with the small hesitations which comes every 500 RPM or so.
Take your foot of the throttle and STFT instantly goes back to (more or less) 0 readings and if you drive normally (no WOT) the STFT will rarely go above -6 or -7 as you by then are already making quite some boost. So the issue is only for WOT and keeping your foot down the whole time.
This to me sounds like a boost leak (past turbo, before Throttle Body) so I’m going to change the hoses soon with new clamps to rule this out. If, however, the problem remains what would be my next suspect, is it Oxygen sensor, boost sensor, MAF sensor?

Looking forward to receive some of your input!

Cheers!

Maurice

Post by **mrbrian200** » 12 Sep 2017, 23:37

Also put the throttle body on your potential list of things to check. This uses the Bosch type with a stepper motor and a couple nylon reduction gears. The internal mechanism can wear resulting in the throttle plate getting sticky at certain positions. Cleaning the throttle body won't solve it: the problem is wear of internal parts. You have to examine live data under varying driving conditions to see if there is a spot or two where the throttle plate position looks to be getting hung up at. On my '06 2.5t the ECU never set a DTC for the actual throttle body. However the ECU was showing both rich or lean fuel trims at different times. In my case bad enough to set fuel trim DTCs and occasionally a DTC for the MAF sensor (which wasn't the problem, it was the throttle body). Fortunately these Bosch type throttle bodies are plug and play/no programming necessary when replacing them.

Maurice EVO · Post by **Maurice EVO** » 13 Sep 2017, 00:43

Hi,

Thanks for your input, honestly I was not having the TB on my list (yet) as I considered this as being one of the last items to fail.
The input I've seen from OBDII data was not directly indicating that throttle plate remained stuck in certain areas however that does not imply that the TB can be ruled out.

However please bear in mind I actually (close to) never see a positive fuel trim, only when I close throttle plate. I'm not sure if the TB will actually only result into negative fuel trims assuming that the TB might become stuck in situation where you actually require less airflow however TB remains open to wide resulting in positive Fuel Trim (lean conditions).

But again thanks for your input, I hope the new tubing will sort my issues and if not I'll make sure to check the TB as well.

Post by **jimmy57** » 13 Sep 2017, 20:06

The intake system from turbo through intercooler up to throttle is very suspect with what you listed. Under boost the air that has been through MAF sensor and has been taken into computations for the corresponding amount of fuel, has leaked away and results in rich indication at oxygen sensor. At low throttle the pressure is very low and leak will reduce to negligible amounts. The MAF value being high also affects turbocharger control valve signal as the ECM expects a "mapped" value range and the leak causing low pressure at boost pressure/intake air temp sensor on the intercooler makes for struggles in the software. All this in addition to knock control and other things you list. You don't mention idling fuel trim, is that in normal range?
Hesitations, surging and likely everything else will go away with air leak discovery and correction. There is a fitting on the turbo inlet with a hose you could trace back to crankcase vent system. That fitting has been known to get stuck open as it is a check valve. It has been a long time since I saw one bad and I think it creates idle fuel trim issues with no lower part load or upper part load trim number irregularities.

Maurice EVO · Post by **Maurice EVO** » 14 Sep 2017, 01:06

@ jimmy57,

First of all I appreciate your feedback.
More or less your findings are equal to mine w.r.t. the boost leak I'm most likely having somewhere between Turbo and Throttle Body.

I've read a whole lot on the check value you are mentioning as well however I also, at least I thought, remember this causing issues with Fuel Trims when idling with warm engine. However when the new tubing is not solving the issue for me this "Check valve" (need to check part # again) is absolutely my next option as this is also quite inexpensive to change out.

To answer your question, I have a STFT of 0,00 and occasionally +0,78 (so indicating lean conditions) when idling with warm engine with all other parameters (which I can extract from OBDII) as far as I know being "normal" :

Fuel Pressure: 3 bar (43 PSI, 300 kPa)
MAF readings: between 2.9 - 3.3 g/s
Engine Load: 10-11%
Timing advance: 9%
RPM: 770
Second oxygen sensor: around 0.45 volts (sadly so I cannot read 1st oxygen sensor)

So therefore I also stated in my first posting that I'm comfortable ruling out vacuum leaks as my STFT is dead on 0,00% with occasionally +0,78% (like once a minute). I've had the car idling for 15 minutes and I watched the numbers so it was not that the STFT during idle was only done for a minute or so, it was that reading all the time.

I've ordered the hoses and new clamps and these should arrive soon, after that I'll mount them on the car and I'll let you know the outcome (lets hope it is a positive one!).

Again thanks for all your (and others) feedback so far, really appreciated!

Post by **MadeInJapan** » 24 Sep 2017, 07:55

Let us know the outcome- my condition was lean and issue was a cracked hose next to a Check valve.

Post by **mrbrian200** » 26 Sep 2017, 10:22

Your idle speed looks a little high. I'd bet it's a little lean. Once intake leaks and PCV system are ruled out, if these problems persist.. together with the unexplained fuel trim issues I think the case is mounting that you should inspect the TB closely.

Note the TB that was having problems on mine was also a MY06 (on an S60). These generally aren't known to fail, however, it is also entirely possible Bosh had a bad batch of these nylon/plastic gears that are used inside these things.
Mine was having problems at 85k (miles).
These types of molded parts are sensitive to variances in manufacturing such as an injection molding machine that's running a few degrees off or variations of the quality of the pellets that come from suppliers.
A sister had an '02 Jetta which used this same basic design TB made by Bosch, with nearly double the mileage I inspected it by taking the plastic cover off out of curiosity. It's up on top on that car and a 5 minute affair to look inside. At nearly double the mileage the one on the VW showed no sign of this type of wear.

If all else has been ruled out or not suspect...

On the Volvo the TB is mounted down below so you'd need to pull it off the car to inspect the internal mechanism. The little metal clips come off around the plastic cover. Take care not to touch/damage the TPS sensor sweeps. Turn the larger middle gear by hand while observing for any points of resistance or sticking both opening and during 'controlled' gradual closing. On mine the wear was at the last smaller gears that drives the throttle plate. The plastic teeth wore down/rounded off with a tendency to bind and stick rather than mesh and turn cleanly.

: tbgears.JPG (73.13 KiB) Viewed 5203 times

1. Look closely- this is the idle stop. It wasn't returning fully closed (idle fast and lean, on mine about 100-120 RPM high)
2. Turn this by hand and feel for points of resistance or sticking.
3. Gears that are worn are under here. Pull up the large gear to inspect.
4. TPS sensor: don't touch this.

Maurice EVO · Post by **Maurice EVO** » 31 Aug 2018, 04:27

Hi there,
It’s been a while since my last post here, however I’m still facing the same problems as some time ago.
Meanwhile I’ve fitted a new MAP sensor (the one on the outlet of the intercooler), cleaned the MAF sensor with MAF cleaner, and pressure tested the parts coming out of the turbo up until the Throttle Body.
Cleaning the MAF and replacing the MAP sensor didn’t do anything so after that I tried to test the tubing an intercooler to check for leaks.
The way I tested was disconnecting the tubing coming directly out of the turbo (that small hose with a bent in it) and disconnecting the tube going into the Throttle Body (which I’ve also cleaned fully). I’ve plugged the tubing with PVC piping and in one of the piping I fitted a valve used for tires. By doing so I could feed in compressed air from my compressor and I’ve pressure tested until 1 Bar (15 PSI). I didn’t dare to go higher and by doing so destroy the intercooler and I figured that 15 PSI would be more then enough to check for leaks.
However the pressure test did not unveil any leaks though, the pressure remained at 15 psi for a full 10 minutes and I also did not hear any leaks (sound of escaping air). So by doing this I can rule out all the tubing (also that metal one) and the intercooler itself.
So I’m a bit running out of ideas at the moment, I could swap the Oxygen sensors (both of them) but I hate to throw parts at it without knowing if it actually failed or not.
I also had in mind to get myself a DiCe with VIDA so I could check a lot more parameters I currently can but again I’m not to sure this will solve my issue.
Are there any suggestions from you guys, again as stated I’m running rich (negative STFT and LTFT) and therefore I do not expect any vacuum leaks as this would normally result into positive fuel trims (lean mixture).
Thanks again and hopefully you guys and girls can help me out!